Peavey Musician 400G

[nickbme07]

Hello Everyone, I have a Peavey Musician 400G on the bench. It came in after being plugged up after sitting for a long while. He said he saw smoke and blew fuse (it was actually videoed). I've replaced most of all the caps and a few resistors that were coming out of spec once pulled out of circuit. All transistors test good and the big ones test good out of circuit. Powering up on the limiter and variac, I get to 110v but NO POWER LED. No DC detected on the 4ohm direct output, but I do get 4v AC when I am at 50v or so mains and it goes down as I bring the variac up (because of limiter). I hook hooked straight up to mains 120v and see 90+v on speaker output. I have connectivity from tip of speaker jack to ground, as well as sleeve to ground. This head had 2 death caps that I removed. I also am only using one side of the AC switch. Any ideas what to look for that is getting the AC on the output? Thanks!

[nickbme07]

Schematic
https://music-electronics-forum.com/...etch?id=868214

[The Dude]

Only one of those caps (the one to ground) is a "death cap". The other is probably for RFI or to help prevent switch arcing. I would have left that one. Check your power rails for AC. If you don't have DC on the output, but do have that much AC, you must have a filtering problem on one or more of the rails. You could even have a shorted or leaky rectifier diode, so check DC on the rails, also. In general, make sure power rails have the correct DC voltage and do not contain AC.

[glebert]

So the 90+V is AC, right? The rails on this should be +/-52V so 90V seems impossible without a catastrophic mishap with the power supply. Do you have a scope?

Also, does this have the Autoformer? I've always wondered what was up with that, it's the only solid state amp I know of with an output tranformer. I think that is why you are seeing continuity from tip to ground on the 4 ohm output.

I think this is the schematic for the power amp.

[nickbme07]

I clipped the criss-cross on the switch and only using one side, so I removed both caps. So I just tested connectivity between hot and neutral on the plug.

[nickbme07]

Yes, it has the autotransformer (8ohm). I am getting the AC on the (direct) 4ohm jack. But great point, not used to that in the SS. The one thing that stands out to me is it seems the only place the main board is grounded is via the speaker jack and hops to PT chassis. The one death cap went from a mains leg to that ground on the jack (which is now gone).

[nickbme07]

lol.. Can I start this over. Sorry about that.. I was about pulling my hair out (again over these solid state amps) and was reading mV.... On 120v mains AC I see it start around 100mV, gets to 110mV and then drops down to around 20mV. When I start up on the variac with speaker hooked up, just 10v on mains, I get some pops and hum. Moving up volts and gets louder.

[The Dude]

You shouldn't bring a SS amp up slowly on a variac with a speaker load. Rails often don't come up evenly. Use the variac only to verify no excessive current draw and no DC.

[nickbme07]

Ah ok, Thanks for that! I was hearing that hum pretty loud at low mains (no more than 10v) so I was hesitant to just plug up to 120v mains with speaker load. With no load, 120v mains gives the mV as shown above, but no DC. I dont want to blow my speaker, but id like to know if I I have no hum. Again, the LED does not come on with 120v mains.

[nickbme07]

I did notice out of all of the transistors on the back, only one of them is a 8039, with the rest being 7939's.

[nickbme07]

OK, so once I was able to see the mV drop on 120v mains, I attached speaker. I hear a very very faint hum. Plug a cable into input 1 and move the post gain pot and get a light pop a scratch, no feedback/change on tip touch.

[nickbme07]

Oh wow, I get feedback on tip change using Power Amp in. She lives!!!

Unplug from Power Amp in and light hum comes back. Plugging in any other 5 inputs have no change. No LED's on channels or main power LED. Post gain on channel 1's pot is scratchy at speaker when turned. Phaser works, changes the scratches and pops. It pickups a guitar using Power Amp in but sounds really terrible, also same with CH1 and CH2 in/out.

I show 16v coming off header of power amp to preamp.

[nickbme07]

So now I will attack this one in two sections. I will have to look further in detail on preamp parts/readings before I report back here. On power amp, it seems hooking up to the Power Amp in should give me a clean signal and it is very distorted and crackly when turning up the guitar. Anything I could look into with the power section to get that going as well? Thanks!

[The Dude]

Did you ever check your power rails? That should be first. An amp can't work properly without a working power supply.

[g1]

And if no channel lights are showing, I would particularly suspect the low voltage rails to the preamp.