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Frankenstein Bass Head, right channel weak and distorted [message #24028]

Tue, 06 October 2015 21:26

IdahoPicker
Messages: 7
Registered: October 2015
Location: North Idaho

Junior Member

Hi all,

I'm new to this forum, brought here by a customer's Kustom Frankenstein head he needs repaired.
I'm rather green at fixing solid state amps, so I could use a few suggestions.

I can find no numbers, nothing printed on the boards, so it's difficult to pinpoint which schematic to use. The closest schematic I could find is the K200 wide panel.
This amp sounds great on the left channel, but the right is very weak and distorted, particularly in the low notes. Looking the circuit over, it appears to me that the problem would be isolated to the right channel pre-amp. Would a problem in the power section affect only one channel?

I appreciate the help!

Thanks!

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Re: Frankenstein Bass Head, right channel weak and distorted [message #24029 is a reply to message #24028]

Wed, 07 October 2015 01:44

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Welcome to the place.

Yes, the K200 that is marked wide panel is for all of the Frankenstein heads. If one side is working, then the problem is in the second preamp only. I assume that the amp you have has no effects.

Because you have one working channel, you can use the working one to compare readings with. I'd start by reading the voltages that are marked on the schematic and see if there are any that are really far off. On a 40 year old amp, the electrolytic coupling caps would be suspect and leaky transistors are always possible. The fact that the low frequencies are most affected, I'd be thinking coupling caps.

Let us know how it goes.

[Updated on: Wed, 07 October 2015 01:45]

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Re: Frankenstein Bass Head, right channel weak and distorted [message #24030 is a reply to message #24028]

Wed, 07 October 2015 06:31

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Greetings to ya!
Every time I get this issue on one of these amps more times than not its due to one or more of the electrolytic caps on the preamp board having gone south!
If you have a Cap testing ESR meter they are easy to fine , but the truth is with the age of these amps all of these Caps on both preamp boards and the reverb board should get replaced, and note that on the reverb board and the rear wall mounted regulator / driver board there are bigger electrolytic caps that need to get replaced also.

If I recall right the schematic states 80uf Caps, but the last one I had on the bench had 90uf, that being said just replace them with 100uf Caps at a 10 volt or higher rating than what's printed on them.
The bigger value Caps are 500uf I believe ?

Many times the two large power supply can filters in these amps are still good as long as the amp got used 4 to 6 times a year to keep them formed and in OK shape .

Post back if you need more help!

[Updated on: Wed, 07 October 2015 06:33]

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Re: Frankenstein Bass Head, right channel weak and distorted [message #24044 is a reply to message #24030]

Tue, 13 October 2015 00:27

IdahoPicker
Messages: 7
Registered: October 2015
Location: North Idaho

Junior Member

Well, it turned out to be a bad capacitor. I tested all of them in circuit, and only one tested below its rated value. Many thanks to you all for the advice! The same customer is bringing in yet another Frankenstein next week, toull likely be hearing about it.

Thanks again!

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