Peavey Classic 120/120 dual mono repair

[Jon Snell]

I would check the NFB loops, If one channel has the output transformer not ground or floating or NFB from the wrong amplifier goes to the other amplifier, it will sound like a fog horn and be uncontrollable.
Look for a broken link on the sub board.

[76jimmy]

Jazz, The 50/50 page looks like the same board as the 120/120, good find.

Jon, I'm an amateur/hobby amp repair guy. I did find the problem was a bad cable that connected pin J200, J204 and J206 to the patch board. I would like to know what a NFB loop or floating NFB is? Like I said I found the faulty component but I still don't know why. Stranger is why each channel operated independently but nut in stereo. I'm only asking to help my understanding.

[Jon Snell]

My apologies.
NFB stands for Negative Feed Back, (known as CH1 or CH2 Feedback in this amplifier, points J300 and J304) and this amplifier uses it to help control the non linearity of the amplifier. It helps to control the back ground noise and overall gain of the amplifier.
If a ground connection breaks then there may be too much NFB and after a certain point, NFB becomes PFB or Positive Feed Back. (Think microphone pointed at the main speaker and a high output whistle/whine is produced).
The amplifier turns into an oscillator and typically "Takes Off" at anything from and including almost any frequency.
If the NFB from one amplifier feeds into the other amplifier, due to poor connection within a connector, (J200 etc), either or both amplifiers will turn into high powered oscillators of uncontrollable power.

I hope that goes a way to explains it for you.