R107 Overhaul 

Looking forward to my next receiver overhaul, I picked the best example from the four R107 receivers I have but found to my dismay that a previous owner had been hamfisted and broken three important grub screws. One secures the outer and two the inner tuning knob. The tuning mechanism wasn't working because the lubricating grease had hardened and the mechanism needed to be completely removed to remedy this. I wonder if the fine tuner needed fixing back in the 1960s, the owner tried to remove it for lubrication, broke the grub screws and then set the receiver on one side... for ever? Certainly everything looks very original, unlike most other examples I've seen. From the lettering on the badge I think this example was made by either McMichael or McMurdo Radio?

 

 Click either picture to see the circuit diagram

 

 I tried to drill out one of the screws but soon found that this was virtually impossible, but remembering how I detached knobs from ancient domestic sets I tried an alternative method. Placing a flat screwdriver blade under opposite sides of the outer knob I was able, little by little, to gently lever it off. It wasn't too difficult and revealed a very rusty shaft. I guess the front of the set must have been damp at one time and this had expanded the screws slightly making them impossible to remove. In a situation like this it's essential to use a well fitting screwdriver blade otherwise the slot in a grub screw will shatter. Once the knob had been removed I was able to work out the next step.

 

 

 
 As the securing screws in the larger knob were completely unusable I decided to use a hammer on the end of the spindle. This worked and the knob just fell off.
 

 The slow motion tuner comprises several parts. There's a large brass slow motion drive assembly which connects to a metal bellows which deals with mechanical alignment inaccuracies, manufacturing tolerances and also provides some tension in the tuning condenser shaft. The drive assembly is held in place by a brass ring that's secured to its inner rim by a couple of screws. To remove the drive assembly for cleaning the bellows needs to be removed and you'll soon discover that the most of the parts are an interference fit and must be juggled around to remove them. This applies to the pointer as well as other parts. The pointer assembly has a collar which is secured to the main shaft by a 6BA screw mating with a tapped ring crimped to the collar. The crimping had failed and it proved very tricky to remove the collar from the main shaft. It seems the resin used to hold all the parts together against loosening from vibration was stronger than the crimping. Detaching the bellows was not easy. This is held in place by six grub screws treated with resin. I dabbed an alcohol solution on the resin but this hadn't any effect so I used a tight-fitting screwdriver which cracked the joints and allowed the grub screws to be slackened. Once the bellows had been removed the pointer could be removed and the brass slow motion assembly carefully manoevered through the front panel. At first sight it looks impossible but after a bit of jiggling it came out. In fact the tolerances are such that the assembly can be drawn straight out if it's exactly square to the panel.

 
   

 The only method I could think of to remove the inner knob was tapping the end of the rusty spindle with a hammer. This worked OK and the whole assembly slid out. You can see the groove in the brass where one of the two grub screws was tightened. The brass ring came off after slackening its two screws. As all the screws were locktighted in place a tight fitting screwdriver is essential.

 

 

 

 The old grease is in a dreadful state and I had to boil the loose metal parts in washing up liquid to remove it. The bearing surfaces above are part of the metal dial backplate and are cleaned in-situ.

 
 

 Removing the damaged grub screws from the knob was impossible but I did drill out enough of the old screws to tap new 4BA holes and fit a couple of new screws. Because the new holes were not now centrally located I had to file the heads of new screws to the same diameter as their threads (electric drill plus file).. ending up with grub screws...

 Above the brass slow motion drive which has the code ZA11840 and serial number 8414 marked on the rim. This example of the R107 dates from 1943 (marked on its block condenser).

 The bellows adaptor which has six securing screws and the spindle coupler. Both need removing before the spindle could be knocked out to detach the seized larger knob.

 

 

 The outer knob which had to be levered off (awkward because it's sunk into the larger knob) and the dial pointer. The latter was also very difficult to remove because its captive nut crimping had failed and the screw was locktighted and access was poor. I had to use a pair of angled pliers to grip the captive nut (in fact it's not a nut but a small tapped circular brass bush not easy to grip).

 Getting smooth action in the slow motion drive was difficult. From the feel of the input shaft I suspect the ball bearings are rusty and splitting the drive looked problematical. I dripped an alcohol solution into the six screw holes used for securing the pressure plates and eventually I was able to turn the output shaft (the one with the peg for limiting the range)... prior to this the output shaft had been seized. Then I dripped oil into the six holes and alternately spun the input and output shafts with an electric drill, turning them until rotation felt unrestricted. It wasn't perfect but infinitely better than when I'd removed it from the receiver. Finally I inserted a lithium grease into the six screw holes and repeated the freeing up with the electric drill then refitted the end pressure plates and adjusted these until the input shaft felt free. Now, when the input shaft is rotated, the output shaft turns properly.

