Kustom Groove Bass oscillating with 69kHz - why ?

[Tilman]

So it seems that the output transistors are not shorted. In this case it should be quite easy to find why it is not working - use generator and an oscilloscope. I think that this is quite typical power amp.
Mark

You think so ?
I don`t see why it is easy to find the place where an 70kHz oscillation is coming from when it is all over the place on the power amps board.
Did I mention ? : No schematic.
I have a generator and an oscilloscope and I know how to use them.
Tilman

[MarkusBass]

I don`t see why it is easy to find the place where an 70kHz oscillation is coming from when it is all over the place on the power amps board.

I thought that you mentioned that the amp does not work any more and I ment finding why it is not working (and not finding the source of oscillations).

Did I mention ? : No schematic.

I hope that this is a typical power amp so it has 4-5 places which should be checked. I will look at it this evening and let you know if I have any ideas how to proceed.

Mark

[Tilman]

Great! Thanks! I am looking forward to it.
Tilman

[MarkusBass]

I am looking forward to it.

OK, I looked at the amp. Seems to be typical. You can clearly identify the components that influence the frequency range of the amp. Starting from the output:
- there is a coil and paralel resistor on the output. Check that they are OK.

- it seems that the amp does not have the Zobel network (at least I dot see it),
- close to every output transistor group (4 transistors) there is a driver transistor with 220 resistor and 150pF capacitor. Check that all these capacitors are OK (or replace them).

- close to the input there are two 10pF capacitors. They are most probably in the negative feedback loop (from the output to the input differential stage),

- there is also 270pF capacitor. It is most probably on the input of the differntial stage (to the ground),
- capacitors close the CA3080 most probably also limit the frequency,
- one of the capacitors close to the input plug may be a part of a low-pass filter,

- I wouldn't worry about the speaker protection, thermal sensor and fan driver,


It would be the easiest to desolder the output transistors and solve all the problems that the amp has without the transistors. But since the is a double sided board, you would need something like Chip quik (Chip Quik: Easy removal of surface mounted devices).
The output transistors can also be a source of oscillations. Often a small capacitor is added directly on transistor leads.
If you don't want to remove the output transistors, I would suggest to increase all these "pF" capacitors that I mentioned above.

Let me know whether you found something. Also can you clarify whether the amp is working, or not at all?

EDIT: I would also check what happens if you add a Zobel network on the output. And whether the enclosure is connected to the amp's ground.

Mark