R109A Receiver Commissioning

 

 I decided to get this old receiver working, so carried out some cursory checks before applying power and noticed that each valve filament has a limiting resistor of 71 ohms connecting to the 6 volt supply except for the first RF stage (either V1b or V3a depending on the variant... mine is very clear because it has V3a written on the chassis). The valve in place is actually an ARP12 so is wrong, but I checked its filament and it was OK, making me believe that the receiver has never been powered up in this state. Fortunately, I have lots of SP61 valves so I plugged in one of these at V3a. I also checked my stock of vibrators. I have two 6 volt 4-pin types so I fitted one of these.

My usual plan when tackling a new project is to find out whether anything really nasty has happened in the past that makes refurbishment too difficult and, being fairly confident after a good look around the chassis of not wrecking anything, I applied 6 volts from a current-limited power supply. The first problem was a lazy on/off switch but repeated waggling got it working. With any valve equipment the heater/filament cold resistance is pretty low so the voltage dropped to virtually zero at switch-on but, by gradually increasing the current limit, the voltage started to rise. At a consumption of 2 Amps the current was oscillating somewhat but the vibrator (to my surprise) had started up and after plugging in headphones and waiting a few moments I heard a whine from unsmoothed vibrator noise.

I found all the switches were lazy but responded to waggling. I connected a long wire and after some further switchery I heard a strong SSB signal around 4MHz. The BFO turned on after some more switch waggling and resolved the sideband signal. Turning the wavechange switch was difficult but it did operate after some persuasion and after waggling I heard lots of strong AM broadcast stations.

 

 
  

 

 Block diagram of the R109A receiver

 What about that 2 Amps of current being consumed? At first, for a battery operated receiver 2 Amps seemed high, however the SP61 consumes 600mA and the other valves 50mA each so that accounts for 950mA. If the HT is 150 volts and let's say 20mA is consumed, this equates to 3 watts and at 6 volts would account for another 500mA, but of course inefficiencies in the vibrator HT supply and the front panel lamp (I checked and the bulb was marked 0.05A) might add enough for a total current draw of 2 Amps.

The next step is to familiarise with the various components and to identify any that need replacing. In doing this I spotted a small capacitor buried in the chassis marked "RS 0.047uF". My guess is that this might be a replacement for C11b (it was), the condenser used for coupling audio to the control grid of V2b, the output valve, even though the original is supposed to be 0.002uF. Another condenser that's been replaced (or in fact parallelled) is either C19a or C19b located in the DC output from the vibrator PSU (in fact C19b). Again the choice is far from the original rating, being 100uF x 250v. I measured this and found it was 68uF but with an ESR of >20 ohms (Note: My ESR meter cannot read above 20 ohms so it could be virtually anything but it should be around half an ohm). I also checked a few other electrolytics. All were either open circuit or a few ohms with no capacitance measurable. None will not stop the receiver working but will result in reduced overall gain and noise on the LT and HT rails.

Unless they fail in a current avalanche mode the majority of the wax-paper condensers may be good enough to leave in place. The exceptions are those used in high impedance areas of the circuit (eg C11b which has already been swapped). No AVC is used in the R109A but there may be the odd condenser associated with the RF gain circuit that will need swapping.

As documentation for the R109 is limited, and for the "A" version, misleading in that the picture purporting to be an "A" is actually that of the "C" version!, I've marked up the pictures below to identify components.

Apologies for the poor circuit diagrams. I'll look out for something better.

 

 

 

 

 

 

 
   

 

The next step was to replace the two HT condensers (or three if you include the 1970s addition). I decided on a pair of 6uF sealed metal-cased capacitors because these had a very low ESR and not electrolytics. I removed the two clips from the chassis because both old condensers are grounded, not to chassis, but to HT minus, which I measured to be about 13 volts. Connecting 6 volt power brought the receiver to life with whine-free audio. Tuning to 80m I heard a decent AM signal, but on 4.0MHz instead of 3.62MHz and strong SSB at higher points on the dial. Clearly the range is skewed. I thought at first it was to move top band further from the tuning limit, but obviously that's wrong because to do that would need the range to be tuned downwards, not upwards. Before tackling any discrepancy in tuning there are a couple of important steps.. firstly the dial mechansm needs to be checked in case it's been misaligned with respect to the main tuning condenser, and secondly I'll need to confirm the whether the IF hasn't drifted or become mis-tuned from its correct frequency of 465KHz. The IF transformers each have two adjustments which are dust cores fitted with brass screws. Initially, an audio power meter can be used and all six dust cores adjusted for maximum output. Once this has been done the BFO can be zeroed to 465KHz. Optionally the BFO could be offset because both 40m and 80m bands use Lower Sideband.

