Help needed, seems my GM36 is down...:(

Hi everyone,

I powered on my GM36 this morning and no freaking sound coming out of it
They only way I can get a "sound" is by setting it on 36W, Master, volume and gain to full and I can gear a very faint distorted sound going out of the speaker. This leads me to believe that speaker, and speaker cable are OK.
I changed guitar: not better.
Changed cable: not better.
Plugged into the bass amp: OK
Line out seems OK (but rarely used so can't say more).
Seems the whole amp part (not preamp) is down the toilet.
The four tubes on the rear are lit as usual.
Nothing lit on the front tubes (the two we can see thru).

What else can I try ?

EDIT: plugging the guitar into the fx return and activating the fx loop (and nothing in the input) will produce a more or less expected sound.

Put a line level signal into the FX Return socket and see if the power amp section is working.  That could be from the FX Send of a different amp or just something from your iPhone, (turned down of course).  If you get sound out as normal then the power amp section is fine.  Then try simply bridging the FX sockets with a standard jack patch cable, (no pedals just Send straight back into Return), and with you playing as normal guitar into input.

This may be just be dirty FX loop socket switches.  When you have nothing plugged into either the Send or Return the switches on the sockets route the signal from the preamp direct to the power amp.  If the switch contacts get a little dirty or corroded, (they often do on any amp), it breaks the signal line and you get nothing or at best a very poor signal.

bordonbert wrote:Put a line level signal into the FX Return socket and see if the power amp section is working.  That could be from the FX Send of a different amp or just something from your iPhone, (turned down of course).  If you get sound out as normal then the power amp section is fine.  Then try simply bridging the FX sockets with a standard jack patch cable, (no pedals just Send straight back into Return), and with you playing as normal guitar into input.

This may be just be dirty FX loop socket switches.  When you have nothing plugged into either the Send or Return the switches on the sockets route the signal from the preamp direct to the power amp.  If the switch contacts get a little dirty or corroded, (they often do on any amp), it breaks the signal line and you get nothing or at best a very poor signal.

Thanks for helping.
The guitar plugged right into FX return works fine.
Patch cable from send to return (and guitar in 'input') will not.

Could be a couple of things, all related to the FX loop (obviously). If the FX loop is active and there`s nothing connecting input to output you`ll get no sound (downside to a series loop). Could be the switch (easy fix). Could be the FX loop itself, somehow derailed and not getting signal from Out to In, which is likely nothing more complicated than having the loop`s card replaced (still not a big deal if it`s under warranty).

Thanks for helping him out guys!

Ok, so we know that the power amp works fine from the return jack onwards, and the preamp works fine up to the Line Out which shares a signal with the power amp input.  After that it goes to the Master Volume control.  In the distorted and quiet state does the MKaster Volume still affect the sound as it normally would?

I would see if there is any way you can tell whether all of the controls are working as they should from the signal you get from the Line Out?  Do you have Presence and Resonance function for example, (they come pretty late in the chain).

I have done some poking around inside my own machine now and I can say that the FX loop in the GM36 is unusual. It doesn't rely on the switches on the sockets, it is switched on and off internally via a relay selecting either the signal on the Send line on the PCB before the Send buffer, or the return line on the PCB. Therefore the socket switches shouldn't take part in the On/Off process. Much better way in my opinion.

bordonbert wrote:Ok, so we know that the power amp works fine from the return jack onwards, and the preamp works fine up to the Line Out which shares a signal with the power amp input.  After that it goes to the Master Volume control.  In the distorted and quiet state does the MKaster Volume still affect the sound as it normally would?

Yes, the Master volume works pretty well and reacts normally to the 1W/5W/18W/36W setting.

bordonbert wrote:I would see if there is any way you can tell whether all of the controls are working as they should from the signal you get from the Line Out?  Do you have Presence and Resonance function for example, (they come pretty late in the chain).

I have done some poking around inside my own machine now and I can say that the FX loop in the GM36 is unusual.  It doesn't rely on the switches on the sockets, it is switched on and off internally via a relay selecting either the signal on the Send line on the PCB before the Send buffer, or the return line on the PCB.  Therefore the socket switches shouldn't take part in the On/Off process.  Much better way in my opinion.

