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. 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, 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. For example 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 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 MNRF839F 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.

 

 

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 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 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 becauseanything 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 is 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 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 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

NE68133-T1B which is equivalent to a 2SC3583

Ratings:-

Vce 10V

Vcb 20V

Veb 1.5V

Ic 65mA

Pt dissipation 200mW

ft 9GHz

Marked as "R33"

 Listening on 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 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 an 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. Before I fired up my spectrum analyser which goes up to 1.5GHz I tried my multimeter turned to volts. Nothing much 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 only worse as the circuit attempted to maintain lock. During testing the emitter voltage was about minus 4 volts and the collector about plus 9 volts although the two 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.
 

 

 Tantalum capacitors are marked with a stripe for their anode connection so this has to be connected to a positive voltage.

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.

 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 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....

 

 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 enough for this level to be usable.

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