Alas, refitting proved to be a problem. It's essential to fit the parts in the correct sequence and it took me several attempts before I realised this. The direct drive from the larger knob relies on smooth action from correct alignment, proper lubrication and correct tension in the mechanism. Both inner and outer phosphor bronze surfaces on the steel dial backing plate must be cleaned and greased and the brass securing ring postioned just right. It was when tightening the screws in the ring that I noticed the thread on one screw appeared to be stripped as the screw kept popping back when it was tightened. I removed the ring and found the thread was fine but the ring was cracked right across the tapped hole. As the ring is an interference fit and an essential part of the mechanism I decided to repair it rather than make a replacement.

 

 

 

 I have a decent large soldering iron so after cleaning the ring surface with fine emery paper I tinned the outer surface and sweated a flat metal strip bent to fit across the break. I have a box of oddments of phosphor bronze strips so used a part of one cut to size and with a hole positioned over the tapped hole. To prevent solder messing the thread I fitted a temporary screw through the hole. To ensure the ring kept its correct diameter I completed the soldering with the ring held in a small vice. Once the ring was repaired I rubbed its bearing surface to ensure it was perfectly flat and fitted a pair of new screws.
 

 During reassembly I noticed the dial plate could move slightly under pressure. If not attended to this would result in a sort of backlash effect in tuning. I noticed two newish looking 0BA brass screws and although seeming to be tight, I found one could be screwed tighter, removing the dial wobble.

 
 
 

 

The outer bearing surface needs to be clean and greased. The opposite, inner surface likewise.

You'll also note the dial locking screw. When refitting the large dial it needs to be positioned to match the gap in the locking screw clamp. In fact it may be easier to detach the knurled nut and plate before you fit the larger knob.

The reassembled parts is shown below. The dial pointer needs to be carefully positioned so it passes cleanly over the dial. The short bar welded to the output shaft extension is used to restrict movement of the tuning condenser. The two screw heads can be adjusted to limit clockwise and anti-clockwise rotation to prevent straining the tuning condenser.

Later, I examined a second R107 with a working tuner and noticed it had a large phosphor bronze washer fitted between the ring and the rear of the dial plate. This is missing on the example I'm working on and would improve the feel of the direct tuner which is a bit sticky because the tuner drive shaft is not precisely in line with the tuning condenser shaft.

 

 During assembly the tuning condenser needs to be positioned initially at full capacity for the pointer to be secured then at minimum capacity for other parts to be secured. There are 6 adjusting screws for setting the proper resistance within the slow motion assembly, 3 front and 3 rear. These need to be adjusted for smoothest running without slippage under load and without any backlash. You can see two of these screws close to the brass ring above. The screws are arranged so that they can be re-adjusted with the main parts assembled if necessary. As the circular brass plate backing onto the larger knob had lost some black paint I removed what was left. The paint can be removed by rubbing with a piece of wood with a good edge so that the soft brass isn't scratched. Now that I've proved the tuning works properly I'll repaint the brass plate later. I'll also need to clean the dial which is dusty and slightly blemished. Because of the way the R107 is constructed, access for this is a bit awkward.

The centre knob now tunes the set extremely smoothly.

 

 I powered up the receiver but, instead of hearing a comforting hiss from the loudspeaker, I heard a slight hum.. then nothing and occasionally the slight hum would reappear. Switching the wavechange knob didn't result in any crackling, just continued silence. The set, like others from WW2, is fitted with a set of front panel test points but, in the case of the R107, these are not intended to be read as voltages with respect to chassis... instead one probe goes to the reference point (=HT+) and the other probe to the test point and thus measures the voltages across 3Kohm resistors which are located in the valve anode feeds.

The designers, or whoever specified this receiver, only used two types of valves in the receiver proper, with a third type used in the power supply. This was done for perhaps two reasons... firstly to aid field maintenance, and secondly, to help valve manufacturers and the logistics chain to minimise overall holdings. Most superhets of this vintage used the 6K8 or ECH35 as a mixer, but the R107 used the same type of valves employed in other functions. The following table lists these. I can't explain why the dashes are used and why the codes didn't use sequential letters for V2 valves as for V1 types. I recommend making a simple lever-tool to help extract the valves because some are located in awkward spots.

