Mark IV 400BH loosing power

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CrazyFinn
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Mark IV 400BH loosing power

Post by CrazyFinn » Tue Mar 28, 2017 1:25 am

Hi,
recently I bought a Mark IV 400BH, which appears to work fine for a while, but then decreases/looses power when warmed up.
When I bought this amp, it has been out of use for quite a while, so it was dusty. I have cleaned the contacts on the PCB's as well as the potentiometers, they seem all to be in order. All the elkos appears to be original and visually ok, no leakage anywhere.

I play the double bass in bigbands and electric bass in a gospel band. So I use both channels, one for standard sound and one for a more funky sound.
My gear is a 68' Precision with standard pickups and a 78' Precision with two pairs of Seymour Duncans. The double bass has standard pick ups.
The amp is hooked up to a 4 ohm 15" speaker.

After playing for about half an hour, with gains/volumes on '5', the power starts to decrease (of both channels) ending up with gains on full and still only power like using gains on a '4'! With the compressor LED turned on as one can imagine.
When cooled down everything is ok for a while.

What would be the most likely fault causing this kind of behaviour? Where should I start to look?

The Stig

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Enzo
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Re: Mark IV 400BH loosing power

Post by Enzo » Tue Mar 28, 2017 1:40 am

When it haappens, poke a plug in and out a few times at the power amp in jack, or the graphic in jack if it has one, or any jack marked insert or effects loop.

CrazyFinn
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Re: Mark IV 400BH loosing power

Post by CrazyFinn » Tue Mar 28, 2017 2:41 am

Thanx Enzo, I'll try that. This one, as I presume all Mark IV 400BH has, separate send/return for effect on both channels.
But I'll try the plugin / plugout next time I'm in our garage.

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Enzo
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Re: Mark IV 400BH loosing power

Post by Enzo » Tue Mar 28, 2017 4:14 am

400BH is the power amp board, the jacks I am referring to are on the front, lower right. The power amp in is lower right, and the preamp out/in is a single insert jack, it is also below the graphic.

Since this affects both channels, I assume the individual channel loop jacks near the inputs are OK and not involved.

CrazyFinn
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Re: Mark IV 400BH loosing power

Post by CrazyFinn » Tue Mar 28, 2017 5:16 am

Ok, now I got it! The 'Preamp' I had find a proper photo to get it. Ok, I'll test as soon as possible.
Thanx again!

CrazyFinn
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Re: Mark IV 400BH loosing power

Post by CrazyFinn » Mon Apr 03, 2017 7:19 am

Tested the "plugin / plugout" 's this weekend, no change.
I also noticed the 'comp LED' being on almost already from start throughout the session.

Cooled it seems ok, but once warmed up ...

What's next to check?

The Stig

CrazyFinn
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Re: Mark IV 400BH loosing power

Post by CrazyFinn » Fri Apr 28, 2017 6:57 am

I have finally been ablo to narrow down the problem. It appears to more 'concentrated' to the upper channel.
With Gain and Volume on full it's just 'n just like the lower channel driven with the Gain and volume on '3'!
With no 'Bright' nor 'Eq' switched on.
Have the amplifiers ends reached the 'best before date'? Or is the failure in the pre amplifier?

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Enzo
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Re: Mark IV 400BH loosing power

Post by Enzo » Fri Apr 28, 2017 11:40 am

The preamp is the front panel, the power amp is the rear panel.

If ne preamp input channel works OK and one fades away, then teh fading one has some issue. Probably a coupling cap dried up, but possibly some semiconductor with a DC voltage problem.

CrazyFinn
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Re: Mark IV 400BH loosing power

Post by CrazyFinn » Sat Apr 29, 2017 5:14 am

I'm starting to have the same feeling myself. The components are EOT, end of time.
Visually they look ok, no swelling or leakage or anything like that.
But they are original, over 35 years old :) .

As previously written, the problem is narrowed down to channel 'A'. However, it's most likely
that, in time, the same problem will occur to channel 'B' too.
So a replacement of most caps is a huge job.

Just wondering how to check them w/o unsoldering.
To replace all of them w/o measuring means a lot of job and shot in the dark.

Anyone with ideas?

The Stig

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Enzo
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Re: Mark IV 400BH loosing power

Post by Enzo » Sat Apr 29, 2017 8:45 am

yes. Isolate the problem. Follow the signal path through that channel to find out just where the signal is getting squashed. The amp may need caps, but chances are only one of them is causing the immediate problem. The procedure is not to wholesale replace a bunch of caps in hopes that will solve the issue. The procedure is to find the issue, fix it, and THEN go about any updates you may have in mind.

n my experience, the little 2.2uf 35v caps Peavey used in that era don't last as well as other caps. But that doesn't mean they are necessarily what is wrong. I check them by their action in a circuit. If you have two channels with similar circuit, you can compare the "good" one with the "bad" one.

Your channel inserts are after the first stage in each. So plug your signal into the return of each to see if the front stage is where the problem lies. Turn the graphic on and off, does that affect the problem? APply a signal and trace along the output pins of each op amp, find where it is no longer strong.

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Re: Mark IV 400BH loosing power

Post by CrazyFinn » Sun Apr 30, 2017 2:20 am

Will do that as soon as I found the schematics. I've asked for one from Peavey.
Thank You, Enzo.

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