Temperature changes and AC128's

[Mike Watterson]

Can you replace the diodes?
Electrolytic capacitors at input side of AC128 or emitters can be noise sources too.

[Crackle]

Sorry guys, I dont know what I was wrong the other day now, I had swapped out tx1 and tx2 and it made no change.
I have just tried the amp with the primary of the audio transformer disconnected and absolutely no noise from the OP pair. (I did this before as well)
Any way I have just swapped TX1 & 2 for a couple of Russian ones and the noise has reduced drastically. I think I may need to look at changing the one I put in tx2 position as I thought these transistors were supposed to be RF ones and not sure how they will cope as being a driver.
   
Thanks for your help

Mike

[boater sam]

That's a familiar looking transistor can, copy of the old Fairchilds?
Would think as its Russian, if it works it will be fine, probably made from recycled T72 tanks.

[Mark Hennessy]

That's good news

The trouble with these old germanium transistors is sometimes they all play up

It would be interesting to try freezing the output transistors again, but with music playing very quietly. I suspect you'll hear a difference as they are cooled...

[Refugee]

There is a nasty looking capacitor in that photo.
It looks like a Callins and going back to the circuit a few posts back it looks like it is in the negative feedback loop.
The Russian transistors are likely to be better at RF and may be getting just enough RF to keep the oscillation down.
Try the old transistors with the capacitor replaced and see if it is better.

[Crackle]

Hi Ref
I tested that capacitor, it is the emitter bypass for TX3, it passed on the Peak tester, but I have replaced so many items on the amp I thought may as well change the last 3 caps as well.

I did the test you suggested Mark, the audio got quite distorted, especially when I froze the other OP transistor. The effect is more noticeable at a low volume than with the volume turned up.
I am leaving the 2 unidentified Russian transistors as they are still quieter than the original OC75's.

I declare the amp ready for operation now, just got to get the rest of the mechanics done.
I started writing up the restoration on UKVRR as it was someone on there who introduced me to the KB Discomatic in the first place.
If you wished to follow it you can see what I have done so far here; http://www.vintage-radio.net/forum/showt...p?t=131587

Thanks guys

Mike

[Mark Hennessy]

Hi Mike,

Thanks for reporting back how that went. That pretty much explains it - transistor gets cold => turns off => doesn't amplifier tiny signals.

I'll take a look at that thread now...

Thanks again,

Mark

[Crackle]

A little update on this thread. I had to change the two bias trimmers, the amp was sounding very distorted, even though I had previously set the bias correctly.
One of the 50 ohm trimmers was measuring 198 ohm and the other about 120 ohm and had an intermittent connection to the wiper.
My recommendation would be to change these trimmers if you come across one in a critical circuit, it is not the first time I have had problems with them in KB amps.

   

Mike

[ppppenguin]

You might also need to change the dilithium crystals

I believe KB abandoned their use as they tended to warp the cabinets