 

 Circuit Code

 Function

 Type

Commercial

 Description
 Monitor Resistor  Test Point

 V1A

 RF Amplifier

ARP34

 EF39

 Pentode

 R6A

 1A

 V2A

 Local Oscillator

 AR21

 EBC33

Double Diode Triode

 R6B

 2A

 V1B

 Mixer

 ARP34

  EF39

 Pentode

 R6D

 1B

 V1C

 1st IF Amplifier

 ARP34

  EF39

 Pentode

 R6E

 1C

 V1D

 2nd IF Amplifier

 ARP34

  EF39

 Pentode

 R6F

 1D

 V2B

 Detector/AVC/AF Amp

 AR21

 EBC33

Double Diode Triode

 R6G

 2B

 V2B'

 Output

 AR21

 EBC33

Double Diode Triode

 R6H

 2B'

 V2A'

 Beat Oscillator

 AR21

 EBC33

Double Diode Triode 

 R6I

 2A'

 V3A

 HT Rectifier

 6X5G

 6X5G

 Full Wave Rectifier

 None

 None
 Reading the servicing details it's apparent that the exact HT voltage wasn't seen as too important, or maybe just varied so much that it would have proved puzzling. Instead valve anode currents are supplied to the fault finders (but expressed as voltages across similar resistors = 3Kohm). This is a really good idea, as is the instruction to monitor the readings and to swap a valve if there was a change.

 

 

 Test Point

 Normal Reading Volts

 Measured Test Volts
 Voltage to chassis

 1A

 15

 19

 165

 2A

 11 or 5

 12

 177

 1B

 11.5

 0

 188

 1C

 16.5

 7.8

 179

 1D

 16.5

 11.3

 167

 2B

 9.5

 12.6

 170

 2B'

 20

 19

 180

 2A'

 0 or 9.5

 7.2

 164

 The usefulness of the table is clearly demonstrated as immediately you can see that the mixer valve isn't drawing any current and there are a couple of anomalies w.r.t. the 1st IF amplifier and the output valve. Another clue to problems are high readings for the RF and LF amplifiers which to me look like leaky condensers. The low value readings might be due to resistors gone high or leaky screen decoupling condensers (or both).

I measured the HT as 203 volts but yesterday it read only 188 volts. Yesterday the HT reservoir/smoothing condenser block was running very warm and today slightly warm. This will be a tightly packed paper condenser (dated 1943) which may or may not recover from its leak... I should really disconnect it because the similar block condenser in my R1155 failed catastrophically all of a sudden... Because of the obvious leak the HT supply will be reduced.

A second fault diagnosis is also possible (and this is interesting) because you can see an HT feed to all the valves (earlier readings).

 Clearly the silence from the loudspeaker can be at least partly explained. Firstly there is no signal into the IF amplifier because the mixer is duff and secondly there is something amiss with the audio section. I can also see that the heater voltage is down a little, reading 12.3 volts at the lamp terminals rather than something slightly in excess of 12.6 volts.

 

 Test Point

 Normal Reading Volts

 Equivalent to Current mA

 Measured Test Volts

 Represents Current mA

 Diagnosis

 Voltage to chassis

 1A

 15

 5.0

 19

6.3

 Fair

 165

 2A

 11 or 5

 3.7 or 1.7

 12

4.0

 OK

 177

 1B

 11.5

 3.8

 0

0

 Bad

 188

 1C

 16.5

 5.5

 7.8

2.6

 Poor

 179

 1D

 16.5

 5.5

 11.3

3.8

 Fair

 167

 2B

 9.5

 3.2

 12.6

4.2

 Fair

 170

 2B'

 20

 6.7

 19

2.4 

 OK

 180

 2A'

 0 or 9.5

 0 or 3.2

 7.2

6.3

 Odd

 164

 I'd like to see if all the receiver features are functioning because this will give me an idea of the length of the overhaul exercise. Once the major anomalies in the measurement table above are cleared up, the receiver should work to some extent on all bands using a long wire aerial. The actual performance will depend primarily on the previous owners expertise.

The R107 has three wavebands.

 Range 1

 17.5MHz-7MHz

 17m-43m

 Range 2

 7.25MHz-2.9MHz

 41m-103m

 Range 3

 3MHz-1.2MHz

 100m-250m

 Here's the crcuit diagram for the R107 split into front end, IF strip/output and power supply. The aerial connections cater for either a long wire or a dipole and the power supply can be driven from a range of mains voltages or a 12 volt battery. The set has three chassis which can theoretically be removed completely from the main chassis after unsoldering groups of jumper wires on tag panels at the rear of the set.

The official repair manual includes lots of resistance and voltage measurements to aid fault finding, however the chief type of fault which will be met some 76 years after its manufacture will not be at all like those for which the authors of the repair manual were familiar. Now, the majority of problems will be due to ageing of resistors and condensers (re-named capacitors at varying times from 1926 to after 1950)

 

 

 

 Here are some slightly better circuit diagrams

 Part 1 (RF)

 Part 2(IF+Audio)

 Part 3(PSU)

 Below you can see the first three duff resistors I found under the chassis. Most of the R107 resistors are of the same style commonly found in receivers dating back to the mid-1930s. They're carbon types and usually oxidation at the junction of the wire ends and the carbon resistor body is the reason for a drift in value. The photos below provide the evidence...

