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K200 B-1 LF oscillation [message #26414] Wed, 29 November 2017 15:49 Go to next message
HRCAS is currently offline  HRCAS
Messages: 1
Registered: November 2017
Junior Member
Hello to all.
An K200 B-1 I've been rebuilding has begun producing a loud LF pulse with an instrument plugged in when I get beyond 50% of throttle. This occurs only on Ch 2. Shorting the 500pf cap (I replaced it when the problem began) between the input jacks is a help. but not a solution. Any thoughts or suggestions would be welcome. BTW all transistors are modern parts.
Thanks,
HR

[Updated on: Wed, 29 November 2017 15:53]

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Re: K200 B-1 LF oscillation [message #26415 is a reply to message #26414] Wed, 29 November 2017 18:30 Go to previous messageGo to next message
chicagobill
Messages: 1821
Registered: April 2003
Senior Member
Welcome to the place!

Please describe the low frequency pulse a little better if you can.

The cap between the two input jacks should be a 0.005uF and shorting it shouldn't make much difference as to the preamp circuit. But maybe I don't understand exactly what you are saying.

The volume control of these amps does two things, one it changes the gain of the input stage and two it attenuates the output of the preamp circuit. If you have changed a lot of parts, please check that you have used the correct transistors and that you have them installed correctly.

If you look at the technical section of the site, there will be schematics for your amp there. Find the one that is for the board that you are working on and let us know which one it is so that we can then refer to specific part numbers instead random part descriptions.

Again, Welcome.
Re: K200 B-1 LF oscillation [message #26416 is a reply to message #26414] Thu, 30 November 2017 06:10 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 3976
Registered: June 2004
Location: NY
Senior Member
Your issue to me sounds like what's most times called a motor boating condition which is most times caused by bad / weak local filters/ electrolytic caps or these items being installed reversed .

When you say channel 2 your talking about the right hand channel I assume?

If your new to doing repair work and your changing out more then one part at a time it's easy to mess things up and install new parts wrong, it's also very possible that new caps you have installed are bad from the get go!
These days I test out all new caps I am installing both on my cap ESR tester and my meter to confirm there uf or pf value.

I you have changed out any Transistors please tell us which ones you changed and what part number you have replaced them with.

Also note that these preamp boards and input jacks pick up there needed ground by being bolted down to the face of the amp, just having them hang there will not do it!

[Updated on: Thu, 30 November 2017 06:15]

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Re: K200 B-1 LF oscillation [message #26424 is a reply to message #26414] Sun, 03 December 2017 02:21 Go to previous message
DeadKoby is currently offline  DeadKoby
Messages: 18
Registered: August 2017
Location: Cleveland, OH
Junior Member
I have doodly squat idea if it will help you at all...

On my Youtube channel (DeadKoby) I fixed up a K200-B1 that had lots of noise on the left side (bright) channel.

If it helps you to have a look at that series of videos, be my guest.


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