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Old 14th September 2010, 03:22 AM   #1
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Question peavey mkvi bass - low output problem

Greetings all,

I've about completed repairs on a MKVI bass head with the exception of a low gain problem.

It came in with a shorted TRIAC, some blown OTs, and some of the driver circuitry needed to be reworked. Seemed like a fairly straight forward repair. Following the cross reference parts list, I replaced all the silicon for good measure, as well as a few resistors. All seemed to be in order so I fire it up and all I get is low gain at maximum volume. What gives...

After studying the schematic, I find that if I short R102 the amplifier functions as normal. Plenty of gain and power with no distortion. This obviously is not the right way to go about things, so I thought I might pick a few brains for some advice.

I've attached a partial schematic of the section I'm referring to.

Thanks in advance.
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File Type: jpg MKVI - Shorted R102.jpg (102.0 KB, 45 views)
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Old 14th September 2010, 04:43 AM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
First isolate the problem, verify the power amp is indeed the whole deal or only part. Use the patch jacks. Is the preamp out signal strong and as expected or weak as the problem? And plug a test signal into the power amp in jack. Is it indeed weak, or does that work OK alone?

We want to make sure we are not trying to modify the power amp input to compensate for a weak preamp output.

But assuming the power amp is in fact at fault, we contimue.

When you replaced "all the silicon" does that include JFET Q105 there at the input? And if so did you use the EXACT part or just an equivalent? Ther is no standard lead pinout for the little JFETs, and some of the subs have to be turned to a different orientation - they are electrically equivalent but have different pinouts. Q105 is a power up mute, it mutes the input by shunting until C105 can charge up through R107 enough to turn off the JFET. COnsidering its function, just remove Q105 and see if that makes the difference. If that restores function, then either the part is bad, or installed wrong, or the gate is not getting turn-on voltage. If that does not make the difference, leave it out until repairs are complete.

Also does all the silicon include the ICs?

U2 is the compressor chip, and it too shunts signal to drop levels to prevent clips. It is parallel to Q105. Remove U2. Is full gain restored? U2 is invisible until the output is at clip level, and increasing signal from there results in increasing shunt action at U2. It works well. COmpressed peaks are much preferred to clips. But U2 is not necessary to the basic function of the amp, so it can be removed for test.

Either way, if one of those is hanging up, shorting that input resistor may overcome it, but that is not curing the disease.

Make sure no leaky or reversed CR102,103 is putting unwanted DC on the input of U1b.

The compressor funtion can be switched out from the front. That won't help if the U2 circuit is hanging up, but when signal is applied to the amp, is this compression light coming on?

And a new 4558 at U1 is a cheap guess.

In my experience, your symptom is most likely a problem with U2 or the JFET. Check those things. If not there, we can look further. I have the complete schematic. Too large to post on this board, but if anyone needs it to play along, we can post it elsewhere and link.
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Old 14th September 2010, 12:18 PM   #3
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A lot ot times people plug guitar amps into the "out" jacks of Peavey stuff instead of the input jack. Thus, U3 might be blown up, and the other half of U3 loads down U1. Try pulling out or replacing U3 to see if it brings the gain up. By the way, starting about 1998 Peavey used JRC4560's instead of 4558's for inputs, they have less hiss.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300
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Old 15th September 2010, 12:51 AM   #4
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Originally Posted by Enzo View Post
When you replaced "all the silicon" does that include JFET Q105 there at the input? And if so did you use the EXACT part or just an equivalent? Ther is no standard lead pinout for the little JFETs, and some of the subs have to be turned to a different orientation.
Enzo, first I want to thank you for the detailed response. It was just what I was looking for.

You nailed it with Q105. After removal the amp sings as it should without shorting R102. I replaced it along with the other parts and apparently the pinout is indeed different. Luckily I have several pieces of that sub part. In my book you never order just one of the small bits for reasons just like this.

I'll correct the orientation and all should be well.

Thanks again.


I don't know the whole story behind the failure. However, I do know the customers son used it for a time. What he did I cannot say. Overall I think the amp had a pretty good run, first failure since original purchase. Some of these Peavey amps have impressive reliability.

I may look into those OPAs. Thanks for that tidbit.
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