The first is R8A which is supposed to be an 80Kohm resistor.. clearly open circuit..hence the mixer is turned off.

The second is the RF amplifier screen resistor R18A, 25Kohm reading over 660Kohm

The third is R3A the 300ohm cathode resistor for the RF amplifier reading 857ohms.

I also found several other resistors wide of the mark, such as R7A marked 400ohm but reading about 850ohms.

 

 

 

 
 As there was no output to the loudspeaker, I plugged in a pair of headphones. These worked, but with a very loud hum. Clearly the block smoothing condenser is not just very leaky, it's lost much of its capacity. The next step will be to remove this and try a pair of new capacitors before stuffing the original block. I'll also check continuity of the speaker wiring and its on/off switch because I see the headphone output is supposed to be derived from the same transformer. I checked this and found the switch was in fact intermittent but after waggling it lots of times it was much better. The block condenser is different from the last one I looked at because it is 8uF+8uF and has three wires protruding from a central grommet in its base. Because of overcrowding in the power supply the condenser is secured by four 6BA screws mating with captive threaded bushes. Once detached I found the two sections measured 17uF and 17nF on one meter and open circuit on my ESR meter. Resistance-wise these measured 470Kohm and 2.2Mohm. Whatever were the readings neither section behaved as a capacitor in-circuit. Fitting temporary new 10uF electrolytics increased the HT from 188 to 270 volts without much hum. The adjacent 4uF block condenser measured 4uF and 0.12 ohms so seems to be fine.

 

 Test Point

 Normal Volts

 Measured Volts

 Voltage to chassis

 New Readings figures (xx) later
 Comment

 Final Volts
 Comment

 1A

 15

 19

 165

 5.9 (52)

 Low

 22.7

 High

 2A

 11 or 5

 12

 177

 15.3 (15)

 High

 6.6
 

 1B

 11.5

 0

 188

 17.4 (17)

 High

 21.3

 High

 1C

 16.5

 7.8

 179

 20.4 (27)

 High

 13.7
 

 1D

 16.5

 11.3

 167

 18.4 (27)

 High

 15.8
 

 2B

 9.5

 12.6

 170

 17.6 (18.4)

 High

 12.5

 High

 2B'

 20

 19

 180

 24.8 (25.6)

 High

 27.6

 High

 2A'

 0 or 9.5

 7.2

 164

 9.7 (11.5)

 High

 0
 

 The voltages at the test points with new resistors and new smoothing condensers are now all on the high side. An obvious reason for this is that the increased HT voltage (from 188 to 270 volts) has increased the leakage through each of the various decoupling condensers (one of which accompanies the resistors feeding valve anodes, screens and cathodes if a bias resistor is present) and adds to the anode current. I measured the test panel voltages again after an hour or so and some had increased indicating worsened leakges.

The next obvious thing to do is to see if the set receives anything. I turned on my signal generator and set it to 465KHz with amplitude modulation and 100mV output and discovered the IF amplifier was tuned to around 459KHz. The IF transformers are all tuned by trimmers, a lot easier to twiddle than dust cores which can jam and break. The initial plan (before adjusting overall IF response) was to judiciously reset the trimmers. After some twiddling (you need to detach three front panel blanking plates to access some trimmers) I got a decent response at the correct frequency of 465KHz. One or two trimmers didn't peak very well (indicating a problem) but the other six were OK. I managed to get the sensitivity down to 5mV before checking Range 1. Setting the dial to 10MHz I found I could hear a test signal of around 90mV with the generator connected to the leftmost aerial terminal and maybe 80mV when connected across the dipole terminals. Connecting a long wire fails to bring in any stations. Clearly the set is very very deaf and I think the best way forward is now to test the valves. Because the heater voltage is a bit low, any lack of emission will be critical. Once the valves are in good order I'll continue component testing. I found all the EF39s were OK but two EBC33s had poor emission so I replaced these but without too much effect. The receiver isn't quite as deaf as it was but still pretty poor. After a couple of hours only Range 1 was working, pulling in several strong signals, and with the BFO on I could hear some 40m CW. Next, I'll need to replace the decoupling condensers and continue checking resistors and figure out why Ranges 2 and 3 are dead.

 

 This table shows the condition of six R107 paper condensers chosen at random from around 20-30 used in the receiver. Resistance measurements showed up as over 1Mohm each but in a test set their condition was very poor. Using a test voltage of 200 volts applied across the selected condenser in series with a 100Kohm resistor a high impedance voltmeter gave the following readings across the resistor. What this means is when one of these is used to decouple for example a valve screen fed by a 100Kohm resistor only about half the theoretical screen voltage will be present at the valve pin. In addition the decoupling quality will be reduced also.

