Help! No sound out of clean channel.

Hi, I recently bought a used tubemeister18. I plugged it and turned it on heard a crackle and the clean channel was gone. The two other channels still work perfectly. All the tubes are glowing but when I press the tube button on the back of the amp the two leds flash a few times and then just one led flashes. I brought it to a repair guy and he said it has something to do with a microprocessor on the printed circuit. I called yorkville sound and they want to charge me $250 dollars to repair it. I'd rather do it myself because getting somebody to repair it the amp will end up costing as much as buying a brand new tubemeister 20. Does anybody know what is wrong with it and how I can repair it?

Hi David.  Sorry to miss you for so long.  For some reason I'm not getting emails when a new post comes in and I do miss the odd one.  Has anyone tried checking out the preamp as an easy starting point?  The flashing LEDs usually just tell you about the state of your output pair and their information does not tie in with preamp valves but give it a go and rule out that area.

First are you able to test the signal out of the Loop Send into any other amp to see if your channels are working that way.  That will tell us whether the problem is before or after that point.

If you aren't able to do that then try swapping the preamp valves, (that's 1 and 2 nearest the input socket).  If the problem changes in any way then one of those valves is faulty.  To get at the valves just remove the top two and the rear middle screws on both end caps.  Leave the front middle one alone, it only holds the handle onto the end cap.  Then loosen the bottom two screws by only a few turns to allow the end cap to flop outwards so the lid can be removed.  There is no need to wear silk gloves to handle valves, that is an absolute myth.  Just make sure your hands are clean before pulling them.  If there is no change then replace them in their original positions and seal the amp up again.  Take care to dress the LED PCB which runs along the top of the perspex front panel so it is fitted correctly and its clips are in place, and don't pinch the power wires feeding it when you put the top cover back on.

These two tests should give you a start as to knowing whereabouts in the circuit your problem lies.  I can see why your tech suggested the uProc board was faulty.  There is a digital circuit controlling the output stage bias current.  It sometimes gets damaged, (though it's not common).  However, it is in place just the same for both channels so that doesn't explain why one channel works and the other doesn't does it?

Are you able to do that?

I'm not sure if you will get notification of my reply to your original question so here is a little more info which may help with this problem both for yourself and others who may come across it in a search. We have a similar issue to this before and it was easy to diagnose, even remotely. First a bit of info.

The valves in the TM18 are labelled by H&K as "T1/2" etc. As a Brit who thinks of these things as they should be ( ), a 'V'alve and not a 'T'oob, I normally think in terms of "V1/2". In order to match the schematics and H&K thinking, we'll stick to their definitions using 'T' but bear in mind 'T' and 'V' are interchangeable depending on where you are in the world. They are used as follows:

T1:A = Input valve shared in both channels in all settings. It's cathode resistor is bypassed to increase stage gain when Boost is engaged.

T1:B = Second stage in Lead channel. Though it is in place all of the time, this stage is bypassed when the Clean channel is selected.

T2:A = Second stage for Clean channel, third stage for Lead channel. This stage is used for all channel/boost settings and its input is selected from the outputs of either T1:A or T1:B for Clean and Lead respectively.

T2:B = Phase inverter for power amp.

So you actually have: Clean channel = T1:A -> T2:A; Lead channel = T1:A -> T1:B -> T2:A

Assuming your problem concerns only the Clean channel and the Lead channel works as it should, giving a little thought to the valve setups above, you can see that the problem is unlikely to be a preamp valve. Both of the triodes used in the Clean channel are also used in the Lead channel so, if it were a valve, any problem in Clean should also show up in Lead. By far the most likely area for the problem to lie is in the switching between the two T1 triode stage outputs into T2:A.

I would like a little more info here if you can supply it.

1) Is the problem the same whether you are selecting your channel from the front panel switch or via a footswitch? If you do not have a footswitch then put a simple jack plug/jack lead into the footswitch socket on the back panel and short out the tip and sleeve. It does not need to be a TRS stereo type, a simple TS mono type will do. With a TS mono plug/lead the Boost will engage but just ignore that. Can you select Clean successfully from either of those setups?

