Bush TR82C - help request

[Crackle]

Check the volume control. Connect a wire from the non earthy end of the track to the wiper.
Check C29 (trader) 8uF
Mike

[ppppenguin]

C29 is the audio coupling capacitor from the slider of the volume control. Its value is utterly non critical. 10uF or 22uF would be OK.

Clip of circuit from Trader 1459 attached.

[TinaRotator]

Thanks for the replies!
Boater Sam - yes, the switch appears to be okay. I can hear something on the earphones when I plug them in, and when I unplug them, there is some kind of sound from the speaker. Both are very quiet and faint, so that I can make out that it's human speach, but I am not able to make out words.

Crackle & pppenguin - c29 now replaced with modern electrolytic 10uF 16V, it still sounds the same. Should I try the big capacitor beind the speaker (C25 on bush service sheet) which is 500uF. Would 220uF do? Thanks for the snippet, I cannot see the traders sheet on the free websites,or even the paid ones.

Crackle, sorry could you elaborate? Wiper as in getting inside of the potentiometer for volume or the middle terminal out of three that are on it?

Have a nice evening all,
TinaRotator

[ppppenguin]

I think the 500u cap is C33 on the Trader sheet. Definitely replace. In fact all electrolytics should be replaced. The modern value would be 470u. 1000u would be OK. 220u will work but give slightly reduced bass. I can't see the voltage rating but even 10V is enough. The original may be as little as 4V rated.

By "wiper", Crackle means the centre tag of the volume control.

[TinaRotator]

The C33 aka C25 is now replaced with 220uF 10V. Penguin you are right about the Voltage, the old one was 6V. I will purchase something with higher capacitance soon.

In the meanwhile I was trying what Crackle suggested and I am absolutely amazed - I was playing about with a jumper cable placing it on different parts and by putting it near TR3 I get loud and clear audio from speaker! I am in absolute awe now . Just listened to a bit of Radio 4 news and I couldn't be any happier.

The audio seems to be best on first leg of the housing for IFT3, but it's clearly audible in two other places. How do I sort it out permanently? Should I just solder a wire and call it a day or should I be searching for what's wrong on the path from Wiper to IFT3?

Many thanks,
TinaRotator

[Crackle]

So if you connected a wire as in the diagram below and there was full volume then it indicates a problem with the volume control.

It may be tarnish on the wiper in the control, try winding the control knob back and forth to see if it clears.
Removing the control and dismantling it by bending the 4 tabs out and cleaning the metal wiper surfaces carefully may fix it.
Dont touch the carbon track in the volume control, though a little grease on the carbon track and metal wiper surfaces can help.

Mike
   

[TinaRotator]

I believe that this is what I did. I also connected the legs 2 and 5 of the IFT3 housing like in picture in attachment and tried to change the volume and it worked. I will try to carefully clean the track now. Thanks a lot!
   

[TinaRotator]

1 minute late to edit...
 I just checked the carbon track and it seems to be relatively clean compared to other parts in this set, picture attached. The beginning and end of the carbon track seem to be stripped of the carbon layer. I darent touch it with a pencil or the graphite paint and unfortunately I have only mechanical lubricants which I think wouldn't be suitable. Is silicone grease the preferred type for this application?
   

[Crackle]

I believe that this is what I did. I also connected the legs 2 and 5 of the IFT3 housing like in picture in attachment and tried to change the volume and it worked. I will try to carefully clean the track now. Thanks a lot!

That would indicate the volume control is working.
Check the diode in IFT3 and the resistance between pins 3 and 1 and 6 and 4 of IFT3.
Mike

[TinaRotator]

Hi Crackle, thanks for your reply.
I checked the resistance between the pins as you pointed out.
IFT3 leg 1 - IFT3 leg 3 reads 9 Ohm
IFT3 leg 4 - IFT3 leg 6 reads 1.4 Ohm

I checked the diode in the Diode mode on the multimeter by putting the positive lead on the Leg 6, which should be an anode and negative lead on Leg 5 which should be a cathode. The reading was 2.3V. I swapped the leads around and the reading was again around 2.2V.

I also measured the resistance between 6-5 and vice versa and it was 5 kOhm both ways. I expected that from the 5-6 (cathode to anode) it would read OL but it didn't.

Is my methodology correct and is this enough to conclude that the diode is not working?

The diode marked CD1 on my service sheet (the snippet of schematic is 3 posts above) is OA70, curiously enough listed as a video detector diode on a spec sheet I found online. I am attaching the datasheet I found. I would ideally get something modern to replace this, will any germanium diode do or should I go for new old stock from eBay?
   
Best regards,
TinaRotator