 100nF = 0.1uF

 Sample

 Marked Value

 Measured Value

 Condenser voltage

 Resistor voltage

 Leakage

1

0.05uF 

 9nF

 108v

 92

 0.92mA

2

0.05uF

 8nF

 138v

 62

 0.62mA

 0.1uF

 163nF

 121v

 79

 0.79mA

4

 0.1uF

 296nF

 27v

 173

 1.73mA

5

 0.1uF

 250nF

 35v

 165

 1.65mA

6

 0.1uF

 155nF

 120v

 80

 0.8mA

 I fitted a sample quantity of new capacitors and, once receiver performance had improved to the point I could hear background hiss and several strongish broadcast stations on the 7-17.5MHz band, I was surprised to still find nothing on the two lower bands. Looking at the front-end circuit diagram shows that wired across the wavechange switch near to the oscillator valve are two resistors marked 25Kohm and 80Kohm (being R4B and R8B). These measured as 32Kohm and open circuit so I fitted new parts. Now all three wavebands are OK. Presumably the local oscillator output voltage was deemed by the designer to be excessive for frequencies below 7MHz when the EBC33 anode voltage was optimum for the highest frequency range (The local oscillator anode feed for Ranges 2 and 3 has R4B in series with R8B but R8A is switched out for Range 1). This explains why there are two voltages given for the front panel test points. Presumably the performance of the EBC33 drops off at frequencies over 7MHz so needs a higher anode voltage to produce an optimum RF level to feed the mixer valve (suppressor grid). In fact, if you study the oscillator circuit you'll see that it's pretty odd. There's no self bias resistor in the cathode and its heater is connected to its cathode. To establish an RF voltage at the cathode for feeding the mixer there's an RF choke shown in the heater connection to pin 2. The description of the oscillator in the technical write-up is a bit strange describing the choke as blocking RF from reaching other parts of the circuit. All things considered... I suspect the final circuitry may have been arrived at by trial and error?

It seems the reliability of the 80Kohm carbon resistor in particular is pretty bad as both R8A and R8B were open circuit.

 

 I decided to just fit 100nF x 500 volt chip capacitors in place of the waxed paper types. Here's one example replacing a 1st IF amplifier decoupling condenser. The lead is solder braid which is flexible to avoid stressing the capacitor body and easy to solder.

Later I used a thin insulated connecting wire taking care to use the same grounding points for the new capacitors.

 

 There are still about a dozen or more old decoupling condensers to replace before the receiver specification can be reliably checked. Also, whilst looking at the oscillator circuitry I noticed that a few screws had been drilled out at the bases of the coil cans. This is probably because the chassis uses a crimped threaded bush rather than just tapped metalwork and the crimp had failed. Maybe this was a standard practice within the manufacturer's factory and the R107 mechanical designer just continued the policy? Alas, the crimping is not too reliable and the bushes can just spin round preventing screws from being removed. The only recourse is to then drill out the screw. I did note that there are official modifications relating to these coils so maybe the problem was encountered when these were being incorporated rather than the last owner drilling the screws, maybe to track down the reason for dead wavebands?

Each time I've carried out a few component changes I tested the receiver. The audio volume is improving gradually but I've noticed that the IF selectivity seems too sharp. As the IF amplifier relies heavily on decoupling condensers I guess that once I've replaced them all the IF alignment will need re-doing. The IF amplifiers are merely tweaked for maximum output rather than stagger tuning (read the Information Sheet by clicking below). I've also noticed the receiver is on the verge of oscillation. Again, this problem will have to wait to be sorted out until I've changed all the waxed paper condensers and replaced all the resistors that are too wide of their marked values.

I finished changing the paper condensers. The receiver is a lot more lively but the IF alignment has moved and I notice there's instability as the RF gain is turned to maximum. I'll next do a final check on resistor values then make sure all the screening is in place before realigning the IF.

The next step was to measure all the resistors. These were mostly high by 30% but this is not too critical in valved receivers so I left all in place. During the improvements to overall sensitivity I'd noticed instability and once all the old condensers had been replaced the instability was too much to ignore. I found it originated in three separate areas, all associated with EF39 valves. Two (the mixer and the 1st IF amplifier) had lost most of their metallizing so I wrapped baco-foil around these and wound bare wire around this connecting it to pin 1 of the valve. This worked fine, comletely stopping the instability around the front end. The third EF39 (2nd IF amlifier) looked OK but the glass was slightly loose and a continuity check between Pin 1 and the metallization showed the wire had broken. The instability here was microphony... tapping the valve produced a howl. Again, I wrapped baco-foil around the valve and connected this to pin1. All the instability had now cleared up.

