Wavetek Signal Generator Type 2407

 This equipment provides coverage from 10KHz to 550MHz via step up and step down push buttons or direct key entry.

It was relatively cheap to buy (under £200 in 2002 when it was 11 years old) considering the build quality and performance.

 The complete user manual can be seen here

 Below is a key to the controls. It's quite user friendly but it needed a handbook to let me know most of the functions. When the power is turned on (button 1) the generator runs through a self test routine which gets more worrying as the equipment ages. I've had two problems so far. One, immediately after buying it and turning it on for the first time, which turned out to be a short-circuit protection diode and the second quite recently which was again the loss of an internal power rail. This was easily fixed although the self test gave a strange fault report which I imagine was because the guys writing the test firmware must have imagined the power supplies would never fail?

The two most frequently used functions are setting the frequency and setting its level.

Direct entry via the keypad "3" is the most useful way of doing either but the up and down arrows "4" can make incremental changes, for example to determine the tuning peak of a circuit.

For example: pressing Freq, 465 then KHz will set the generator to 465KHz. Pressing Lvl, 5 then uV will set the output to 5 microVolts. Once this has been done you can set the output to CW "Mod Off", AM, or FM setting the modulation depth as required. The button marked "5" switches the output on or off and "Rate" sets the modulation to either 400Hz or 1KHz.

All perfectly logical. Also, there's a menu feature which lets you set global parameters such as the display brightness.

A store feature lets you keep routinely used output settings.

 

 Although synthesised generators are jolly stable and accurate they are not always as useful as an old fashioned analogue equipment when aligning a radio.

In the case where one needs to tune a generator to a radio for example an analogue type is just twiddled until a signal appears but a synthesised type often is tricky to use. I've often listened for clicking noises which is the generator locking to its frequency and increasing or decreasing the settings until the clicking gets louder and louder. Once you're hearing fairly loud clicks you can increase the frequency resolution until the clicking changes to a signal, a bit messy.

When I was testing my DST100 receiver the Wavetech broke down. It took me a little time to discover the fact because the receiver has a few intermittent faults. When AM was selected the received signal kept cutting out for brief moments before re-appearing almost instantly, but connecting the generator to an oscilloscope showed it was the Wavetech not the receiver. RF output was OK but switching on modulation resulted in an intermittent RF signal. The manual points to modulation problems being in a specific area, that of the AM/DIV module so that's where I started. After removing two dozen screws the top cover can be detached revealing at the side of the unit the view below.

 

 

 
 The AM/DIV module has two circuit boards mounted in an aluminium sleeve which can be removed after unplugging various RF plumbing, including one at the rear of the unit connecting to a power amplifier module. At each end of the sleeve you need to remove the end plates by taking out their four fixing screws plus unscrewing the nuts securing the RF plugs.

 

Below, pictures of the circuit board carrying a lot of the final RF circuitry. Modulation is carried out using a mathematical process controlled by one of the units two microprocessors, but as with all the circuit boards in the signal generator local power supplies are fitted in order to provide accurate voltages. The top view does have a clue to the fault I'm investigating but the underside view reveals clearly what's happening.

 

 

 Here's the fault: three dry joints at the pins of a power regulator transistor Q905 an MJE253. Although the designers included a hole for fitting a heatsink, this wasn't fitted so after replacing the solder I fitted a small brass bush which should help keep the transistor temperature down a little. In fact, as I tracked down faults later on I found the overheating was due to chemistry. Everyone knows about Mullard AF117 problems but I for one hadn't heard about Motorola transistor problems. Read on....

 

 Back in business after the Wavetech had gone successfully through its diagnostic tests. After using the Wavetech on and off for a week or so I noticed a sizzling noise on the output. I looked again at the repaired output board and found another dry joint and a discoloured resistor. I resoldered the joint and fitted a new resistor. After assembly the sizzling noise was still present. The equipment cannot be tested by usual means so I decided to detach all the circuit boards and apply 18 volts to each. This is the standard supply to all the boards and on each one a 15 volt regulator provided on-board power. Starting with the output board I noticed the new 68 ohm resistor I'd fitted was very dark in colour and sure enough, powering the board showed the resistor was getting very warm. The 18 volt current was over 400mA and across the 68 ohm resistor I measured about 11 volts. This represents a power dissipation of about 1.75watts although the handbook states it's a half watt component so something is wrong. The chief feed from the resistor is the collector of an MRF571 RF transistor and this measured as a pair of diodes on my transistor tester. I substituted a BFR90 for the MRF571 and the resistor ran cool with 11 volts on the transistor collector intead of the previous 4 volts, but after reassembling everything there was still a sizzling noise on the RF output. Maybe the transistor had failed due to a problem with its components and this is still giving trouble?

 

 A little more about this saga. Having diagnosed a faulty drive transistor I eventually decided to splash out and buy a proper MRF571 (see above) because the output wasn't quite right. It was difficult to put one's finger on the problem, but using a scope I determined the RF was jumping around amplitude-wise and frequency-wise. I found a source of the rare device and ordered it. In fact it came as a set of six transistors from Poland although I'll probably never use the other five (but see later). After dismantling the assemblies I fitted the new transistor and the RF problem seemed to have cleared up... or had it?

 

 Above right to left...driver transistor Q900 (note the crossed zeroes which look like eights!), with Q901 bias set for Q900, Q905 current regulator for Q902, and RF output Q902. Q903 sets the bias voltage for Q902. Below Q905 (centre) is a voltage reference diode LM336Z-2.5.

 
   The MRF839F is a variant of an MRF839 fitted to a Type 319-07 base. This is a view of the underside showing the heatsink.

