Problem with WA 88

[bob orbell]

Hi all, I have a problem with my WA88, first off it is one of the very first to come from WD at Great Gidding, so must be 12-15 years old, it has given very good service apart from some valves glowing red hot and taking the capacitors with them, a simple fix apart from the cost of new KT88's, (think it has done this four times). The problem is, after it has been on for two or more hours, the sound from the right channel get less and the high frequencies are less, after it has cooled down, all is well again, today I touched the output transformers and the left was warm, the right was much cooler, all four power valves were very hot as normal, I have checked all voltages many times and all is good. I don't yet have my new scope, (mid April) so I wonder if anyone here has any thoughts, could it be the output transformer damaged by the above faults, when cold resistance readings of both are much the same. Please help. BOB

[Richard]

Hi Bob, If it were mine I'd lower the current through the KT88s a bit with 560R cathode resistors. If you don't like it or it doesn't help it's easy to swap them back.

[bob orbell]

Thank's Richard, I will try that change of value, but do you think that I may have damaged the output transformer, the 470 resistors are quite new, as are valves and C7/8. BOB

[bikerhifinut]

Bob,

A quick head scratch and I figured if it's not too difficult to do, you could swap the OPT tx's across and see if the fault follows them?

A.

[Richard]

Yes that would check out the OPTX but first swap over the L and R inputs when the problem starts to eliminate a problem with Pre or source.

[bob orbell]

Hi Andy and Richard, I have thought of swapping the OPT's, I may try that. Richard, I have done all the obvious like swapping inputs, speakers, valves etc. even sources and a different preamp, I also have a KEL84, configured to one input and that is good for many hours, so the problem is with the 88, Andy's suggestion seams to be the only option left. BOB

[bob orbell]

Hi all, spent yesterday afternoon changing the OT's from left to right, yet to have a good long listen, so can't yet say much, as I fitted the transformers back in, I added earth wires from the earth post to the fixing bolts of the OPT's, the small hum on the left side has gone and the amp is almost completely silent . I am going for a cycle ride this morning, but will have a good listen this afternoon. BOB

[bikerhifinut]

Good luck Bob.

I read the earth wire from the OPT bit with interest. I have a niggling hum from the LH Channel on my WD88VA, wonder if that would work on mine? I had wondered if it was due to a slight imbalance on the LH side due to the LED indicator being driven from the cathode of one of the LH O/P Valves? But maybe its a grounding issue.

How do you do it? A star washer on a mounting bolt or do you have to scrape the lamination stack to get continuity?

A.

[bob orbell]

Hi Andy and all, star washer under the nut, no fibre washer at the top. After a 22 mile cycle ride this morning, got back and had a shower, switched on and sat down to have a listen, no poxy difference, , it's back on the bench, I have now changed the coupling capacitors for new one's, I don't think there is anything wrong with the old, think I will have to wait for the scope. BOB

[bikerhifinut]

did it follow the transformer bob?

if not then i guess the heartening thing is that your expensive trannies are ok.

Ok daft question time, forgive me if you have done it. Could it be a valve breaking down on the affected side?

If not I suspect a capacitor breaking down when it gets warm, even new ones can be duff. Or a resistor but you said all the DC conditions are fine which would rule that out.
My suspicion is a dodgy electrolytic?

Good luck and keep us posted.

A.