TM18 Lost output after 2 hour

Hi, I have an H&K tubemeister 18 that recently quit working.

I was playing (about 45 minutes), put it in standby for a while (also about 45m), came back and played for about 5 minutes then it just cut out - absolutely no sound.

I connect my guitar directly to the amplifier. The amp's speaker output is connected to a 16 Ohm Celestion Greenback.

The effects loop was sending -> SA Nemesis delay -> Strymon Flint -> Effects Return

I tried 3 different cables and two guitars but I got no output - not even a slight hiss from the speaker. I put the TM18 in standby, removed the effects loop pedals, tried switching the power soak around, still no sound.

So I let the amp cool off, and opened it up. I replaced the rightmost 12ax7 and turned it on - amp works again.

But just to be 100% sure, I put the original 12ax7 back in and the amp still worked. wth?

Any ideas about what could be going on?

The tubes in the amp are original, so quite a bit old, and I've got replacements on order. I'm wondering if old, flaky tubes will behave like this or might I have a more serious problem?

Thanks

Hi BF, welcome to the forum, hope we can help.

It's not unknown for valves to behave like that.  The first step would be to replace them for testing as you have done and that seems to have fixed things - but it's not going to be as simple as that.  You say it was the rightmost valve you swapped out.  I will assume that is looking from the front and that would make it V1, the preamp valve.  That is the 1st Stage triode which is always used and the 2nd Stage triode which kicks in for the Lead channel only.  If that had been the whole source of the problem then it would leave T2 and the output valves in place and working properly.  So the amp would be working right from the 2nd (Clean)/3rd (Lead) gain stage onwards and this would produce some audible hiss at the speaker.  You say that the problem made the amp completely silent and that doesn't sound like a preamp valve problem to me.

You can test this when the amp is in that faulty state by putting a cable into the Loop Return and just touching the tip.  The amp should still hum and buzz when you do.  Putting the other end of the cable into the Loop Send of another amp should make it playable via the other amp's preamp.  I would try this if the problem reappears.  You could also try using the Loop Send into another amp's Loop Return to show whether the preamp is in fact faulty and silent.  On the other hand the valve change may just be the fix you need, though if it had been the problem it would most likely have left other diagnostic signs there which you could have found if you had known a little more about what to look for.  (That is in no way a criticism of you, it's just a fact of engineering life.)

If the amp has never had a valve change then it is probably a good thing to do this now and start diagnosing with a new set of valves in place but I will give my usual warnings!  Valves do not "wear out" as most people in the music business will tell you they do!  Without that myth no one can make a living selling you new ones whether you need them or not.  If they work within their design capabilities and are not abused in any way, (and overdriving them is not abuse, rough treatment, overvoltage and current misuse are), they should last decades.  There are always some which will die early, this is just statistical in any engineered product.  They are the rare ones which were faulty or at least weak after manufacture.  It happens just as it has always happened even in those golden Days of Valve Yore.  In reality the number that genuinely do give up early is minimal but the anecdotes fly around the internet when one does die and they are hyped up by numerous supporting reports of others failing which are not correct and off we go, another myth is born.

Do not fall for any "change them out every 18months, they are getting weak by then" advice, it's rubbish.  Change them when they actually give you a problem and you can diagnose it as a genuine valve issue.  Do you change your car's fuel injectors every 3 years "just in case"?  Do not fall for any "the Namikoshi designed Mullard reissue 12AX7 has unbelievably glowing transparent mids like the setting sun reflecting from Ayers Rock and waterfall silky creamy highs redolent of the mists surrounding the gorillas of Mt Kilimanjaro" reviews, they are utter tosh!  Do not fall for any "just made available, discovered in the back of a restored sealed government WW2 research establishment, NOS unused valves from the 40s, when they really knew how to make decent valves" crap.  Most NOS components of all types should be stored in the waste bin.  Modern components of all types are much more reliable, accurate, and without much sound implication, particularly in our guitar world of normal 5-10% distortion.   They are to be preferred despite the frenzy to get PIO caps made by Hobbit craftsmen from The Shire at a time when the Dwarves from under The Mountain were still digging out Oxygen free iron and sending it to the Wood Elves for plating and blessing.  Does anyone really believe that we have lost the knowledge that our forefathers in valve manufacture had?  Or that we cannot produce such good quality materials as they did?  Or that we cannot generate as hard a vacuum as they could?  Or that our modern production machines are less accurate than theirs?  How do these people think that a better machine with better materials does not produce a more consistent and reliable product to specification and to a sensible price?  At the very least it could even be argued that modern musical gear is designed and developed using modern components, so the sound and reliability the designer wants to build in is dependent on using those modern components during replacement.  People may like the bragging rights of boasting how much their valve set cost them to others who value that fact and judge them on it, but it means nothing in the real world.