IF realignment was now straightforward because the instability had cleared up and the receiver sensitivity had increased enormously, receiving broadcasts even with the RF gain turned down to zero. The IF bandwidth switch seems to do what it's designed to do whereas previously not much happened when the switch was turned (probably bad condensers).

Any remaining problems? There are lots of relatively minor problems. I noticed the outer knob of the tuner doesn't rotate when the fine tuner is turned but this didn't really matter (in fact this makes the slow motion tuning extra smooth). AVC doesn't seem to work properly and turning AVC on or off doesn't really change the volume of broadcasts so that needs checking. My guess is there isn't enough voltage being produced by the AVC diode. Maybe there's a bad condenser or an open circuit resistor that I missed in circuit, or even a bad diode in the EBC33?

I added a set of test panel readings entitled "Final Volts" in the table above (note that because all the old resistors are high in value these monitor voltages will also be on the high side. For example.. take V1A, if the monitor resistor is 30% high it will be 3,900 ohms so a 15 volt reading across 3,000 ohms representing 5mA anode current would now result in 19.5 volts)

During testing I found that receiver gain varied by a significant amount if the centre IF transformer was pushed slightly to one side so I temporarily put an elastic band across this and the adjacent tansformer. I spotted this because when gain was down the lower trimmer had no effect on IF tuning. Just before I finished for the day both the lower headphone jack sockets from where I pick up the output for an external speaker failed mechanically (oddly, both 4BA fixing screws securing the socket to the inner front panel seemed to have sheared). Ordinarily, fixing this this wouldn't be much bother but the weight of the R107 makes manipulating the chassis very awkward.

Below is a view showing the power supply module partly detached from the main chassis. The R107 has a set of three modules designed to be removed from the main chassis.. however, complete removal requires lots of wiring to be unsoldered. By removing four 0BA brass screws, and the knob from the headphone volume control, the power supply can be moved back from the front panel sufficiently to examine the securing screws for the headphone jack socket. It's held in place by two long countersunk 2BA bolts secured by a nut and a locknut. I found something very strange. Instead of proper countersunk bolts two standard bolts had been fitted with their heads filed down to allow the module to fit properly. Unfortunately, too much metal had been removed and the modified heads had squeezed through the panel allowing the double jack socket to have be pushed backwards by a tight-fitting jack plug. The socket is a bit special because it needs to be fitted in place behind a front bezel and the metal panel in order for headphone jack to be located correctly.

 

 

 

 These are the two bolts. Both were very difficult to remove because what remained of the heads had pulled into the jack socket body and jammed solid laterally but turned freely preventing the nuts from being unscrewed.

 

 

 

 

 

Below the double jack socket back in place at the upper right of the picture. I needed a cramp to hold the heavy power supply in place while I refitted the four securing screws (the heads are visible through holes in the chassis).

 

 

If the audio filter is selected by its switch the receiver goes completely silent because there's no filter fitted. At first I thought that whoever last had this R107 removed the audio filter can, but reading one of the 1946 technical papers it seems that because these audio filters were problematic, a number of R107 sets had been manufactured without the filter fitted in order to expedite delivery. I recalled that a similar type of filter in my R206 hadn't worked because the paper condensers were in an absolutely awful state. There was a general manufacturing problem with filters of this type because they needed very high tolerance condensers which were not readily available. Because the whole circuit is missing I'll need to find a couple of suitable coils to make a new filter. Looking at what remains of this area of circuitry, I see C10C couples the anode of the AF amplifier to the grid of the output valve and the reading across R6H (3000 ohms) is a trifle high (27.6 volts instead of 20 volts) indicating a possible leak in C10C which is placing a couple of volts of forward bias on the valve raising its anode current from 6.6mA to 9.2mA. I tested C10C but this had absolutely no leakage so the valve itself may not be within spec. Then the penny dropped... a drifted monitor resistor was probably giving the high reading (ie. 38% increase in the 3000 ohm resistor would give the 27.6 volt reading from the correct the anode current).

When I'd first started the overhaul the audio level was very poor so I added a 100uF decoupling capacitor across the cathode resistor of the audio output stage. This dramatically increased the audio level but of course affected the negative feedback so I'll need to step back and see if I should remove the extra capacitor now that there's plenty of receiver gain. Interestingly the output valve is a mere EBC33 whose anode is designed to run at around 6mA and 250 volts which is about 1.2 watts. In class A the output would be around 0.5W. In the R206 there's an EL32 running at 3.5 watts output for a decent loudspeaker volume. In practice I guess the use of the EBC33 may account for some of the audio distortion I can hear on strong broadcasts?

