Thanks Enzo :cheesy:

Okay maybe I should be embarrassed here, but I'd been running this amp on a bulb limiter. I removed the bulb limiter and modulation gone!....amp plays great. Anyone care to comment on why this would have happened? Guess the reduced voltage on rails produced incorrect voltages on the components and the amp didn't function as designed, although it produced some good sounds except for the bass modulating/clipping? ..... I remember the same thing happened with a Peavey Renown (after replacing output transistor) the amp sounded crummy and I thought I hadn't fixed it until I removed the limiter and then the amp sounded good    

Hi!

Well, no matter what channel I use in this amp, if I crank the bass on any channel the amp modulates on and off rapidly. I can crank the amp without bass and all sounds good but once I dial the bass in in there is a big modulating problem--like a loud bap,bap, bap sound that continues for quite some time until the sound dies out.  Power supply unable to keep up with low frequencies? Any thoughts? I replaced the output IC but same problem....anyone experienced this before? Suggestions? Thanks.

I also ran the amp through another speaker and the same problem exists.

Also hear clicking sound from power amp when this happens--relay?

Friend picked up amp. It will have to do in its present state--I invested a ton of time on this one...as you say better minds....

My conclusion. One fader cuts out at a certain level. Tried Deoxit and lithium grease but no luck. Other than this fader issue and the loud pop/sometimes squeal at start- up the amp sounds good. Found another SS post that talked about large inrush current being normal for some amps--the Renown has gone as high as 12 amps. It has been suggested to put a standby switch on the speaker, wait for the inrush current to subside, then turn on the speaker. I put a thermistor (7.5 ohms) on primary and that reduced the inrush current to an average of two amps but still get the loud popping. Tried the cap/resistor across AC switch but still heard pop/siren sound. So, basically after 5 seconds this amp is very playable (minus the fader issue of course). .....so is it fair to say that the 6800uf caps are creating this large inrush current and the amp is amplifying the poppingl/siren sound produced by the cap and its mechanical imperfections during inrush? And if that is normal then can we say that the speaker standby switch is a simple effective solution?   

Should I try replacing the op amps? Are they expensive?  TL072CP. Internal shorting?

Thanks gents. Pried sockets half way out and all the way but no difference. Took out preamp board and saw limiter pot was loose so resoldered. Used deoxit on all pots. Noticed that when I turn on amp light bulb limiter stays on for awhile then goes normal. Subsequent turn ons and the light only flashes momentarily and all appears normal. Something at cold startup is eating lots of current--but, as I say, lamp dies out, amp works, and bass sounds good through amp....siren sound occurs when limiter bulb is normal(unlit)..hmmm

Thanks. Still the siren sound. Tomorrow will take the boards out and check for cracked solder joints. I'm sure others have experienced the siren sound I'm talking about... I'll also check The caps. Maybe try applying heat to components to see if I can duplicate the siren sound. .

Thanks Dr Gonz.

Notes. Plugged guitar into PowerAmp, no siren sound.

From Left to Right  along  bottom of Circuit Board we have 7 OpAmps, From Left to Right, along bottom, A,B,D, F,G, H, I
When Siren Sound starts there is Distortion present on third OpAmp (D) from left but not first two (A and B), but when I put the Scope on pin 7 of OpAmp D the siren sound stops.

The following tested when Siren Sound starts: All pins distorted on F and G. On He, 5 and 6 are not distorted but pin 7 is. On I (last one on right) 5,6,7 do not distort, but all other pins do.

On Quad Comparator, no pins distort except for OUT2 and OUT 3. What exactly is it's function in this circuit and where is it shown on the schematic?

Distortion (with siren sound) all across faders. I've tried to trace backwards from the distortion using the traces (hard to see but possible) and have had some luck but I then I feel I'm missing something.

The OPAmps aren't numbered so I find it challenging to figure out which is which. Any insight to which is which?

Is this siren sound a cap charging late, then later on losing its charge and re-charging. Do caps make a siren sound? Is this a case of a cap going bad.

-15Volts/+14.9 at all op amps. Scope the rails  during siren sound but they appear stable.

Single black band? A resistor?  Also, a pic of circuit board.

Here are some schematics. I numbered them as 1,2,3 because I kept getting error messages.

I noticed that turning the third EQ knob all the way to left  immediately kills the sound. I will post schematic and check all other knobs to see if they have any effect on killing the sound. Away for weekend so will post schematic on Monday... I'll also scope rails then...thanks!

This seems to be happening before the EQ but I'm still unable to find the source. Something thermally triggered? When the siren sound starts the opamp voltages stay the same. It sounds like a charging cap is getting into the audio signal  . Any suggestions? Does this sound like a cap issue?

Below is a quote from a website which got me thinking (it was in response to a vox amp making a siren sound when turned on, the same sound my makes)...so I checked the zeners in the power supply and they checked out okay. Measured the ESR of the caps in the 15V supply and they are good...15V at Opamps....tried chopsticking components, pushing down on circuit board to see if maybe some poor solder joints...hmmm

"It's one of the 15 volt regulators in the power supply. They go into a cycle of thermal shutdown, turn back on and charge up the filter capacitor after the regulator....Hence the weird noise. Some do it randomly others do it in regular cycles.
There are two regulators, the LM7815, positive 15 volts and the LM7915, negative 15 volts. I always change both with 1.5 amp rated regulators even though it's usually only one of them causing the problem (always the one that's not hot to the touch, which shows that it's not working). These is the only parts causing that noise, not a capacitor, op amp or anything else. They supply power to all the audio op amps, other analog circuits, and depending on model, they also supply bias power for the tube."