Mysteriously Mute Maestro

[Joe]

I've posted about this before as it was my first attempted restoration. Since then our membership has grown so I'm hoping that there may be some new ideas available. Besides that the original thread got very messy so a re-cap seems in order.

The set in question is a Pilot Little Maestro T105. When I first got it the dropper was o/c. There was no sign of previous fiddling as all the joints still had anti-tamper paint on them.

I changed all the capacitors, except for the low value ones and one half of the smoothing can, and most of the resistors.

When switched on there are a few 'static' noises but it won't tune into anything. If I connect a crystal ear-piece to pin 2 of the UCH81 I can tune it in which makes me think that the wave change switch, ferrite aerial and tuning capacitor are all in order.

The output stage is also in working order as an MP3 player or whatever connected to it sounds great. This however has been one cause of past confusion as the published data states that the output valve is a UCL82 when in fact it is a UCL83 and this number is stamped into the chassis.

Injecting a circa 4 Kc/s signal into just about anything causes it to come out of the speaker.

I have checked what I have done against the circuit diagram on multiple occasions and also against the photographs I took before I started work on it. I also found a photograph of another restored example and can't see how mine differs from that either.

All the coils seem to have continuity so I am lead to the conclusion that the fault lays either with one of the small capacitors, a valve or that the alignment is a mile out despite there being no sign of it been tampered with.

The valve voltages are as follows (digital meter, relative to chassis):-

V1 (UCH81)

Pin 1 = 86V
Pin 2 = 0V
Pin 3 = 0V
Pin 6 = 150V
Pin 7 = 0V
Pin 8 = 142V
Pin 9 = 0V

V2 (UBF80)

Pin 1 = 85V
Pin 2 = 0V
Pin 3 = 0V
Pin 6 = 145V
Pin 7 = 0V
Pin 8 = 0V
Pin 9 = 0V

V3 (UCL83)

Pin 1 = 63V
Pin 2 = 6V
Pin 3 = 0V
Pin 6 = 65V
Pin 7 = 178V
Pin 8 = 145V
Pin 9 = 0V

It must be something obvious but I think I've looked at it for so long that I can't see the wood for the trees.

I should warn potential helpers that my knowledge is still small and I don't have large stocks of spares.

Thanks in advance

- Joe

[tiv48]

hi jo the fact that there is 140 volt on pin 8 of the uch 81 (triode anode) is wrong should be 75 volts this trode is an oscillator thats not oscillating i would look at c11 (110p) and c 13( 470 p)
rob t

[Joe]

Thanks, Rob. They should be easy enough to get, or maybe even find.

Cheers
- Joe

[Steve_P]

That voltage on Pin 8 seems rather high. The sheet has 75v here! Check the socket, in case the pin isn't making contact. Also, make sure the tuning cap isn't shorted. Clean the wavechange switch thoroughly,

Have you got a signal generator? If you have, the IF of this set is 470kHz. Does this come out OK.

R4 should be 47K. Pin 3 of the valve should be decked. Trimmers C8 and C9 could be shorted. Check C13, C12, C11.

Try a replacement valve as well.

Cheers,

Steve P.

[tiv48]

just as an aside find L3 and check for any resistance from any of its pins to chassis (earth )there should be none.its not unknown for the trimmer caps (c8)TO GO LEAKY because of contamination or fiddling
rob t

[Aerodyne]

Joe, your meter should give a negative kick when applied to the grid of the oscillator, pin 9. Set to volts. No kick, no oscillation. That should narrow the search down to related components. The triode anode voltage is far too high, indicating no oscillation. It should be around half that voltage. Judging from the 150V on pin 6, your HT level is down about 20 volts too. That won't help the local oscillator, though it isn't likely to be the cause of the problem unless your valve is low in emission.
-Tony

[Joe]

Thanks, lads. Here's a up-date.

I found a ceramic capacitor marked 471K and fitted that as C13. This made no odds.

I managed to find a polystyrene one marked 121J which I tried as both C11 and C12 (trying not to think about the voltage rating) but this made no difference either.

There's no sign of earth faults on L3.

I don't have a signal generator and can't see there being anything amiss with the wave change switch or tuning capacitor given that it works fine as a crystal set.

I tried Tony's test on pin 9 and the AVO needle did move backwards SLIGHTLY on the 2.5VDC range. It wasn't enough to measure so I put the digital one on and that read 0.3V positive, as did the AVO when I tried that again. This seems like it either changed as the set fully warmed up or the polystyrene cap had had enough.

I'm starting to suspect the valve. When I first looked at this set and found the o/c dropper I got in touch with a professional restorer with a view of purchasing a replacement. I also mentioned that this valve looked very silvered within but he replied that this was nothing to worry about and it's a 'milky' appearance that is a cause for concern. I've since seen quite a few valves but have never seen anything else approaching this though. Here are some photos so see what you think. Needless to say I haven't got a spare.

   

I certainly wouldn't be surprised if it is faulty as the resistors I replaced were so far out it could well have been working well outside its rating before the dropper went.

Thanks for the help, it's nice to at least have the area of difficulty narrowed down.

- Joe

EDIT: I should have said this before but I'm working with an ERT Service Chart though also have access to the R&TS version.

[Aerodyne]

I believe the silvering is due to HF gettering during production, even though these valves also use an active getter. I wouldn't concern yourself over the silver but I do think the valve is very likely to be the culprit. Pity you haven't got a spare. Anyone care to help?
-Tony

[Joe]

I've just been looking on e-Bay (and wishing I hadn't after seeing the prices!) and there's a photo of a boxed one that's nearly as 'bad' as mine.

I dare say I'll come across another to at least test it with as radios seem to keep coming my way and I've got a definite preference for AC/DC ones.

Thanks
- Joe

[OldTickTock]

Joe,

your 's FOC, pm me your address as I lost it from before.

Chris