Peavey CS800X 50 watts per side max?

[booj]

peavey_cs800x.pdf

The same thing happens on both sides, with the DDT switch on or off. With the level control maxed, the amp will put out about 14 volts with a 600mv signal in, with or without a load. As soon as the input goes over that level the amp drops out to no output. When I bring the input signal level back down to 600mv or less the output comes back. the power supply remains constant at about +/- 78 volts. Other than the board under the fan speed limiting resistor, R103 being kind of toasted looking, no signs other wise of distress. Amp looks clean inside. Is there a circuit that can shut down the power amp with a bad signal input?

[Jazz P Bass]

So the +/- 78 Vdc supply is good.
Is it ripple free?

How about the +/- 15 Vdc supplies?

Have you tested both the Stereo & the Bridge modes?

Pay particular attention to how you have the inputs wired.
http://peavey.com/media/pdf/manuals/80300991.pdf

At full power, unclipped, you should see 45 Vac at the outputs into an 8 ohm load with a 1.4Vac input signal.

[booj]

Thanks for your response, Jazz! I'm getting <60MV ripple on both +/- main supply all conditions, and +15.x and -13.9 volts with negligeble ripple. On the inputs, there are no plug-ins, both PL-2 buttons are out, DDT is defeated and it's in stereo mode.

[drewl]

You have the amp hooked up to a speaker or load?
What is the output unloaded?

I think I had a similar problem years ago that was caused by a bad preamp or limiter chip.

Another cause could be bad output drivers, measure the emitter resistors to see if any are open.
But if both channels are cutting out at the same point its probably something in the preamp.

[J M Fahey]

Enzo sometimes said that the limiter may be faulty but since it's not actually needed, it's an external add on, he just bypassed it for testing.
Don't exactly remember how, but the post (as well as Tons of Enzo knowledge) must be lurking somewhere all around us.

[Jazz P Bass]

That -13.9 Vdc seems odd.
Have you measured the Vac ripple on the two '15' volt supplies?

[booj]

Hi Drewl, it does the same thing whether there is a 4 ohm load on it or not. I'm thinking yes, a bad limiter or preamp chip also. I wouldn't think the emitter resistors would be open if both sides put out 50 watts though?
JM, that looks like what I'm trying to do, or at least that's the direction I'm headed for now.
Jazz, yes it does seem odd, but I've seen it before. Like I posted the ripple on the preamp supply is negligeble.

[Jazz P Bass]

Like I posted the ripple on the preamp supply is negligeble.

So what is common to both amps?

[booj]

I'm thinking that too, Jazz. That would logically lead to the power supply. Just because the voltages I can see aren't changing, that doesn't mean they aren't changing somewhere else. I found one thread related to a limiter circuit, but no real information, and a certain Q6 on a drive board, but there is no Q6 on this amp. I guess I'll have to basically take it apart and check as I go. I'm afraid it will start working and I won't know what happened. ahh well, here we go!

[Jazz P Bass]

How about a funky contact on the stereo/ bridge switch?

Try it bridge mode & see what happens.
No load & then with a load.

[booj]

Found it. After re-flowing the solder on the 400 ohm 20 watt fan speed limiting resistor and replacing C50, a .22uf / 250v cap that looked toasted due to proximity with the 20 watt resistor, I put it back together and tried it. It would just sit there and oscillate on and off. Then I lifted the board up and started another inspection. I found a red/yel wire with a lug just dangling in the air. I also saw a handwritten note saying red/yel wire by one of the P.S. capacitor lug connecions. Problem was, there was only room for the two white wires I had previously connected. I took a flashlight and inspected the chassis and sure enough found a spade lug "U" connector lying loose in the chassis. Hooked it up along with the two white wires and it tested ok at full power into 2 ohms.

[J M Fahey]

WEIRD that somebody went as far as pulling a spade connector/wire.leaving a handwritten note ... and not replacing it