GEC BC5050 receiver.

[DonaldStott]

The attached schematic extract shows C3 0.05uF (highlighted with a RED circle) and due its place in the circuit am I correct in assuming that this needs to be a Y2 type?

C1 is a silver mica cap and will be left well alone.

[Station X]

In a word "No". Class X capacitors go across the mains supply and Class Y capacitors go between the mains supply and chassis. Neither consideration applies here.

[DonaldStott]

Thanks Graham - succinct and clear.

So a 0.05uF 630V polypropylene will suffice?

[Station X]

Yes, but it'll probably be 0.047uF or 47NF.

[DonaldStott]

As I mentioned in Post #12 above "Normally with caps I would replace then test one at a time but as we had an internal short between one of the anodes and the filament in the UL50 rectifier valve I'm not taking any chances!"

So I've made slow but steady progress with restoring this set and have replaced the waxy caps, a few resistors and the electrolytics. All the previous 'work' was removed and the new electrolytics were connected using solder tag strips with the -ve of C17 and C25 going to the chassis. In Post #7 above Lawrence noted that for C24 the " -ve connection is to the center tap of the HT secondary winding" and this is how the new electrolytic was connected.

With the shorted U50 being replaced with a 5Y3GT, very kindly supplied by Alan (snowman_al), I was fairly confident with my work and proceeded to a first power on but with the lamp limiter of course. Thankfully the limiter went through its normal cycle of a bright bulb initially which dimmed as the valve heaters warmed up and gradually brightened again - relief all round, no short!

BUT - no audio from the speaker whatsoever although there was some some scratchy noises from the volume control and a loud beep when touching the probe of my Velleman 'K7000' Signal Injector/Tracer to the wiper. To me that indicates that the audio output stages are working but nothing else at the moment? After about 10 minutes I did notice that both V5 (rectifier) and V4 (audio output) were warm to the touch while all other valves seemed to have not warmed up at all?

One of the things I noticed way back at the start of this restoration but didn't mention was that several of the valves seem to have been substituted? We know that V5 is now a 5Y3GT, V4 should be a KT61 but has no markings, V3 is correct i.e. a DH63, V2 should be a W61 but is a 6K70 while V1 should be a X61M but is an ECH35? I have Paul Stenning's 'Valve and Transistor Data DVD-ROM' so will check if the substitutions are correct before moving on to check valve pin voltages while tracing the signal path with my Velleman to find the point(s) of failure.

[DonaldStott]

Initial checks show that for V1 the ECH35 is a suitable substitute for the X61M but for V2 the 6K7G (not 6K70 as detailed above) is a suitable substitute for the W63 but perhaps not the W61?

For the W61 the Valve Museum has these details: -



And these for the W63:-



Differences in Vs, mAa, mAs and gm - to what extent are they critical?

[DonaldStott]

Working my way through the valve pin voltages and so far so good with V5 and V4 (values in brackets are from Trader Sheet 925) :-

V5 (5Y3GT)

Pins 2 and 8 (Filaments) : 268V AC (265V AC)
Pins 4 and 6 (Anodes) : 276V (279V)

V4 (KT61)

Pin 3 (Anode) : 247V (253V)
Pin 4 (Screen) : 209V (201V)
Pin 5 (Control) : 0.01V
Pin 8 (Cathode) : 4.8V (3.7V)

Moving on to V3 (KT61) and there is a rat's nest of components and wires connected to these pins which will take a while to penetrate! I have found, however, that R18 which should measure 680kΩ is actually 0.5Ω so clearly needs replaced before we go any further! This resistor is difficult to get at to extricate and replace and I'm always impressed as to how these sets were assembled in the first place.

R18 is in parallel with a 200pF mica cap and is associated with the waveband switch, S5 in particular, so not sure what this component does and what effect almost no resistance will have - see schematic extract below:-

[cathoderay57]

R18 which should measure 680kΩ is actually 0.5Ω

Probably you have taken the resistance measurement with S5 closed, hence only 0.5Ω. S5 being one of the wavechange switches is closed only on SW. It forms an audio filter that affects higher frequencies presumably intended to aid intelligibility of SW signals. If you switch to LW or MW the resistance measurement should reflect something close to 680k. Jerry

[Station X]

Reading the notes in the service sheet, R5 should only be closed when the set is switched to shortwave. Try measuring R18 with the wavechange switch in all positions.

EDIt, Cross posted with Jerry.

[ms660]

Originally Posted by DonaldStott

V5 (5Y3GT)

Pins 2 and 8 (Filaments) : 268V AC (265V AC)
Pins 4 and 6 (Anodes) : 276V (279V)

WRT chassis the voltage on the filaments should be DC.

WRT chassis the voltage on the anodes should be AC.

Trader sheet gives 279 Volts DC and 265 Volts AC respectively.

Lawrence.

[cathoderay57]

Sounds like the audio section is fine (Post #25). AFAIK the Velleman only generates audio frequency and so isn't going to be much use for injecting a signal into the IF stage. If you don't have an RF sig gen then the most sensible next step is to measure the anode and grid2 voltages of the IF valve (6K7G) and mixer/oscillator (ECH35). Jerry

[DonaldStott]

Apologies Lawrence - got my Anodes and Filaments transposed!

