Tubemeister 18 Troubleshooting - Lots and lots of hum and not much else

Hi guys!  I need your help troubleshooting my H&K Tubemeister 18 Head.  I bought it untested from an auction and I think I have learned my lesson.

I am getting hum on both channels whether or not a guitar is hooked up.  Volume knobs do not affect the hum volume.  I visually inspected all caps and resistors and I am not seeing any bulging/burn marks/bad solder joints.  I can hear the guitar over the hum, but it sounds horrendous.

The TSC meter on the back is blinking six times on the right and 11 on the left and isn't really improving after I replaced the OEM chinese EL84s with a matched pair of JJ's.  The lights on the TSC are kind of leading me to believe that their may be something wrong with the TSC unit.  I tried swapping the tube positions and I would think this would cause the blinking patterns to flip, but that is not the case.  

What am I missing here?  Does this sound like a mosfet issue?

Hi Todd. Bad luck with the auction buy, I've done the same thing myself in the past. On the other hand I've had some ridiculously lucky buys too.

What you describe does sound as though it could be the bias control circuitry but there is a fair bit more to it than just those MOSFETs. There is an entire digital control setup to consider too. The fix could mean checking out a lot of the voltages around that area and interpreting them correctly and that would be pretty technical - and could even be dangerous if you aren't experienced! There is close to 400V floating around in there! If it turns out to be the digital side then there is no fix I'm afraid. It means replacement of the Bias TSC Eco board and H&K will not supply parts to ordinary owners. You have to go through a recognised repair shop.

Is there any chance you could get it to a good tech for a little diagnostic work? He should be able to tell you what is the extent of the damage with just a little work. You would need someone who knows more than the "I learned to fix amps from Jim who works in the local baker's, he's a real electrickery guru!" type you sometimes find as this will not be just a "everyone knows you just replace all the valves and caps then it must be a solder joint on the PCB" type of fix. Even replacing parts on spec after guessing what could be wrong could easily turn out to be an expensive waste of time. You need someone who can tell whether that area is working as it should and that requires proper training and experience.

Hi Thatguy,
did you ever get this fixed? I have a TM 36 that has a very similar problem, loud hum, sounds like 100Hz but not sine, more like triangle or square wave. Volume controls don't affect it, it's still there with all master volumes fully down.
Cheers
Hugh

Well, I had a poke around and read a lot on forums and I have found Q24 Mosfet that controls tube T7 bias set up is virtually short D-S. Ive ordered a replacement and will let you know if it fixes it.
Thats a very complicated auto bias setup. I was tempted to bypass it and unstall simple resistors and pot to adjusrt it manually.

Did you catch this current thread Hcardle? It may have some info that may be useful. TSC MOSFET Replacement

Well that did the trick.
Hope you got yours fixed.