troubleshooting an Ampeg VH-140C

[spy]

I'm troubleshooting an Ampeg VH140C and I have a problem with the Right (Chorus) channel. The Left (Dry) channel works perfectly. But the right has a noise (hisss) on the signal. It appears after 10 seconds when you open the amp.
Here is the schematic for anyone who would like to help.
With the scope I checked the noise and it's from the output until the collector of Q4. Not at the B and E of Q4 and not at the power supply rails. But it's everywhere else! All the resistors, diodes measure ok and the transistors too. If I'll apply a signal on the input, I have it at the output with the noise on it. It's pretty weird... any help?!

[Enzo]

A noisy transistor won;t test any different from a quiet one. if you have a noise signal at one end of a transistor but not the other end, than I suspect that transistor of being noisy.

[spy]

Thank you Enzo! It seems that Q4 is the faulty transistor!
I'll test it by change it with the left's channel Q4 and see what happens...

[spy]

Unfortunately it wasn't Q4.
How can I find out which transistor is the faulty without change them one by one? There are other 5 transistors around plus the power output pair! Is there any chance to be another part, like a cap or a diode for example?

[Enzo]

Certainly it can be other things. Are you talking Q4 in the power amp or Q4 in the preamp. The one in the preamp doesn't have ECB, so I assume power amp?

Isolate the problem. PLug something into the power amp in jack for that side. Does that kill the noise or does the noise remain? if the noise remains, it is in the power amp, but if that stops the noise then the noise is from the preamp.

Likewise, ther shoul;d be a preamp out jack for each side. Send that signal to another amp for a listen. Is the noise present on that signal?

[spy]

I'm refering to Q4 that it's in the power amp section. The problem is definately at the power amp section. After Q1 and Q2, there is a resistor in parallel with a cap, R12//C10. Before these components the signal is totally clean after these and everywhere else (Q4 until Q11) it's the noise. Except the supply +/- 40V rails!

The amp has two preamp out and amp in, one for each channel. I plug the signal generator at the Amp In of the Right Channel where the problem is. The Left channel is totally clean!

EDIT: The right channel, where the noise appears, the components are numbered Q104, instead of Q4 or C111 instead of C11. But I follow the numbers of the schem...

[spy]

Today I changed Q8 and Q9 with no success!! I'm afraid that at the end I'll change all the transistors and the noise will be still there!

What about the protecting diodes D10 and D11. Do diodes produce noise? The noise is about 2.5Vp-p...

[spy]

Anyone to help?

Would a freeze spray help me to find the faulty-noisy part? I haven't use a freeze spray before....

[Enzo]

It might. If an amp works OK at first, then after it warms up it does something wrong, then freeaze spray might point out what is thermally sensitive.

[spy]

Enzo, do faulty capacitors become noisy? The board have some poly in not so good condition and they are between big 10W resistors...

[Enzo]

They could. Almost any part can get noisy.

[spy]

ok thanks! I'll start to change the parts one by one!