TM36 Low volume

I have a TM36 here which seems to have low output volumes.  I can turn the master and gain up all the way on lead channel no problem.  It would be good for bedroom volumes but from what I understand this amp should be loud.  I measured the AC voltage on the output leads of the output transformer when playing fully cranked.  Picture 1 is where I measured with those leads plugged in of course.  Now on the PCB it says 8 ohms, is that referring to the speaker impedance or the impedance/resistance of the transformers secondary?  Now if Im only seeing 3.5VAC being outputted from the transformer would we say the amp is only delivering 1.5 watts?  (3.5^2) / 8.  or am I oversimplifying this to hell.  So my guess is bad transformer perhaps.  The clean channel gets much louder than the crunch and lead channel.  The amp sounds good but just low volume.  I tried jumping the fx loop no luck there.  I will also include pictures of some solder joints.  Let me know if you would consider these suspect joints or not. Thank you.

Hi Element2211 (Tina?).  Sorry to take so long to get to you on this.  I hope you haven't done anything with this that could be "destructive" in the meantime.  And you are right, this amp should be LOUD!

You say you have jumped the Fx Loop. by that I think you mean looping it with a signal cable.  Good work.  I wouldn't expect it to have any real effect as there is more going on here.  As a step beyond that test try putting the Fx Send of another known good amp into the TM36 Fx Return.  If you can't do that simply put the cable into the Return and touch the tip with your finger.  You should get the usual buzz, just not as loud as with your guitar input.  If it works then you have a good power amp and a faulty preamp.  Now try the opposite, put the TM36 Fx Send into the good amp's Fx Return.  If that works then you have a good preamp and a duff power amp.  If they are both bad then we have something more fundamental like power supply.  "The clean channel gets much louder than the crunch and lead channel" sounds more like a preamp problem to me but that isn't certain.

Do you have a known good 12AX7/ECC83?  If so then try swapping your preamp valves with it one at a time making sure to replace the original valves in their original socket after each test.  Next try this odd little test.  Is this fault there with all settings of the power soak?  When you have the power soak set on 18W or less the inner pair of valves is taken out of circuit.  So simply swap the outer pair of EL84s with the inner pair.  KEEP THEM IN THEIR ORIGINAL PAIRS AND DO NOT GET THEM MIXED UP!  Try it again with the power soak on 18W or less and see if things change.

There you have a couple of easy things to try at least.  Don't worry about complex measurements in the live circuitry yet.  Simple tests give you a lot of useful information which can narrow things down immensely.  Perform those tests in turn and get back to us with the results.  Report them fully and clearly.  Too many people don't listen to advice and just assume that testing like this wouldn't be useful so they don't come back with the correct info to help further.

Oh, and one other thing.  What does the plectrum test for the state of the output valves tell you?

Ok I will not be able to get to it for a week or so. Those are some good tricks. I can def run another preamp into the HK poweramp. Just out of curiosity though would you consider any of these solder joints to be suspect. An amp tech touched up some joints on another amp I had and it came back sounding twice as good.

Element2211 wrote:Just out of curiosity though would you consider any of these solder joints to be suspect.

Honestly?  First pic - nothing wrong at all these are just spade connectors used everywhere and the resistor joints are perfect from this side.  Second pic - Absolutely perfect from this side. (I would guess you would be worried because these are the power soak resistors and therefore they must get hot?  No they don't.  Expansion leading to the legs buckling and solder crystallisation don't occur here.)  Third pic, not highly likely but possible if you read what I write later on.  The poor conditions are only around the valve bases.  Those valve bases look very cruddy to me and could be cleaned but that says nothing about the condition of the joints under the muck.  It simply looks as though someone in the past has tried to bulk reflow those joints and not cleaned off an inordinate amount of flux after they finished.  That flux can be corrosive to some items and attracts other crud to it over time.  I have never seen a H&K amp PCB in that condition before without "human intervention".  It's another good argument against the mojo driven hand wired approach.  A careful clean with the correct solvent would be good but probably not necessary.  Reflowing the joints wouldn't hurt if you feel you must to sleep at nights, but YOU MUST BE ABLE TO MAKE AT LEAST AS GOOD A JOB AS THE ORIGINAL ROBOTS DID!