 After replacing the temporary BFR90 with a new MRF571 it turned out the problem was still present albeit not as bad as before. Why was it that sometimes the RF was as clean as a whistle and at other times infuratingly crackly? By experimenting I discovered that any RF output greater than 137.5Mz was faultless but below that I found the RF was sort of wobbly. Time to re-read the repair manual and study the circuit diagrams. This explained the reason for the transition frequency. Above the frequency of 137.5MHz the RF amplifiers, feeding the output socket via a programmable attenuator, are driven directly from a VFO but below that a mixer is involved. This mixer combines a second RF signal of precisely 512MHz with the VFO to derive any frequency from 10KHz to 137.5MHz. This being so, any problem in the mixer circuit or in the generation of the fixed frequency of 512MHz could be responsible for the crackly output.

As the board carrying the mixer was already on the bench I checked this first. Looking at the mixer circuitry (below with shield removed) showed that it was possible for a faulty component in the selection process to result in a bad RF output from the mixer. For example a bad capacitor in the filtered connections from the microprocessor outputs might be responsible so, if the mixer was being turned on/off by noise on the selector line this might show up as a crackle on the mixer output. With difficulty I removed a metal cover from the mixer circuitry but everything seemed to be OK.

Time to study the repair manual circuits once more. These are poorly scanned in my copy making it a time consuming job to identify the parts (not to mention the use of a crossed zero making a nought look like an eight). Before tackling the 512MHz oscillator which is on another circuit board I looked at the RF amplifier transistors. I'd already replaced the driver transistor because it had a leak of a couple of hundred ohms between its collector and base so I looked at the output device. This is Q902, an MRF839F fitted on a heatsink. It has an odd-looking base listed as Type 319-07 and I'd noticed several years ago when I had a problem with crackly RF output that the bias circuitry had been running very hot leaving the Q905 power transistor dry-jointed. I'd resoldered it and for a short time all seemed well, but why had this happened?

The designers of the equipment surely would not make the basic error of not fitting a heatsink to the Q905 transistor? Maybe this overheating is related to my current RF instability problem? My circuit schematic is pretty poor but I eventually reproduced the circuit on a sheet of paper. The manual tells me the bias circuit performs two functions... to set the operating point for the MRF839F and to set its collector current.

 

 I applied an 18 volt supply to the board and measured the various points in the circuit. The RF power transistor collector was sitting at 5.5 volts and I couldn't figure out why, so began to unsolder components. All the capacitors were fine and all the resistors were OK. Maybe the bias setting transistor was duff? I lifted off its connections to base and emitter and found absolutely no change... very odd. At this point the penny dropped. The MRF839F was faulty. With some difficulty I removed the transistor and discovered it had the same fault as the driver transistor. There was a leak between the base and collector. It measured 284 ohms and this resistance didn't look like a diode because its value was exactly the same in both directions when checked with an ohm-meter. The 284 ohm leak was turning on the transistor through self-biasing and pulling down its collector voltage. Q902's resulting collector current of around 300mA through Q905 together with the latter's increased emitter-collector voltage was resulting in a dissipation in Q905 of around 3 watts and because there isn't a heatsink Q905 was getting really hot.

Even though Q902 had a bad leak, the balancing circuitry in the Wavetek was managing to maintain the correct RF output to the attenuator, but was the leak responsible for the crackle in the RF? Perhaps the output from the mixer was lower than that from the straight-through VFO thus pushing up the demands on Q902?

Before proceeding further, I fitted a 2N3866 in place of the MRF839F to test the theory. The collector voltage correctly measured as 12 volts but that type of transistor doesn't fit easily due to space considerations in the metal case for the circuit board and a 2N3866 is anyway not capable of running enough power output.

A search of the Internet revealed the MRF839F was a pretty rare beast with only two far-east suppliers with stock and a UK source quoting over £55 plus postage and VAT. I commenced the ordering process for one of the cheaper options but found that delivery was quoted at 20 to 40 days by airmail. Maybe it's pigeon post? I didn't place the order as I would not be home at the delivery date and I didn't fancy supplying credit card information to a Chinese company I'd never heard of before.

 

 

MRF839F ratings

Vce 16V, Vcb 36V, Veb 3.5V, Ic 0.6A

Dissipation Pd 10W @ 110 deg C

RF out 3W 806 to 960MHz

This and its stablemate the MRF371 had both degraded during their lifetimes by developing a leak between base and collector. Maybe this is due to tin-whiskers which grow from the material inside the case over a long period eventually bridging electrodes?

 Back in the workshop I carefully removed the MRF839F and checked it with a transistor tester... it read "two diodes". Now.. many years ago back in the 1970s I had a really expensive RF power transistor with a similar fault. Our Plessey rep (also a radio ham) had given me a free sample of their latest VHF power transistor... a huge device worth a huge sum of money. I made a 2m linear amplifier from it but during experiments I'd killed it. To cut a long story short I mended it.

Can I repair my MRF839F? As far as measurements are concerned there's a fixed value resistor between the base and collector. This had been the exact same fault with my exotic free sample. My reasoning was that being an NPN device I could connect a negative voltage between base and collector and fuse open the short because the device can tolerate quite a high Vcb without damage. I set up my power supply and connected the positive output to the collector pin and the negative supply to the base... set the current limit to 50mA and gradually increased the supply voltage. All that happened was an increasing current then constant drain of 49mA from 13 to 18 volts. I adjusted the current limit to 500mA and tried again. This time, once past about 12 volts the current dropped then rose again then, as I increased the voltage, it suddenly dropped to zero.

Maybe it worked or maybe I'd fused a connection to an electrode? I connected my transistor tester and it said "NPN transistor", "gain 17"... so I removed the 2N3866 and soldered back the MRF839F. I applied the 18 volt supply and the collector voltage measured 12 volts.

