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Discussion Starter #1
Hey all,

I don't usually post here with hat in hand although I presently have a particularly nasty repair situation on the bench. It's a H&K Statesman with EL84's. The initial problem was a buzz when the amp is on the clean channel.
I was able to trace the problem to the OD section. Unfortunately, another problem has surfaced which is causing much frustration. There's a FET on the grid of the first gain tube where the switching for the channels goes in. It's leaking and now causing the clean channel to shut off. It's function seems to be either for setting the bias to the grid but far more likely, it's a clamping FET. Either way it's connected to -33V, ground and the input grid of the tube. Of course, the original FET is no longer produced. I tried a replacement FET but to no avail.
I contacted Yorkville as they're the distributor, but they were no help at all.
Does anyone have a source, or the schematic for this?

Many thanks,

Richard
 

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Discussion Starter #5
Thanks sammyr! That helps solve part of the problem! Unfortunately, it doesn't help with the line buzz issue. The search continues.....
 

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I'd try calling yorkville. Guy Beresford (of the late GBX amps) runs the service department there. You can reach him directly using the company directory or calling the service department and crossing your fingers. He can probably get the schematic for you or if not find the person who can. I had an old GBX amp come in for a repair last year and I needed help identifying the ratings of a transformer and I found him at Yorkville and he was kind enough to help me out.
 

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Discussion Starter #7
When I talked to Yorkville, I was put through to Steve Das who wasn't interested in giving me the schematic. He put me through to Steve Gooderham who gave me a bulletin regarding hum that had nothing to do with what I described. I basically stopped there.
 

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You probably know this already, but FETs vary in their "on" drain-source resistance, some having less than 100R "on" resistance, and others being well above 600-1000R. Maybe an assessment of what the FET resistance needs to be might help identifying a modern functional equivalent?
 

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Discussion Starter #9
True Mark. Just to update, I had tried an enhancement FET first and that definitely didn't work. I have finally found a simple N-channel FET that seems to be doing an adequate job.....now onto the channel buzz. Thanks! R
 

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Discussion Starter #11
I'd try calling yorkville. Guy Beresford (of the late GBX amps) runs the service department there. You can reach him directly using the company directory or calling the service department and crossing your fingers. He can probably get the schematic for you or if not find the person who can. I had an old GBX amp come in for a repair last year and I needed help identifying the ratings of a transformer and I found him at Yorkville and he was kind enough to help me out.
Got a hold of Guy....he helped me out! Thanks sammyr!
 

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I had a feeling Guy would help you out. When I called him I had a GBX solid state combo from the 70s that he designed and made. I had tracked down a schematic but the transformer values weren't on it. Did some digging and tracked him down and he was pretty incredulous when I explained why I was calling but in the end sat with me on the phone and went over the schematic and helped me figure it out.
 

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Hey Nonreverb. I've specifically signed up here to ask if you're able to share your fix to this problem. Seems as though I've run into the same problem and any help would be greatly appreciated.

thanks,

Chris
 
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