Solder joints do not go bad anywhere near as often as internet scuttlebutt would have you believe.  The "resoldering all the joints" fix option is generally one which is grabbed onto as a last resort when no other ideas are apparent and it often leads to things being fixed simply for the moment because they have been disturbed.  The faults often come back in a short while, but no one ever posts that to have happened of course.  Hence the problem is still believed by other readers to be a bad joint which was fixed with a manual soldering.  I've worked in the electronic industry all my life designing, building and maintaining hitech circuitry in a number of critical industries.  I've never known a major problem with any solder joint either hand soldered or wave/flow soldered.  How many billions of pieces of electronic equipment are there out there which work day after day in a huge range of conditions without any problems of that sort?  Your amp is made to the same standards as pretty much all of those.  And so low a percentage fail because of this issue it's untrue.  The reason it can happen in guitar amps is usually because of one of three things.  These are the heat involved when using valves, the high voltages involved and the insistence that "hand wired" is better.

1) The valve bases suffer from conducted heat from the valve up above which can alter the solder structure in the joint.  Note I specifically said conducted heat.  By all means reflow these if you suspect a problem.  I have had that issue with a Marshall JVM205H I use where I have had to resolder one valve base on one occasion.  This amp seems to have a propensity for conducting heat down its pins.  I diagnosed the fault down to one particular valve base pin by using good testing technique so I knew that definitely was the issue.  The air temperature in the case is usually not the problem people think it is particularly in the H&K metal case designs.  Everyone who doesn't really look into the figures bleats about the overworked power soak resistors and their heat generation.  It's utter bol... tripe!  There is extensive info here on the forum on experiments we carried out to measure real temperatures and we found it to be a great piece of design in that way.  Modern components will work quite happily up to about 90degC at least, most well above that.  Yes, even electrolytics are happy with that and, if you look at their published specs, have lifespans of many thousands of hours of use at those temperatures!  The idea of replacing all of the caps in an amp as a maintenance measure is a standing joke to those who really know their components.  It's a way of generating more money in the maintenance industry and shows a bad set of diagnostic skills.

2) The high voltages do put weak joints and connections under more stress.  With good design and good PCB implementation this shouldn't be an issue but sometimes bad component choice or poor soldering methods, (back to hand wired again), can allow a problem to arise.  The one thing I do not like about modern methods is lead free solder.  This requires a higher soldering temperature and does not wet the joint as easily.  I understand the need for it but I always recommend that amateur manual maintenance/repair is carried out with leaded solder which mixes in with the original and gives it easier properties.  As long as it is only a joint or two there is no real issue.  I always use it myself for projects at home which are non-commercial.

3) No tech I have ever met has ever produced a bad joint in his hand wired work!  Yet they still do occur.  Go figure?  It's natural and statistical.  Just as PCBs will always have a very small occurrence of the problem so will hand wired gear.  The question is, are you better trusting a human tech who may be distracted, tired, rushed, having to jump from job to job, or even plain lazy or lackadaisical or unskilled, or are you better trusting a soldering machine which, once it is set up correctly, repeats and repeats the same process without any tiredness or loss of concentration followed by a number of quality control methods?  I know where my money lies almost every time.

Element2211 wrote:An amp tech touched up some joints on another amp I had and it came back sounding twice as good.

 You should expect it to of course as that is why you gave him some of your hard earned money and we must hope your tech really did find that problem in the joints he repaired.  However, the idea that it is therefore necessary to treat all amps as though they have a huge potential to solder joint failure is wrong.  Of course there are occasions when that could be a cause but it is not very likely at all.  Almost every other cause is more likely.  With good diagnostic technique a tech should be able to test and prove the fault right down to the very joint he says it is in!  Sadly the idea of reflowing a significant number of the joints if not all is often seen as something that they can charge time for and which "might just" solve the problem.  If it does then quids in.  But as I said, how often it comes back because that wasn't the genuine fault is another matter.