 

Feeling fairly confident I reassembled the Wavetek and turned it on. After a major repair job it's very comforting to see the self-diagnosis to report all is well. I turned on my monitor receiver (Icom IC7000) and set the Wavetek to 137.5MHz. Perfect.. a rock solid carrier at 137.500MHz.

I detuned the Wavetek to 137.4MHz and tuned the receiver... drat.. the rock steady carrier was now warbling. Turning from SSB to AM, I could hear a sort of crackly sound so the duff transistor wasn't the reason for the fault after all. I suppose the Wavetek checks the RF level and cranks up the internal RF levels to compensate for a ropey amplifier transistor so even with a failing part the output is guaranteed.. obviously up to a point.

The noise on the received signal seems slightly less than before, but then again there was never any noise above 137.5MHz, but I'd assumed that the mixing process increased the stress on the MRF879F maybe because of reduced RF input level, but no.. the problem must be in the circuitry associated with the incoming heterodyne frequency (the local oscillator feeding the mixer)? Checking the manual, I see this oscillator runs on a fixed frequency of 512MHz and is derived on the LO/REF board so that's next on the list...

At least the much more complex VFO RF signal is OK because anything above 137.5MHz, where this is used without the 512MHz local oscillator, is rock steady.

The local oscillator is not just a simple crystal oscillator multiplied up because this might result in lots of spurious images (harmonics etc), but instead a 512MHz phase-locked loop oscillator whose output is divided down and referenced to an internal stable source of 10MHz. However, being a phase-locked loop means any solder joint or dodgy component in the loop (or its power supply) might result in an intermittent lock problem. The scratchy crackly noise when listening on AM suggests a dry solder joint or perhaps a bad capacitor. I might add that the problem isn't severe enough to result in loss of lock for long enough for this to be detected, but of course it's not nice to hear a scratchy crackly test signal when aligning a receiver...

There seems to be a couple of options available before removing the circuit board. First I can unscrew the RF cable carrying the 512MHz local oscillator and see if what emerges is crackly and secondly check the internal reference output which is 10MHz and see if that is crackly. So here goes...

Well I pulled down the Wavetek from its shelf and before pulling the LO/REF board, I first checked the 10MHz reference signal which is supplied via a BNC connector on the rear panel and found it was rock solid (in fact it's derived from a 10MHz crystal.. although it might have been a bad component in its circuitry) so next I'll check the LO signal itself. I unscrewed the coaxial copper link carrying the LO signal and decided first to inject a 512MHz signal from my other decent signal generator into the mixer board. I turned on my Marconi TF2008 but I noticed the dial was marked up to only 510MHz or 2MHz short (Sod's law), so instead I set the TF2008 initially to 170.666MHz and then to 256MHz. Both resulted in a cleanish 512MHz AM signal on my Icom monitor receiver although I suspect this receiver similarly uses 512MHz in its circuitry so although I got a clash and some heterodynes the RF output signal was crackle-free. This then clears the output board with its mixer and points definitely to a noisy 512MHz local oscillator. I connected the 512MHz output from the LO/REF board to my monitor receiver and heard similar heterodynes as before, but I could also hear the dreaded background crackling so I switched off, unplugged the module and extracted the LO/REF board shown below. This is neatly laid out but a quick examination revealed no obvious dry joints or burnt parts.

 

 Above, the LO/REF board: Top right is the 512MHz RF output and below this the reference oscillator circuitry. On the left of the board are the phase lock loop logic chips with some microprocessor-driven control circuitry. Below these chips are the three on-board power supply regulators. Turning the board over reveals a large selection of surface-mounted parts, some quite exotic but including a pair of tantalum chip electrolytics.

I then noticed some discolouration around the RF output socket.. shown below. Soldering can often look messy when seen close-up but dry joints, apart from really bad ones, can only be seen with a strong magnifier. That discolouration around the 512MHz output socket looks like a candidate for noise if it's conductive.... but nothing looked unusual when I checked with an ohm meter so I just cleaned it off.. another red herring.

 

 The next step was to power the board and see if there were any clues to crackling. I connected three power supplies, +18V, +8V and -18V. which are stabilized by three regulator chips to +15V, +5V and -15V. Each supply line drew a nominal amount of current and some parts were warm but not hot. I looked for the phase locked loop and found this on the underside of the board in the form of a number of tiny chip transistors of which the oscillator is marked "R33", a microwave oscillator transistor type NE68133-T1B, rated in the upper GHz plus two others (BSS88C) which are used for biasing and switching the oscillator on and off. The R33 uses striplines formed by the printed circuit and various chip components plus a pair of 10uF 25V electrolytics paralleled with low inductance chip capacitors, presumably for bypassing local noise on the +/- 15 volt power supply lines.
 

 

The 512MHz phase-locked oscillator transistor Q603

"R33" is an NE68133-T1B which is equivalent to a 2SC3583

Ratings:-

Vce 10V, Vcb 20V, Veb 1.5V, Ic 65mA

Pt dissipation 200mW, ft 9GHz

There's nowhere near enough room on the case for the full type number so it's just marked as "R33"

 Listening on my receiver for 512MHz indicated the oscillator was turned off, but by adding a 1K resistor connecting the OSC on/off switch line going to pin 13 of IC604 (a 74HC574N latch) to 5 volts I was able to turn it on. Again, I had a problem because the monitor receiver uses an internal 512MHz oscillator, but at least, amongst the heterodynes, I was able to detect that the oscillator could be turned on and off and, more importantly, I could hear that the signal wasn't too clean so I looked for anything that would give me a clue to the crackly output.

