Tubemeister 36 dead

A few years ago i had accidently stepped on my midi board cable and it got pulled out of the midi jack on the rear of the amp.The amp went dead.No lights,no sound,none of the push button lights will light up but i do a glow in the tubes.TSC shows absolutely nothing,no lights there either when inserting a pick.I've tried inserting my guitar cable into the return jack to see if there's any audible hum coming from the power section but still get nothing.I thought maybe a fuse since the midi board is phantom powered.Fuses look o.k. but i need to check them for continuity.The 2 inside i didn't feel comfortable with as they're close to the capaicitors.The fuse at the power cord i would assume is o.k. as the tubes do have a glow so they must be getting power to them.I've been meaning to get it checked out but the closest repair shop is about an hour drive away and i just put it off for years.Searched the forums here and havn't seen anybody else have similar issues.I have a feeling it's fried somewhere inside and it'll cost as much to fix as another used one would.Is it worth even getting it looked at or should i consider it toast.Hopefully someone here may have some insight but it doesn't look like the TM forum has been very active lately.

Hi Toddmc. Yes, the forum is a lot quieter nowadays and less of a community I think. People move on and sometimes the idea of a technical approach to things doesn't sit well with many guitarists who prefer to believe in the power of common knowledge and internet magic over solid provable engineering fact. I hope it still offers some useful advice for the H&K models as there is nowhere else that seems to know anything about the H&K brand. They aren't mainstream really.

It sounds like you might have a pretty serious problem. There is no reason why pulling the plug out of the midi socket should produce any effects like that. Even the MIDI voltages are not an issue as MIDI itself is designed to be tolerant of short and open circuits. It really sounds like what has happened is that in stepping on the cable the plug has been levered downwards badly and upset something in the amp in a big way. Is there any other sign of damage on the outside of the amp? Is the MIDI socket distorted or pulled away from its mounting screws in any way? And, as you have already opened it up, was there anything that looks obviously wrong inside too?

Firstly I think you should check out those fuses. Don't worry about electrocution. It's very straightforward to be safe. Turn off the amp, (a moot point now), take out the mains plug and leave it for about 5 minutes to be sure. The HT caps you were a bit threatened by are genuinely something to be wary of. As we say in the game, "they do bite"! A word of warning here. They are not like the mains, they are DC rather than AC and that is potentially more dangerous. An AC voltage will shock you but throw you away from the contact as it is constantly cycling back to 0V. A DC voltage will make your hand muscles spasm tightly and grip onto whatever they are near and you can't let go as it is continuous. However, any decent design will have drain resistors applied across the caps to prevent just this problem. These are high value resistors which do not affect the circuit action when it is running but, as soon as power is removed, they steadily drain the charge out of the capacitors pulling their voltage down to close to 0V. After a few minutes there will be no voltages to speak of in the amp so you would be perfectly safe to remove those fuses. If you do it straight away not having run up the amp for a few days there will be absolutely no danger of those caps holding any charge. If you remove them with an insulated electrician's screwdriver or even a plastic chopstick and just pop them gently up out of their holder you will be able to pick them up without any contact. A pair of insulated tweezers or pliers will allow you to drop them back into the holders again and you can push them home with the screwdriver. Just take sensible precautions and be careful which you obviously are. So just remember - MAINS PLUG REMOVED, FIVE MINUTES MINIMUM WAITING TIME.

Check them one at a time, don't remove both together so they can get mixed up, and do not rely on your eyes. It is not uncommon to find a fuse blown inside an end cap, even right at the point where the fuse wire is actually soldered to the cap. Even with a break in plain sight it can be almost impossible to see it. You need to check it for continuity. This can be with a fairly basic multimeter on the "diode" or a resistance range. These only cost a couple of pounds nowadays and are almost an essential to the guitarists maintenance kit. I always carry two to every gig. A simple Heath Robinson alternative way of doing it is to get a torch, take off the battery cover, turn it on, put the fuse in contact with the end of the battery chain and use a piece of wire to connect it to the terminal of the torch completing the circuit. If the torch doesn't at least flicker under this setup then the fuse is dead. Of course you do always have the option to just replace the fuses with new ones regardless and see if it makes any difference.

I would start there and see if anything shows up or the amp springs back into life. I wouldn't hold up too much hope of a complete fix but it may be just possible. Try that and get back to us and we can see where to go from there.