A forgotten or unknown fact is that circuitry is actually designed so it removes or minimises the effects of any differences in replacement parts.  That is one of the reasons for employing feedback for example, and it is a good one.  This is in order to accommodate a wide production spread in components and to not tie the unit into any one specific type of that component.  That is good engineering!  And the main target of this approach is the active components, that is the amplifying parts like semiconductors and valves.

As an engineer I can genuinely tell you, it's all myth and legend.  There is absolute minimal difference if any between all makes of the same valve in decent well designed circuitry, (like your TM18), and reports of massive differences in tone are nothing but BS and self-delusion.  Blind listening tests show this to be true, people cannot tell any difference when they do not know which they are listening to!  Pay the silly money for exotic claims if you must but it is just as good to start with a set of plain old JJs or EH or anything in that middle layer and see what your sound is like.  You will end up buying a new guitar with the money you save by paying £15 rather than £40 per valve every 18 months for no other reason than to be in the "Mojo Muso Club".

bordonbert wrote:Hi BF, welcome to the forum, hope we can help.

It's not unknown for valves to behave like that.  The first step would be to replace them for testing as you have done and that seems to have fixed things - but it's not going to be as simple as that.  You say it was the rightmost valve you swapped out.  I will assume that is looking from the front and that would make it V1, the preamp valve.  That is the 1st Stage triode which is always used and the 2nd Stage triode which kicks in for the Lead channel only.  If that had been the whole source of the problem then it would leave T2 and the output valves in place and working properly.  So the amp would be working right from the 2nd (Clean)/3rd (Lead) gain stage onwards and this would produce some audible hiss at the speaker.  You say that the problem made the amp completely silent and that doesn't sound like a preamp valve problem to me.

I was actully thinking 'rightmost' from the rear of the amp. Sorry I should have been more clear. I attached an image with the tube I replaced highlighted.

If V1 is the rightmost (from the front tube) then I guess this is V2 I replaced. Which would be a real head-scratcher for the total-silence.

However, I understood that the TM18 used an op-amp for the first gain stage and that's why it has only two 12ax7s. That was in the Anderton's review and only briefly mentioned so maybe it's incorrect.

The total silence thing seemed weird to me too, since I'd expect a bit of noise from the power tubes.

Is there any significance to that fact that both red LEDs were solid-on in the back when the amp was having trouble?

bordonbert wrote:
You can test this when the amp is in that faulty state by putting a cable into the Loop Return and just touching the tip.  The amp should still hum and buzz when you do.  Putting the other end of the cable into the Loop Send of another amp should make it playable via the other amp's preamp.  I would try this if the problem reappears.  You could also try using the Loop Send into another amp's Loop Return to show whether the preamp is in fact faulty and silent.

I don't have another amp with a loop, so won't be able to put into practice any of these tips.

bordonbert wrote:
 On the other hand the valve change may just be the fix you need, though if it had been the problem it would most likely have left other diagnostic signs there which you could have found if you had known a little more about what to look for.  (That is in no way a criticism of you, it's just a fact of engineering life.)

<snip>

If the amp has never had a valve change then it is probably a good thing to do this now and start diagnosing with a new set of valves in place but I will give my usual warnings!  Valves do not "wear out" as most people in the music business will tell you they do!  Without that myth no one can make a living selling you new ones whether you need them or not.  If they work within their design capabilities and are not abused in any way, (and overdriving them is not abuse, rough treatment, overvoltage and current misuse are), they should last decades.  There are always some which will die early, this is just statistical in any engineered product.  They are the rare ones which were faulty or at least weak after manufacture.  It happens just as it has always happened even in those golden Days of Valve Yore.  In reality the number that genuinely do give up early is minimal but the anecdotes fly around the internet when one does die and they are hyped up by numerous supporting reports of others failing which are not correct and off we go, another myth is born.

<snip>

Do not fall for any "change them out every 18months, they are getting weak by then" advice, it's rubbish.