I tried my 100MHz oscilloscope at various points but its bandwidth was miles too poor to see 512MHz, although I could see some intermittency as the trace kept intermittently bobbing around in time with the crackling. Before I fired up my spectrum analyser which goes up to 1.5GHz I tried my multimeter turned to volts. Nothing much to see except I found both emitter and collector connections to the R33 (at the marked ends of two 10uF capacitors) each intermittently varied by up to ten millivolts. Measured volts would jump around, say 3.72V.. 3.68V...3.81V.. etc so I tried moving outwards to the other ends of the emitter and collector feed resistors (R626 and R618) and found these voltages were rock solid. Dabbing R33 with my finger produced much the same effects on voltage readings only worse as the circuit attempted to maintain lock. During testing the emitter voltage of R33 was about minus 4 volts and the collector about plus 9 volts although I noticed their decoupling capacitors, C656 and C649 (the other ends of which are grounded) were each connected the same way round. This is a bit odd. As the capacitors are surface-mounted tantalum types they carry a stripe to indicate polarity (similar to that on a surface-mounted diode cathode) however, the capacitor stripe marks the positive connection, so clearly, although C649 is OK, C656 is back-to-front!

 

 

 A tantalum capacitor is generally marked with a stripe (or a dot) for its anode connection so this needs to be connected to a positive voltage otherwise reliability will suffer. In this case the voltage is fairly low, but place a high reverse voltage across a capacitor and it's liable to fail quite dramatically.

I measured the capacitor with an ESR tester which gave 9.4uF with a resistance of around 0.5 ohm although this figure kept changing by up to 0.2 ohms or so each time I checked it.

Next I connected a variable current-limited voltage across it monitored by my multi-meter switched to mA. With the striped end positive and a voltage of 1 volt the current through it read at the limit set by the power supply which is wrong for a good capacitor. As the voltage and current limit were increased the capacitor failed (changing to a resistor) at around 5 volts. Rechecking on the ESR meter showed the capacitor was now defunct.

This example (C656 on the schematic) degraded because it was decoupling a negative voltage connected to its anode. It had lasted a long time before failing because only -4 volts was present and its rating of 25 volts had made it fairly resilient.

 Checking with a multimeter, with the oscillator off its emitter is some 12 volts negative and when on is about 4 volts negative. This means that when the output frequency is greater than 137.5MHz, although current limited to an extremely low value, C656 carries 12 volts with the supply negative connected to the capacitor anode. I removed C656 and fitted a 10uF electrolytic. The emitter and collector voltages were now rock steady. Was the back-to-front tantalum capacitor a fault in design or assembly and for how long did the capacitor last before starting to fail? Alternatively, could the capacitor have degraded to the extent it drew DC current resulting in a change in the operation of the bias transistor circuitry and polarity reversal across itself?

Because I wasn't sure exactly what the true voltages would be when the equipment was fully reassembled I decided to use two tiny 22uF 16volt electrolytics series-connected as a non-polarised pair in place of C656. This solves the problem of the voltage changing from positive to negative and back once the equipment is fully reassembled and with the mixer in/out/in service. The clearance between the metal sleeve and the circuit board dictated the maximum diameter of the new capacitors as no greater than 4mm.

I fitted the LO/REF board in its sleeve and reassembled the rest of the Wavetek and turned it on. I could hear the 512MHz signal turning on when 137MHz was selected then off when 138MHz was selected. The good news was that when I listened to both the 137 and the 138MHz signals at 1mV output they registered at exactly the same level on the monitor receiver S-meter and both were perfectly stable with no crackling....

The Wavetek was now up and running, fault-free... for the time being; but as electronic engineers are aware, once an equipment reaches a certain age, just like humans, problems will occur at an increasing rate. What's termed the "bathtub curve" rules in the world of reliability and my Wavetek is now climbing out of the bath.

 

 Above you can see from the button markings the various features. Readings can be swapped between volts and dB. Changing frequency can be done either by direct entry eg. Freq, then 110, then a press of the KHz button. Output by pressing Lvl, then 1000, then pressing the mV button.

Altering frequency is achieved by pressing Freq which activates a cursor under the frequency display. The cursor can then be moved by the left and right Cursor buttons to rest under the desired significant figure which can then be increased or decreased using the up and down cursor buttons. The same facility is available for output level or modulation. Minimum output is 0.1uV and the equipment is screened well enough for this level to be usable.

 I must have used the Wavetek loads of times before it started acting up again. I'd noticed that when it was turned on, if there was a radio on in the workshop there would be dreadful crackling while the auto diagnostics were running which settled down after a short time to be followed by intermittent lower level crackling.My immediate guess was a bad RF output transistor so I hunted around and discovered an identically packaged example coded SRFT3034 that was cheap enough to gamble on it being OK for replacing the original MRF239F. In the meantime I invested in a new signal generator. I say "new", but in fact it was labelled as a "barn find" but the fact it was (a) clearly going to be cheap and (b) made by Hewlett Packard convinced me it could be got going despite the seller's comment about "plugging it in and nothing lit up"...Read the exploits in commissioning this. I decided to tackle the Wavetek (again)... but before fitting the new transistor to clear the latest crackling noise, I carried out some basic tests. When I last used it I was aware of a warning message intermittently popping up in the display and this generally corresponded with a disturbance in the RF output so I checked and discovered, through loads of button pressing, the clever diagnostic hardware and firmware was certainly picking up a problem but wasn't keen to tell me what it was.