The amp looks practically brand new.The midi socket looks fine and no apparent damage.Nothing on the inside looked burnt or buldged like a bad capicitor on the boards and the tube sockets all looked good.That's what got me,i can't tell you how many times i've had things like this happen over the years,where someone else or myself accidently yanks a cord from somewhere.First time i ever had an issue.I bought this amp off Ebay used and had it about a year before this happened.I didn't play it much though because it lacked so much bottom end.I was in the process of running an EQ in the effects loop and trying different combinations of stuff when i stepped on the midi cord and it went dead.Up to that point it had worked fine with no issues.I'll check the fuses with a multimeter.If they're good,i'll probably just post it on Ebay for parts and maybe i'll get a few bucks for it.If not i'll keep the tubes and chuck it.The only place that works on tube amps is about an hour away.The last time i checked his site,he wanted a $200 deposit up front.If it needs anything,which i'm sure it would,then that's added on.If it can't be fixed then i'm out $200 unless he reimburses some of the deposit.I'm not real sure,but i feel i' already spent $600 for the amp and midi board.Putting any more into it will just be costly.I'll check the fuses and post what i find.

I do think it is possible that your HT fuse at least may be blown. If the cable yanked the amp sharply it is very possible that something was shaken about so badly that it popped that fuse. The valves themselves can be prone to that. I think you are taking a very pragmatic and sensible view on this. If the fuses are good then something else is wrong. With a $200 deposit, (that sounds really scandalous to me but I'm in the UK with a number of repair shops within sensible driving times), the idea of funding a repair is very questionable.

Let us know what you find with the fuses and how things go on from there. There may be more advice we can give if we know a little more of what is going on. For the moment just roll with it, and by the way, if the footswitch MIDI unit is a H&K FSM432 you should sell that separately and get at least a portion of your money back. Don't just throw it in with the amp for spares. They work with any of the H&K TM and GM and Black Spirit series amps and I'm sure there must be a market for them around you. There are a few on ebay at the moment, £125 used in the UK and £300 new in the US. (Yes that is £ not $).

It lives! I finally got back to the TM 36 and the problem was the HT 800ma fuse.I removed the fuses and checked with a multimeter and sure enough the 800 ma fuse was blown. I looked at that several times in the past using a maglite and a magnifying glass and it looked fine.No apparent burnt marks on the glass and the metal filament looked intact. I wouldn't have found it without taking the fuse out and checking for continuity. Thanks for the help. Now i need to play it and put it through it's paces and hopefully it doesn't throw any more fuses.

Haha! That's great news Toddmc. Another amp back to life and in use again always gives a warm feeling. Glad it was as easy as just replacing the fuse, and it is good to hear that it wasn't an obvious thing.

As I said earlier, you can never trust your eyes with fuses which look good to go, you always need to check for continuity to be sure, or simply replace with a new one as a test. HT fuses can blow for a number of reasons. Valves very commonly take them out if they have a problem so keep your eyes open for any issues that may be valve related. Don't be paranoid about that, you probably won't need a new set of valves because of this. Sometimes a simple mechanical jolt can do it, either by damaging the fuse direct or causing a surge from elsewhere which means the fuse blows. And of course, you must remember that fuses spend their life passing currents and those currents cause the fuse wire to heat up considerably and cool down rapidly as the current varies. Eventually this takes its toll and the fuse becomes brittle and will eventually snap due to old age.

Blowing at switch on is a common occurrence. So, they can blow due to nothing at all wrong with the amp. That means that you would be fine to swap out a blown fuse and try the amp with new, just don't keep on running up an amp with new fuse after new fuse, a second blown fuse suggests there is another problem as the root cause and it needs to be investigated. And NEVER EVER replace with a fuse of a higher current, (higher voltages ratings are not an issue it is only the current which counts in fuses). It is perfectly feasible that you may have a problem with the amp which increases the current above its correct level but not high enough to blow the replacement fuse. Its your life on the line then! And, I'm sure I don't have to point this out to anyone but... under no circumstances should you ever "temporarily" replace a fuse with any other object such as a nail or aluminium foil as that totally removes the main protection for both your amp and your continued existence in this mortal plane.

Now, just as you say, get on and play the Hell out of it, that's what it was made for! Merry Christmas.

It blew the 800 ma fuse again.All was going well and i put it to work.I played through it several times and decided i would revisit the midi function. I have the H&K FSM 432 MK2 midi foot controller. It can take a 7 pin midi cord and uses phantom power or i can use a 5 pin midi cable and use a power supply. I plugged in a 7 pin cable to the head,turned it on,and it went dead almost immediately. Lit up for half a second maybe. I replaced the fuse again and then tried an old Yamaha midi controller that uses a 5 pin cable. Everything working as it should. Then i tried the H&K midi controller with the power supply and 5 pin cable. Everything still working as it should. So i'm at the conclusion that either the midi cable is bad or the midi socket in the amp has a short,something maybe got bent when it got yanked on and out of the amp. i can buy another cheap 7 pin cable and test again to rule out the cable. The midi input is buried under the pc board for the rear panel output jacks,fx loop,etc.and hard to see if anything is bent or going on in there. I'll need to read up some more to see how hard it is to get in there or if i can test the midi input with a multimeter.I know there's a pin that supplies the phantom power and a ground pin. If it'll work o.k. with a 5 pin cable then i can live with that but i would still like to know exactly where i stand and where the problem lies if i decide to sell the amp down the road.

