KT88 Intermittent problem

[A.N. Beal]

Dear Folks,
Can anyone advise on a strange intermittent problem in my KT88 XL amplifier? Most of the time it works (and sounds) fine but sometimes (most commonly at switch on but occasionally after running for several hours) the left channel is very quiet (but not silent) and the right channel is louder than usual. There is no hum on either channel and there are no hisses, crackles or other 'problem-type' noises.
I find that if I turn the volume up and down quickly a few times, the problem is solved: the right channel drops to normal volume and the left channel comes up to normal volume and it then runs perfectly for the remainder of the listening session. However this 'cure' only works if there is a signal going through - turning the volume control up and down without a signal going through does not have any effect.
I assume that it is probably a slight dry joint or poor connection somewhere and expect that I shall have to open it up some day when I have time and poke about to try to find and fix this. However I would very much appreciate guidance on the most likely place(s) to look to find it. Based on the above description of symptoms, has anyone got helpful suggestions?
Regards,
Alasdair

[John Caswell]

Hi Alasdair,
Intermittent problems can be the very devil to find.
Valve substitution is the starting point. If not successful then--------
I suggest that you start by turning it upside down and setting it in a immovable position, removing the baseplate, connecting it to the power line, speakers, signal etc then powering it up, setting the volume to a sensible level.
Try to get hold of a plastic knitting needle and, with one hand in trouser pocket, try prodding the various joints starting with input sockets - volume control - ECC83 grids pins 2&7, anodes pins 1 &6 plus associated wiring and components on the tagboards, then follow through to the 5687 and finally the KTT88 then report back here.

John

[A.N. Beal]

Dear John,
Thanks for the advice. Needless to say, when I opened up the amplifier and connected it up it did not reproduce the fault. I poked around the relevant connections at the front of the amplifier with a plastic chopstick (quite a nice tool for the job) but no found no signs of problems. However I did find something not right when I poked the input relay board and I tracked this down to a dry joint to the right channel 'tape in' socket. I don't know whether this could have caused the problem. However I have resoldered it, put things back together again and see how it goes. Thanks for your advice.
Regards,
Alasdair

[John Caswell]

Hi Alasdair,
Plastic chopstick hmmmm good choice.
I have to admit it does sound like a dry joint, and as I said they can be the devil to find often taking days/weeks.
Do you have any test equipment of any sort?

John

[A.N. Beal]

Dear John,
Sorry to be slow to reply. I don't have any test equipment apart from a multi meter, a mains tester screwdriver and pair of old speakers to connect to the amplifier when poking about with the plastic chopstick. However having identified a dry joint on a tape connection socket I resoldered the joint - and since then the amplifier has run perfectly, with no crackles, no funny noises and no odd changes in volume. It appears that the tape socket joint must have been the problem - strange. However at least it is fixed. Thanks for your help.
Regards,
Alasdair

[John Caswell]

Hi Alasdair,
Well that is good news, I am not sure what the mechanics of the tape socket dry joint fixing the problem are, but possibly caused a relay to operate when not required.
Any how good job.

John

[A.N. Beal]

Dear John,
I spoke too soon! After a week of completely trouble-free listening, I had slight crackles on the right channel at switch-on yesterday - but no further trouble. Then this morning I had loud right channel and very faint (but not silent) left channel just like before - and (just like before) a quick 'turn the volume control up and down' restored everything to normality.
It seems that the dry joint on the tape input socket may not have been the only problem after all. Oh the joy of intermittent faults!
I am very busy over the next couple of weeks and so far the problem has only recurred once, so I shall probably wait a while to see if it gets worse before thinking of having another go with the plastic chopstick. When that happens I shall let you know what happens.
Best wishes,
Alasdair

[bob orbell]

Hi Alasdair, I have been watching you thread and think that your problem all points to the volume potentiometer, it can in no way be the tape socket, do you use the amp. with the volume control at max?, if you do, remove it, if not try a very small squirt of WD 40 at the spinal end, and let it run in, or replace, I have had a lot of success with WD 40 on noisy pot's, and it is safe to use on electrics, after all it can be used on car high tention leads, and they run at up to 50,000 volts. BOB

[John Caswell]

Hi all,
Sorry Bob, No don't use WD40 it leaves an usuitable residue, use Isopropyl Alcohol (IPA) NOT India Pale Ale!!!!!!! or preferably a specific contact cleaner.

John

[pre65]

I knew someone who swore WD40 was good for aching knee joints, but I would not recommend it for that either.