Indeed, checking every controls seems the way to go so here it is:
* Volume and Gain do nothing at all, neither does the boost.
* The channel knob does very little, it influences the volume output, as it should but nothing else (normal since guitar is in fx loop return)
* there's nothing in the fx loop send, no fake jack or anything.  Shouldn't it prevent the return if there's no send ?
* All effects are working: reverb, delay and the 4 on the Mod Type/ Intensity knob. Since those knobs also do Gain/Volume, knobs are not the culprit.
* Presence and Resonance seem to work just fine.
* Line out seems actually UNresponsive to volume change. Can't test XLR though.
* What would happen if tubes in the Pre-amp was dead ?
* Noise gate doesn't do a thing at all.
* TSC does as following, * being light on and - being light off:

  **** x10
  -***  x1
  -*-*  x2
  ---*  (sometimes, 1x)

Does TSC manage the pre-amp tubes too?
I'm not quite sure those on the front did get slightly orange but if it is the case, mine certainly do not anymore, at all. The four on the back though seem fine.

I think that's all I can say for now.

Thanks.

EDIT: MIDI communication works fine with the GrandMeister Remote Utility (btw, thanks to its coder, much appreciated):
It reacts to any command. Playing with the commands in a vain hope it would do domething useful, I can confirm that if the signal is strong enough (compressor and overdrive ON), I can hear a tiny bit of signal from the speaker. Nothing useful but I can hear it.

AHA! That sounds like you may have a problem with the heater supply to at least the preamp tubes. This is very likely to be a "take it to a tech" fix I'm afraid. The only things I can suggest are, remove all of the valves and refit them in their same sockets to friction clean the contacts, and if you feel confident enough then open it up underneath and disconnect all of the PCB header plugs in turn then refit them. If you feel confident enough to try this let me know and I'll post instructions on the right way to remove the baseplate, (warranty issues). I can't get our search functions to find anything at all so I'm assuming they are as useless for everyone. I know I posted instructions on how to remove the baseplate but I'm damned if I can get any hits at all for anything I search on. If anyone else can find the instructions and post a link it would be appreciated.

For the record, preamp tubes are biased to a fixed level by the components around them and this should be stable. They do not need rebiasing or controlling like output valves do. They are build and forget effectively. That's why valve swapping with inappropriate types screws with their correct biasing.

bordonbert wrote:AHA!  That sounds like you may have a problem with the heater supply to at least the preamp tubes.  This is very likely to be a "take it to a tech" fix I'm afraid.  The only things I can suggest are, remove all of the valves and refit them in their same sockets to friction clean the contacts, and if you feel confident enough then open it up underneath and disconnect all of the PCB header plugs in turn then refit them.  If you feel confident enough to try this let me know and I'll post instructions on the right way to remove the baseplate, (warranty issues).  I can't get our search functions to find anything at all so I'm assuming they are as useless for everyone.  I know I posted instructions on how to remove the baseplate but I'm damned if I can get any hits at all for anything I search on.  If anyone else can find the instructions and post a link it would be appreciated.

For the record, preamp tubes are biased to a fixed level by the components around them and this should be stable.  They do not need rebiasing or controlling like output valves do.  They are build and forget effectively.  That's why valve swapping with inappropriate types screws with their correct biasing.

Ouch, well I kinda expected it.
A simple tube refit should not be a warranty issue I suppose. That I can certainly do.
I will open it and try to see if it does anything.

Thanks again. Will update as needed.

No, if there is a problem with the heater current to the valves it should be a warranty issue. The valve heaters are all in parallel so a fault in one should not affect any others, (unless it's a short). If you have more than one valve with no heater then it implies there is a fault elsewhere causing it rather than in a valve itself. It's very unmlikely to be a simple fix, it may be power Tx or PCB damage for example, but I would have thought it should be covered under the warranty. That's why you have to be VERY CAREFUL to get the right procedure to open up the amp!!!! Do it wrong and you'll void the warranty.

The end caps are the start point, and they must NOT be completely removed. You simply remove both top screws and the rear screw completely. Don't touch the front screw it only holds the handle on. Then loosen both of the bottom screws by a turn or two, NO MORE! The end caps will then be able to flop outwards and allow the top to be taken off for valve changes. Underneath there is now some slack to remove the 10 baseplate screws and wiggle it sideways to remove it. The secret is to not loosen the end cap bottom screws too much as they hold the "void warranty" device in place. A turn or two is fine. You should also put the original valves back in place as that is a potential warranty issue too unless you had them replaced by a professional tech, (and let's face it, how many people do that nowadays)?

If you are have trouble removing the base plate then maybe this video will help you with any anxiety you maybe experiencing before you attempt the procedure:



All joking aside, Bordon is the best man for the job in helping you to resolve the issue from the consumer side, if he can.

Hahahahahaha!  That's just a tad too close to home Hwystar. He's actually like a highly technical American Stanley Unwin. (If you know the Small Faces album Ogden's Nut Gone Flake you will hear him talking Unwinese during Happiness Stan). That total bollocks you posted sounds so plausibly intelligent it's like being back in lectures at University. We should get him to give some of the advice for us, he'd make it sound clearer than we sometimes do.