<snip>

People may like the bragging rights of boasting how much their valve set cost them to others who value that fact and judge them on it, but it means nothing in the real world.

I've ordered some replacements already. I think tubes wear out based on cathode emission and, as I understand it, should be a pretty long course - like thousands of hours. I would guess rough handling and transportation are probably the biggest killers of tubes in standard practice.

I don't go for all the NOS hype or guitardom mythos about magical tubes... I just ordered some reasonably priced matched Sovteks for the power and Tungsol for the preamp. Less than $50 to replace all tubes (and I get two spare EL84s)

Hopefully tonight I'll have time to try and reproduce the issue.

Thanks for your suggestions. If you have any other ideas to test let me know.

As a side note - it seems weird the TM18 manual doesn't have a tube diagram in it. Does anyone have a link to the service manual or something more descriptive/useful?

Thanks

+edit to attach image, sorry newbies can't create links for a preview in the post +

Ok, so it is the V2 you replaced, no problem.  It could be either the 3rd Gain stage at fault or the PI stage which is a single triode in your TM18.  (Or I suppose you could be very unlucky and have a problem with both? )  Even though they are late in the chain I would still have expected a little noise to be still there.  I wonder if something has been wrong with the PI triode stage and the TSC has realised and cut off the output to protect it.  That would lead to the 2 LEDs being on.  If that is the case then your valve swap would solve it.  Time should show that.

You are right, the H&K TM and GM series has a solid state buffer/overdrive/distortion stage at the front end.  This is an extremely well designed circuit block which ensures you get the best match to your guitar, prevents input overload and it runs from absolutely transparent through assymetric (that's even harmonic) overdrive right to full blown symmetrical clipping, all controlled by only your guitar and pedal level pots.  People get outraged when they find out that there is solid state circuitry in the H&K "true valve" amps.  The tone generating heart of the amps is genuinely true pure valve but they use solid state circuitry to enhance the valve stages and to wrap the control sections so they work just to do the job they are intended to and don't alter that basic character of the valve stages.  It's a hi-tech and very effective way of doing things.  The only thing which stops people like Marshall and Fender from doing it this way is the "Old Guard" who refuse to accept that an opamp used in an ultra clean stage does not make any change to the signal whatsoever and therefore cannot "suck tone" or any other ridiculous expression for "ruin the sound of the amp".  Why anyone listens to these "gurus" rather than looking for the facts and using their own common sense and intellect is beyond me.  As a TM user you should play around with using the volume control on your guitar with teh amp set to Clean with low Gain all the way through and seeing how that changes the balance of the basic sound even before you get to play with the following controls.  You can't get a good clean tone if you are hitting that input stage with a massive signal and using it as a Tubescreamer type of circuit and, if you get the level right, you can thicken up the sound in overdrive in the same way as any leading pedal.

Tubes don't really wear out with cathode emission if you mean the process of pushing out electrons in normal use.  They are made to do that all day and they do it easily under normal circumstances.  There are two main problems usually quoted as to why they wear out, cathode stripping (which may be what you mean) and cathode sputtering.  The first occurs when the anode/cathode electrostatic field is so high that it rips the oxide coating from the cathode surface.  The second occurs when +ve ionised gas particles with much higher mass than electrons are accelerated to the cathode and crash into it damaging its surface.  Neither of these is an issue with the type of valves, (radio receiving types), that we are using and that includes the output valves too.  The field strength is never anywhere near strong enough (4MV/m) to strip the cathode and our valves never work at kV levels near saturation for sputtering.  The killer is usually cathode poisoning due to poorly implemented standby switches, (that's most of them!)  Any Standby switch which removes the HT from the anodes while leaving the heaters on is killing your valves by cathode poisoning!!!  The good old Volume control is much better than a Fender/Marshall old style standby switch once you understand that the idling normal use does not wear out your valves.  For the record H&K amps do not have that problem as they have a much better design of Standby system.

If you are interested this is an excellent site for all things valve, hosted by a man who is a genuine expert in the field.  The following page on his site goes into this subject a little more.  The Valve Wizard!  If you click the Home link in the top left corner there is a whole site of design info on each aspect of our guitar valve amps.

As to the Service Manuals, H&K is very careful about letting these out into the wild.  We respect their wishes and won't post them anywhere in the site, after all it is their bread and butter.  If you dig around you can find a few of them however.

