Peavey 5150 EVH clean sound only?

[pontiacpete]

Isn’t this amp suppose to have a huge overdrive sound? It came in with a shorting tube, taking out 2A fuse on the HT. One screen resistor opened. Now with a new quad and all good screen resistors it’s making a huge loud clean sound but not dirt. In the 1st channel Rhythm the crunch button doesn’t do anything. THe post gain pots do something but it’s uneven as you sweep across the distance of the pot, and produces a lot of hiss all the way up on both.
Does anyone have any suggestions on what to do here?

[Jon Snell]

It could be relay related. If the relay doesn't change state when commanded, it won't change pre amp stages.
It could also be lack of input signal or a high value anode load resistor on one of the 12AX7s.
Could be a noisy low gain 12AX7 ... I have had bad connections on the plug and sockets joining the boards cause weird things to happen.

[tedmich]

yes the 5150 is HUGE gain with a Soldano cold cathode clipping stage and several more. Its got to be a switching problem, I'd check the JFETs and relays.

[pontiacpete]

Thanks for the response on this issue. Sorry it’s taken so long to get back to what I found. Which is not much. I went through all the relays. I started by changing one that I thought was not switching. And I thought that made a difference. I thought. Then I thought another wasn’t working and changed that one. And again I thought it made a difference. Then I just changed the other two and the jfet and another transistor since while I’m throwing parts at this may as well. I fully realize that this is not good approach to a repair. I had, previous to this shennanegans, checked the voltages on all the preamp tubes and found voltages on all plates and cathodes I think are providing what I believe they are suppose to, but this is something i need to double check.
But there is something weird here, on both ‘post gain’ pots, when dialing from 1 to 10 the volume is higher at one and between 3 and 5 it dips lower and there is some distortion.
I would expect that this amp would be able to provide a master volume like effect and be able to get a fully overdrived dirty signal at a low volume. But it still isn’t doing that. And in the 1st channel the crunch button isn’t doing anything even though I can see voltages move on one of the relays when using it.

[g1]

Q7 is the Fet you replaced? Check for DC on the post gain pots. Schematic attached.

[pontiacpete]

I changed the Jfet J174 with the same. There is no DC on either pots. I was just playing it again and it sounds really good, I’m getting some overdrive/distortion now, but the weird thing on both post gain pots is still there. The volume dips down at about 3 and comes back up at about 4 and after that it gets louder. I keep wondering, why would the volume be greater on 0 than at 3 on the post gain pot.
It’s almost like it doesn’t really matter because it sounds really good and certainly loud.
Ah the crunch button does nothing, too. That’s also bothering me.

[The Dude]

Check all the caps in that tone stack right before the post volume controls. I've often found the volume issue you describe caused by shorted or leaky caps.

[pontiacpete]

I wish they printed the component designations on the board. It took a while but I found the caps associated to the tone stack, 4 of them. The two small caps are the resistor looking type. All of them show no signs of leakage or shorts or open circuit. I put the scope on either end of each of them and a signal appears. Put the meter at both ends and no voltages.The pots also check out ok. And the mid is connected to ground at one end.
But the tone controls don’t seem to do much at all. A slight volume difference can be heard when rotating the each pot. Not much of a difference in tone though

[pontiacpete]

OK I solved the tone control problem. The triac MAC97a was in backwards. My fault probably, unless it was put in backwards before I changed it, and I copied the way it was in place. But still the issue with the post gain pots remains. And of course the crunch control doesn’t do anything. I have to check that switch again.

[Enzo]

I wish they printed the component designations on the board.

That is what they make layout drawings for

[pontiacpete]

Yes it would be nice to have a layout.
Can’t open that file.
I’ll give them another call this week.

[Delta362]

I couldn't open Enzo's upload either. Came up as invalid file specified. I see that error from time to time when attempting to download a file from here. Not sure why.

pontiacpete , try this file.

[pontiacpete]

Thanks for posting this. Now this is strange, I had CR23(MAC97a) the right way after all. I couldn’t see the trace under C55 and that one end of the cap does go to pin 1 of CR23and the other end goes to pin 3.Strange thing is that now the tone controls are working with it in backwards, they weren’t when I had it in the right way. I guess this means i need to first put it back in the right way and then figure out way the tone controls do nothing and all the other stuff.

[g1]

Thanks for posting this. Now this is strange, I had CR23(MAC97a) the right way after all. I couldn’t see the trace under C55 and that one end of the cap does go to pin 1 of CR23and the other end goes to pin 3.Strange thing is that now the tone controls are working with it in backwards, they weren’t when I had it in the right way. I guess this means i need to first put it back in the right way and then figure out way the tone controls do nothing and all the other stuff.

All the CR23 triac can do is turn the 'clamp' Fet on and off, which mutes the whole amp. If something is going on with the tone controls, I think it is just a coincidence that it occurred when the triac was reversed.

[Enzo]

OK so now you have the layout drawings, but I need to clear up my thing, so does THIS file open?