I rigged up a test, connecting my scope and spectrum analyser to the Wavetek. The first thing I noticed was the indicated RF output on its display was miles different to that displayed on both test equipments. To be sure, I switched everything to volts to line up with the (high impedance) scope's display. An indicated output of 0dBm, now reading as 224mV, showed as 146mV on both the SA and the scope, so the Wavetek is reading high by around 5 or 6dB. I inserted various fixed attenuators and saw the same discrepancies. I even swapped coax leads to no avail, so it appears something is wrong.. perhaps the RF output transistor is short of gain since its "repair", or had even developed another internal short? I also found during these experiments that the amplitude of the RF output jumped around and when it did so the signal was sometimes accompanied by transient signals either side of its frequency. Little by little I pinned it down to a similar problem already found and repaired.. see above. The Wavetek includes some mighty clever self-adjusting alignment procedures and I tried one. The RF output at 13dBm can be adjusted within an alignment routine to the correct level using cursor keys, but adding the maximum adjustment of +6dBm still left the output short by about 5 or 6dB as seen on my spectrum analyser.

Basically the lower the frequency of the RF output, below 137.5MHz, the less reliable it became. So, the problem looks like a repetition of the bad 512MHz signal used for mixing down to the frequencies below 137.5MHz. In summary then, there are two problems. Not enough RF power into the attenuator at all frequencies and a dodgy circuit responsible for generating frequencies below 137.5MHz. At least I'm now familiar with the circuit boards likely to be involved. Having just tackled extensive repairs to my HP8640B, it's interesting to compare this with the Wavetek. The HP uses loads of complicated mechanical switching to achieve much the same results as the Wavetek's microprocessors. In theory the Wavetek should last forever compared with the HP whose mechanical contrivances wear out. The Wavetek isn't lasting for ever and it's beginning to let me down a lot. Why is this so? I believe for a few reasons.. firstly design errors, secondly manufacturing errors and thirdly unforseen ageing problems... but none significant enough to have been apparent for many many years.

The first step was to tackle the MRF239F (again). Was it leaky? I measured the resistances and found (in-circuit) 2.4Kohm between collector and base so carefully detached it. It still had a leak of 2.4Kohm just as it had measured in-circuit. Previously the leak had been a few hundred ohms and this had cleared, but if it's an internal structural failure, maybe tin-whiskers, it could recur so a new transistor is the next step.

 

 The question is whether the new device will work in place of the original MRF839F? The type is SRFT3034 which is equivalent to the Motorola TP3034. Click on these links to compare the ratings. Obviously the new device has a far higher RF output capability and it's clearly a much beefier chip (35 watts versus 3 watts at 960MHz) with a correspondingly higher output capacitance, but as the Wavetek is rated up to only 550MHz the new device might be suitable.

Checking the detached device with a transistor tester revealed "two diodes" so clearly it's not ideal. I (again) applied a current limited DC voltage across the base to collector with base negative. Vcb can be 36 volts max but with the current limit gradually increased to 400mA the leak finally disappeared once the potential across base and collector reached minus 19 volts with current reaching 220mA. I noticed the stud was fairly warm so weird things must have been going on inside the case. At this point the transistor tested as an NPN device with a gain of 8. It seems my repairs are resulting in progessively less gain which might be to do with a different problem to that of tin whiskers?

It's not uncommon for RF power transistors to use a set of multiple junctions forming say four tiny transistors wired in parallel. Each of the four contrubuting a quarter of the output power. Could the reverse bias repair be fusing open one of these parallel junctions?

Anyway, as a comparison, the new transistor checked out as an NPN device having a DC gain of 37.

 

It's worth a try and if it fails then I can put back the original device and see how that performs... twice repaired.

I fitted it but the RF was down by circa 10dB or so on the the twice repaired 839F was later put back.

At this point I hadn't considered a second fault. Later, it seems the new power transistor might have been usable after all.
 

 Having tried the new PA transistor and failed to improve the low output (in fact making it worse) I found the intermittent fault was still present (which at least means it isn't due to the leaky PA transistor). The fault is very similar to the one I discovered about a year ago which I'd cured by replacing a 10uF tantalum capacitor. To recap.. the RF output drops intermittently with crackling by about 20dB accompanied by "Status" appearing on the screen. From the evidence on the spectrum analyser the frequency of the local oscillator changes slightly as well as the output changing in amplitude. In fact two things are happening. The amplitude of the output signal drops by 20dB and the frequency might (but not always) shift by a MHz or so (perhaps an oscillator losing lock?). The circuit boards are all inside metal sleeves and mounted in pairs making it difficult to work on them when powered up, but by experimenting I found the problem, like previously, was a fault in the 512MHz local oscillator (LO) circuitry. I was able to switch between 100MHz, when the LO was running and 200MHz when it was switched off and find the intermittent only affected the lower frequency.

When I was looking for the fault the last time it occurred I didn't have a suitable RF source with which I could interchange with the LO, but having recently repaired my HP8640B, I used this in place of the LO. With it set to 512MHz and greater in amplitude than -10dBm the Wavetek worked perfectly, mixing the RF to the desired output. No intermittent was visible, but switching back to the internal LO re-introduced the problem.

 

 I checked one 10uF tantalum (C649) which proved OK. I also monitored the +15 and -15 volt supply lines which also appeared to be OK. Access to the powered board is tricky but eventually, I found some voltages that were varying when "Status" appeared on the display. With the metal end plate detached from the pair of circuit boards within their sleeve I could just see the circuitry dealing with the 512MHz output. The most significant reading was the end of L601 which intermittently switched between 6.13 and 8.15 volts (with +15 volts stable). The circuit diagram shows IC605, an MSA-0204 amplifier whose output uses L602 to block RF. A resistor, R622 of 475 ohms is used to define the power supply and bias to the amplifier and a pair of capacitors C640 and C642 are used as decouplers. The +15 volt supply is used to power IC605. The two voltages indicate the current drawn by IC605 is either 19mA (normal=6.13 volts) or 14mA (fault=8.15 volts). The manufacturers recommended operating range is 18-40mA. Interestingly a second MSA-0204 (IC614) is used to feed the Internal Reference output socket so it's fairly straightforward (a spot of careful desoldering) to swap the two amplifiers if indeed the LO amplifier is bad.

