Legend rock n roll 50

[pontiacpete]

This amp is doing something strange. The volume drops to less than half and is very distorted. The weird thing is that it drops out when I just slap or play something on the guitar strings hard. Then it comes back on after a little bit of time, like anytime between few seconds to longer. For the longest time I thought is was a connection issue on one of the boards. I re-soldered both a couple of times. I’ve chop sticked and pulled on components but the only thing that makes it do the volume drop is hit the strings hard or some notes or chords hard..
It seems to be a problem in the power amp board because when jumping the pre amp over to another amp it works fine but inserting into the power amp in the problem is there.
I’m at the point where i think it is something inside a component that’s loose. Is this possible?
Does anyone have an idea on where to go with this, I’m at a lost?
Also, is there a schematic for this? I’ve been able to find anything to fit this.

[mikeydee77]

Could it be speaker cone jamming up?

[pontiacpete]

I’m using one my own speaker cabinets.

[Mick Bailey]

It could be that one half of the output is dropping. Scoping the output would quickly identify this. An internal break in a component is possible, but my first check would be to see if there's a circular crack around a component leg especially one that carries more current. Check the emitter resistors, output transistors etc. Another possibility is it could be a thermal fault with a component, which may respond to freeze spray.

[Jon Snell]

http://www.prowessamplifiers.com/sch...d_Roll_50.html

[nosaj]

What is the health of the power supply caps?
nosaj

[g1]

I’m at the point where i think it is something inside a component that’s loose. Is this possible?

I have had components that have broken legs but are glued down to the board cause similar issues. The only way I found them was heating up the solder joint and finding the leg moving freely while the component stayed in place.

[Chuck H]

It seems like something may be causing a slower than normal charge to peak voltage capability for peaks. This could be a flaw in the filter cap assembly or, more likely, a flaw in the resistance in the power supply increasing the the time it takes for the filter caps to charge. That's where I would look first.

[J M Fahey]

DO-NOT-GUESS

Inject 100mV 1kHz at the input, in this particular case (you want to drive power amp HARD) set all tone controls to10 and drive it hard, to clipping and beyond.

If it does not cut on its own, help problem by chopping boards, jacks, pots, and components one-by-one , while it´s still at full power.

Agree with Mick, you may very well be losing half the waveform, scope will tell which.

Bench instruments were invented for a reason.

If visuals were enough, Eagles would be best Techs in the World.

[pontiacpete]

Thanks for all the suggestions. Here’s what i have so far. The filter caps are original. I have monitored them when the amp volume drops. When I inject a signal and scoped the output and I get this wave. I’m not able to get the drop off of sound like I can when using the guitar. It seems to go into the dropped mode right away, perhaps it’s oscillating.
I also took some pictures of the bottom of the board and the two other resistors between the output transistors.
Resoldered the power board again, paying attention to anything that might move freely like G1 mentions.

[Helmholtz]

Looks like bursts of HF oscillation.. Unfortunately one can't see the scale settings.
If you expand the time base you should be able to see the HF itself.

[pontiacpete]

I also tried the freeze spray with no reaction from any component.
How’s this picture?

[nosaj]

Looking at the circuit board IMHO I would just replace the filters. Cause right now your driving the car with some old dryrotted tires. And a blowout won't be pretty.
nosaj

[pontiacpete]

I just put some new main filter caps in and still not working properly.

[nosaj]

So how do your voltages lineup with the schematic? What does J2 pins 2 and 3 show before and after oscillations?
nosaj