2) Does the front panel channel LED turn on and off as it should when you select channels by either method? If it does then that shows the selection relay control circuitry is working as it should and that the relay coil is intact.

It is possible, (I think most likely), that the selection relay has bombed out on you. It could be that its control circuitry has a problem but that is less likely and the LED action should show you that. It could be that there is a broken joint somewhere in the feed between valve output and relay, (unlikely despite internet "common knowledge" that everything soldered has to be resoldered every year or two! ). It could be that a component has died in the feed between the first valve T1:A and the Clean second stage T2:A.

Complete those simple tests if you can and we can go from there.

Is it possible to test the selection relay?

The answer is yes, but it depends on what circumstances you are able to test it Teeb.  If you have the ability to open the amp up and apply voltmeter test probes to measure voltages on the PCB then, of course, that is a very doable task.  This means working close to LETHAL voltages, yes, genuinely deadly.  Not everyone has the experience necessary to do that safely which is why work of this sort is usually best done by experienced techs.  It is foolhardy at the least to overestimate your own skills in working with voltages of this level of threat.  Of course, if you genuinely have that experience then it is within your grasp to perform what is necessary.  I have offered the correct warnings now, it is up to yourselves to assess your skill level and proceed or not.  If in doubt don't!

From outside the amp my guess is it will be impossible.  The relays are miniature PCB mount types FRT5-12V.  On the older Tubemeister 18 they sit on the mainboard just behind the Red Lead LED.  They are easily available online cheaply.  That said, it would be silly to start pulling them and replacing them without first knowing that they are the genuine source of the problem.  There are other things which could cause them to malfunction.  However, if you have a multimeter which reads resistance then pin 1 and 10 should read 1028Ohms, (1.028k).  1 and 10 are the relay coil terminals and are the two on the opposite sides of the Index line on the top end of the relay body.  This is always approximate so anything around 1k would be fine.  This should be measured with the amp switched off and unplugged of course.

While this next point does not apply to our current problem, (I'll explain later), the most obvious cause of a selection malfunction is the Footswitch setup.  The front panel Channel switch directly controls the action of the relay by shorting one end of its coil line to ground.  This includes a ballast resistor and the LEDs.  The line from the switch is routed through the switch wafers of the Footswitch jack socket on the rear panel.  This is so the footswitch can take over control from the panel switch when it is inserted.  If channel selection is not working then start by simply fitting a footswitch and testing, or just insert a jack plug in the footswitch socket and short out the Tip and Sleeve.  See if that brings it into action.  If it does then the socket wafers most likely need a clean with proper contact cleaning solvent spray.  (NOT WD40!)  This is unlikely to be our problem as the Clean channel is selected when the relay is deactivated which is the default position with the switch open and the control line floating anyway.  I have only put this explanation here to perhaps help with other selection problems for people who come across it in the future.

If the coil resistance doesn't show up a problem, the way I would test this myself is to use an old auto mechanic's trick.  It isn't for the inexperienced.  Open the amp by removing the bottom panel, locate the relay, then put a LONG screwdriver or an insulating rod of some sort - NOT SOMETHING CONDUCTIVE - onto its case with the other end pressing against your ear.  If the Channel switch is activated you should be able to hear a slight "tick" as the relay kicks over.  Mechanics use this trick to trace sources of noise in engines.  You are very unlikely to hear this under normal circumstances as the relay is too small and quiet.  That means putting your head near the open amp in a way that you can't see down into it and precariously resting the tip of a screwdriver on one of the circuit components.  It isn't as easy as it sounds, don't go in there and do this if you do not have experience of working inside high voltage equipment!!!

If you should have access to an oscilloscope and can use it then testing is easy.  You simply look for a signal on pin 7 and see that that signal is transferred to pin 8 when the Clean channel is selected.  Likewise look for a signal on pin 2 and check that that is transferred to pin 3 for the Clean channel.

How does that sound?