Should be:-

V5 (5Y3GT)

Pins 2 and 8 (Filaments) : 276V (279V)
Pins 4 and 6 (Anodes) : 268V AC (265V AC)

And thanks to Jerry (cathoderay57) and Graham (Station X) as S5 was indeed closed i.e. on SW with knob turned fully clockwise.

Measuring again on LW and MW we have 740kΩ which is slightly high (about 9% over).

[snowman_al]

The X61M is fine subbed by an ECH35.
The 6K7 should 'work' in place of the W61, they have the same pin out and are variable mu types. Might not be perfect, but enough to prove the set works, as long as it (the 6K7) is working.

Ignore the 740kΩ variation, it won't make any difference.

[DonaldStott]

Thanks snowman_al - all points noted.

Meantime I’ve had a bit of a breakthrough after chasing down a loose connection and changing a few resistors - the radio burst into life but I’d love to know what changed? Relief all round!!!

Reception on MW is ok but very noisy while LW only has a very distant BBC Radio 4!

Now that I’ve got basic functionality I can move on to improving reception, volume and sensitivity - any suggestions with this GEC set? As this radio has been ‘got at’ I’m wondering if the Phantom has been twiddling with the alignment?

I really thought I was stuck but ripping out all the previous ‘work’ was clearly the way to go.

[DonaldStott]

Following a family trip to London and a nasty bout of Covid I'm now feeling as if I can get back on track with this GEC set - fatigue and brain fog permitting!

As I mentioned, I'm working forward from V1 checking the voltages simply because this valve and V2 are easier to get at - the underside of V3 is a real 'rat's nest' and will wait until I have more patience!

I did manage to check V1 (X61M) and the voltages are all bit high (figures in brackets are from Trader Service Sheet):-

Pin 3 (Anode) : 150.7V (146V)
Pin 4 (Screen Grid) : 81V (64V)
Pin 6 (Oscillator Anode) : 92.6V (74V)
Pin 8 (Cathode) : 0.0001V (N/A)

Top Cap Grid : -2.0V

I'm unsure to what extent these values are problematic or what I can do if they are? I've changed all the waxy caps in and around V1 but none of the resistors - most of them are over value but within 10-20%?

My internal AM aerial is about 30 feet of wire pinned along the picture rail - anything I can do to improve that? Please don't mention that I need an external aerial as this is a non-starter!

[cathoderay57]

Hi Donald, those voltages look acceptable to me; how does R4 LW reception compare to other valve sets that you might have? You might be able to improve the LW signal by carefully adjusting C28 trimmer. Sorry to hear you have had Covid and glad to hear you are feeling better. Cheers, Jerry

[DonaldStott]

Originally Posted by cathoderay57

Hi Donald, those voltages look acceptable to me; how does R4 LW reception compare to other valve sets that you might have? You might be able to improve the LW signal by carefully adjusting C28 trimmer. Sorry to hear you have had Covid and glad to hear you are feeling better. Cheers, Jerry

Thanks Jerry for your kind words.

R4 LW reception is much poorer (fainter and distorted) to other valve sets that I have so I'll have a quick look at adjusting C28 but don't want to make things worse!

As I mentioned in Post #25 above "As this radio has been ‘got at’ I’m wondering if the Phantom has been twiddling with the alignment?"

[DonaldStott]

It's time for one of those Laugh Out Loud (almost) moments as the young people would say!

Looked out my plastic trimming tool (nylon) and touched it to C28 just to ascertain how loose it was or otherwise. No more than about an eighth of a turn and then back again.

Result - I have now lost R4 LW completely!

Does that now make me 'Apprentice to the Phantom Twiddler'?

[cathoderay57]

Hi Donald, that's odd. Going back to MW, apart from background noise, are signals received at reasonable strength? Background noise is pretty normal these days, unfortunately, coming from LEDs, routers and other power supplies around the house. On LW check the soldered connections to C28 particularly if there are any fine wires connecting the solder tags to L4. Are either of the wires hanging loose? If there is any oxidation or corrosion on the C28 trimmer screw, washer or contact electrode then what you have experienced sometimes happens. Try gently turing the srew back and forth to clean up the contact and see if you can get recpetion back. Jerry

[DonaldStott]

Originally Posted by cathoderay57

Hi Donald, that's odd. Going back to MW, apart from background noise, are signals received at reasonable strength? Background noise is pretty normal these days, unfortunately, coming from LEDs, routers and other power supplies around the house. On LW check the soldered connections to C28 particularly if there are any fine wires connecting the solder tags to L4. Are either of the wires hanging loose? If there is any oxidation or corrosion on the C28 trimmer screw, washer or contact electrode then what you have experienced sometimes happens. Try gently turing the srew back and forth to clean up the contact and see if you can get recpetion back. Jerry

Thanks Jerry.

MW strength and reception are reasonable, usual background noise, just seems to be a problem with LW.

Had another listen today and R4 LW is just about audible at full volume which is reassuring as I thought I had lost it completely? Given the measured V1 voltages and the fact that I have reception on MW I'm assuming that the local oscillator is working and the frequency changer is doing it's thing?

I've already cleaned the wave change switch with contact cleaner but will go back and have another go.

I'll carefully examine all wires and connections associated with C28/L4 and for any oxidation or corrosion on the C28 trimmer screw, washer or contact electrode.

Might be time to connect up the 'scope (using my isolation transformer of course) to try and find where the LW problem lies?