 

 There are several methods of aligning an IF amplifier, really depending on the test equipment you have. Years ago a wobbulator using a CRT may have been pressed into service. Another method was to meticulously and individually adjust each IF amplifier stage according to detailed instructions to give the desired overall response.

Any receiver using a crystal filter is a special case because the crystal resonance characteristics would dictate the final IF which may be up to a kilocycle away from optimum.

With a simple receiver like the R109 the easiest way to align the IF strip is to inject a 465KHz AM signal into the aerial connector and, with an audio wattmeter or an old AVO set to AC volts connected to the headphone socket, tweak all the IF coils for maximum audio output. This is my preferred method, however I like to complete the alignment using my DSA815TG spectrum analyser which will show up any odd effects.

This "A" version of the R109 has no AVC so careful manipulation of the signal generator output and the volume control is necessary during the initial set-up. In fact, although it's called a volume control on the panel, the R109A control is actually an RF gain control.

You can see opposite that all the IFT primary coils (L7) are identical as are all the secondary coils (L8).

The BFO (L9a) is adjusted by zero-beating it against a 465KHz CW output from a signal generator.

 RF alignment looks to be straightforward once I've identified the trimmers and dust cores. The R109 and the A version differ slightly in the labelling of the tuning condenser sections. Much like the WS19, trimmers for the higher range, once adjusted are not touched because these are used in parallel with trimmers for the LF range. Sufficient adjustments are provided by trimmers and dust cores for aligning both ranges to conform to the start and end of the dial markings.

IF alignment was not easy. I turned on my Wavetek digital signal generator and set it to 465KHz AM around 10mV and connected it to the R109 80 ohm aerial connector and ground expecting to hear at least a weak off-tune signal.. but nothing, so I increased the output and only when this was 1000mV did I hear anything. This was a rough signal that didn't vary as the Wavetek tuned up or down, so I turned on my old Marconi analogue TF2008 and tried this. Unlike the digital Wavetek, the TF2008 tuning knob can be rotated backwards and forwards around the expected frequency to determine if a signal can be heard. Success... I heard a single sharp tuning point in the right vicinity so turned on a frequency counter and checked the TF2008 output. It read 453KHz which was so low that my button pushing of the Wavetek to around plus/minus 5KHz hadn't been enough. Maybe, if I'd used FM it would have worked?

Having discovered the R109 was nicely aligned to 453KHz (there was a single sharp response) I plugged in the Wavetek, tuned it to 465KHz and set the output to 100mV AM then tuned the two cores in IFT1 which peaked a weak but tuneable note. Moving to IFT2.. this also tuned and I could reduce the input somewhat. Finally IFT3 cores both unscrewed to tune in the 465KHz signal. By now the RF input level was sensible and the audio wattmeter plugged into the headphone jack socket was peaking nicely. After another couple of rounds I was happy that the IF strip was OK. Next, the BFO which had worked previously to resolve SSB. This must have also been set to around 453KHz and sure enough, switching off modulation and unscrewing the core of L9a a very long way allowed me to zero-beat the BFO to the Wavetek.

What must have happened in the past? Without an accurate signal generator one can tune to a strong signal, then tweak each IF core in turn to increase the audio level. Then retune the receiver slightly to again add some volume, then return to the IF strip and continue tweaking. I imagine that the gain of the IF strip will increase slightly as it's tuned lower in frequency so repeated twiddling will increase the recovered audio, but of course you'll end up miles (several KHz at the IF is miles) away from 465KHz. The final tweak must have been retuning the BFO.. easy enough.. just screw in the core and it's now compatible with the wrong frequency.. 453KHz. Does it really matter what the final IF response is? In the case of the R109 it's not greatly important except the dial readings will be out somewhat, and if you tweak the local oscillator to fix this then continue to tweak things you might be quite happy if your interest is the 40m and 80m amateur bands. Aligning the receiver to these bands would be easy but over the whole of the two ranges you would probably end up with good reception at one end and poor reception at the other end.

