Tubemeister 18 test point voltages chart?

Hi everyone! is there a chart (or maybe someone notes) for all the TP (test points) voltages into a tubemeister 18? I have an issue with the amp of a customer. he accidentally "run" his amp without a load into the output jack for a while (he was on his headphone, say to me that his amp was sounding fantastic into the headphones.... no load...), but when he realize, he plug his cab, but sound is terrible, big HUM... and on the standby mode, he seem to play well. Just like another post that i've read here. (tubemeister 18 playing in standby mode). I want to check all voltages on the TP, but i can't find any informations on them. Someone? Thanks Dominic

Hi Domlamp. We have a strict policy here that we will not share anything which could be classed as H&K's intellectual property. They do have to make a living after all and their schematics and supporting details are their bread and butter. If you dig around a bit with Google you can actually find what you need on other sites less stringent than ourselves. The H&K team have always been very helpful to us when we ask them for technical info and we don't want to lose that goodwill.

If you have anything more specific let me know and I'll do what I can to help.

Re your initial problem.  I'm assuming that you know your way around the inside of a box with lethal voltages in.  And I do mean lethal.  Please don't play around in there if you are inexperienced and "only looking for a friend"!  Your continued playing guitar on this side of the greatest divide is more important than fiddling with any amp.

Running the TM18 without a load should not be a problem as long as the internal power soak is set to silent.  Of course your client unfortunately didn't take care of that and the TM18 does not have the automatic sensing of its bigger brothers.

The obvious first step is to change out the output valves for a known good pair.  I would be surprised if that were the problem but it should be done.  If that makes no difference then you should find 390V on JP8, the output transformer primary centre tap.  This should be fed through to the anodes with only a few volts lost if the Tx is ok.  The screen grid voltages at R92/R93 should be about 380V.  If they check out then you are looking for something more complex.

It sounds like it could be a problem around the TSC autobias.  There is a high power MOSFET in each of the output valves' cathodes.  This is controlled by the TSC digital circuitry through an ordinary BJT transistor.  It isn't unknown for these to be damaged and require replacing.  I've just checked on ebay and they are available from around £4.  I would check out the voltage around R89/R91 which are the cathode resistors, and Q14/Q15 the MOSFETs.  These are driven from Q7.  At idle you should find the voltage on the two resistors should be about (1.5R x Bias Current), around say 30-50mV.  (I will be honest, I have never actually measured that to see what the TSC sets the bias current to.)

IF YOU ARE SAFE TO DO SO give that a go.

bordonbert wrote:Re your initial problem.  I'm assuming that you know your way around the inside of a box with lethal voltages in.  And I do mean lethal.  Please don't play around in there if you are inexperienced and "only looking for a friend"!  Your continued playing guitar on this side of the greatest divide is more important than fiddling with any amp.

Running the TM18 without a load should not be a problem as long as the internal power soak is set to silent.  Of course your client unfortunately didn't take care of that and the TM18 does not have the automatic sensing of its bigger brothers.

The obvious first step is to change out the output valves for a known good pair.  I would be surprised if that were the problem but it should be done.  If that makes no difference then you should find 390V on JP8, the output transformer primary centre tap.  This should be fed through to the anodes with only a few volts lost if the Tx is ok.  The screen grid voltages at R92/R93 should be about 380V.  If they check out then you are looking for something more complex.

It sounds like it could be a problem around the TSC autobias.  There is a high power MOSFET in each of the output valves' cathodes.  This is controlled by the TSC digital circuitry through an ordinary BJT transistor.  It isn't unknown for these to be damaged and require replacing.  I've just checked on ebay and they are available from around £4.  I would check out the voltage around R89/R91 which are the cathode resistors, and Q14/Q15 the MOSFETs.  These are driven from Q7.  At idle you should find the voltage on the two resistors should be about (1.5R x Bias Current), around say 30-50mV.  (I will be honest, I have never actually measured that to see what the TSC sets the bias current to.)

IF YOU ARE SAFE TO DO SO give that a go.

many thanks!! I've already ordered 2 MOSFETS to replace the one in there. I'll let you know how it goes. Dominic

Hope "teaching granny to suck eggs" didn't irritate too much Dominic. It is always difficult to pitch things right when you don't know the capabilities of the people you are talking to. You obviously know what you are doing and I needn't have worried. Keep us up to speed here with what you find, it always helps others.

hey! Finally, it was the 2 MOSFETS in the amp that were bad (actually, just one of them, but i've changed the 2!) and now, the amp is alive and playing like a charm!! My symptoms were (sorry, i'm french, so...)

-The amp coming in for a checkup. The person said to me that he was running the amp in 18W mode without a load and played in his headphones. he realize this when the amp stop working. How many hours did he played like that? i don't know...
-He said that now, the amp make a hum, loud pops and, strange, when played in standby mode (not supposed to have any sound), he heard a partially clean sound, like no problem... When he disengage the standby mode, HUM, noize....
-He tried to replace all tube with good one (bought a new set of JJ EL84 + 2 x 12AX7), but this does'nt change anything in the problem.

Solution:

-I've test some voltages into the amp, plate, screen, cathode seem to be all good. Ground seem to be OK too. I've tested (into the circuit) the MOSFETS like i've saw in one post make some years ago by a user and one appear to be dead on the resistance tests that i've make. I've replaced them and WOW!! The amp is now working like a charm!!

I want to say a big THANK YOU to Bordonbert who help me a lot into this, providing me such good tips and helpful solutions for me. have a nice day everyone! Dominic from Quebec, Canada.