R208 Refurbishment 

 I needed to make a replacement test probe for my spectrum analyser because I'd mislaid my usual one when recently rearranging the workshop. I wanted to check the alignment of an HQ170 receiver on which I'd been working and whilst waiting for delivery of the parts I looked at my R208. I removed the receiver from its case but before plugging it into the mains supply checked the mains input panel which gives the connections for 100 to 250 volt input. Surprisingly this was set to 210 volts which I suspect was the voltage tapping used by its last military user. Although the set has been in the hands of at least one radio enthusiast since then, pound to a penny no-one ever looked under the cover to check the mains setting. I would imagine that at least one valve is low in emission from being over-run, but nevertheless I plugged it in and checked for life.

See the user manual

 

 Although the receiver responded with crackles when the wavechange switch was operated there was very little background noise except when the BFO was turned on. I fitted a 20dB attentuator to protect my signal generator and connected it to the aerial terminals. The set was clearly not only a bit deaf, but the local oscillator didn't appear to work fully across each waveband. On the 10-20MHz band I needed 100mV to produce a response at something like 29MHz and at other frequencies in that band I suspect the oscillator wasn't functioning. Maybe a bad 6K8 mixer/oscillator, or a resistor gone high/condenser leaking... or both.

In the band 20-40MHz I could hear rough responses (at least on the lowest band the signal, although weak, was clean), both main and image signals with 10mV input. One of these, I can't say which until I've read the technical spec was stronger but not much stronger than the other. On the 40-60MHz band I could hear a signal at 58MHz with maybe 5mV input, but this needed more input voltage as the frequency was decreased and soon disappeared at below 50MHz.

The frequency coverage (10MHz to 60MHz) of the R208 is a bit peculiar, but could be explained by the fact that many WW2 communications receivers were limited to 20MHz, and a few reached 30MHz, but apart from specialized aircraft receivers, the low VHF band was inaccessible. A simple trick to test the local oscillator is to place a wire aerial from another communications receiver near the chassis and look for the presence of the R208 local oscillator. By using an S-meter you can quickly see whether the signal is good or bad, (for example fading out). I used this method to diagnose my RA17 extreme deafness. A low emission EF91 VFO was cutting out before the tuning condenser reached full mesh. A replacement valve fixed the RA17. If your monitor receiver doesn't exactly match the band being checked you can tune to a harmonic of the oscillator.

According to the handbook the intermediate frequency is 2MHz and the local oscillator is on the high side of the incoming signal on all three wavebands so, for example, an input of 25MHz requires the local oscillator to be 27MHz putting the image at 29MHz. Both the 25MHz and the 29MHz signals will be heard with an input of 25MHz but their difference in level will be a function of the tuning and tracking of the RF amplifier coils. Equally, one can hear and therefore alter the tuning of the receiver to 25MHz by inadvertently setting the local oscillator to 23MHz.

Rather than test the valves I'll first check the voltages at the test panel. These should be close to the values shown in the handbook.

 

 Here are a couple of sets of measurements.

 

 V1A Anode

 V2A Anode

 V2A Cathode

 V1B Cathode

 V1C Cathode

 V1D Cathode

 V2B Anode

 Ideal values

175v

188v

1.9v

2.6v

0.5v

8.1v

205v

 First Test

122v

139v

1.9v

1.4v

1.2v

5.6v

142v

 Second Test

149v

171v

2.4v

1.8v

1.4v

7.7v

175v

 I checked the readings at the test panel (see First Test) and Anode V2B' reading was about 142 volts (= the HT voltage), and varying. No doubt the hum from the speaker was connected with this variation so I wired a 22uF 400v capacitor across the metal-cased condenser C1B and the hum dropped considerably and the HT rose to 175 volts (see Second Test) Clearly the old condenser is past its best. Looking at the circuit diagram there are two similar condensers C1A and C1B acting as reservoir and smoothing respectively for the HT. C1B was bad so where was C1A? This is located underneath the power supply chassis (below) which is fastened to the main chassis by four 2BA screws. To detach the power supply it's convenient to remove first the horizontal bar between the front panel and the rear frame. This done, the power supply can be unscrewed and turned upside down to access the reservoir condenser. I used the trick of connecting the condenser in series with a resistor across a variable HT supply whilst monitoring the current by measuring the voltage across the resistor (in this case 20Kohm). With 200 volts applied the current drawn was initially pretty low but as I watched it rose steadily to about 1mA. Increasing the voltage to 400 increased the current to 4mA. It then very slowly dropped and stuck at 3.7mA. I then tested the smoothing condenser. This gave roughly the same results so leaving them both disconnected, I wired a couple of new 22uF 400v electrolytics as reservoir and smoothing. The HT was then 175 volts, still a bit low.

Here's a view under the power supply chassis.. Many of the parts are for 6 volt battery option using a vibrator. The previous owner had removed the Mains/Battery switch and some of the wiring and the vibrator.

 

 Under the main chassis there are a further two metal-cased condensers. These are 2uF and rated at 350 volts. I tested these and to my surprise, both were in excellent shape with only tens of microamps leakage at 300 volts. Both measured very close to their marked values of 2uF with an ESR reading a small fraction of an ohm so I left these in place.

