Peavey Century 200 Series

[spartadrive_in]

I have this old head dated from '78 that works when it wants to.
It always powers on but sometimes the sound will be dead. Play for a few minutes then it dies off, starts off dead, or will play for an hour and nothing wrong.
I've already been in there and I see no visual explanation. No cracks in lines, cold joints, or loose connections. I brought out the DMM and did some voltage probing and value reading. Everything checks out.

Does anyone know where in the circuit it might be so that I can focus on an area. I have the schematic if you need to see it.

THANKS

[52 Bill]

Sound like the usual power amp/fx in/out jack problem. Try cleaning the switching contacts on the fx and/or pre-out power-in jacks.

[spartadrive_in]

There is no FX in.
So then you think the jacks are dirty? Would that reallly cause a loss of sound. I mean I jiggle cords around and reinsert them. Wouldn't that at least create a in/out/in/out sort of sound you know?

[52 Bill]

Dirty break jacks are a very common problem with all amps. A lot of it depends on where and how the amp was used. The metal contacts on the jacks become oxidized or coated with smoke, tar, nicotine, grease, etc. and then the connection from the preamp to the power amp opens up and no more sound.

The next time the sound dies, if your amp has a pre-amp out and a power amp in, try plugging in a cord in here to bypass the jack's switch contact. If the sound comes back, then you have found the problem.

[Enzo]

The B series Century has op amp ICs and it does have teh power amp in jack, so do take 52's advice. The older A series Century has no such jacks. In that case I'd be looking at the connections between front (preamp) and rear (power amp) panels

[spartadrive_in]

The B series Century has op amp ICs and it does have teh power amp in jack, so do take 52's advice. The older A series Century has no such jacks. In that case I'd be looking at the connections between front (preamp) and rear (power amp) panels

This one is a bit older so it DOES NOT have ICs. I also have the schematic of that one and I did see those.
My amp doesn't have the power in jacks. Just Booster and Footswitch.

What do you mean by front (preamp) and rear (power amp) panels. the actual panels that hold the knobs in the front and the power plug and all in the back?

thanks.

[Enzo]

Isn't your preamp circuitry on a board on the front panel of the head, while the power amp is all on the rear panel of the head? If you removed the wooden cabinet, there would be two metal panels (and a transformer either bolted to the rear or mounted to the floor). There would be a cable between front and rear that carried power and ground to the preamp and a signal from the preamp back to the power amp. The boards aer mounted to the panels.

You can operate the power amp alone by disconnecting this cable. Clip a signal source directly to the appropriate pins on the power amp board. If that is stable, then the trouble is in the preamp or the cable. If trouble remains, then the power amp is the culprit.

The cable probably has connectors at each end, so look for a female pin spread too far open.

Does it do anaything if you whack the top of it with your balled up fist? Seriously. Looking for loose connections. For example a broken off inductor in the output bus. Or broken lead or solder on a large cement power resistor.

[spartadrive_in]

Yes they are mounted on boards just like that (its a head).

By signal source then you are saying to totally bypass the front panel and just maybe solder a guitar cable to that input on the power board? so directly to it so i can hear sounds? (if thats the prob)

[Enzo]

No, not solder!!! CLip wires!!!

I don't recall which power amp assembly is on that amp, probably the "200 series." But in any case it is too old to have a layout drawing. So I cannot direct you with landmarks. But the power amp is complete and self contained on the rear panel of the amp. It will operate without the preamp panel connected.

The cable to the front should have a pin for ground, a pin for power supply to the front, and a pin for input signal. If I wanterd to work on the power amp alone, I would determine which pin is the signal input. Then I would connect a test signal to that pin and ground and should be able to hear that from the speaker.

WHich pin is the signal input? Turn on the power amp, connect a speaker, then touch each connector pin with a small screwdriver or some other metal object. The one that makes hum is the signal input.

Now with a steady signal flowing through the power amp, you can probe around and wiggle things to your heart's content. I'd use a tone from a signal generator on my bench. Or maybe some music from a CD player or similar. Here I usually use the FM radio on my bench stereo.

[spartadrive_in]

I did this earlier and I never heard anything. DOes that mean it is a power amp problem?

WHAT NEXT>

[Enzo]

If the power amp has power applied, a speaker connected, and some signal applied to the input, it should make sound from the speaker. I use the metal object touching the input method for go/no-go tests since I don't care what it sounds like yet, I just want to hear if it makes output, so that is as good a signal as anything. If not, then you need to troubleshoot the power amp as usual. PV will send you the schematic if you request it from their parts department.

But your problem was intermittent I thought. SO I wanted you to check for loose pins on that cable copnnector - pins spread open too far.

[spartadrive_in]

I did check for loose pics, all seemed well. It doesnt work more often then it does work. Actually, when it works it's like whoa it worked. you know?
I have the schematic, I'll put it up when i get home tonight for you to see.
THANKS!

[Enzo]

What power amp is that? What does the power amp schematic page call itself?

[spartadrive_in]

The last page. 100 watt poweramp for 100 and 200 modules.

[Enzo]

Ah, that one. We have discussed it around here recently I think. Gently determine if any of the six small wires are broken free of that driver transformer on the board.