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Old 12-31-2012, 12:29 AM   #1
NJAtrophy
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Join Date: Mar 2009
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Peavey Mark III Head Trouble: Looking for Help

So I picked up a Peavey Mark III Head from a guy off craigslist dirt cheap. Figured something would be wrong with it, but for the first few weeks it sounded great. No issues, awesome. But now I'm starting to run into some problems.

Sometimes when I turned the amp on, it didn't make any sound or just a really low crackling. In order to get it to work most times I had to crank the pre- and post- gain knobs to +6. I was getting concerned I was going to blow my speakers out, but I figured it wasn't that big a deal at the time. Now the sound cuts out while I'm playing...if I can get it to make sound. And so the cycle continues.

I thought it might be the inputs at first, but then I realized it couldn't be that because of the whole cranking the gain knobs deal. Now I'm starting to think it might be gain pots, but I have no idea.

I really need some help here. I've got gigs coming up and I'm short on cash. I have almost enough knowledge to do it myself so I'll give it a go if you know anything.
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Old 12-31-2012, 01:22 AM   #2
Sliide90027
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First I will confess to having ZERO experience with this Amp or this sort of thing.

From what I have heard about this Amp, it is a workhorse. Can take the road and deliver, even being dragged behind the equipment truck. Some people will abuse such things, and they will be faithful to a point.

It sounds to me, without monitoring sound quality from a direct out or effects send, that the output circuit has issues.

One never knows if the prior owner had it running cabinets with mismatched or low ohm ratings. Something like that will damage the Power Transistors, and if those are starting to go, then you are looking at a repair.

The fact that this is a worsening situation and not consistent. I am thinking that this may well be the place, as this was the second most likely failure (behind blown speakers) when I was running sound and playing bass for a touring band. The second most likely place where wear and tear of a performance was occuring.

I used to replace Power Transistors on Peavey CS-800's, before every show it seemed for a while (the 18" Gauss speakers were melting them as they thirsted for MORE POWER!). The parts were not that expensive, the process of finding blown transistors (as opposed to failing, one will need rating specs and competency with a voltage/ohm/Multi meter) was easy, removal and replacement with the Heat Sink compound and mica inserts (oblong clear waffer plate with 4 holes, one at each end and two tiny ones in the center) was easy.

You should probably at this point not get too bummed and get the unit checked out. Chatting with an actual tech may be helpful.

Should you choose to pull the transistors, unplug the unit. Locate the transistors in the power section, there are probably 4 of them (Sometimes they are under a plate monted on the exterior back of the amp, oblong with two screws, it will have two prongs or leads that decend into the unit, like a plug but much more delicate.

Pull one... If I recall correctly, using the ohm/resistence/continuity setting on the Multi Meter, touching + and - together gets 0, so touching + to one prong and - to the other should net something between 000 (signinal passes through) and 1 (meaning broken circuit). Test each Prong to metal face (where prong extends from) of the transistor, just as you tested Prong to Prong.

If I recall correctly, if you get a 1 on any of these three tests, you have a fried transistor.

Replace as you removed it. Use Mica insert/spacer and the Heat Sink Compound, the Compound really made a difference for my applications.

If you choose to take this mission you will be now involved in the final stages of your amp operation before the speakers. With one brand new transistor in hand, you could make an educated guess at performance degradation of any particular individual part or parts.

If this is not it, then it is far deeper in the components.

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