KECL82 debugging

[Tumwad]

In the early 2000s, I built the KECL82 valve amp kit from World Audio Designs. It worked fine for a while, but then it started to show partial muting in one of the channels. It would be fine at switch-on, but after a few minutes, one channel would reduce in level, giving an unbalanced sound. It was always the same channel, but I can't recall which.

There's four valves in it, and I swapped them around to see if the problem changed sides, to no avail.

I checked all the test voltages at various circuit points as per the assembly manual, and these were all at or very close to what they should be.

It got tucked away in a cupboard shortly after this, but has anyone got any suggestions for next-stage diagnosis? I was out of ideas when I tucked it away.

[John Caswell]

Hi,
If as you say all the voltages are close to what they should be then it is time for some substitution. For a start I would have a look at C6/7 and C12/13/14/15 as these going low capacity or o/c will cause the gain to drop but no affect dc levels.
I cannot remember if the KECL82 amplifier is PCB or hard wired, but just "bridging" the component with a new one will often show up the faulty one. If it PCB handle the PCB with care they were not all that brilliant around that era.

John

[Tumwad]

Thanks, the hunt will be on for the assembly manual soon. I know it's around somewhere....

[Desmodromic]

Originally Posted by John Caswell

Hi,

I cannot remember if the KECL82 amplifier is PCB or hard wired, but just "bridging" the component with a new one will often show up the faulty one. If it PCB handle the PCB with care they were not all that brilliant around that era.

John

I can confirm that it is a PCB based kit.

On my example, one or more of the caps C8-C10 had failed, and I lost sound on one channel. These caps are fairly close to the valves can can get a bit hot. I replaced all 4 with 0.22μF, 630v Amtrans AMCO caps and it's working well now.

[Desmodromic]

Originally Posted by Tumwad

Thanks, the hunt will be on for the assembly manual soon. I know it's around somewhere....

I believe that Greg should be able to send you a copy of the instructions.

[Greg.]

Yes, if you can’t find your copy of the build instructions and diagrams, PM me your email address and I’ll forward a copy to you.

[Tumwad]

Thanks all!

I know the assembly manual is on a shelf somewhere, but which shelf...

[Tumwad]

Originally Posted by Desmodromic

I replaced all 4 with 0.22μF, 630v Amtrans AMCO caps and it's working well now.

Where's a good supplier for these?

[John Caswell]

Don't just change things willy nilly approach it systematically and change one component at a time.

John

[bob orbell]

As John say's, find the fault first, of course, if you change every component it may work, but you will never know what the problem was, if these capacitors are plastic something, I doubt they will be the problem. Bob