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Need help with Kustom K200B-2 [message #12251]

Mon, 19 October 2009 15:42

doctor_t
Messages: 4
Registered: October 2009

Junior Member

Hi guys,

I have a K-200B2 with a very weak and distorted Reverb channel (PC303 board). I cleaned all pots/switches/jacks to no avail and managed to rule out the Preamp/Boost section, but I'm working from a set of K200B PA schematics. (i.e. Reverb circuit is not identical and I don't have ANY info on the vib/trem...) All effects/switches on this channel "work", but as stated, volume is very low and signal is very distorted. Clean channel works fine.
Has anyone run into this problem before? I don't have access to the PC303 schematics, so any off-the-cuff advice would definitely help!

thanks,
Tom

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Re: Need help with Kustom K200B-2 [message #12269 is a reply to message #12251]

Thu, 22 October 2009 01:40

doctor_t
Messages: 4
Registered: October 2009

Junior Member

Specifically, here's what I've done so far and where I'm stuck...

(1) Ruled out the power amp (PC703) since the other channel sounds fine.

(2) Output from PC303's preamp section checks OK. I "tapped" it directly to another amp and it's just as loud & clean as the good channel. This would imply that something else in the PC303 signal path is killing volume & distorting.

(3) Checked all tantalum caps for shorts, they appear OK. (I don't have a good way to test for open caps, though)

(4) All transistors measure a reasonable Vce voltage and seem to be "on". (except for the 2 pairs of 2N3567/2N3638 footswitch control transistors, which I believe are normally off)

(5) One PET8004 transistor (in what I believe to be the tremolo section) measured a lower Vce, maybe 0.9 V... But I replaced with a known good part and got the same results.

So I'm stumped... especially since the problem appears to lie in the reverb/trem circuitry and the effects themselves still work fine. Any ideas???

Tom

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Re: Need help with Kustom K200B-2 [message #12270 is a reply to message #12251]

Thu, 22 October 2009 05:55

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Hi.PM me your email adress and I will email you a pc303 schematic.
Since it sounds like you have no o-scope to test with and have to use another amp ou might try to narrow things down by taking the signal from the input marked side of the reverb and feed it into the other amp. If that signal is clear than you know that the problem lies on the recovery amp(output) side of the reverb pan.
Coupling cap C336 would be one to check along with any of the other electrolyitc type caps in that part of the circuit, a shorted cap passing DC voltage down stream and messing with the bias of, and or a shorted transistor can sometimes sound like the same problem.

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Re: Need help with Kustom K200B-2 [message #12271 is a reply to message #12251]

Thu, 22 October 2009 13:23

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Tom:
You've done a good job in trying to isolate your problem.

When you took the output from the preamp section, where did you tap off the signal?

When you get the schematic from Steve, you'll see that the basic preamp feeds to the bright/boost circuit, then to the vibrato circuit and then to the reverb section.

If you tap off the preamp signal further along the audio path, you may be able to find the stage at which the signal distorts.

Bill

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Re: Need help with Kustom K200B-2 [message #12272 is a reply to message #12271]

Thu, 22 October 2009 15:20

doctor_t
Messages: 4
Registered: October 2009

Junior Member

Thanks for the info Bill! Glad you pointed that out... From the PC303 schematic Steve was kind enough to send, I now see that I wasn't checking the signal post-boost, as originally thought.

Back to the drawing board then... although now I'm in a much better place to finally figure this out! Will be sure to post what I find,

Tom

[Updated on: Thu, 22 October 2009 16:39]

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Re: Need help with Kustom K200B-2 [message #12303 is a reply to message #12272]

Fri, 30 October 2009 22:16

Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA

Senior Member

I have a B2 for sale if you want to use it for parts (though it works perfectly, not sure how attached you are to yours yet). It'll be up for sale as soon as my A4 arrives =)

Born on the Baaaaayou (Ok, in Massachusetts)

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Re: Need help with Kustom K200B-2 [message #12629 is a reply to message #12251]

Sat, 02 January 2010 00:11

doctor_t
Messages: 4
Registered: October 2009

Junior Member

Hi guys, sorry this reply is overdue. I just recently solved the problem and wanted to post details, in case it might help someone else at some point.

I ended up handing the amp off to some friends of mine with far more Kustom experience... and it was a good thing too since they caught something I never would have... On the PC303 board, Reverb input driver Q317 had been replaced at some point and was installed BACKWARDS! (collector and emitter reversed) Magnitude of Vce still measured OK though, making it a real pain to track down, even for them.

Once Q317 was corrected they also identified another problem, this time in the Power Amp board. Bad/shorted input transistor Q700 was sapping about 6dB of the total output. Still, only a minor headache compared to the above!

Thanks again Bill for the advice, and Steve for the schematic help! Not even my tech pals had one for PC303 and we would've been lost without it.
Also, thanks for the offer Shaun; won't need a parts B2 but I hope it finds/found a good home since it's working!

Tom

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Re: Need help with Kustom K200B-2 [message #12630 is a reply to message #12629]

Sat, 02 January 2010 07:54

Shaun_Musings
Messages: 323
Registered: April 2009
Location: Reading, MA

Senior Member

Well, I'm glad you got the problem fixed! I actually sold the B2 to someone that is now selling it on CL. Vultures. Anyway, I now have the elusive A4 head that I wanted. Now I just need to find a really nice Rickenbacker...

Shaun

Born on the Baaaaayou (Ok, in Massachusetts)

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Re: Need help with Kustom K200B-2 [message #12632 is a reply to message #12251]

Sat, 02 January 2010 10:07

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Glad to hear its back up and runnning for you Tom!

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