My experiment last night:

1. Play amp for 10-15 minutes
2. Put in standby 45 minutes
3. Return to amp - DEAD, No sound
4. Turn off amp - 30 minutes
5. Turn on amp - works again
6. Standby 10 minutes
7. Play - works

I repeated steps 6-7 four times but the amp never broke again. I'm suspicious of how the amp handles being in standby.

Yes, that is an odd response isn't it.  Thanks to TSC the H&K Standby is not the same as the traditional one.  Traditional Fender Bassman based amps have their standbys remove the HT from the valves thus putting them to sleep.  They leave the heaters on to keep the valves ready for when the HT is reapplied and the valves instantly come back to life.  The only problem with this is that when a valve is left sitting in an emissive state, (with its cathode hot and champing at the bit to throw out electrons), but with no current flowing through it, the cathode slowly grows a layer of high resistance "skin" between the base metal of the cathode and its oxide coating.  In time this increases the resistance of the cathode to the point where the valve becomes useless.  This is the "Cathode Poisoning" I am always warning against.  It is far far less destructive (and costly) to leave the amp on with the Volume turned right down than to put it into Standby if this is the way it works.  One way to prevent it is to leave a trickle current running through the valves which will not act as a sensible signal but will prevent the growth of the resistive layer.  Some amps nowadays have a high value resistor bypassing the standby switch to do this but there are still other drawbacks to overcome, not least of all safety ones!

Now for a techy bit, feel free to slide on by. With the use of H&K's TSC setup the Standby works a little differently.  The Standby switch acts on a MOSFET in the cathode circuit of each output valve to take control of its anode current.  Amplification in the valve is prevented but the HT is left in place with hopefully a small trickle current allowed, (perhaps).  If it is not coming out of Standby as you say then there is something worryingly wrong there.  I would hope it could be something simple like a bad standby switch or a poor terminal contact in the line or I would suspect the controlling circuitry in the cathodes of having a fault.

In the TM18 it appears to work like this.  When the Standby switch is closed, (Standby is off and amp is being played), the 17VAC supplied by the mains transformer to the LT +-22V and +-15V power supply section is also connected through a 1k5 resistor to the Standby switch and on through a diode direct to the gates of both MOSFETs where a paralleled capacitor and resistor smooths it out to a DC voltage.  The resistor serves to drain this cap down quickly when needed but while it is active it cannot prevent the voltage from growing to a preset level.  As this DC voltage climbs the MOSFETs are turned on and allow the valve cathodes to pass current.  The TSC reads the current through the valves via a small 1.5R resistor between each MOSFET Source and ground, (much as you read the bias current as mV across a 1R cathode resistor when biasing a traditional valve amp).  It can read the current in real time and uses that info to control a transistor which also drains the DC voltage on the MOSFET gates thus turning them off.  The balance of producing that DC voltage and draining it down based on the current flowing keeps the MOSFETs turned on to the correct level at all times.  When the Standby switch is opened, (Standby mode is engaged and the amp is silent), the 17VAC is disconnected and the MOSFET gate DC level drains away quickly thanks to the resistor turning the MOSFETs off despite what the TSC controlled transistor tries to do.

If the circuit kicks back into life when the amp is switched off for a while then it must be working in general.  Can you try to reproduce it and see what the TSC LEDs on the back indicate as you go from Standby to Play?  If you see a flicker from one of the LEDs it may indicate that one particular valve is trying to work as it should until the TSC quickly detects that something is wrong and shuts it down for safety to match the other.  If there is absolutely no hint of this we have to hope that both MOSFETs are working correctly and the problem is somewhere else in the system. So... (the word of today!)

1) Use the amp for a while until you are confident it has been long enough for it to maybe go wrong.
2) Switch it to Standby while you watch the LEDs.  They should both immediately turn on together.
3) Leave it for long enough for it to perhaps stall in Standby.
4) Switch back from Standby to Play while you watch the LEDs and look for any hint of a flicker in either of them.

