Crown PS-400 power amp is oscillating on channel 2

[Professor Bizzt]

Hello,

I am hoping someone with experience on Crown PS-400 power amps could help me solve this problem.

I have a Crown PS-400 power amplifier with a very high frequency ultrasonic oscillation problem on only one channel (channel 2) that occurs with no load connected to the output or anything connected to the input.

I am using a variac to significantly lower the line voltage and keep amp draw below about .5 amp during these tests to protect the output stage. All tests were done with the Stereo/Mono switch in the Stereo position.

If I remove U1 (8-pin DIP dual op amp used for both channels at the input), the oscillation stops. If I short the inverting input (pin 2) to ground, the oscillation stops. If I short pin 2 and pin 3 (non-inverting input) together, the oscillation stops. This happens whether or not I open-circuit the global feedback loop.

I have checked almost everything and cannot find any defective parts. The source of the oscillation is pin 1 of U1 (channel 2 op amp output). If I put a 22pFd cap in parallel with C202 47pFd compensation cap for U1, the oscilation stops. If I remove this extra cap, the oscillation does not return unless I power down the amp and bring it up slowly again with the variac.

Is it possible that a 47pFd compensation cap is not big enough for this application?
If so, why is it that channel 1 does not oscillate when it uses the same size compensation cap?
Is there another explanation?


Thank you, Professor Bizzt

[Professor Bizzt]

Hello everyone,

The "22pFd" test cap that I was paralleling with C202 was not 22pFd, it was 221pFd. Sorry about my mistake here.


Best wishes.....

[Professor Bizzt]

Hello all,

I soldered a 200pFd cap across C202 and the oscillation stopped. I reassembled the amp, calibrated it, and power tested both channels with a 1kHz sine wave. I then had a look at the small-signal 10kHz square wave performance at the outputs.

Channel 1 produces a nice looking waveform with no overshoot and no ringing and exhibits very slight edge rounding. Channel 2 has a very small amount of overshoot, but is very quickly damped. So this amp still needs more work as far as I'm concerned.


Thanks...

[simon7000]

Crown amps behave differently when powered up slowly than when switched on to full voltage! I have seen it a couple of times. When under warranty Crown service just changes the entire PC card!

I would check the output loading RC network. Parts may have changed value, if not just try a bigger cap or less resistance.