Peavey Mace VT Problems

[ric44]

First off I'm new here but have been searching for Mace info and the search brought me here. Im for certain no amp tech but do dabble in electronics and like to repair my own stuff if possible. I bought this amp last summer as a fixer upper. It will work to some extent but has very little power and a somewhat scratchy sound. The tubes all light and look good however I think only 4 of them really match. I did research the forum and have done some limited testing. However when I plug my guitar into the power amp I have nothing-dead.... I also have nothing when I plug into the automix on front. I have no led channel lights. The chassis looks good and all the large caps look to have been replaced fairly recently. The long blue slim cap also has been replaced but I noticed evidence that the old one they replaced had blown and left some telltale burn marks on the board under it . I did the chopstick test on chassis and had no noise or loose connections. Also both of the large transformers look ok and there is no burnt smell . I hope perhaps Enzo could remark on this since he is so knowledgeable on these amps. Any help is appreciated. Thanks, Rick.

[Enzo]

Blue slim cap? Sure that isn't a 400 ohm 10 watt resistor?

You mention inputs that do not work, are there any that do?


In any case, first, plug a cord from preamp out to power amp in. ANy help?


Some voltage checks:
Plug a cord into the preamp out jack. At the free end of the cord, measure voltage from tip to sleeve at the plug. Is ther 15v there? Now plug into the power amp in jack and do the same measurement. 15v? We DO NOT want 15v there, we want zero volts DC there, but if you find 15v, then you have a shorted diode or bad op amp, and it will cause symptoms you describe.

Power tubes, remove them, power up the amp, and measure DC voltages on the socket pins. Do you get about +500v on BOTH pin 3 and pin 4 of every socket? And about -55v on every pin 5?

And all those ICs run on +/-15vDC. so are both 15v supplies getting to the ICs?

[ric44]

Yes please excuse me I think it is a resistor (slim blue). I do get some (louder) sound from the normal channel jack. I also get sound from the 2 effects jacks. Give me a bit to measure voltages. Thanks

[ric44]

Preamp out showing no voltage at all measured at tip of cord. Power amp in shows no voltage either. Will take a bit to check sockets.

[ric44]

Also the transformer on the right (looking at chassis from behind amp) is cool to the touch even after its been on a while. Transformer on left is warm .

[Enzo]

Don't worry about transformers.

[ric44]

Tube sockets all check good- voltages good.

[Jazz P Bass]

However when I plug my guitar into the power amp I have nothing-dead.... I also have nothing when I plug into the automix on front. .

I find this part very odd.
Why would the amp work at all?
The Power In jack passes the preamp signal or doesn't.

As to the Automix, I would check Q1 transistor.

[ric44]

I'm not sure I follow you on this ? I plugged a cord from the preamp out jack to the power in jack and had my guitar plugged up front in the normal channel jack. I got sound.........i could unplug the cord from preamp out-power in jacks and I still had the very same sound from the speakers. Is the preamp out jack supposed to have 15v present ?

I find this part very odd.
Why would the amp work at all?
The Power In jack passes the preamp signal or doesn't.

As to the Automix, I would check Q1 transistor.

[g1]

No there should not be 15V on any of the loop jacks.
The odd part is when you say if you plug your guitar directly into power amp in jack, you have no sound. If this is the case then there should be no sound when you use any of the other inputs either.

[ric44]

No there should not be 15V on any of the loop jacks.
The odd part is when you say if you plug your guitar directly into power amp in jack, you have no sound. If this is the case then there should be no sound when you use any of the other inputs either.

Well I just reaffirmed and checked. I have no sound thru speakers at all when plugged into the power amp in jack. I also do not have sound when using the automix jack. i DO have sound when using the normal jack and either or effects jacks .

[The Dude]

...The odd part is when you say if you plug your guitar directly into power amp in jack, you have no sound. If this is the case then there should be no sound when you use any of the other inputs either.

g-one,
He may be feeding such a low level that it's barely audible going directly to the amp? Just a guess, but I agree with the gist of your assessment.

[g1]

I was thinking maybe it needs a real hot signal from the preamp to overcome some power amp problem.

When you get sound through the normal jack, it is very weak? Do you have to turn the volumes up high?

[ric44]

I was thinking maybe it needs a real hot signal from the preamp to overcome some power amp problem.

When you get sound through the normal jack, it is very weak? Do you have to turn the volumes up high?

Yes I have to run the volumes way past where they would normally be to get sound . Also when I plug into the automix jack I get no sound but both normal and effects channel leds light up. I have also found that the pre amp out jack is kinda loose and wobbly because part of the threads are gone and the nut is not holding it securely to the chassis.

[Jazz P Bass]

Forget the Preamp out jack.
It's a single wire jack. (ie: there is not a normally closed contact to 'pass' signal like the power amp in has).