That's about all I can think of to test to see where the problem may lie.  It may be that it is a very rare glitch and it may not occur for a long time. However if it becomes a real regular problem, I would guess it is not something you will be able to fix yourself.  It will mean opening up the TM18 in the bottom chamber, (where the BIG voltages live), and investigating there. And it may even indicate a TSC problem which may mean changing out the TSC board.

bordonbert wrote:Yes, that is an odd response isn't it.  Thanks to TSC the H&K Standby is not the same as the traditional one.  Traditional Fender Bassman based amps have their standbys remove the HT from the valves thus putting them to sleep.  They leave the heaters on to keep the valves ready for when the HT is reapplied and the valves instantly come back to life.  The only problem with this is that when a valve is left sitting in an emissive state, (with its cathode hot and champing at the bit to throw out electrons), but with no current flowing through it, the cathode slowly grows a layer of high resistance "skin" between the base metal of the cathode and its oxide coating.  In time this increases the resistance of the cathode to the point where the valve becomes useless.  This is the "Cathode Poisoning" I am always warning against.  It is far far less destructive (and costly) to leave the amp on with the Volume turned right down than to put it into Standby if this is the way it works.  One way to prevent it is to leave a trickle current running through the valves which will not act as a sensible signal but will prevent the growth of the resistive layer.  Some amps nowadays have a high value resistor bypassing the standby switch to do this but there are still other drawbacks to overcome, not least of all safety ones!

Now for a techy bit, feel free to slide on by.  With the use of H&K's TSC setup the Standby works a little differently.  The Standby switch acts on a MOSFET in the cathode circuit of each output valve to take control of its anode current.  Amplification in the valve is prevented but the HT is left in place with hopefully a small trickle current allowed, (perhaps).  If it is not coming out of Standby as you say then there is something worryingly wrong there.  I would hope it could be something simple like a bad standby switch or a poor terminal contact in the line or I would suspect the controlling circuitry in the cathodes of having a fault.

I'll try to dig up a proper schematic sometime to look at this. I've read a little about the TSC circuit.

bordonbert wrote:
If the circuit kicks back into life when the amp is switched off for a while then it must be working in general.  Can you try to reproduce it and see what the TSC LEDs on the back indicate as you go from Standby to Play?  If you see a flicker from one of the LEDs it may indicate that one particular valve is trying to work as it should until the TSC quickly detects that something is wrong and shuts it down for safety to match the other.  If there is absolutely no hint of this we have to hope that both MOSFETs are working correctly and the problem is somewhere else in the system.  So... (the word of today!)

So one of the things I noticed when doing a 'manual check' was the right LED, looking from the back, blinked about 4 times more than the left. According to the bottom of the amp, the difference between blinks should be less than 4. This is something did when I first plugged in the amp and haven't thought about it again until now.. maybe relevant.

Anyway, I did get some new power tubes in the mail today and replaced both. The new tubes are 'NOS Sovtek' - I know, I know tube-mythos. I got them 4-matched for $32 on the 'bay so thought it can't hurt

Popped the new power tubes in and I haven't had the problem again. I'll try to remember to watch the LEDs when going in/out of standby.

That's about all I can think of to test to see where the problem may lie.  It may be that it is a very rare glitch and it may not occur for a long time.  However if it becomes a real regular problem, I would guess it is not something you will be able to fix yourself.  It will mean opening up the TM18 in the bottom chamber, (where the BIG voltages live), and investigating there.  And it may even indicate a TSC problem which may mean changing out the TSC board.

I'm not scared of the 'big' voltages, I've got a decent electronics background and a healthy respect for safety. I diagnosed and repaired my old Egnater Rebel 20 which was shipped with the wrong IC used in effects loop.

thanks again for the help & cheers

You're more than welcome BF, glad you got it stabilised.  It's good to know that you are properly set up to work safely in there.  Sometimes the warnings are really for others who may come across the thread and think they can just have a go too.  That's kind of the same  reason I keep on pummelling the "mojo myths" idea, you never know who will read it in the future.  I know it gets a bit old for regular members but, hey, you've got to spread the genuine facts somehow.

One of the difficult things about any work on the TSC circuit is that the majority of it is in the programming of a microcontroller.  That does all of the hard work and the bits we actually can fix are basically a handful of very basic and reliable opamp and transistor drivers and the control MOSFETs themselves.  When you do get a genuine problem in the TSC it usually means a board swap out as it is impossible to repair the uProc setup.  Fortunately faults in the TSC seem to be relatively rare.