For the R109 to work correctly, as the designer intended, providing a uniform response across each of its two ranges, 465KHz must be the IF. This is because many of the components in the front end were selected to conform precisely to the choice of 465KHz, in particular the tuning condenser and the oscillator padder condensers. Of course component tolerances and ageing will somewhat degrade the final results but correct alignment will always be the best option.

   

 Now for the front end alignment... there's some vagueness about this because the trimmers and coils for R109 and it's "A" variant may have different locations. The first step was to free up the wavechange switch and to clean its contacts. A few squirts of switch cleaner worked quite well and good enough to proceed with alignment. Clearly, although I can receive signals, it's very noticeable that the background noise across each of the two ranges varies considerably indicating poor alignment. At every point on the tuning dial it's essential that the oscillator, mixer and RF amplifier coils are all precisely tuned. This is made possible by alternately tweaking the coil cores at the LF end of each band and the trimmers at the HF ends. By doing this several times you eventually find a constant background noise when tuning across the dial, and more importantly.. dial readings will be correct.

The LF range was miles out and it became obvious that the various cores and trimmers had been haphazardly twiddled during the life of the receiver in much the same fashion as the IF adjustments. Little by little I managed to line up each range (not easy because the positions of the coils and trimmers for the two ranges on this "A" model are switched around). Some adjusters were so far out I had to connect the signal generator directly to the top cap of the RF amplifier.. bypassing its tuning coils. The LF range tuned something like 2.5 to 4.5MHz instead of around 1.8 to 5MHz so that the receiver wouldn't initially tune either my 2MHz or my 5MHz test signals. The HF range oscillator wasn't quite so bad, but couldn't at first tune my 12MHz test signal so I had to initially use 11.5MHz to get the oscillator lined up to the dial markings. I've marked up the locations of the coils and trimmers, each with the relevant dial setting for adjustment in this  picture. Note: Four coils and one trimmer are hidden away.

I finally finished the front end alignment. Very easy once I'd identified the correct coils and trimmers. The hardest part of the job was seeing the dial which isn't illuminated. There are three or four electrolytic condensers that measure zero capacity. The only problem I can find is perhaps not enough reverse bias so that minimum volume isn't low enough. In this R109A, designed primarily for CW reception, the volume control was omitted and a new RF gain control fitted in its place (although still labelled as volume control). Not only is minimum voume too high, but when listening to SSB you can't reduce the RF sufficiently to clearly read it. My guess is a bad condenser.

 

Above, the PSU parts responsible for smoothing and establishing the correct HT, bias and LT for the R109A. The condition of the wax paper condensers is typical of what's to be found in WW2 equipment. The reason for this investigation was to sort out the rather low value of the bias supply. Access to these components is extremely awkward unless fairly major dismantling is undertaken, however I took a short cut by unscrewing the upper rectifier and the bias rectifier. Then I unscrewed the two 6BA screws securing the upper terminal strip (self-tapped but locked in position as are the rectifier nuts). Then it was possible to bend the end plates sufficiently to withdraw the upper HT rectifier to access R4j, C12h, C12j, C20a and C20b.

After fitting four new capacitors and a pair of new 270Kohm resistors (R4h + R4j)I tested the receiver, expecting to find the audio control reduced the output but, alas, the set was even more sensitive and the minimum audio level was much louder. I heard very strong amateur sideband on the 60m band, but so strong was the signal, even with minimum RF gain I couldn't resolve it cleanly. Looking at the circuit diagram, the problem must be C4m. The bias voltage marked VB1 below reads minus 15 volts with the RF gain at minimum and minus 17 volts at max. C4m (at least) must be the leaky suspect?

 

 

 This is a simplified circuit for the manual gain control arrangements. I've not included the HT components which share the same transformer shown in the previous drawing of the power supply.

 I removed and tested C4m, which was indeed a bit peculiar. Over the years I've found that some old condensers not only leak when HT is applied but some seem to cycle between leak and no leak (or little leak). I guess this is because of a virtual almost-short-circuit within their innards. C4m was in this category so I left it out of the circuit and made measurements without it in place. As I'm unsure of the working conditions of the J25 rectifier I swapped this for a standard 1N5402 silicon diode. This gave me a potential at its anode of minus 70 volts (VB w.r.t. chassis). This feeds R1k, a 1Mohm resistor (actually marked 910Kohm) and then R10a, a 1Mohm potentiometer to chassis whose wiper feeds the control grids, via R1e, of V1e, the 1st IF amplifier and V1f, the second IF amplifier each via a 1Mohm resistor (R1c and R1f actually marked 820Kohm). Remember that in these bias circuits, virtually zero current should flow through the control grid circuit unless via a leak to ground.