The AVC switch was a bit intermittent but after operating it several times it improved and I could then see the AVC line correctly switching to ground with AVC off. However the audio with AVC on was poor and got much worse the stronger the signals. Measuring the AVC diode output at R2E (250Kohm) showed minus 2.2 volts with the strongest broadcast signal tuned in. I then noticed I'd missed a decoupling condenser (C11K) which is located hidden away against the inner surface of the front panel. I snipped the positive lead to C11K and the AVC jumped to minus 5 volts. I then fitted a new 100nF chip capacitor in its place and checked the AVC line. With the IF response set to wide the AVC was now reading over minus 7 volts and the audio was now clean and undistorted. Turning the AVC switch to off resulted in receiver overload (as expected) with the RF gain pot now controlling the audio level.

I checked some SSB signals on 40 metres and by adjusting the RF gain I got very good results but I then noticed the slow motion drive for the BFO wasn't working because of dried grease.

Alignment of the front end was next on the agenda and the table below lists the RF coil details. From the inductance values you can see that the oscillator is always meant to be on the high side of the incoming signal. As with all superheterodyne receivers it's sometimes difficult to identify whether you're tuned to the "true" signal or its image. A simple test is to use a local communications receiver to listen to the R107 local oscillator. That way you can confirm you're not aligning the set either on its image, or as I disovered recently with my R208 the true frequency at one end of the tuning range and the image at the other!

 

 Waveband (Range)

 RF Amp Grid

 RF Amp Anode

 Mixer Grid

 RF Oscillator

 Oscillator Tunes

 Image

 17.5MHz-7MHz (1)

 L1A=1.6uH

 L4A=1.6uH

 L4B=1.6uH

 L7A=1.5uH

17.965MHz-7.465MHz

 18.43MHz-7.93MHz

 7.25MHz-2.9MHz (2)

 L2A=10.2uH

 L5A=10.2uH

 L5B=10.2uH

 L8A=8.4uH

 7.715MHz-3.365MHz

 8.18MHz-3.83MHz

 3MHz-1.2MHz (3)

 L3A=60.4uH

 L6A=60.4uH

 L6B=60.4uH

 L9A=40.5uH

 3.465MHz-1.645MHz

 3.93MHz-2.11MHz

  The coils are mounted in four metal cans, oscillator (nearest the rear of the chassis), mixer, RF amp and Antenna with the highest frequency coils nearest the chassis to reduce wiring lengths. Coils are adjusted by 2BA brass screws (held in place by nuts) with cores (these are said to be brass rather than iron dust) and the trimmers are air-spaced ceramic condensers. Adjustment of the coils is carried out by tuning the set to the lowest marked frequency on each of the three wavebands, then tuning to the highest marked frequency and adjusting the trimmer for maximum output. By repeating this exercise a few times the dial markings should correspond to received signals. Years ago I aligned my R206 and found one particular coil was too high in inductance so I had to change the iron slug to a brass slug. Inserting a brass slug will reduce the coils inductance and thus it was possible to track the receiver correctly. I found this same problem with the R107. Several RF coils refused to peak at the low end of the three bands. Before going further I need to confirm I'm not adjusting the set to tune the image (or possibly the coil cores have come adfrift from the adjusting screws, which I've heard is a common R107 problem)

Before going further I need to resolve an intermittent issue. I'd noticed that sometimes the receiver would suddenly lose loads of gain. I found two points of note... if the third IF transformer was tapped the gain would improve (or reduce) and on one occasion whilst pondering the fact that the lower trimmer had no effect I tapped the screwdriver in contact with the trimmer and this had dramtically increased the audio output. Clearly something is amiss. I removed the transformer and detached the lid.

 
 Two coils resonated at 465KHz by their trimmers. Note... no metallic contact by any components to the can.

 

 

 

 Above... four 6BA bolts screwed into the two plates and countersunk into bakelite

 Above (top)... the two metal plates used for grounding the IF can

 Looking at the circuit diagram the IF circuit above is shown as T1C and in one (official... see Part 2 (IF + Audio) link below the circuit diagrams) schematic there's an error with a connection missing ... which I corrected. The circuit diagram can be seen correctly in the second example (shown above). The circuit has a pair of tuned 465KHz coils inductively coupled but with a bandwidth switch joining their earthy ends. No matter whether the bandwidth setting is normal or narrow, both coils should tune using their respective trimmers. Oddly, both technical documents I have exclude C26 and C24 but as these are in place and both coils do tune correctly some of the time, the intermittent fault must lie elsewhere. The first step is to check the values of the trimmers C26L and C26F and the three fixed condensers C24E, C23C and C24F. These do seem OK. Also the replacements for C5K, C5L and C11F. There's also C12A which is a tiny 2.2pF coupling condenser, external to the cans, carrying the IF signal from T1B (which at 465KHz has a very high impedance of something like 156Kohm).