I seem to be taking a big swing at techs here. I'm really not!  Many of them are great at their job and very knowledgeable about their electronics from the standpoint of working with repairing and maintaining a vast array of gear on a day to day basis.  Many have a fantastic knowledge base of models and common faults which I don't have as I don't do their work regularly.  However I do have a different expertise and perspective and I know that, in the music industry, far too many are just guys who fell into the business without any real training and just picked it up from forums like this or other "gurus" who "took them under their wing".  It's easy to bullshit to someone who wants to believe that you are an expert and is impressed by ideas of "mojo" and doing things "by feel". There is no substitute for good solid engineering knowledge and skill in our field and our work is a factual based thing which can always be proven true or false, not one of opinion which might or might not be correct and is based on having magic among your senses.

A good tech is worth his weight in gold but I wouldn't want to hand over any of my gear to the guys in the second group.  I don't believe in electronic elves who sneak into the workshop during the night and fix the amps for poor old overworked underpaid gurus. Don't believe anything you are told in this field, (my own advice included), without first looking into it for different opinions or, more correctly as opinion shouldn't come into technical arguments, conflicting facts.  The facts are always out there and anyone who is upset by you saying "I know you say that but what about..." is most likely a self styled snake oil guru and one of the BS merchants.  Listen, think, question!  Facts, facts, facts.  It's not really rocket science, but it is definitely not mojo magic!

thank you for your detailed reply. Yea thats good to know, the valve bases look like they were resoldered given all the flux on the board. So generally speaking you wont see flux stains like this coming from the factory? One of the reasons I posted a pic of those joints is I that am of the understanding that your joints should end up looking shiny even without wiping the flux off, perhaps they lose their lustre after use, or the flux boils up a bit, or maybe they used cheap non leaded solder.

A note on my amp tech: he did more than touch up a few joints, he replaced one or more resistors to make the tubes run cooler, but there was no cap replacement just a lowering of the bias range, but I can attest the amp came back sounding like a whole new thing. I have another amp of the same exact model that sounds kinda dead to me so I was wondering if it were bad solder joints because it is biased correctly, it is operational, it just not have the balls for serious heavy metal crankage like the one my amp tech serviced. I resoldered the tube sockets but no change, still sounds kind of dead or muffled, to my standards anyway.

Hopefully this weekend I can use the effects loop to help identify the problem on the HK.

Sorry for the delay. I tried swapping the inner power tubes with the outer ones with no difference. I tried many combinations of 12ax7 tubes in the preamp, I have a lot of those laying around. I ran the send from the HK to the return of a Marshall artist 3203. With the master of the lead channel on the HK cranked it was still tolerable to the ear when running through the marshall. I then took the send from a Crate Blue Voodoo and an Ibanez Toneblaster and ran that into the return of the Marshall. This produced much higher volumes, ear drum blowing types of volume. I then ran the Crate send into the return of the HK and this produced much higher volumes than would be normal with my tubemeister. So it appears you were on the right track it seems like a weak preamp. Any suggestions on where to go from here?

Hi again. Ok, so you have proven by simple logical testing that you have a preamp problem and that the power amp is working fine. Good work. And by replacing the preamp valves with known good ones you have ruled those out as a problem. If it isn't the valves then you are up against it in terms of fixing it yourself I'm afraid. It's fairly likely that it will turn out to be something which will need resoldering or replacing which means soldering too. Working on PCBs like the H&K ones needs a set of skills and tools which the average person doesn't normally have. Do you have access to a decent soldering iron, (definitely not some random poker with a tip like a carriage bolt), and any skill in soldering technique?

You say that the problem is a lack of volume. A couple of questions leading on from that.

