by bordonbert Thu Aug 26, 2021 8:51 am
You're welcome. Apologies if I seemed to "teach my granny to suck eggs", I had no idea whether you were an experienced user, (which you are), or a total beginner. Even with your experience it is sometimes useful to go into more detail than you need as others may come across this who aren't as knowledgeable as yourself and it would help them too.
So, speaker cable is ruled out, that's good. Stick to the new one for testing where you can. Next, it is just possible that the output socket may be the problem. As an experienced user you will know how to clean that. It would help to rule that out as a possible issue.
The Redbox working definitely points to the fault being after the output transformer so inside the PS/Speaker Out section. This is viewable through the bottom of the amp. You have ruled out the possibility of an open circuit component in the power soak. Any single component is specific to only a single setting or two, there is none which affects all three active settings. The fault is there with all settings including 18W, and 18W is straight through with no components in the signal path at all therefore no faulty component is breaking the path on that setting.
Knowing that you know your onions, the next step is simple. Have a look inside the base, through the ventilation slots, and you should see the PS components. In the TM18 there are 8 of them, 6 large rough coated resistors and 2 inductors which look like slightly smaller shiny resistors. The main resistors are large and grey while the inductors are a little smaller and more light blue. They are all mounted in cut-outs on the edge of the output PCB. Do any of them look scorched or damaged at all? I have attached a quick pic of the PCB, and one of the layout with the relevant components marked on it. While the components are not the problem, if they look badly scorched it may show that the PCB tracks have been stressed too. Most heated components will discolour with time, you can see that with the inductors in the picture, a little darkening is not a problem but any signs of burning or flaking could be an issue.
If they do look a little scorched then how do the PCB tracks you can see look?
This really looks like it comes down to: Faulty PCB, (burned or broken master track between transformer terminals and speaker socket.); Faulty PS selector switch; Faulty Speaker Socket; Something external to the amp in the speaker line is faulty.
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- H&K_TM18_Rear_Board.jpg
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