Hi Spin81. I'm really sorry I missed this for so long. For some reason I'm not getting emails from the system to let me know when anything is posted. Anyway, assuming you haven't fixed it in the meantime, on to your problem...
The background:
This smells like either valves or relays, and most things of this nature come down to valves. The H&K TM/GM series basically works by first adding or swapping valve stages into the overall path to give different gain options in the circuit for the different channels. You have 3x 12AX7s. Each of those has two triodes inside it. Usually, (I would say pretty much definitely here), H&K use them as 2x preamp valves and the third for the phase splitter in the power amp. The 2x preamp valves give you 4x triodes to play with. These are used as 3x gain stages and a DC Coupled Cathode Follower which offers no gain (x1 voltage) but allows for better driving of the following stages like the tonestack and adds a unique distortion mechanism into the mix.
The Clean channel is usually one triode from each of the first two 12AX7s with the second triode in the first valve (
) unused. The Crunch may be the same with extra gain added by altering component values or it may add in that unused third triode between them. The third will definitely be used for the Lead channel. The valve swaps are obviously dependent on the valves themselves working but also on the relays used to select which options are needed, (as are some of the other component changes). The relays/switching elements are all high quality miniature sealed types or JFETs and don't usually give problems. It is a no-brainer to go for the valves as your first area and see if that sorts things out. If it's a valve it is an easy fix. If it is a relay then you need to get it done professionally.
The test:
To test the valve theory, do you have access to a spare known good 12AX7? If you do then swap it for each of the first two 12AX7s in turn. If it is a valve problem you will get the amp up and running immediately. If you don't have a spare 12AX7, (not even from a different amp), then just swap your existing 12AX7s around, (the one under the metal can and the one next to it). You won't cure the problem but it should shift noticeably from one area to another. If the nature of the problem changes markedly then changes back when you reverse the swap, you know you have a valve out. Make sure you end up with the valves back in their original places, sometimes valves can be selected to be best suitable for a specific job.
The advice:
If you find a fault then just replace it and, for my money, for God's sake don't spend a month researching which valve manufacturer will make your amp sound like Eddie Van Halen's 5150s and swallowing any old bull that people hand you. It just doesn't work that way. There are only extremely subtle at best differences between valves despite what people, (either sellers of valves or their "white line followers") will tell you. That much has been clearly and irrefutably demonstrated in the most precise and inarguable way now, and that was by someone who was trying to show that there were differences! I can show anyone who is interested the proof, nothing to do with me but properly conducted tests with good results and a completely distorted and incorrect analysis of them afterwards. You can make your own mind up. A good old JJ is the valve of choice for most of us here. It's cheap, is easily sourced, and does the job perfectly well. (Isn't that what tone controls are for?) There is also a case for sticking with a Chinese made valve for the second one as the Cathode Follower has a very hard life and Chinese valves are shown to take that abuse much better.
There isn't much you can do to test the relay action and I don't have access to a schematic for the TM36 to advise you further I'm afraid. There is a distinct family similarity between the way the TM36 and GM36 do things but they are definitely very different amps to work on. I used to own a TM36 but moved up to the GM and sold it. My advice is, if you haven't already, work on the valve swaps first then get back to us with the results.