The decoupling condensers are pretty substantial 70+ year-old components and quite unusual but swapping these might help reduce the demands on the HT rectifier. I tested the electrolytic condenser at the bottom of the picture (clearly not original 25uF 12vw component) and found it was marked 50uF and measured 51.87uF with an ESR of 0.39 ohms which is remarkable as it will date from 1958.

While I had the chassis upside down I checked all the resistors. All were high by varying amounts, but apart from a couple were not too bad but as none were open circuit or dramatically high I left all intact.

 

The next test was to investigate what appeared to be the local oscillator dropping out, and sure enough whilst listening on another receiver, the oscillator was giving S9 over about a third of the two higher ranges and for only a quarter of the lowest range. Id already swapped the two 6K8 valves around so it's likely the problem may be low HT or a bad component in the 6K8 oscillator. It's quite possible the previous owner had changed the mains transformer tapping to deal with this?

  
 I shelved the local oscillator problem for the time being and decided to clean the dial because it was impossible to read due to tarnishing and I needed to read the markings in order to align the set. The Muirhead dial was best removed for this job. It's held in place by a screw securing the drive output to the tuning condenser shaft. I had to use a torch to see the screw head which needs to be turned to a convenient point for slackening. You'll need a long thin screwdriver to access the screw. The dial mechanism has a tab which locates in a metal block but before it can be pulled off the small plastic bandmarker needs to be slackened (one screw is sufficient, allowing it to be turned out of the way of the dial). Once the assembly is off four screws are removed to free the metal dial which can then be cleaned with Brasso. Once cleaned it was easy to see the numbers and gradations.

 

 

 The dial, partly cleaned before deciding to remove the metal plate and get rid of all the tarnishing. This meant removing the salmon-coloured finish to produce a shiny blemish-free surface.

At some time it might be a good idea to apply a matt finish to help with readability.

 

 While sorting out the dial I noticed that the RF gain pot wasn't connected. I swapped over the wire from the centre where it was soldered to the ground wire and connected it to the end of the wiper. This made the receiver totally deaf because someone had not only disconnected the pot but they'd completely removed the resistance wire so the wiper was rubbing on plastic.

It soon became obvious what had happened. The pot must have seized and broken its winding. This may have been shorting to the chassis so had been removed. Why not replace the pot? Well the pot is directly in front of the coils for the RF amplifier so there wasn't room to get it out. Two of the coils had been broken from their mounts in an effort to remove the defunct pot but still there was half an inch too little space so they'd given up.

The solution was to break off the spindle. This is brass rather than steel so was easily broken off with a large pair of pliers. I was then able to extract the remains and fit a replacement. I had a small WW2 US pot small enough to fit in place. The new pot is 5Kohm rather than 2Kohm so I fitted a small resistor to shunt the resistance down to the correct value.

 

 

 

 

 I'm probably revisiting something that may have happened back in 1958 when Sputnik 1 was in orbit?

The RF gain potentiometer had failed but there wasn't enough room to remove and swap it even though the two coils directly behind it had been forced from their mounts. The securing screws for the coils can only be accessed once the whole RF front end chassis has been unbolted from the main chassis and all of its cable harnesses unsoldered. A daunting task...

Maybe the resistance wire had become jammed in the wiper and shorted to surrounding metalwork so the wire had been painstakingly removed?

The solution was to break off the brass spindle, giving half an inch extra clearance, which enabled the pot to be detached.

Below, the new potentiometer installed and wired up.

Did it really take 60 years to fix?

 

Above is the coil pack. Left to right RF amplifier, Mixer input and local oscillator and top to bottom, 40-60MHz, 20-40MHz and 10-20MHz.

Beehive trimmers are adjusted at the HF end and the coil slugs at the LF end of each range.

I connected a long wire aerial and initially checked the lowest band 10 to 20MHz because this was completely dead, whereas the two highest bands were active over a small dial rotation. I suspected the wavechange switch and after spraying it with switch cleaner and rotating it backwards and forwards a few times the lowest band sprung into life and loads of broadcast stations appeared.

I turned on my signal generator and after 20 minutes twiddling the lowest band was nicely aligned, as was the IF strip at exactly 2MHz. The first IF transformer had an intermittent but as I looked for a means of removing it I noticed its two fixing nuts were very loose. I tightened these with a 4BA nut spinner and found the intermittent had magically gone away.

Now that the dial was clean I could read the dial markings I was able to see that the oscillator was cutting out at 33MHz and 53MHz on the two higher bands. This might be a bad 6K8 or a low HT voltage although the latter isn't too bad now that the HT is cleaned up. I removed the RF amplifier and the mixer/oscillator valves and checked the valveholder pins. Anodes and screen voltages were all present, reading 195 volts. As the HT voltage is 175 with all valves in place, I suspect the HT rectifier (a full wave bridge) maybe has more resistance than ideal and is adversely affecting the HT rail. This tends to support the reason for the mains selection tapping of 210 volts. I might change the setting to something in-between... say 230 volts rather than 240 volts. Before that I'll swap any leaky decoupling condensers because I imagine the oscillator problem could well be due to poor decoupling.

