Peavey Century 200 Series

[spartadrive_in]

Problem with Treble

Here is another problem that has been present but since the amp wasn't even working at the time I left it out.

The amp works great now. It hasn't busted at all. The problem is when I raise the TREBLE knob to about 10 o'clock or more I get a lot of squealing. What could cause this?

Thanks.

[Enzo]

It is unstable, who knows why. COUld be lead dress, could be lack of ground at the input, etc.

[spartadrive_in]

A friend of mine suggested that maybe the pot was replaced with the wrong value. Would this be a valid reason? I have yet to open it up and measure it.

[Enzo]

Wouldn't be my first choice, but it takes just a second to find out. SHould be able to turn the knob all the way up.

Do the DC levels around the final couple stages of the preamp come close to the schematic? Does turning that control affect them?

See those 2uf/35v electrolytics at the emitters of the emitter follower stages? The third, fifth and seventh transistors in the top row? They may be tantalum on the board. On the righthand end of them - on the schematic, I have no idea how they face on the board - there should be no DC. Is any there? They may be leaky and that is upsetting things.

Across the top of the drawing, see those 100uf/25v electrolytics? They filter the +23, +21, +19 volts for the transistors. These are decoupling caps. If one or more of those is dried out, then the stages may not be decoupled well enough.

I assume it screams even without the speaker - scope the speaker out or measure AC volts there without a speaker but with the treble cranked. if it is there, then scope or AC measure the voltage at each of those caps. There should be darn little ripple there, and if you see AC voltage increasing and decreasing as you turn the treble up and down into screaming and out, then those caps are not doing their job. A lack of decouplign can make an amp unstable.