Your LED blink count was on the edge, though many of us here will have seen valves which show up out of spec then within spec a bit later.  Valves are of course weird and wonderful devices like that and they do sometimes just drift.  You have done the sensible thing though and changed them out for new, it can't hurt.  And please don't think I have a down on most of the valve marks out there.  The completely wrong idea of the "equivalent" series of 12A*7 which aren't is a different issue and is a definite "shouldn't really do it".  The differences between manufacturers models is something we should argue about.  The believers should be trying to prove their claims that they can "hear" a difference.  If they can genuinely hear it you can measure it!!!  Hence, we should try to base our arguments on factual engineered observation and measurement - - - and no one ever does.  Why hasn't this shown to be true by now if it is so obvious?

Put a valve stage on the bench.  Plug two different types into it.  Measure any damned parameter you want to.  Show me how it is different for the two valves.  Then I will joyously applaud your work, loudly proclaim my fault and be a believer myself.   I can set up double blind listening tests to show that people can't tell the difference between valves, (and caps too which I have done many times), when they don't know which they are listening too.  And that is, they can't identify any difference at all between them, not that they can repeatedly hear a difference but not attribute it to which valve.

It's only the unjustified "cachet" that some of them get which drives their prices up into the stratosphere which I feel is wrong.  If people are being influenced to pay much more for no good reason then I think that is plain wrong.  It's good marketing but it's morally wrong!  If you get a good deal on them at a fairly standard price then snap them up, they will do the job probably as well as most others.  But twice the price (or a lot more) just because they come out of a different loading port in the factory or have had some vague new magic process performed on them which has questionable effect?  It doesn't make sense.  "If God didn't exist we would have to invent him".  That's certainly what too many guitarists do in the sense of the ultimate gear.

I would recommend with your NOS Sovteks that you monitor the TSC bias very regularly.  Fleabay is no longer the place of bargains that it used to be.  There is a very slim chance that your valves are not new at all, or that they are unused for a very good reason.  That is the risk we take when we buy from a faceless nickname on a soulless site, (caveat emptor!), and I willingly take that risk too at times.  If you save say 40% over a period and one deal in 10 goes wrong you are still quids in.  Just check out the matching of your pairs very regularly just in case.

The Egnater Rebel fix was a good bit of work, hope it was a board replacement and not a surface mount IC swap!  I've had to do 'em but I'll never love 'em, through hole wins every time for me!  Anyway, enjoy your new amp and work those valves hard.  Let us know how it behaves after a while, just like Beverley Goldberg, we do worry you know.

bordonbert wrote:The believers should be trying to prove their claims that they can "hear" a difference.  If they can genuinely hear it you can measure it!!!  Hence, we should try to base our arguments on factual engineered observation and measurement - - - and no one ever does.  Why hasn't this shown to be true by now if it is so obvious?

EXACTLY!

We have plenty of tools to measure the difference, but nobody does it because it's like (a) manufacturers make more money and (b) the "Apple Club" - people begging to be up-charged for the rights to claim exclusivity.

I would recommend with your NOS Sovteks that you monitor the TSC bias very regularly.  Fleabay is no longer the place of bargains that it used to be.  There is a very slim chance that your valves are not new at all, or that they are unused for a very good reason.  That is the risk we take when we buy from a faceless nickname on a soulless site, (caveat emptor!), and I willingly take that risk too at times.  If you save say 40% over a period and one deal in 10 goes wrong you are still quids in.  Just check out the matching of your pairs very regularly just in case.

Very true, I'm feeling pretty good they match perfectly (according to TSC) out of the gate but I'll keep my eye on them.

The Egnater Rebel fix was a good bit of work, hope it was a board replacement and not a surface mount IC swap!  I've had to do 'em but I'll never love 'em, through hole wins every time for me!  Anyway, enjoy your new amp and work those valves hard.  Let us know how it behaves after a while, just like Beverley Goldberg, we do worry you know.

It's been a few years since I did it and the amp is now gone, but I think it was just a through-hole voltage regulator.

The buffer they have in that amp uses an dual-sided op-amp with V+ and V- rails, not just using a divider to offset ground and fake the negative rail. But they used the same part to generate V- as V+ (e.g. two LM7805s instead of one 7805 for V+ and a 7905 for V-. I assume it was a pick & place error, but being from China and through-hole they could be hand placing those boards too.

Anywa, pretty easy swap really, it was the diagnosis I was most proud of because when investigating an otherwise unmodified factory built device, the completely wrong component is *not* what I expected