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K50 Trem/Vibe not working [message #15543]

Wed, 20 April 2011 12:38

El Músico Loco
Messages: 12
Registered: February 2011

Junior Member

Hi,
I was wondering if anyone has any ideas where to start with the tremolo/vibe section. It is turned on/off by the Intensity knob and switched between trem/vibe by a switch on the speed knob on the front panel. I have checked the switches and they work. I was hoping someone has had this problem and knows a good staring point.
Here is the schematic,
Thanks,
Chris

http://i852.photobucket.com/albums/ab86/pigyboy_2009/K50-Page1.jpg

http://i852.photobucket.com/albums/ab86/pigyboy_2009/K50-Page2.jpg

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Re: K50 Trem/Vibe not working [message #15544 is a reply to message #15543]

Wed, 20 April 2011 16:31

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

What is the amp doing now? Is there any effect at all?

I'd suggest starting at the circuit around Q17 and Q18. If the oscillator doesn't work there will be no effect. Check the three transistors. Check to see if any of the caps C28, C29 and C30 have broken loose from the board. If the oscillator is working you should be able to read the output voltage at the input to Q19.

If the oscillator is working and there is no effect, then check all of the small diodes CR1-3.

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Re: K50 Trem/Vibe not working [message #15545 is a reply to message #15544]

Wed, 20 April 2011 18:11

El Músico Loco
Messages: 12
Registered: February 2011

Junior Member

Thanks bill,
I just figured it out. Someone had replaced the intensity pot which has the switch on the back with onw with a n/c switch; instead of opening the path to ground by turning the switch on it was shorting it to ground effectively turning the effect off by turning it on. Happened to have a push/pull cts pot here that will work. All in all the amp sounds great now with the new input fet and the trem/vibe & reverb all running. It is extremely quiet though it still has it's ungrounded, long original cord and I am using a stepdown transformer here in Spain to supply the 120v. Learned a lot with this one.
Thanks for the help
Chris

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Re: K50 Trem/Vibe not working [message #15548 is a reply to message #15543]

Thu, 21 April 2011 12:09

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Glad to hear you found the problem. I see a lot of gear that has been worked on by others, some good and some not so good.

Usually when you first open up the amp you can tell if it was worked on. Then from the way things have been replaced, you can get a feel for how good the earlier work was. If it looks all hacked up, with parts installed every which way, you immediately know that you have to check everything that looks like it was touched earlier, because you just can't trust what the earlier guy did.

I once got a K200B-1 chassis that was really hacked up. The outputs were bad, so someone replaced them with any transistor they had that had a matching TO-3 case. There was even a Germanium PNP from a car radio in there. Needless to say, the board was also cooked as the drivers were also ruined by the earlier repair.

Any way congrats on the fix.

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