 

 What fault condition could cause a higher voltage to appear at L601? Besides the amplifier chip itself, a bad decoupling capacitor C642 is unlikely as a leak would reduce the voltage not increase it. The most likely problem, assuming the amplifier current increases with RF drive, is the circuit to its left (above) has an intermittent fault. This would include Q603/Q604 and Q605. Of interest is C656 because I'd changed that because the original had failed (see above). A prime candidate is a second 10uF tantalum C649, but I'd already removed this and testing proved it was OK. Next, I'll measure the various points around those three transistors.. awkward because of the board position.

Not to be overlooked is the rather nasty possibility that the digital control circuitry is responsible. There are two connections: an RF sampling connection at R617 and a feedback connection to the varicap diode CR605. A complication of course is the fact that being a closed loop, a fault in the oscillator will cause an error correction signal, and a faulty error correction signal will produce a shift in the oscillator.

 

 I carried out more tests. Monitoring the LO with both the spectrum analyser and my UHF receiver set to SSB so I could monitor the stability I checked various voltages. I found again the when the output was clean IC605 output was about 6 volts and when bad this rose to about 8 volts. It was difficult to check the RF input to IC605 but it did appear to be more stable than the output which led me to believe IC605 was bad. I also managed to check the input to IC605 with some difficulty because of access. The input voltage read 0.9 volts but increased to over 2 volts when the fault appeared. This discrepancy seemed to me to point to a bad amplifier chip because a change of a volt and a half at its DC blocked input (C644) with a more or less constant RF input was not logical. I switched off and removed IC614. This chip provides an output labelled INT REF which I take to be the 10MHz internal crystal and is only used in rare circumstances. I can replace this chip in the future if need be by a modern device such as the BGA622 which gives 15dB rather than the 11dB of the original chip.

 

 

 It was not too easy removing the pair of MSA-0204** devices without using excessive heat because of ground connections, but I managed without damaging either chip. Checking the DC parameters I found they matched fairly well with no significant different forward voltages, however, once IC614 was fitted in place of IC605, I switched on and discovered the UHF receiver produced a much stronger signal than before and there were no crackles during Wavetek warm-up. The spectrum analyser showed a clean steady signal at 512MHz with an amplitude of -16.8dBm. I left the system soaking for ten minutes then connected the LO output into the PA board. Setting an output that required the local oscillator, I was able to see a clean 100MHz signal with a clean LO on the receiver. Changing to 200MHz (where the LO isn't used) resulted in a clean 200MHz output with the LO correctly off, then back to 100MHz resulted in the same results as before. No instability and no crackling but, with the attenuator set to 0dBm, the output read -19.6dBm. The RF level is so low compared with the displayed reading that it definitely needs sorting out. ** Note that the Wavetek parts list quotes IC605/IC614 as MSA0304 but they are both marked "2" indicating MSA0204.

 

 The next step is to tidy up the changes I made to the LO board then reinstall this, remove the PA board, refit the MRF839F and check the RF output.

All done.. the new PA transistor was clearly not correctly biased and had given -20dB for a setting of 0dBm. I fitted the original (repaired twice transistor) and the output improved reading -10dBm with the setting still at 0dBm. I found that increasing the output from 0dBm raised the output slightly but continuing further resulted in the output dropping, instead of rising (very strange), and with more and more harmonics showing up. I'm putting this down to the PA transistor which, although now showing as a normal NPN device, had a low indicated gain of around 7, but as it costs a lot of money and the fault may be something entirely different, I'm hesitant to buy a new one.

Basically the Wavetek is now usable but on the understanding that its output is 10dB less than indicated and shouldn't be used beyond 0dBm, but I'm not happy with that so I'll bash on and try and resolve the issue when time permits.

Before condemning the old power transistor, or perhaps looking for a cheap alternative if I can definitely pin the original down as faulty, there are further tests I need to carry out. The user manual is unfortunately silent on certain essential (useful) things such as RF voltages and power levels around the circuit. My guess is that these are not well enough defined in terms of what they should be to assign precise numbers. The reasoning is this... tests dictate one measures the output power to see it accords with the attenuator setting. All internal adjustments to set output and linearity are made through firmware by the operator from the front panel. These should ensure that throughout the circuitry the gain setting levels are suitable for providing the correct output but with the important consideration that there are no faulty parts. For example the operator can add up to +6dB to the output to make the measured output equal to the RF output power shown on the display. That 6dB could account for loss of nominal power due to say component ageing but hopefully not bad components. However, there's an advisory warning to check that harmonic levels are below 26dBc (in other words harmonics from a perfect Wavetek design are not expected to be much better than say 30dB down). That means that boosting gain by a significant amount to mask a bad component would be likely to degrade harmonic suppression and thus refer the user to look for a fault.

In my example the RF output is about 10dB down on what it should be (and possibly more or less than this at other frequencies I didn't check). I did increase overall gain to pump up the output by 6dBm and I did notice harmonics were then very bad. There are two checks I need to carry out. Firstly I need to measure the input into the attenuator by disconnecting the cable and checking the power into 50 ohms. I need to do this in order to confirm the attenuator isn't responsible for the loss in power output. I could also connect my HP8640B into the Wavetek attenuator to check the attenuator settings. The reason for this is to confirm there isn't a damaged component in the attenuator itself.

