Anode fuse keep blowing TM18 head.

Went to play the other night the tubes were all glowing, but didn't have any sound.  I located the fuse that was blown and replaced it.  Then tried with speaker off and power tubes removed.  Each time the fuse blown.  Any other suggestions that I could try before the head sits storage as I can't afford to have a tech look at it at this time.

Could it be my power I have my other heads plugged into a Tripp Lite ISOBAR8ULTRA Isobar Surge Protector.

First thing would be to try substituting a borrowed set of EL84s.  Could be an output valve gone west.  Hope it isn't the output Tx though that is very unlikely.


EDIT:  It's just registered you wrote "power tubes removed".  Which fuse is blowing?  There are 3, the mains fuse, FU1 200mA and FU2 315mA.

Fu1 200mA

Yes that's the HT fuse.  It's the +ve line to all of the valve anodes and the output valve screen grids.  You have a set of rectifier diodes and the chain of HT capacitors all hang on there.  You've opened it to check the fuse, can you see any sign of a problem while you're in there?  Look for charred or darkened components and capacitors with the can top popped outwards.

I'm not convinced that the testing wihth the output valves removed is valid as this amp has special active auto biasing circuitry.  Can you borrow a working pair of EL84s to substitute as a test?


EDIT: For those worrying about delving into a valve amp with 390V floating around, there is a 300k drain resistor in the HT line! Give it a little tim e and the HT is brought down to near 0V.

Put new tubes in and the fuse blew as soon as I came out of standby.

Well done for testing with valves. Sorry it didn't help at the moment, (actually it did really as it showed the fault was not the valves themselves). I'm afraid it sounds as though there is something very wrong there. I think your only option is to take it in for testing to a tech. It's impossible for any of us here to get to the root of the problem reliably without having the amp in front of us on the bench. There's also the danger that it may cause more damage fiddling with it in a faulty condition.

I was able to find a schematic of TM18 head. Checked a few resistor in the power soak section and diodes that come are part of the Fuse circuit.


The problem is what the schematic is calling out for doesn't match the color bands on the resistors.
R10, 11, and 12 are all the same value.. Meter reading 33.2Ω on all three of them. The schematic states that they should be 100R/5W the color code are saying they are 10.1 unless I recently became color blind.

Here's a picture of resistor - Reading it right to left as Brown, Black, Brown, Gold, and Black, but there isn't a black in the 5th position when trying to determine value. So I'm saying it's silver even then the 5th band isnt' the multiplier band it's the tolerance band. Unless there isn't a tolerance.


R7, R8, and R9 all within tolerance.

These look like the ones sitting in the PCB cutouts which are the power soak resistors.  These should be 39R, 15R, 33R, 3 x 100R.  That picture looks like your 15R - Brown, Blue, Black giving 150 with gold muiltiplier x 0.1R.  The last band looks like it's brown, 1%.

The R10, 11, 12 are metering spot on at 33.2R (100/3 - 33.3R).  I think you may be reading the 5 band resistor code wrongly.  The first three bands give the figures, the fourth is the multiplier and the fifth is the tolerance.  Should be Brown, Black, Black (100), Black (x1R), Brown (1%).  Does that tie up?

I'm pretty sure, barring odd short circuits, that the power soak area is not where your problem will lie.

I'm pretty sure I'm reading it correctly plus I used http://electrodroid.it/app

R7, R8, R9 are reading spot on and the color codes match.  The Picture is of R10 and R10, R11, and R12 as I'm reading it Brown, Black, Brown, Gold, and Black.  My eyes aren't what they use to be either.  

I'm assuming you have a copy of the schematics.

I also checked diodes D7, D8, D9, and D10 all check good.  I also check voltage to the fuse from the transformer 380VAC.

Well something is wrong as it meters up exactly as it should, 3 x 100R in parallel. Never mind.

Do you have a couple of 12AX7s to swap them out and try it with the EL84s removed? That way we could rule out a valve problem.

Have a look at C31, 32, 35 ,36, the power line smoothing caps. Do they all look ok? They are clustered around the two fuse holders on teh main PCB. There are a few other electrolytics there too, check them visually. Look for either a domed out top or some sort of leakage or colouration around them.

