Seeking UK engineers - fx loop oddity

Hi all,
New to the forum but have had a TM36 for a couple of years. It has a strange issue on the back panel that's a bit variable; If there's something in the effects loop with the loop switched on, or nothing plugged into the loop with it switched off, I get some crappy crackling noises, variable volume, and it doesn't sound at all nice. I thought I had bad cables or connections somewhere but nothing seemed to make any difference. Then when I tried putting just a short lead in the loop and switching it off, the sound came reliably back to normal. There's also something strange with the reverb but it's less common so I've forgotten what it does!

A very experienced and successful local engineer has given up on it, saying there's a problem internal to one of the chip boards ( I may be using the wrong language), and I need an H&K specialist.

Anyone in the UK know who I could send this to? JHS as distributors are not responding, I didn't get it from a shop and the nearest dealer is miles away so I may be posting it anyway. (I'm in the south west).

Cheers
Sig

Which of the loop sockets did you put your short cable into to stabilise it Siggy? Was it the Send or the Return? Have you tried shorting the loop out with a short cable taking the signal from the Send and putting it straight back into the Return?

As far as we know there is no special switching for the Fx Loop. It should just be utilised by putting a plug into the Return socket which opens the switch for the socket's tip and allows the signal to be passed forward from the cable tip rather than internally from the preamp section.

I would have recommended cleaning the switch leaves on the loop sockets as a first try. There's very little else which could cause this with your symptoms. You could also try removing the 16 pin SIL connector W1 on the Rear Board and JP1 on the Main Board and refitting them. It's possible that these have got dirty and just need refitting to clean them up. I don't know where the tech was getting the internal board stuff from!

Hi,
Thanks for your reply. There's definitely a switch on the loop - button on the back panel to switch the loop on and off, and you can also do it by midi or footswitch. I've had another go and now it just seems to be noisy when the loop is switched on, whether or not there's anything plugged in!

hey ho.

Siggy

Sorry Siggy, of course there is a switch on the back.  I was confusing it with the TM18 which doesn't have one.  You would think I would know that having a TM36 of my own here!

It comes down to the three most probable areas plus a fourth very tenuous one; the switch could be bad, the Return socket switch leaves could be bad, the interconnects between the boards could be dirty.  Then there is your mysterious reverb problem.

In the TM18 the reverb module is a small pcb sitting on the rear board alongside the sockets and switches and seems to plug onto a line of vertical pins mounted in the PCB.  I would have thought the TM36 would share the same module.  This could be another area where it gets a tad corroded and causes an intermittent problem.  The input to the reverb module is taken directly from the signal line immediately after the Return socket.  I wonder if that module could be causing the noise on the signal line?  It should be possible to rule this out by simply removing that board temporarily.

Both the reverb module mounting and the board interconnects only need to be split apart and refitted to clean their contact surfaces.  I have noted above which plugs are relevant to your problem but the designations were for the TM18.  It shouldn't be hard to see which ones are right for the TM36 by inspection.

The socket is a fairly easy fix, it's a common thing to have to clean jack sockets from time to time, their switching often seems to get unreliable with age.  As long as you can get in at them it's just a case of carefully slipping a strip of card between the leaves so you don't bend them permanently and sliding it backwards and forwards a few times against their natural springiness.  Never use anything abrasive like sandpaper or emery board to clean them!  A good contact cleaning solution is useful but is by no means necessary unless there is heavy corrosion or pollution despite what everyone nowadays says.  A drop of meths on the card can do the job in a case like this.  From the drawings I have seen it looks like the sockets are the standard open leaf type which you find everywhere.  If you have to do it without opening the amp up then it is necessary to simply flood the socket with a contact cleaner spray and insert a plug a few times to open and close the contacts.

Now, the switch seems to be our most likely culprit I'm afraid.  The symptoms you describe with the loop on and off with cables in and out seems to point to it.  If it is the switch your engineer friend is on the ball, the switch is mounted on the rear PCB and would need to be replaced if it cannot be cleaned with a contact cleaner spray.  I don't know the physical design of that switch but it may only need a spray of cleaner into its innards to solve this, but of course it may be a sealed type where that is not possible or at best very difficult.  And in any case it would require opening up the amp. Personally, I would have that PCB out for a look but I deal with this sort of stuff regularly and you may not be a techy type.  If it is an available type it would not be an impossible task to replace it.  Other than that it means replacing the whole rear board.

Unfortunately all of these areas really require you to break down the amp to get at them if they are to be addressed easily.  I don't know how old your amp is but there could be warranty issues to consider.  If you get your gear like me on the second hand market it isn't a problem. If you are not used to working inside amplifiers, particularly valve ones with LETHAL voltages knocking around, you would not be advised to do this yourself.  The official dealer in the UK seems to be John Hornby Skewes in Leeds just as you said but I thought other good repair shops should be able to get spares for it.  I know you are based in the South West and only H&K can tell you whether there is a repairer they support in your area.  My own thought was that H&K assisted most legitimate repairers when asked but I could be wrong on that.  It might be that your engineer gave up on it when he found that H&K made it difficult to get spares if you aren't an official agent.

I would now try getting onto H&K direct on their Facebook page to ask for advice on how to proceed.  That is always more public than an email which is useful and gets a much quicker response.  You can find them by searching for 'H&K Product/Support' on Facebook.  There is a guest posting area on the left to start a thread for your own query and I would just ask them for advice on where you can send the amp to for repair explaining that you have tried JHS and got no response.  They probably can't really advise much more than I can about fixing it online.

Do that then get back to us and keep us up to date.  We'll always do what we can to help even though we are entirely unofficial on this forum.

Anyone other Brits offer advice on this for Siggy?

Wow, thanks for your fantastically comprehensive analysis. I'm not techie enough to try this myself, so I will now follow your advice about finding some more advice from H&K / JHS.

I'll let you know how it goes.