# Long shot request



## nonreverb (Sep 19, 2006)

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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## bigboki (Apr 16, 2015)

you didn't;t post exact model.
Try here?
Free Hughes Kettner Diagrams, Schematics, Service Manuals :: Schematics Unlimited


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## nonreverb (Sep 19, 2006)

It's a STM Quad EL84. It isn't in that list. I checked there already.


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## sammyr (May 7, 2013)

schematics for h and k stuff is pretty hard to find. I did some digging and found someone had drawn up the switching schem for the statesman. at least I think that's what it is.


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## nonreverb (Sep 19, 2006)

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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## sammyr (May 7, 2013)

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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## nonreverb (Sep 19, 2006)

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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## mhammer (Nov 30, 2007)

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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## nonreverb (Sep 19, 2006)

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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## mhammer (Nov 30, 2007)

I have some J113s (the kind that DOD uses for switching because they have a really low "on" resistance) if you need any.


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## nonreverb (Sep 19, 2006)

sammyr said:


> 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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## nonreverb (Sep 19, 2006)

mhammer said:


> I have some J113s (the kind that DOD uses for switching because they have a really low "on" resistance) if you need any.


Thanks mark. I wound up using an MPF102 which seems to work.


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## Lincoln (Jun 2, 2008)

NETWORKING at it's finest hour.


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## Chito (Feb 17, 2006)

This place just rocks.


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## Granny Gremlin (Jun 3, 2016)

I love a happy ending.


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## nonreverb (Sep 19, 2006)

Granny Gremlin said:


> I love a happy ending.


Not quite yet....but hopefully soon.


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## nonreverb (Sep 19, 2006)

Granny Gremlin said:


> I love a happy ending.


Not quite yet....but hopefully soon.


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## sammyr (May 7, 2013)

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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## nonreverb (Sep 19, 2006)

I've figured it out....sort of. I've found a work around anyway.


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## ChrisShooter (Jun 4, 2018)

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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## jb welder (Sep 14, 2010)

If you put in @nonreverb like this, it will get his attention.


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## nonreverb (Sep 19, 2006)

Well, you might be surprised to know...at least in my case, it was the damn hum balance control pot. For some reason, probably cost, they used a hum balance control that was completely inadequate for the task at hand. It was basically melted internally from the current load. What threw me off was there was some control and the pot didn't feel bad when I was adjusting it....Once I got it out it was obvious.
Try replacing it with a same value pot but much bigger. I enlarged the hole where the adjustment is and mounted the balance pot there. From there I hard-wired it to the chassis. Hope this helps.


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## ChrisShooter (Jun 4, 2018)

Thanks @nonreverb . I will definitely pass this info to someone more competent than myself when it comes to this stuff.

Appreciate it.


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