10.1 x 3 = 33.3

But the schematic shows them in parallel!

I've been trying to post the power soak area but the system isn't letting me communicate with the forum easily!!!!!!  Are we having server problems at the forum end?  Has anyone else had problems with speed recently?

You are absolutely right there, there's no doubt about that, but have a look at this:

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...and the PCB layout shows you aren't reading those 3 resistors in series as you assume, one to the other to the other, you are actually reading each end of the 3 connected in parallel.  Have a look at the left two.  Their opposite ends are connected by a PCB track which also connects to the third.  And the other end of the third goes off to connect to the middle point of the first two.

bordonbert wrote:I've been trying to post the power soak area but the system isn't letting me communicate with the forum easily!!!!!!  Are we having server problems at the forum end?  Has anyone else had problems with speed recently?

You are absolutely right there, there's no doubt about that, but have a look at this:

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...and the PCB layout shows you aren't reading those 3 resistors in series as you assume, one to the other to the other, you are actually reading each end of the 3 connected in parallel.  Have a look at the left two.  Their opposite ends are connected by a PCB track which also connects to the third.  And the other end of the third goes off to connect to the middle point of the first two.

I know.

I'm now worried about those resistors as I think you are right and they are 10.1R.  I hadn't imagined they were that clear for you.  There's no mistaking them, they're Brown, Black, Brown, Gold.  And if that is 10.1R then that is loading the thing with 3.3R not 33.3R when they are engaged.  I've just checked out my TM36 but it has a much more complex power soak setup with more resistors of different values.

How old is your amp?  Has it always worked perfectly well until this problem?



EDIT: As to the previous post, I meant your own photo of your PCB shows them in parallel.

bordonbert wrote:I'm now worried about those resistors as I think you are right and they are 10.1R.  I hadn't imagined they were that clear for you.  There's no mistaking them, they're Brown, Black, Brown, Gold.  And if that is 10.1R then that is loading the thing with 3.3R not 33.3R when they are engaged.  I've just checked out my TM36 but it has a much more complex power soak setup with more resistors of different values.

How old is your amp?  Has it always worked perfectly well until this problem?



EDIT:  As to the previous post, I meant your own photo of your PCB shows them in parallel.

I purchased this used back in 2012. It worked great for almost 4 years. Then I moved it to a different location and played it for about a month without my effects loop connected. Then after I had everything straighten out I hooked up my FX loop (Reverb and Delay) and that's when it went silent.

Good. So it has worked for a good length of time without problems. Then I would suggest there is something strange about those 5W resistors but that isn't what is causing the problem. I think we have to assume that, as we can see they are Brown, Black, Brown, this must represent 10 + another 0 = 100R. I have no idea why they aren't following the usual 5 colour code but there is no doubt, you have 33.2R on your meter and they are all 3 in parallel from your photo of the PCB, hence they must have values of just under 100R! The only other way to check them is to disconnect an end of one of them and meter that in isolation. My money is firmly on 100R!

As I said, I can't see that causing the problem you are having. We are looking for something much more immediate at switch on. If that were the problem it would only show itself when you engage those resistors with the power soak. If they then loaded the output in some strange way I would expect the protection to kick in and remove the drive. Best forget them and look elsewhere until the problem is solved and the amp is working, then go back to it.

I'm looking at my replacement fuse and they are fast blowing. The parts breakdown says it's a T200mA and isn't "T" for slow burn?

Yes, 'T' does designate a slow blow fuse and that is important.  The smoothing caps will take a surge of current when they start charging up.  This drops off quickly as they charge but it can be above the 200mA for a significant time.  Really we are sailing close to the mark with 250V fuses here but getting higher is very difficult.  250VAC is 354V peak and the line voltage at this point in your TM18 is 390V plus a few volts lost in the diodes.  It's not desparate to use 250V, everyone does, but a little more leeway would be good.  I would get a good supply of them if you want to continue diagnosing the problem yourself, they are cheap enough and you are likely to go through a few.