I did this and found the output from the PA transistor with the output set to 0dBm was much the same whether or not the attenuator is used.

A second test is to monitor the auxiliary output to see if this is similar to the main RF output in terms of harmonics. The Aux o/p is established early on in the overall amplifying process where it passes through several amplifying stages to the driver and output RF transistors which have both previously given trouble so I wouldn't be surprised to discover yet another fault in that area. I unscrewed the Aux o/p cable and discovered the RF level was -11dBm with low level (well within spec) harmonics, so there's a fault in one or both amplifiers.

 

 Here's a comparison between the Aux and Main outputs: 100MHz uses the 512MHz LO and 150MHz doesn't. This seems to prove that the problem is in the two RF amplifier stages, one or both of which must be distorting the sinewave so, due to limited access, perhaps the best solution is to bench test the output board using bench power supplies and a signal generator (knowing that the likely RF input will be something like -11dBm and the output needs to be OK up to the max output level of 13dBm).
 

 Frequency

 150MHz

 300MHz (2nd)

 450MHz (3rd)

 600MHz (4th)

 Aux o/p

 -11dBm

 -41dBm

 low

 low

 Main o/p

 -7dBm

 -19dBm

 -37dBm

 -48dBm

 Frequency

 100MHz

 200MHz (2nd)

 300MHz (3rd)

400MHz (4th)

 Aux o/p

 -11dBm

 low

 low

 low

 Main o/p

 -7dBm

 -19dBm

 -33dBm

 -45dBm

 What next? Well the construction of the equipment prevents easy fault finding because the circuit boards are fitted in metal sleeves and RF connections are made by miniature solid copper coax. My idea therefore is to remove the output board and test this on the bench. To do this I need a set of three power supplies, +18V, -18V and +8V, relying on the on-board stabilisers to produce the required +15V, -15V and +5V rails. Having identified the RF path I can feed its input from a signal generator. The RF path comprises no less than seven amplifiers. The final two being the Motorola RF power transistors. Prior to the first of these I see a feedback path which must be to stabilise the RF input so that the final RF output fed to the attenuator is at the maximum RF level of +13dBm (which is conveniently 1 volt RMS into 50 ohms). As I observed earlier, the manual as far as I can see is silent on specific RF levels but it seems reasonable for the two power amplifiers to provide a fixed gain over the full range of the generator. By gradually increasing the RF input to the amplifier chain the input to the final two RF amplifiers should follow the input until the feedback kicks in to stabilise this. Having measured this RF level I can then check the RF levels at the first and second transistors. Not only measure the fundamental frequency level, but also the second harmonic level as this seemed to be a problem. My guess is that the input to the final amplifier pair will be free of excessive harmonics and these will be getting produced either in the first or second final stage.

Below is a rough sketch of the Output PCB RF path with a spec of the MSA0304 (click to see). The MSA0404 is similar but has a 7.5dB gain.

The MA0304 has a maximum gain of between 10dB and 12dB so the theoretical maximum gain up to the feedback loop is between 47.5 and 55.5dB, say 51dB, but in practice maybe something like 25dB would be the designer's aim?

The RF output measured at the AUX output was -11dBm and the RF output into the attenuator should be a little over +13dBm to take account of attenuator losses. I've indicated on the drawing in red below the sort of levels I'd expect, but these are pure guesses. The two output transistors will be operating in a linear fashion so perhaps less than say 40% efficient?

 

 

 Above you can see the RF PA driver stage on the output board showing the MRF571 and above its 68 ohm collector resistor. This transistor has a DC base bias arrangement which includes a small transistor Q901. One puzzle is the condition of the resistor which is specified as 0.5W.

I then carried out some tests having wired +/-18V and +8V to the board from bench supplies.
 

 With my spectrum analyser connected to the RF output connector I tried various RF input levels at 20MHz to IC810. The results look very odd and support my previous findings re-harmonics. The drivers are producing decent gain and the PA seems to work well at a very low level also producing gain which changes to attenuation as the drive increases. The main reason for this was the observed severe increase in the amplitude of harmonics. This can be explained by the PA being overdriven.

 INPUT

 -30dBm

 -20dBm

 -10dBm

 0dBm

 +10dBm

 PA input

 -28dBm

 -18dBm

 -6dBm

 +6dBm

 +14dBm

 PA output

 -17dBm

 -8dBm

 -5dBm

 -16dBm

 0dBm

 Gain

 13dB

 12dB

 5dB

 -16dB

 -10dB

Because of the condition of the collector resistor accompanied by a hot resistor smell, the first step was to look at the MRF571, Q900. For a start I removed the collector resistor and fitted a physically larger 56 ohm resistor as that's the value given in the manual. Why was the original resistor burnt? Although the MRF571 base-emitter-collector junctions looked about right on my multimeter, I removed the transistor and found it had failed with a bad collector junction.

 

 

 This transistor was replaced about a year ago and as the seller was offering them in sixes I fitted a second from my remaining stock of five. As it gets pretty warm and possibly this had caused the demise of its predecessor I glued a brass washer to its top to lower its temperature. I then continued RF tests. From the table below you can see each stage produces some gain and between each stage there are some losses due to components used for coupling the stages and for matching. The overall gain appears to now be 20dB. I kept the input signal well below that which produced the previously noted overloading. IC801 has a gain of 11dB, Q900, the MRF571 has a gain of 8dB and the PA has a gain of 8dB. Losses total 7dB.

