KT88 Intermittent problem

[colin.hepburn]

Originally Posted by 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

Oops I have been drinking the wrong stuff john Ha just kidding I use Servisol super 10 seems to work fine for this pot cleaning think

[John Caswell]

Hi all,
Colin wrote "Oops I have been drinking the wrong stuff john Ha just kidding I use Servisol super 10 seems to work fine for this pot cleaning think"
Yep thats the right type of stuff. Farnell, RS, and CPC all do something similar.

John

[bob orbell]

OK, I will give a tin of this a go, Thanks.

[Richard]

Yes another vote for Servisol 10. This has a small amount of lubricant in it which is left behind when the solvent evaporates.

There are others which contain no lube and may be better for other jobs.

A point to watch when doing pots is to aim a short blast onto the tracks from any holes in the back of the pot. If you use too much or flood it in from the spindle end the thick spindle lube will be washed out into the pot and when it dries the pot will feel poor and lose it's nice smooth action.

[A.N. Beal]

Dear Friends,
Thanks for the further suggestions. I can't give more feedback because I have been away a bit and also the problem is not happening enough at present to warrant the time to try to fix it. However I am not convince that the problem is a noisy potentiometer - it always adjusts volume smoothly with no crackles or other problems, so I am reluctant to squirt anything into it (alcoholic or otherwise) at present unless the evidence starts to point more firmly in that direction.
To recap, the first sign of the problem is if slight crackling occurs on the right channel during warm up; this disappears after a short time and both channels are then as quiet as normal. However when the amplifier is fed a signal, the right channel seems to be louder than usual and the left channel is quieter than usual but still clearly audible and undistorted. Changing sources or turning the volume control up and down a bit has no effect but a quick turn up to full volume and back down again clears the problem: the left channel comes up to normal volume and right channel reverts to normal. My guess at present is that the cause is a marginal connection or short-circuit which usually works OK and is cleared by a blast of full volume when it gives trouble.
As it isn't bad at present it may be a few weeks before I have another go at solving it but I shall let you know if and when I have more news to report.
Regards,

[bikerhifinut]

Short of reflowing all the solder joints on the affected channel, If its a dodgy joint the only other way is to poke every joint with your chopstick....................
The "Cure" you suggest does point to the volume pot, don't think that just because its an expensive Alps pot and that its not crackly when rotated that it cant have an intermittent contact.
I say this because in the scrapbox I have a motorised Alps pot that was doing my head in as i was losing the signal at a certain point at the travel of the wiper. But not every time.
I'd be tempted to swop it out for a cheapie 50-100k pot and see if the fault reoccurs. We aren't worrying about ultimate sound quality at this stage of the diagnosis.
Another thing and I am not sure if it's been covered, have you discounted the possibility of a dodgy ECC83, or (less likely) 5687? I have also had issues with ECC83's especially old mullards going "Crackly" but not every time they were switched on. They usually display the fault by making a "rustling" sound when powered up. Whanging the level up might be causing a microscopic movement in the electrode structure that temporarily clears some sort of intermittent problem? Checking if its a 5687 is easier, just swap em over and see if the fault follows the channel.
These faults can be so frustrating, it took me ages to diagnose a dry joint on my WD88 on a coupling capacitor, it was only when I metered the voltage on the capacitor lead out wire that I discovered it as I'd been probing on the tags.
what sort of coupling capacitors are you using by the way? I have to say that I swear by the original Grey rectangular polyesters or the polypropylene soniqs jobs as being totally reliable.

Andy.

[Phil Y]

As I don't think it has been specifically mentioned, I would suggest you check the wiring (soldering) on the volume pot itself. Volume changes sounds like it could be a bad earth or other connection. Check both ends, at the pot and where it connects to the rest of the circuit.

Phil.

[bikerhifinut]

Originally Posted by Phil Y

As I don't think it has been specifically mentioned, I would suggest you check the wiring (soldering) on the volume pot itself. Volume changes sounds like it could be a bad earth or other connection. Check both ends, at the pot and where it connects to the rest of the circuit.

Phil.

Well spotted Phil, sometimes we forget to look right through the signal path.

A.

[A.N. Beal]

Dear Folks,

After all this time I think I have finally solved the problem. I decided that if the problem was somewhere in or around the volume control, then rather than trying to precisely track down an intermittent, which might take for ever, I would 'take the bull by the horns' and replace the volume control and all its connecting leads. At least this would either fix the problem or else eliminate a lot of possible suspects from the enquiry.

I have did this very carefully yesterday. In the course of it i checked adjacent connections and found a dry joint on the front end of signal lead for the left hand feedback connection. I put it all back together and it has worked perfectly ever since, so I am cautiously optimistic that the problem is solved. However ... I have a sneaking suspicion that maybe the problem was actually in the feedback lead connection, not the volume control. This would tidily explain sudden jumps in volume. Maybe I have therefore replaced more than I had to - curses!. However if what was an irritating problem is finally fixed then I'm not going to complain too much. Because of the help you gave, I thought it would be useful to give you feedback (!) on how the problem was finally solved.
Best wishes,

[bob orbell]

I will stand corrected on this, but think if the feed back is disconnected, then the volume of the disconnected channel becomes louder. BOB