1) Is there any overall general change in tone along with the loss of volume? I mean, we know that the level is low but is the quality of the sound different in any other way such as losing all of the bass or by being very muffled with no top end? When you turn down an amp at the Master Volume you expect a change in tone of course but we all know roughly how that sounds. Is there a change other than that at the moment?

2) Is the problem the same on all channels? If you have a condition where for example the Clean channel was loud but the Crunch and Lead channels lost volume then it could be an indicator of where to look.

3) Do all of the preamp controls act in the correct way? Check out each channel in turn by using its Gain and Master control to see if they work as they should. Also check out the two sets of Tone controls to confirm that they both work as normal.

The amp definitely looks as though it has been worked on in the past. The amount of flux around all of the valve bases shows that. All decent commercial PCBs, (H&K are good high quality), are cleansed in a solvent bath after population (stuffing) and soldering to remove all flux residue. While odd spots sometimes creep through, the amount yours has would be rejected immediately. If you have the correct setup there to do it well you could try resoldering the preamp valve bases. (Remove the valves first and don't keep on putting in more solder as it can run through those joints and build up on the back where you can't see it. They just need melting for a second then leaving to solidify. DO NOT BLOW ON THEM TO COOL THEM!) I would leave the power amp valves alone, they are fine even though they look untidy. Don't cause a problem where there is none at the moment.

It does look as though there has been a fault in the past and an attempt has been made to fix it by soldering all of the valve bases just as I said in the long post above. It hasn't worked, just as I said in the post above! I'm afraid this is looking as though you may well have a more complex issue which you would need to chase through the circuitry with a scope or at least a workshop amp. Anyway can you answer these questions and we can see if they suggest any other possibility?

So ive done some more tests.  There seems to be issues in both the preamp and power amp sections now.  The tone controls on the clean channel seem to work fine.  On the dirty channels though, the treble and bass controls do not seem to do anything.  The mid does change the tone, when going from 0 to full you can definitely hear a change as well as a slight increase in volume and presence.  All the gain and volume knobs seems to work fine.  

So before when I swapped the outer power tubes with the inner tubes I did not test the tsc afterwards.  I tested it a couple days ago and got some strange results.  13 26 13 12 blinks were observed.  I then swapped the outers with the inners and got the same result. So apparently it is not a tube issue.  I then looked at the pcb board and how it was screwed down to the mounting posts. The pcb board looked warped so I took a few pictures and will post here.  I then backed off the screws and did a tsc test again with the same results.  It does not seem right to have a pcb with a bow in it.  I figure this could stretch the traces or possible crack a solder joint.  I kept the screws backed off over night and ran some more tests today.  Now im getting 21 13 12 13 blinks on the tsc with the screws backed off and tightened down.  I then swapped the tubes again and got the same result.  So a few days ago one of the inner tubes was blinking 26 times, now the outer tube next to it is blinking 21 times, both results observed with inner and outer tubes swapped back and fourth.  So the power amp seems to be acting fishy, we have the volume issue, and the tone controls on the dirty channels are not working properly.

Wow.  That PCB is not right.  There are a number of things which ring alarm bells for me here.

First, the PCB should be dead straight just as you say.  I can't think why it would be distorted like that unless it has been damaged by very rough handling or has been fitted wrongly so the valve bases are not in place and are fighting against the PCB screw mountings..

Second, the valve bases are a mess with regards to their soldering.  That amount of flux all round all of them is definitely not a manufacturing issue.  They do not come like that from the factory, these have been worked on by someone not very experienced in soldering.

Thirdly, the outer electrolytic caps in the picture, (the big black cans) are upright during manufacture.  Yours have been bent to one side and not restored to upright.  That in itself isn't a huge issue but it does show that some work has been carried out.  I suspect those caps have been replaced as they should sit flat and firm down on the board.  If that is correct then you have to ask yourself why?

