Peavey MK 3 Musician... is it dead?

[jsonmfntidwell]

Hello all, I'm new to this forum, I am needing a little help here if it's is out there. I have a MK 3 Bass head and I love this tank of a beast... and today happened across a MK 3 Musician, the man I got it from simply said that it was not working. I brought it home and first, checked the fuse in the back. It was good, then took the front housing off the amp. facing the inside of the amp, the left side of the reverb was disconnected and there was a wire running from the front part that was not hooked up I found where it went, facing in would be the back left of the aplifier. I hooked both of them up and checked the other 2 fuses in the back to make sure they were not blown. *NOTE* did all of this before plugging it up at all. Then plugged it up, and it powers on, and can hear it power up in my speaker cab. It is not making any noise with a guitar hooked to it unless you turn it WAYYYYYY up and then it's barely a whisper at best. I am not a tech by any means but I'm not dumb or lazy, I just don't know where to begin. Any help would be greatly appreciated. Thanks in advance.

[Jazz P Bass]

Peavey Mark 3

Run a known good cable from Preamp Out into Power amp in.
If any of these jacks are at fault, this will prove them out.

Run Preamp Out into a working amp.
Run a working amp Preamp Out into Mark 3 power amp In.

I have attached the schematic & the user manual.
The user manual has a block diagram , near the end, that may help you localize the problem.
Good luck.
John G

[jsonmfntidwell]

UHHHHHH...

I have tried all of the listed options.. and oddly I have not gotten a peep out of it. all cables have been checked and double checked. I said I was not dumb.. until I looked at that diagram.. all of a sudden I was looking at greek.. I was hoping to fix this myself but it looks maybe to be beyond me. I am guessing that it's in the pre amp section of the amplifier though, because running into the power amp plug it sends a "power surge" to my speakers with a loud thump as if plugged or un plugged with volume up. would that be a correct asumtion? I understand that running the good pre out of the other amp into the power section of this one should have produced noise if the power was working.. so I'm still kinda stumped. and the pre of this amp into the power of the other amp should have worked if it was the other way.. frustrating. I am gonna spend some time with it today and tinker, and do some studying and some research. if anyone has a magic pill that these old peaveys like, lol... I'd sure take one.

[Jazz P Bass]

Peavey Mark 3

Hey, I did not mean to overload you with that block diagram.
There are a lot of hints there on what should be to function properly.
A lot of the jacks show a normally closed condition. If any of there are contaminated or open circuit, the signal path will be broken.
If you ran Preamp Out to another amp & it did not work, then there is something wrong with the preamp.
Running a patch into Power Amp In should be done with the amp off when inserting the plug.
When you go in this socket there is no volume control. Pure power amp.
The fact that you got a thud tells me the power amp is trying to work.
A guitar signal (100mv) will not drive it to full power. But it will produce some volume.
That is what the preamp is for (among other things).

[Enzo]

I wonder if you actually did ALL the tests Jazz P suggested, because from your reply, you think it is one part because the jack makes a pop. I don;t hear you saying music came out either half of the amp.

The idea was to plug a cord into the preamp out and send that signal to some other amp for a listen. Does the preamp signal sound OK through some other amp? That tells us if the preamp works.

Likewise plugging some other preamp signal into the power amp in jack tests your power amp. Pop or thump is only part of the story. Does the powr amp all by itself amplify a signal? Hell, you can just plug your guitar into the power amp jack - does that make sound?

If the preamp out and power amp in are the same jack, then you will have to plug the cord in the lack only half way for one of the tests.


And if you get a pop or thump every time a plug goes in or out of the power amp jack, that makes me suspect a protective diode is shorted at one of the jacks.

[jsonmfntidwell]

well...

well guys, I've tried everything all over again. I can take my working MK3 bass amp and hook it up straight to the power section, it plays, it's a properly working amplifier. I have ran the pre from the bad amp to the power of the good amp, no good. I have ran the pre from my good one to the power of the bad one and nothing. I have ran into the power on the bad amp and it's not working either. the only time I can get anything out of the "broken amp" is to plug a guitar into it and turn it all the way up, hooked up in the "joined" section of the channels. that of course makes both of the channel lights light up. if I have a cable hooked into the power section of the bad amp, when I remove it... a very loud pop comes with that. but not when I plug into it. just on removal. and I still have a power light of course. but other than that.. it's not doing anything.

[jsonmfntidwell]

update

I just took the amp in the kitchen, took it completely apart, all connections, blew what little bit of dust, and cob webs out of it. plugged everything back in nice and tight. put just a touch of 3n1 on a 1/4 plug and ran it in and out of all sockets, plugged it up and it is making amazing music... I guess chalk this one up to age and sitting. like I said, it came an older man. he runs a pawn shop. it's probably been sitting in the back for a few years and God knows where before that. maybe a little bit of moisture or corrosion???? anyway, I thank you so much for your help. I have a MESA single Rec. lets hope nothing ever happens to it, but if it does... I know where to turn.