The only way from here without better facilities as far as I can see is to take everything out, all valves, and run it up.  If it blows then you know you have eliminated the valves as the cause and it most likely lies in the passive components.  Then fit them back a section at a time, each preamp valve in turn then both the power amp valves, and see which one brings it back in.  If you find inserting a preamp valve causes it, make sure to use that suspect valve in the other slot to see if that then stops the fault.  You need to know if it's the valve or the overall gain stage around it which is at fault.  At each stage a fuse could go so you need a few of them to continue.

I'm not sure what your experience is, you appear to have equipment around which gives you a chance of getting there, but if you are not used to working on high voltage circuitry live like this I would say plain don't!  Remember, we aren't talking about a little arm jerking jolt here, we are talking about lethal voltages!!!  At the very least it is easy to cause more damage than you currently have.  Don't be a hero and don't bite off more than you know you can chew.

As a curiosity, what is the story on this component?

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That looks a bit curious to me!  It's a 1k 2W screen dropping resistor for the EL84 next to it.  It should be Brown, Black, Black, Brown and looks as though it is.  From the other picture where the boards are fitted it looks to have been left high on the board to the point where the 39R power soak resistor above it has had to be bent up to clear it.  Do you know anything about it?  Has it been a replacement at some point?  Have a look around that area to see if there is anything obviously wrong, components coming close to touching the case for example.  If it could be a problem and if there is space it may pay you to gently bend that to the left to take it down nearer the board then ease the power soak resistor down and out from the board too. (If you think it may need that don't be too heavy handed or you'll end up having to replace them.)

If you can get it up stable with a working fuse in it and all valves back in place you could try measuring the voltages down the power line.  You do not need to have the PCBs out for this if you just trace check out the Test Points on the PCB.

I have no clue why that resistor is like that.

I have work with high voltage circuity before and are very cautious when I do so. I put my left hand in my back pocket when I do.

I will try removing all valves when I get replacement fuses. My last one I blew yesterday when trying the 12AX7's.

Success!!! It was the fuse being fast blow.  

I should had verified them off the bat.  The package states slow burn, but they are not.

Haha!  Good news.  Pleased it turned out to be such a simple fix.  Now enjoy your TM like it was meant to be enjoyed.  You will find quite a bit of useful insight on this site.  Don't rule out the Grandmeister forum as well as a lot of the info is applicable to both.

If you do fall into a money pit in a little while I would think of the GM36!  It's only £200 or so more in real terms if you consider what you could sell your TM for.  I went down that route and, while I still love my TM, the GM is a much more rounded amp to use with more balanced preamp circuitry and better voicing in my opinion.  And the midi side is awesome!  Everyone says "I'd never use it", until they get their hands on it and see what it can do.

Anyway, get back to rocking and keep us informed as to whether you have any other problems.



EDIT: I'm still curious about those damned power soak resistors being strangely marked. Should be 100R and they seem to be banded 10.1R. If anyone reads this and can offer any thoughts I for one would welcome it. See the pics on the first page for what I'm talking about.

Hello from Italy!

I'm having the same issue with my TM18... First time I sent the head back to the seller and have to say they gave a very good service, but the problem was just the Blown anode fuse which they've replaced. After less than two hours of use at high volume, the fuse broke again... The amp was plugged into a 4x12" 16Ohm Marshall Cab... any suggestion?

Thanks a lot
Fabio

Let's check the simple things first.  What is your voltage supply in Italy?  Is it 220V?

Are you using a slow blow fuse Fabio?  You absolutely must!  A fast or quick blow will not be able to handle the current peaks for long without giving up the ghost.  The way you describe it as going after a couple of hours of use sounds very like that could be the problem.

The slow blow is denoted "T" so it should be "T200mA" in the HT line and "T315mA" in the LT.  Also make sure they are high voltage types though that is not really critical in this case.  You usually get 250V rated even though the HT is higher.

Thanks Bordonbert,

Absolutely yes, I've blown two fuses, the first one was the OEM and the second was the one replaced by H&K service. We do have 220V and the fuses are slow blow (T) 250V type...

Should I use a fast blow type, I don't think the filament would survive the switching on from the stand-by position...

I do have a 112 H&K cab but often plug the amp to a 4x12 marshall (16Ohm) large cab... could the impedance be on the extreme side? I would measure the cab which is not mine,
heard that often the tag impedance differs a lot if you measure it...