Most of the decoupling condensers are large black things which, surprisingly, all tested as perfect. I changed a 75Kohm resistor that measured 92Kohms but this didn't fix the local oscillator. I may use an external HT power supply to see if this works...

Below, I've indicated the various measurements made (with the HT measured at 182 volts). Not easy because the underside of the valveholder is buried under the coilpack and it was easier to stick a short length of wire into the socket with the 6K8. I tried various examples of the valve and ended up with one that tested 100% on my AVO valve tester. The voltages are what might be expected and imply insufficient feedback to maintain oscillation. The lowest band works over the whole dial but you can see the local oscillator amplitude is dropping at the LF end (ref. the grid voltage). The cathode bias increases as the oscillator amplitude drops because the triode is drawing more current (going from circa 2.6 volts to 3.1 volts). Also, the triode anode voltage drops as the oscillator amplitude drops ( from 125 volts to about 114 volts and then 81 volts when it's stopped oscillating)

 

 Waveband & dial setting

 10-20MHz... 10MHz

 10-20MHz... 20MHz

 20-40MHz... 20MHz

  20-40MHz... 40MHz

 40-60Mz... 40MHz

 40-60Mz... 60MHz

 Local oscillator

 OK

 OK

 Stops at 33MHz

 OK

 Stops at 53MHz

 OK

 6K8 cathode

 2.5v

 2.6v

 3.1v

 2.5v

 3.1v

 2.7v

 6K8 triode anode

 125v

 116v

 81v

 120v

 81v

 114v

 6K8 g, g1T

 -0.8v

 -2.5v

 +2.7v

 -1.9v

 +2.8v

 -1.2v

 6K8 anode

 178v

 179v

 182v

 178v

 182v

 173v

When the local oscillator has maximum amplitude the grid voltage is minimum (ie. -2.5 volts) and when it stops the grid voltage increases (ie to +2.7 volts).I disconnected the R208 HT feed at the HT choke and measured the current at 66mA then connected an external HT supply. This was adjusted to provide 250 volts and the receiver drew about 70mA. The internal HT supply provides about 180 volts. The local oscillator did manage to get down a little further with the increased HT but only improving to see 46MHz. At 200 volts this went to 47MHz, but I need to get the oscillator down to 38MHz so a further study of the circuit diagram is needed to try and identify a rogue component.  I checked all the resistors around the frequency changer and the condensers (decoupling, grid coupling and feedback) for value plus any leakage and all were fine. Shorting out the cathode resistor slightly increased background noise level but only affected the local oscillator cut off point by a few KHz. I suppose the next step is to change the feedback condenser C12A from 150pF to say 250pF and see what effect this has...
 

 Changing parts in the R208 local oscillator area is tricky because of the position of the valveholder. Its buried beneath wiring and wedged between screening plates and the wavechange switch. As I studied access I noticed someone had been here previously because there was an earth solder tag with solde but no component or wire. Also, although the grid blocking condenser and the feedback condenser are both supposed to be identical, one was correct and the other (the grid coupler) was 200pF instead of 150pF. Oddly that was going to be my next step... Instead I fitted a 125pF and put a 330pF in place of the 150pF feedback condenser. Checking the 20-40MHz range proved that this had absolutely no effect on curing the problem. Looking at the oscillator on my spectrum analyser however showed something rather odd. The range of the oscillator was OK although cutting out with the tuning condenser two thirds closed. Maybe the padder is too high? This was accessible so I checked it and found the 0.002uF condenser measured 1800pF. Thinking about this I reckon even at 1800pF it must be too high so I fitted a condenser marked 830pF. To my surprise the oscillator amplitude remained steady and tuned 21 to 40MHz. After twiddling the slug and trimmer this ended up at 22-42MHz... pretty well perfect but very puzzling.

Looking at the dial markings it seems the tuning range for the highest band is marked 40-60MHz but this only cover two thirds of the dial. This means that the padder has to be checked so that the frequency coverage matches the dial markings. The lowest frequency is indeterminate as the dial markings stop at 40MHz. Although I aligned the 10-20MHz band I'll recheck this with the spectrum analyser because its very easy to flip from the main RF input to the image.

That oscillator padder change from 2000pF to 830pF for the 20-40MHz band is really odd. The implication is the tuning condenser has too great a range. Maybe I should check this? Physically, the tuner has three identical sections and the user manual confirms 3 sections each providing 110pF swing. Maybe some calculations are necessary to indicate what's happening?

After some rough calculations I worked out that the 830pF padder capacitor would work perfectly given around 0.47uH for the oscillator coil and, for the RF amplifier, a coil of about the same value would tune 2MHz lower. The answer must be in the oscillator coil inductance. If this is different to the original, perhaps from ageing (ie. a change in characteristics of its former or the iron dust core) it might explain what's going on. Another explanation, a rather slim possibility, is that the wrong coil was fitted to the oscillator circuit in the factory. Next I'll look at the 40-60MHz band...

 

 in progress

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