by bordonbert Thu Jun 08, 2023 12:47 am
Firstly, this is a TM20D topic and there is a specific forum for the TM20/40D. Why not put it in there? I have moved it to that area but please try to keep things where they are more easily found. We keep specific forums for each amplifier group as that makes it easier to find info specific to your own amp. Remember this info is also there to help others who may be searching as they have the same problem. We don't know your level of knowledge so this reply has to assume you know only a little. Apologies if that means I will "teach my granny to suck eggs" as it may help others who come across this and who know next to nothing. Please be patient and don't be miffed by that.
Ok, on to the problem. Murph has highlighted neatly something we all forget - maintenance. Things like valve socket and plug/socket terminals really should be cleaned periodically. They are a source of a lot of noise (and lack of noise) issues. Given time if they are left alone the connecting surfaces will corrode and the contact will become a tad intermittent. Good sockets are designed with a slight wiping action as the contacts are opened and closed which keeps them clean to a degree. However it never hurts to apply a little tender loving care to keep them at their best.
The only thing I would say is that alcohol is only an "ok" cleaning medium. It really is worth buying a specific contact cleaner solvent. These are usually spray cans and are formulated to do the job specifically without damaging surfaces or any other components in any way. I don't mean that alcohol would necessarily do any damage but it potentially could and it is a simpler solvent which may not remove all forms of contaminant. For example, it will remove many greases and oils but it will not dissolve oxide formed on any corroded metallic terminal surfaces. Anyway, use whatever you have to clean everything he says and, if you have it specifically, also spray contact cleaner into your jack sockets and work a plug in and out a few times to clean the mating contacts inside.
Remember jack sockets usually also have switches built in which operate when you put a plug in. The switch contacts are out of the line of the plug tube but will be cleaned by the spray too. Give a couple of seconds solid spray into the mouth of the jack socket, a proper cleaning spray will not damage anything internal, plug in and out a number of times and then leave the amp to dry off for a few minutes before running it up again. Fx loop sockets are very prone to causing problems when their switch terminals become dirty. It is very common to have the signal routed through those switches when there are no plugs inserted and a bad contact there commonly causes the signal path to be completely broken. You can always test whether you have a dirt/noise issue with the Fx loop sockets by simply putting a plain patch lead into both and bypassing the switches. If the noise goes away the switches are the cause.
Now, you say that you don't hear anything via the XLR socket to your recording or through the headphones. That doesn't sound good to me, it's unusual. It means that it is unlikely to be a plug/socket problem nor is it going to be a valve problem. If it were either of those it should be audible as you monitor. The Redbox output is taken directly from the output terminals of the output transformer so any noise created anywhere within the amp's signal circuitry would be in the Redbox signal. Can you just clarify where this noise is actually being created so we are absolutely clear? When you say it is a mechanical noise do you mean it is like something physically tapping together which would still occur even if you are on silent running? Or is it actually a sound coming out of an attached speaker? Could it be just the parts of the metal case expanding and eventually giving and moving slightly against each other?