TM20 Boost won't shut off

Hey friends!

I was at rehearsal tonight rocking away as usual and all of a sudden my boost wouldn't shut off with the HK foot switch. The LED on the foot switch for the boost wasn't lit up, either. I was still able to switch between clean and lead with the foot switch. Rebooting didn't fix it (hey, ya gotta try!).

I unplugged the foot switch so that I could use the button on the front to push the boost button but it didn't respond. I can switch between channels by pressing the clean or lead buttons on the face of the amp, but pushing the boost button doesn't turn it off, despite the button making a clicking sound like the other channel buttons.

Is this the right way to manually turn off the boost?

I tried a different foot switch with a different cable and it also would not switch the boost off. Anything I'm missing? The internet seems to redirect me to the user manual when I search for this sort of thing and nothing in it seems to give me an answer.

Thanks, all!

John

Hi Jlampson.  That doesn't sound hopeful, you have done all of the sensible things that you could have at the time.  If a different known good switch and cable doesn't sort out the problem then it has to be internal.  You could try completely ruling out the footswitch by simply plugging in a bare TRS (stereo) jack plug or a TRS 3-core cable and shorting out the ring to the sleeve, (that's the other two leaving out the tip).  If it doesn't switch then you know you can forget a footswitch fault.

Other than that there isn't a lot I can recommend that is easy.  Make sure the footswitch socket on the back is clean, I mean inside.  If the socket contacts are dirty it will lead to problems in switching.  Remember there are two types of contact on that jack.  Each function, Channels and Boost, doesn't just use the plug contacts, they also use the switching contacts which open when a plug is inserted.  Those switch contacts route the front panel switches to the circuitry when you are not using a footswitch so they need to be clean too.  When you insert a plug these open and the footswitch takes over through the plug contact wafer for the tip and ring.  So you need to have clean plug contact surfaces AND socket switch contacts too.  You will need to spray it with a proper contact cleaner and push a jack plug in and out a few times.  Do this with the amp OFF and don't switch it on until a few minutes have passed for the fluid to evaporate!

You would need a meter to check the next thing, what is the voltage on the ring of the footswitch socket with just a bare plug or cable connected to it?  If the TM20D is the same as the older TM18 in this area, (my guess is it should be), you should see about +22V between the ring and sleeve, remember that is when there is no switch attached, just a plug or cable.  If you have a meter handy that would be helpful to know but if you don't then I'm afraid there is nothing else you can check out.

So clean the socket well with cleaning spray and a plug.  Then measure the voltage on there if you are able to.  Then I'm afraid you need someone to look inside so a friendly tech should be brought in on it.

Thanks for all that! One notable thing that happened was the boost did shut off at one point, then I plugged in the foot switch and could control it, but then the boost would turn on and off still randomly, even while on the clean channel.

I'll run through what you shared but will likely have to find a service tech. Thank you!

With that intermittent fault description that does sound very like a socket could be just giving up proper contact.  I would suggest you do that cleaning procedure in the Footswitch jack.  You will need to get a contact cleaner spray to get inside the socket and make sure you give it a fair bit of fluid and plug your cable in and out a fair few times while there is still fluid sloshing around the contacts.  This is not an area of the amp which usually gives trouble in the H&K amps but back panel jacks are a known source of issues in amps in general.

Let us know how the cleaning goes on.  If you don't want to do it yourself then you might even find a tech who would do it for you without logging it in as a "full price" job if you ask him nicely.

Thank you! Will try later today and report back!

Alright, it took me a bit to work in getting some contact cleaner, but here's the scoop:

The last couple of days, things seemed to actually be working okay with the footswitch, weirdly enough. I had noticed that the end of the footswitch cable seemed slightly corroded, but I've worked with worse.

I sprayed the contact cleaner and inserted the footswitch cable a number of times to "work in" the cleaner". Brought it back in and hooked it up and it seemed to be working okay, until.... (video for evidence)

https://photos.app.goo.gl/TcsiX4K5PvryJfeN6

The TLDR is I think I need to schedule repair to make sure there isn't something else I'm missing on this. I have another head I can use in the interim.

Sorry for the delay, been on holiday for a week or so.  Blistering Aegean sunshine and back to grey British blaahhh.  What a come down.

That vid is a great piece of work Jlampson, it helped a lot to see the problem first hand.  It really looks like it may be down to dirty contacts somewhere along the line.  That unreliable switching action can easily be caused by a dirty contact presenting a highish resistance rather than a clean short as it should.  With the footswitch inserted those switch contacts are not in circuit so the problem goes away.  It shows that the problem could be in the switches of the footswitch sockets as I explained earlier.  When there is no footswitch in place these are required to connect the front panel switches to the control circuitry.  They are then putting that high resistance into the control line and changing the switching level to "just on the brink" rather than way below what is necessary to give reliable action.  When the switch is open and there is an infinite resistance the dirty switch makes no difference.  When the switch is closed and there should be a 0V level on the line the poor circuitry is seeing a higher than normal voltage which may be right in its "confusing zone" where it can't really tell whether you want it to be low or high.  Hence your confusing switching action on the channels.

If you have already taken the amp to a tech I think that is a good move as long as you don't mind paying someone to clean this up for you.  It's a simple enough task - once you know that is the genuine cause and you have a little experience in fixing this sort of thing.  When you don't you can be fly fishing in the dark I'm afraid.  He will recognise the problem immediately and either cure it or rule it out as the cause.  If the second is the case we assume, of course, he will find the true cause and fix that.

Make sure to let us know what he finds, it may just help someone else who has the same problem.

Oh my goodness, so sorry I forgot to respond on this. Yes, the jack for the footswitch had become corroded. I'm not quite sure how, though. The tech was able to track down some diagrams and replace the switch and it's been working fine and sounding great. Thanks for the suggestions along the way!

No problem Jlampson.  Glad it turned out to be the simple fix I thought it should be.

These contacts do corrode over time, particularly if it lives or works in a smoky or damp atmosphere.  Do you gig a lot of sleazy underground dives?    Fixing it is usually a simple process.  You just get a can of proper contact cleaner spray and flood the socket with it then repeatedly push a plug in and out for 10 seconds or so.  DO NOT USE NORMAL WD40 WATER DISPLACEMENT AND PENETRATING OIL!!!  They do a contact cleaner in their range but make sure not to just get their original, and don't use anything with silicone in it.

In a good design of socket the switch wafer opening/closing action has a slight flexing movement built in.  You can just about see it if you look carefully as you slowly insert a plug into a normal type of jack socket.  This creates a wiping action to the wafers as they come together or break apart and that acts to scrape any corrosion off every time you work the plug in or out.  If there is too much build up you can need a solvent to break the surface and lots of it to flush the problem away.  The cleaner is not corrosive in any way and there is usually only a microscopic amount of problem material so the inside of the amp is not harmed by any that drips through, it just evaporates.  If you have given the amp a cleaning like this it's a good idea to give it 10 minutes or so before you fire it up.  Flammable solvents don't mix with high voltage electronics!  It's great for amp sockets and switches, and the same on all of your guitars' sockets, switches and pots too.  A can is a good investment for any guitarist for your toolkit.

Remember, we are usually talking about a micro thin layer of corrosion or grease or a few tiny grains of very fine grit here, not a thick layer of gunk.  There usually isn't even anything you can see with the naked eye.  It can happen to any of your sockets which use those switch wafers so it's a good bit of maintenance to do on all of them every few years.

Down to using the amp now.  Give it some stick!