Aha!  Ahahaha!!!

This would seem to point to damage around your MIDI socket or maybe the PSU or something simpler...  The original problem was caused by your tripping over the MIDI cable wasn't it?  Now the problem only occurs when you access the power line of the MIDI cable.  Have you tried a different 7pin cable?  It may be the cable that is damaged not the socket.  MIDI cables are notoriously cheap and flaky, even the good ones!  We have had a plethora of problems over the years which come down to MIDI cables.  Many more than any other type of cable in fact.

As you have a multimeter, with the amp in working condition measure the voltage coming out of pin 6 and 7 of the MIDI In socket, (6 should be +ve).  They are the two on the extreme ends with the gap between them.  A pair of paper clips is a great tool here but be careful. You should find +22V, at least for the GM36 and they are both using the same footswitch unit so the voltage out should be the same.  Now, looking at the GM36 schematics I can see that the MIDI socket is actually fed through its own fuse.  I did not know that!!!  This is not the one you are changing which is the 800mA fuse which should be labelled FS2 on the PCB.  FS1 should be 400mA.  This is the same in the US and UK as they are after the mains transformer so voltages are not different.  The MIDI fuse is labelled as 200mA on the schematic and is a special type.  It sits on the PCB right next to and outside of the MIDI In socket.  I don't think this is likely to be blown as you report the MIDI system works when you only use the 5 pin alternative.  That should mean that the controller chips are still ok and therefore their PSUs are ok too.

Can you check out the MIDI cable, swapping it out is best but even a check for shorted pins may help, and measure the voltage on pins 6/7 of MIDI In?



Hahaha!    Sorry, I got excited with your great diagnostic work and didn't read enough of your post.  You already have these steps in hand.  Good work!  Get back to us with the results.  

Just looked into the MIDI fuse, (Littlefuse RXE020-60V/0.2A), and it is a resettable type.  It actually looks like a circular ceramic capacitor.  It trips and resets itself 1000s of times so it would seem to not be the problem, but we should keep it in mind.  Any component can go faulty.

And to add a thought.  MIDI is specially specified and designed so it is immune to shorts on any of its lines.  Circuit designers know musicians in that respect!  If it could happen in a parallel universe then a musician somewhere will do it.  This includes shorting everything to everything else.  So MIDI is designed from the ground up to be immune to those problems.  With that in mind it is hard to know what is going on here.  I am beginning to suspect you may have cracked the PCB and the problem is not actually in the MIDI circuitry alone.  We are pretty near the Power Soak lines here though the fuse which is blowing is for the LT line.

Been busy over the holidays and finally got back to the amp. It was a bad midi cable. I ordered another midi cable to troubleshoot the midi input on the amp and cable and so far it's working fine and hasn't blown the fuse again. I'll be sure to give it a workout several times just to be confident. I did wiggle the cable around at the midi inputs on both the amp and pedalboard just to make sure there wasn't a bad connection or bent input contacts. No issues.Time will tell. Now i can move on to another amp issue. I recently bought the H&K VS250 tube power amp. It has a few issues that i should be an easy fix. The amp will switch back and forth from on and standby mode. Also a few scratchy pots where volume cuts out a little. I removed the top cover and was suprised not to find a dead animal in there. Dirty is an understatement. It looks like it was left in a dusty barn for 20 years uncovered. Anyways i plan on cleaning it up and going over the pots and switches with deoxit. Then relubricate the pots. I can remove most the filth with a chip brush and some compresssed air for the hard to get spots.I think my issues are just due to dirty pots and switches.

Great to hear that was such a simple fix Todd. As I think I said before, MIDI cables always seem to be notoriously cheap and unreliable, much more so than instrument or patch cables! I have to say that it is a surprise to me that the cable should have this much effect. MIDI hardware has been designed to take into account all manner of abuse to its connections. They knew musicians when they drew up the spec! All of the feeds are from lines fitted with current limiting resistors and inputs are opto-isolated with current limiting resistors which are a necessary part of that area working at all. The only thing I can conclude is that the FSM432 22V feed must be the culprit and is shorting to something else. This is outside the MIDI design remit and would override the normal safeguards.

Anyway, my brain is rambling again. I reckon it is a case of Ockham's Razor, "the simplest explanation is usually the best one." Keep us up to date if it shows up again and say a little prayer for the H&K designers who made their circuitry so robust.