The 70 volt bias supply VB will have a maximum grid bias level of 35 volts (VB1), being half the total due to the two equal 1Mohm resistors forming the load (R1k + R10a) with the wiper at the live end of the pot. Let's assume that 70 volts is the correct value.. but 35 volts appears to be inadequate to cut off V1e & V1f.. so what's wrong, because looking up the spec of the ARP12 I see this should cut off when its grid is minus 20 volts so minus 35 will be plenty. Following the circuit from R10a this goes to the two valves already mentioned.. V1e and V1f whose control grids are each buffered by 1Mohm (R1c & R1f) actually 820Kohm resistors) and decoupled, by 0.1uF condensers (C4f & C4j..no doubt leaky). The control grid of the Mixer (V1d) is coupled to the anode of the RF amplifier through C7g, a 150pF condenser. Could that also be leaky? No, I checked V1d top cap and it was sitting at minus 1.5 volts.

Now comes a practical problem.. both V1e and V1f control grid decouplers are inside the IF screening cans so will be difficult to swap if they're leaky. Fortunately, some guesswork can be taken out of the investigation because I can simply measure the voltages at V1e and V1f top caps. If these track the RF gain control wiper then the decoupling condensers are OK. Well.. the top caps read minus 10 and minus 11 for V1e and V1f which sadly means bad condensers inside the 1st and 2nd IF cans. I say "sadly" because the IF cans are soldered to bases screwed to the chassis and to get at their innards means a lot of dismantling, particularly to get at the 1st IF can in the centre of the chassis. Is there any option, bearing in mind I only need to turn down the audio volume (or reducing IF gain in order to resolve SSB)? Possibly there is a simple solution... I initially said that the bias voltage was minus 35 volts due to the load imposed by R1k and R10a, so what if I reduced R1k from 1Mohm to say 220Kohm? The bias voltage would move from minus 35 to minus 57 volts. Each of the two top caps is at minus 10 volts and so the loss in the respective bias resistor is about 20 volts so we need to double the bias supply at the volume control pot from 30 volts to 60 volts to bring each top cap to minus 20 volts (assuming this is the cut-off voltage). In this state the loss in each bias resistor will be 40 volts and the bias will be minus 20 volts. This means that bridging R1k to bring it down to say 200Kohm will do the job of reducing the audio to a low enough level. Adding 220Kohm in parallel with R1k should work.

A second solution also looks possible. Bearing in mind that the available HT from the same transformer winding feeding the J25 rectifier is around 150 volts, then this level of voltage might be available as a negative bias voltage? Looking at the circuit diagram, you'll see that the J25 is fed via a pair of condensers, C12c & C12h which isolate any DC connection to the HT circuit. In fact the bias voltage VB is referenced to HT- or chassis via the two resistors R4h & R4j. These are equal in value, so the reference point at their junction will be about half the theoretical maximum available from the transformer... or as I measured previously 70 volts. In fact using the J25 rather than a modern 1N5402 this works out at nearer 60 volts. What then, if R4j was smaller than R4h.. say if I shunted R4j with 1Mohm? I think the bias voltage VB1 might be close to 40 volts using the J25 rectifier. The figures are a bit woolly because we're dealing with indeterminate leakage currents in the bad condensers C4f & C4j, however with a little experimentation swapping or shunting R4h & R4j the volume control (or IF gain control) will enable SSB to be resolved.

In summary, using J25, which appears to work satisfactorily, the measured loss from it compared with a 1N5402 is about 10 volts so, in the first method for fixing the problem of too much overall gain, I might need to use in place of R1k, not 220Kohm, but maybe 180Kohm? The downside of this is that I might need to decouple the bias supply a little more to compensate for the lesser smoothing effect of the lower value resistor. Time to try the two solutions...

I found that bridging R4j with 180Kohm and R1k with 330Kohm enabled VB1 to establish a range of minus 56 to minus 63 volts which brought the minimum RF gain down to an acceptable level. With these additions I measured the DC voltages, VB4 & VB5 at V1e and V1f to be minus 14 and minus 11 volts respectively, which seems to be sufficient negative bias to protect ones ears. Incidentally, on that subject, the crash limiter which uses another metal rectifier, works a treat.

 ongoing

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