 

 Test point

 IC801 I/P

 IC801 O/P

 Q900 I/P

 Q900 O/P

 PA I/P

 PA O/P

 Signal

 -48dBm

 -37dBm

 -43dBm

 -35dBm

 -36dBm

 -28dBm

 

 The next step is to figure out why the PA is overloading. I measured its base voltage at 0.7V which seems low. The emitter resistor is shown as a pair of parallel 17.6 ohms so given a Vbe of say 0.6V the collector current must be very low at circa 10-12mA. To develop 1 volt RMS at 50 ohms which represents 20mW, given the transistor is operating in Class A (say 30% efficient) you would need a supply voltage swing of more than 20 volts. The collector is sitting at 7 volts hence the transistor will be saturated once the drive exceeds say 0dBm. Saturation=square wave rather than sine-wave, hence harmonics.

Clearly the PA circuit isn't working as it should. My first suspect is the RF power transistor itself which had only a single-figure gain when I tested it out of circuit, most likely because I've repaired it twice!... look back in this write up..

But.. what else could it be? I checked the various voltages, in particular around the PA bias circuit. I found several anomalies which of course could be due to a bad PA transistor, but my first alternative suspect was a tiny high-precision 2.5 volt zener diode (LM336 Z-2.5) as this had only 1 volt across it. I detached it and tested it, using my Peak zener diode tester, finding much to my surprise it was a "2.5 volt zener diode", so back it went.

 

 Next I looked at the power transistor used as a series pass regulator for the PA base bias. Bearing in mind it was a PNP device, its electrode voltages didn't entirely look right so I detached and tested it. The test meter reported "Two diodes" so I looked around and found a suitable alternative, a TIP125, and fitted that. Now, it's base, emitter and collector voltages made sense (ie. base and emitter now differ by around 0.6 volts) and the adjacent precision zener voltage correctly measured 2.5 volts. The PA collector now measured around 13 volts and an RF input at IC801 of -20dBm gave me an output of -5dBm. Varying the input to IC801 produced a linear response at the output socket (see below) so hopefully the problem is solved. The maximum rated output of the Wavetek is +13dBm so the final test giving +14.4dBm is fine.

The twice repaired MRF839F seems to be OK after all...below the latest results IC801-Q902

 

 RF input

 -70dBm

 -60dBm

 -50dBm

 -40dBm

 -30dBm

 -20dBm

 -10dBm

 0dBm

 RF output

 -55dBm

 -45dBm

 -34dBm

 -25dBm

 -15dBm

 -5dBm

 +4.5dBm

 +14.4dBm

 Gain

 15dB

 15dB

 16dB

 15dB

 15dB

 15dB

 14.5dB

 14.4dB
 

 

  Above, the faulty PA bias regulator transistor. This is a PNP MJE253 rated at 100V and 8A.

After fitting the circuit board back in the chassis I checked the output. The RF is now pretty clean with harmonics more than 35dB down and also fairly accurate in terms of the attenuator setting versus spectrum analyser display. Most signals are within 2dB, many less than 1dB, and with only very low frequencies, for example 10kHz, showing more than 2dB discrepancy. Switching from +10dBm to the maximum output of 13dBm actually gives 16dBm. My guess is that, like the HP8640B, the Wavetek may have extra circuitry for outputs at this level. Of course, I've carried out lots of repairs to the RF path but haven't touched any of the preset adjustments, indeed if there are such things. There is a strong possibility that there are pots or even firmware settings, for the feedback circuitry to maintain a flat response across the frequency range.

I looked in the Wavetek manual and then remembered that, instead of pots and internal twiddling, one must use the firmware to carry out calibration. During my first attempt to correct the low output level when set to +13dBm I'd cranked up the power level as high as it would go and failed. That's why, now the RF amplifiers are working correctly, I'm getting +16dBm. The power level is faithfuly being boosted to the power I'd set earlier.

All I now need to do is follow the complete procedure (click to read this) and the RF output should be dead flat from 10kHz to 550MHz at all RF output levels. It should be easier than described in the handbook (using a power meter) because I can use my DSA815 to monitor the output. In fact the latter method will be more accurate because the power meter method unavoidably adds together the fundamental and harmonic powers. The manual describes what is called "Autocal" which turns out to be not exactly "auto" because no less than thirty-one manual adjustments are required. Basically, you need a power meter (or with some extra fiddling a spectrum analyser) to check each of the 31 frequencies determined by the firmware. At each frequency you have to use push buttons on the front panel to set the power output to precisely 0dBm. Say you use 6 button presses per frequency.. you'll need 186 button presses to set the output level, plus 31 further presses to move across the list. "Auto" becomes therefore 217 button presses.

I just completed the level and linearity "auto" test. It probably took at least 20 button presses per each of the 31 pre-assigned test frequencies then a further eight button presses for around 21 pre-assigned power settings. My count is therefore 788 button presses plus the date.. a further 12 because I mixed up the month and day. That's 800, but the thing is now well nigh perfect to within 0.5dB for any frequency at any RF level!

The way it works is roughly as follows...Level Setting: a frequency is shown on the signal generator (SG) display, say 8MHz. The level is supposed to be 0dBm. If the spectrum analyser (SA) says the level is -2.6dBm you press the SG up button 0.1dB per press till the display reads 2.6, at which point the SA will show 0dBm. You then press the right arrow to move the frequency to the next one, say 16MHz and so on. Linearity Setting: the first SG display shows +13dBm at 300MHz so you check the SA level, say +14dBm and press the down arrow till the display shows 1.0dB at which point the SA level will be +13dBm, then the SG right arrow which brings up +12dBm etc etc....When you're using the signal generator you see the raw output which is followed within a second or two by the modified level which has been corrected by the look-up table generated by "auto" calibration. What would we do without microprocessors? Below the test bench with repaired HP86430B and Wavetek 2407 back in place.

 

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