Looking at the construction of the transformer there's a puzzling fact. Grounding of the outer can is done through two metal plates held in place at the base of the can by four countersunk 6BA screws. Four further 6BA screws are used to connect these plates to the receiver chassis. My first concern is a build up of oxide between the plates and the can plus poor electrical connection between the eight screws and the metalwork. Sure enough.... see below.

The use of a 2.2pF coupling condenser is, in itself, probably a sound design technique as long as there are no problems. However, if the outer metal can became isolated from ground its action will change. One effect is its change in capacitance to ground and it's likely to have a change in value greater than 2.2pF so will shunt a varying proportion of incoming RF to ground. What about the other three IF cans? Well, these do not have a tiny 2.2pF condenser associated with them, so an unearthed can will not have the same effect... maybe just the odd crackle.

 

 

 

 Above.. inner surfaces of the two metal plates that were supposed to provide earth bonding between the end of the IF can and the chassis.

Not only were these badly oxidised but the red paint used in the factory to confirm the various parts are correctly installed has leaked between the metal surfaces making conductivity worse. The plates are secured by small 6BA countersunk screws whose heads are sunk into the bakelite sheet holding the six solder tags, not the metal of the can. Ordinarily screws with star washers would be used and these would make good electrical contact between their threads and the metal can. As it is, once oxidation began, electrical continuity would suffer resulting in T1C can being electrically isolated (to varying degrees) from the chassis. Earthing of the can was intermittent and could be influenced by wobbling the can. I added an elastic band but, although that helped intially, failed to eliminate intermittent operation later. When the can was wobbled and grounded, signals were around 20dB stronger than when the can was wobbled and floating. Perhaps the change in gain was due to a change in parasitic capacity between the ends of the 2.2pF condenser (=136Kohm @ 465KHz) and chassis? This would shunt a different amount of the RF signal from T1B to chassis. For the theorists... if the parasitic capacitance varied due to the IF can not being grounded by say 100pF (=3.4Kohm @ 465KHz) then the attenuation of the incoming IF (voltage) signal would be 34dB. Not only that but the parasitic capacity might prevent the lower coil from being resonated (which, apart from a distinct loss in loudspeaker volume, is exactly the symptom).

Cleaning the plates with a fine emery paper showed that the metal wasn't even flat. When the plates were cut (back in 1944) the metal deformed resulting in very little metallic contact between the plates and the inside surface of the can especially if a plate with a concave surface faced the can. In fact one plate was in this position with the other having its convex surface facing the can. I also cleaned away oxide from the inner surface of the can with a small rotary wire brush before refitting the refurbished plates. This cleared up the annoying intermittent at T1C.

Below a picture showing the overhauled R107 complete with a full complement of chip capacitors in place of the original 30 or so wax covered 0.1uF (C11)and 0.05uF (C5) condensers.
 

 I reset the IF amplifier to precisely 465KHz using my spectrum analyser. At first, tuning the first transformers appeared to have no effect on the observed 465KHz signal until I realised the RF gain setting set to maximum was causing the final stage to oscillate at exactly 465KHz. Backing off the RF gain sorted things out and I was able to adjust the IF response approximately equally balanced about 465KHz. I found that either increasing or deceasing the RF gain setting made the peak IF response move higher or lower in frequency by a KHz or two. I also found the amplitude of the local oscillator varied considerably across the two shortwave bands. This was probably because I was measuring the amplitude at the control grid of the mixer, which is affected by its tuning condenser, so I'll repeat the measurements at the suppressor grid of the mixer where the output of the local oscillator is injected. Sure enough, when I monitored the local oscillator directly it remained ostensibly constant across each waveband.

Later, I swapped the final IF amplifier valve with a ropey metallising for that serving the first RF amplifier with perfect metallising. The instability that occurred with maximum RF gain cleared up.... then not long afterwards the audio at the loudspeaker dropped considerably. I put my finger on the grid of the output valve with very little effect so I fitted an EBC33 in place of the old AR21. The audio reappeared and normal service was resumed. As a final check I measred the HT voltage. This had risen considerably from when I'd first tackled the receiver and was now reading 300 volts primarily due to getting rid of 30 leaky condensers. When I first turned on the R107 the HT was 203 volts, then dropping to 188 volts when the smoothing condensers failed... then after some decoupling condensers had been swapped it had risen to 270 volts, then after some more, 288 volts and finally 300 volts after all wax condensers had been swapped.

I feel inclined to now close the overhaul exercise and move onto something new even though there are still a few things to clear up.

 Read the R107 Information Sheet published in 1942 (This gives you alignment information etc)

 

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