Hughes and Kettner 100 watt head,intermittent,then total loss of power

[rumpelstiltskin]

Hi all,got a 1991 Hughes and Kettner 100 watt 'ATTAX' head and 4x12 cabinet.This transistor model has one preamp valve.I was playing it one day and the power just went,i could 'just' hear the guitar very faintly.Then suddenly it will come back to full power,then it goes really faint again.If i pull the guitar lead out of the amp it just crackles,where i think usually you would get a hum??Its now got to the point im just getting a very,very weak undistorted (even on lead channel)sound even with amp turned all the way up.I took out the chassis and the valves shining bright,there's no whistles or anything from the amp.Anyone any ideas?Cheers.

[J M Fahey]

Very probably a bad jack or cracked solder in its legs.
Take a look at it with good light , the PCB, use a loupe if necessary to look for cracks or resolder its connections to the board.
Also clean (with DEOXIT or a similar product) the loop jacks and the headphone jack.
Squirt a little into them and work a plug in/out a few times to clean the contacts.
Also *carefully* beat the board in different places with a wooden chopstick or a plastic ball pen or similar, to check whether there´s something loose.
Your problem looks like a bad contact somewhere, but of course you have to find it first.
Good luck.

[joecool85]

Juan is right on the ball, follow his suggestions and you'll almost surely fix your amp.  Let us know how it goes.

[rumpelstiltskin]

Ok,i've thoroughly checked circuit boards for cracks,in darkness with an extremely bright light shining from the back of the boards.I've used electrical cleaner on all the jack plugs and sockets.Tapped the circuit boards gently and there's no rattling.Amp doesn't crackle now with the lead taken out the amp.If you turn the amp all the way up it's like you can hear the guitar very faintly in the distance?When the distortions all the way up on the lead channel you can hear it like it's in the next street if that makes sense?It's annoying knowing that the sounds there but it's so weak!What symptoms would a duff valve give?(it only has the one!)As i said before,the valve is brightly lit and isn't whistling etc.

[J M Fahey]

Write HK, give them your model and serial number and ask for the schematic; they usually send it to you.
Anyway, just to save some time, plug your guitar into the effects loop return or power amp in jack,
Any luck?

[rumpelstiltskin]

Getting sound no problem out of the return,nice clear and loud like it should be.You'll probably know more than me but i take it you should just get a clean sound connecting into the return?:-)Excuse my ignorance!

[J M Fahey]

First test OK.
Now plug your guitar in the usual front panel input, *and* plug a known good guitar cable from loop send to loop return.
Any news?

[rumpelstiltskin]

Amp works perfectly connected like this!

[J M Fahey]

OK.
Your Loop Return jack is dirty/rusty/worn/weak or all together.
Squirt a little contact cleaner (or at least some WD40) in it, work a plug in/out 10 times until you clean it; best is to replace it with the exact same type if you can get it.
Check online catalogs (Mouser - DigiKey - Antique Electronics - etc.)

[rumpelstiltskin]

Thanks for your help,really appreciate it!