So, small update: I did open the GM, just so I could check the valves as to be sure not to do anything voiding the warranty.
Nothing to report. I also checked with my phone (flashlight) to see if any visible component was showing a sign of burn or anything else but nope.

So I guess it will need to go through the repair chain.

Any idea about the turnover time? I'm in europe so I guess it should not be too big.
I'll probably buy a 2nd hand TubeMeister 5 for the time being. I do NOT want to play on the Roland Cube 15x lying somewhere

If anyone in europe has a TM5 for sale....

In fact these are similar if not same symptoms. Difference - I can hear the fx in the fx loop. The channels all change , very faint. Plugging directly into return does nothing. No sound

Replaced all the output tubes. Still same. No sound when direct into return with loop engaged.

Thibsie, maybe I read too fast, but I thought the output side was working, and bordonbert stated heater on preamp tubes may be shot. No, TSC does not manage pre-amp tubes. If you have one spare 12ax7, you can rotate them around, hopefully finding a bad one. One bad tube can short the heater circuit - or maybe you have just have one bad triode in a 12ax7.

Hey there,

It took time but I brought my GM36 for repair. No ETA yet.
Hope it won't be too expensive (second hand, no invoice so... screwed !)


In the mean time, I got a Laney Cab10 which is a darker but my cab being too bright makes it about right

I miss my GM, though. No real crunch on the Laney but with my Cort ES335 copy and its P90.
I want my GM back !!

Hahaha!  We've all suffered from this at some time.  I once christened it suffering from "The GMs".

"First you play the GM,
     then the GM plays the GM,
            then the GM plays you!"

'bout right?    Shucks cuzzins, I should be writing country lyrics.

How did this pan out for you? Im having the exact same issue and im wonding how bad the sting is gonna be $$$. I miss my gm36 and i just dont know who i can trust to work on it in wny.

Hi guys,

Sorry I completely forgot to update the thread about this issue.
The first reseller bitched for weeks about H&K not taking care properly etc and said 'no way I send the important to importer for repair this need to be done in Germany blabla'.
I finally took the amp back and try in a 'big' shop around.
No question asked. About one week (less I think) total. Cost between 100-150€ for repairing an amp which was of course out of warranty and never bought in that shop.

So, Very happy with the support actually, both shop and importer. The only detail is they didn't bother to put the back little metal/spring thingies that keep the valve in place.
They changed middle pre-approval valve and changed the left black block for a bigger one.

Works nicely enough now
I still have problem but midi related so unrelated. I'll post on dedicated thread.

HELLO!  

This morning i had EXACT issue as you...  Was panicked.. Wedding gig for a good friend of mine tm!  So... Started looking through this forum with no solutions to the issue that i could try myself..  I did the obvious by starting with tube swapping..  I grabbed two 12AX7 tubes from my 5150 (also not functioning!!!!  power tubes wont fire!) and inserted..  Boom!  Full performance once again! I'm quite very relieved and this is a cheap and simple fix...  I recommend trying this b4 going and spending $$ on a tech who looks out for his own business and can give you any explanation needed to make a few extra $$.   Hope it helps..

Cheers!

Grim Latimer

Hi Grimmy.

Swapping out valves in turn for a known good one is a really good simple thing to try in the first instance though it isn't the solution for everything of course and you do need to apply your intellect to see what could be causing your own set of symptoms. We still don't know if the original problem actually was a heater issue or not. As I think Namklak said, it could well have been caused by a shorted heater actually in one of the valves, hence the fix when that valve was replaced. But the "black block" was replaced also, a transformer maybe? So how could the power amp still be working.

Always be careful when you say "my symptoms are exactly the same". Sometimes the tests we ask you to perform are to show whether the fault is in one area or another and that may not be obvious if you are non-technical. Speaking for myself I find it very frustrating when I give someone a few simple checks to report back on and they try the first one, give me a garbled report as to what they sort of found and ignore all of the rest. What is it that leads people to believe that "my amp has no sound coming from the speaker" has an obvious easy solution that we can tell from that info alone?

It's easy to assume that you have exactly the same problem as someone else but in truth it generally means you just share something like "no output when the amp is played" and other aspects you aren't noticing are very different. For example, were your heaters in your preamp valves dark as Thibsie's said his were? The mystery is how the preamp valves could be cold but the power amp valves were not. Certainly with the power amp working via the Fx Loop Return the PI valve V3 must have been lit. I wonder if it may have just been a single cold valve but Thibsie just couldn't see the heater glow in the other visible preamp valve.

Anyway, glad that a valve swap fixed your own problem Grimmy, you did well to catch it yourself. And it's always best to keep a couple of spare valves in your gig bag even if they are only the ones you swapped out last time you changed them all "for maintenance".