Finally, look at the terminals of the preset pot in the top right.  You can see there is a solder blob on the left hand leg.  Why is that there?  It looks as though there may have been some mod or temporary fix applied at some time.  It may also be nothing and just be where the iron caught the terminal while soldering the valve base nearby but it is another suspicious factor.

I'm afraid it is really looking like you need a decent tech to look into this and fix it.  The symptoms are beginning to add up to problems which you can't sort out easily with resets or simple fixes.  Do you know anything about the history of the amp before you bought it?  Did you get it from someone who you know has had it from new or was it a "steal" on ebay?

lol well I hope to be a good tech myself. I have an electronics degree and soldering experience, which does not mean shit but I have some knowledge anyway. This is why Im tackling this job. Its not necessarily that I dont see problems here its just that I want to pick your brain to learn from you. What I may do now is take the pcb out, resolder the tube sockets, take my esr meter to those caps, check that trimer pot, and maybe check the gain of all the op amps. What I want to know is how the pre amp connects to the power amp, because remember the power section works well with the effects loop return, volume wise anyway. What I need to know is why the pre amp is so weak, bad op amp maybe? Now if I = V/R, a drop in resistance to the power tubes may be causing the tsc to up the voltage to the tubes in question. Thus we get the 26 and 21 blinks on the tsc test. The question is what are the chances that bad solder joints or bad components would cause a drop in resistance, would not a bad joint generallly cause an increase in resistance?

I think you are off the mark with your analysis of the problems.  Forget the caps and opamps!  They would be a secondary problem.  You have a working amp so the caps are at least good enough to get it there.  Worry about that later.  Practically, opamps do not suffer from a fading of performance, they either work or they don't.  Again, they are working at least well enough to run the amp.  Worry about that later.  And there are NO magic opamps in our line of work.  Despite what the hacks butchering pedals may say, the truth about opamps is that their effectiveness does vary massively, just NOT in audio work like this as long as the original design work is carried out correctly.  Opamps give poor results only when they are driven outside their range of characteristics.  That means that as long as you make sure to take care of a number of factors such as, their drive and load impedances stay within the correct limits, they are operated within their frequency range, and you never allow them to clip internally (there are excellent simple ways of preventing this which are adopted in a few good quality amps but ignored in the vast majority) you cannot tell the difference between even the humble TL07x series and more exotic ultra expensive types.  For the circuit designer the array of types is there to select the most suitable when the parameters like low impedance drive and high frequency response or low offset requirements need to be met.  The H&K designers have done their work well.  They have designed good opamp stages and the opamps they use are working well within their capabilities.  You will gain nothing spending time swapping them out then trying to convince yourself you can hear a difference.  It can't be measured so it can't be heard unless there is a blue moon and a unicorn sprinkled pixie dust into your ears, and I can point you at a high level paper, (a long paper which critically compares many opamp types right across the range), where this fact has been put to the test and found to be true.

For the others out there who disagree with this I would suggest you come in on the discussion and ask for the evidence before you make the usual "utter bollocks, everybody KNOWS that they make a difference" argument.  You can then make a fool of me by showing me where that evidence is wrong or perhaps point me to other evidence which shows, categorically and factually, that there is a difference in a well designed circuit.  Most pedals and many amps are NOT well designed circuits!!!

So to the TSC.  It works by measuring the voltage across a small (1.5R) resistor in the cathode of each power valve.  That voltage is a direct measurement of the cathode current in each valve.  Also in the cathode circuits above each resistor is a MOSFET.  The TSC processor is monitoring the voltages across the resistors individually and controlling the gate voltage of the two pairs of MOSFETs on each side of the push pull output circuit to set them to a preset level of bias current programmed into the TSC system.  That's why there is no bias adjustment in the H&K amps.  It maintains a good overall balance between the two sides of the output stage.  This has to take place intelligently to ignore the part of the cycle where there is massive output current, hence the processor controlling it.

