Hacker RP33 Autocrat

[Skywave]

O.K. Joe. I'm very pleased that you didn't take that post personally. As I said, all I want to do is to help you get this fixed - but perhaps more importantly (and I do not wish to sound condescending nor patronising) - contribute to your learning curve. After all, I know that you yourself have said (in one of your posts on this forum) that transistors are something you don't fully understand. And I feel that that puts you in a club that has many members - and, it must be said, to a certain extent, myself.

Anyway, I'll conclude with this suggestion. Get the right parts, fit them and let us then know how things turn out.

Aside.
If you are going to spend £ buying those transistors, it may be as well to get several, since the chances are that one day you'll go down a similar path as this one. But then you'll be able to say "Ah-Ha! I recognise these symptoms. I know what's causing that . . . . " and simply get on with it.

Experience is a wonderful thing - but it's often hard earnt.

Al.

[Mark Hennessy]

I tried Alan's idea with mixed results. A cap between the collector and emiter of T4 and/or T5 sent it utterly Deolali

Alan said collector-base, not collector-emitter. I'm not surprised it took off - effectively you were asking it drive the caps in parallel, and amplifiers don't like driving capacitive loads (like Quad ESLs).

When you connect between collector and base, you are introducing negative feedback at high frequencies, and effectively slowing down the silicon transistors to act like slow old germaniums... Actually, all transistors have internal capacitance between base and collector, and this is one of the factors that determines the speed of the transistor.

As I said before, emitter followers are especially prone to bursting out in oscillation, and this tendency is strongly dependent on collector current, as you've demonstrated. Putting ~100 ohms in series with the base (can be done above the PCB - pull out the base and fold it up and joint it to a vertical resistor) is much more elegant and effective for an EF circuit. Not surprisingly, these resistors are known as "stoppers" - you'll note that term in the most recent document I've sent you

Give it a go...

[Joe]

I settled this set's hash months ago by building a new audioboard based on a TBA820M. This works very nicely and cost less than I sold the wrecked original for!

   

- Joe

[boiss]

When I got my Sov2 I had similar probs with distortion and instability, distortion was caused by breaks in a couple of Pcb tracks near the pads and instability in AM by the main decoupling cap.
Once I had a Sony KV1300 tv, which used to blow around 20 transistors when the PSU packed up, £60 per hit, after the second time I built a new Analogue PSU.
I've often wondered what the real prob was, Sony reckoned a noisy thermistor.....
Back to the Hacker, I would have kept the original board and sorted it later or sent it to Mark !!!!!!!!
Mike.