Some solder faults which increase the resistance in this area would have some effect.  If the 1.5R resistance increases due to its joints then the TSC will be thinking that the current is higher than it really is and it will overcompensate to bring it down.  It would settle at a lower current than it thinks is present.  It is possible that bad joints in that section could be at fault and, as you say your soldering skills are good, it wouldn't hurt to correct them.  I would say that it wouldn't be a good thing to just go over the whole board as most people advise.  "If it 'aint broke don't fix it" is a good maxim to follow.  It's much better to actually diagnose what the fault REALLY is than to just assume you might fix it by doing a few blanket things and then hoping you got it when the fault goes away.  I've seen faults come back a short time later too many times with that approach.  Remember the days when we "fixed" our televisions by knowing the right place on the side to give them a whack with the flat of our hands?  That ALWAYS "fixed" the problem didn't it?  (For a short while anyway.  )

Look at your TSC readings.  You said you got (13, 26, 13, 12) when you first checked.  Then you swapped the outer and inner pairs and got the same result.  Then you left it overnight and retested and got (21, 13, 12, 13).  That's good!!!  Compare the two sets of readings.  Have you not spotted that you swapped the right hand pair and the left hand pair over and the results went with each valve exactly - eventually.  You seem to have one unstable valve which reads 26 and 21 but all of the others have behaved exactly as they should.  The delay in testing may be important.  It may take time for the TSC to recalibrate itself to the new valves but it eventually seems to settle down to what it should be reading.

Try swapping the left hand pair with the right hand pair keeping the inners to the inner and vice versa.  I'll describe that a different way to be sure.  Swap the inner valve pair with each other and then the outer valve pair with each other.  You still have the same pairs working together but they have been changed over in a legitimate way to something you have not tried.  Now test the TSC reading a couple of more times with switching the amp off and on each time.  If it makes no difference then wait overnight and retest.  As I said, it looks like the TSC takes time to recalibrate itself which is not surprising.  Assuming you still have the valves in position from your (21, 13, 12, 13) test, after swapping 1 with 4 and 2 with 3, I would expect you to get (13, 12, 13, 21-26).  If you get pretty close to that then you have no problem with your TSC.

My advice is to fix that PCB mounting problem first, it is obviously badly wrong so there is a serious fault in the mounting.  That should be an easy fix.  You should find the boards connect together via small jumper cables between white sockets.  You can't get them wrong as they have different numbers of terminals and they run pretty much close to each other.  They should just gently disconnect without too much serious pulling and pushing.  And no, these are not cheap connectors despite what they look like, they are good quality industry standard!  Don't worry about them.  The H&K amps have drain resistors across the power supply caps so, measure the voltage across them before you go in and you can be sure the voltage will stay down and be safe to work on.  It should take only perhaps 30 seconds to come down to safe levels once the amp is switched off (and unplugged of course).

It takes a lot of work to get yourself into the mindset of only fixing what you know is broken and trusting what isn't.  As a budding tech you need to come to terms with that, one way or the other.  Are you going to fix amps according to what is broken and really needs fixed or just go by what every "guru" tells you you should be doing without any supporting proof that it has any validity?  If you like the sound of the amp when it is fully working, there are a lot of reviews to show you how that should sound, then why assume that esoteric "common knowledge" weaknesses just MIGHT be there and that you can improve on the H&K design team by swapping out their faults?  Remember that "common knowledge" is just that.  It's knowledge common to everyone out there who knows absolutely sweet FA about amp design, components, and what makes an amp sound good or not.



EDIT: And, for what my personal opinion is worth, an electronics degree DOES mean shit! You've started your career the right way, with a good sound foundation of engineering knowledge and principles and, presumably, training in how engineering is at the core of amp design and function. Don't swap that logical critical approach for a reliance on what untrained people on the internet who don't really understand in depth tell you is truth. ALWAYS check out any claim for yourself and be critical of it. (My own included!)

give me a bit, Im studying tube technology while I work on an old sears 5xl amp.