Wd kt88.

[bob orbell]

Help !! I need some help please, my WA 88 ( one or the first from Peter C ) has developed a fault, I switch on and begin to play music, after about 2 minute's the left speaker fades slowly away, and remains very quite, there is no crackling, no hum, I have reversed all valves but the problem remains on the left, I have substituted the WA 88 for a Quad 405 and all works good, voltage checks on the bench show as they should, where should I look next ? Bob

[Phil Y]

Perhaps a cathode decoupling electrolytic gone open circuit?
I think that would reduce gain but leave DC conditions unaffected?
Similar for an open circuit coupling cap.
One for an oscilloscope ideally.

Phil

[bob orbell]

Thanks Phil, I have a scope but not really sure how to start, a completely dead channel would be more simple. Bob

[Phil Y]

Hi Bob,

With the amp opened up and powered, you will need a signal source of some sort, CD or tuner will do if you don't have a sig gen.
Have it feeding both channels as if in use, then just scope various points. First stage anode initially perhaps and compare L & R channels. Working forward through the amp, you should find a point where the channels suddenly differ.

Have the scope set to AC coupling and initially say 50v/div and 50mS/div or something. The settings are not critical as long as you get a "picture" and you can adjust them to taste once you are happy with what you are doing.

Obviously you will need a dummy load on the speaker outputs.
Does the amp have an input volume? if not some sort of volume control external might be wise, a CD player straight in might be driving the amp rather hard!

Phil.

[bob orbell]

Thanks Phil, I have signal gen. too, so will use that. Bob

[bob orbell]

Panic over, checked the input resistance this morning, right channel 100K, left channel fluctuating between 33K and 69K ohms, a small blob of solder on the input socket almost in contact with the ground outer body creating the problem, melted the solder and wiped away, 100K rock steady, I wonder if the blob was creating some sort of capacitance ? Bob

[A Stuart]

Whisker from lead-free solder?

[bob orbell]

I don't use lead free solder A, it was strange to see the meter fluctuating at such a fast rate . Bob

[A Stuart]

Spooky! Can't see it being capacitance. Why would the blob start to move progressively, one day out of the blue, anyway. Thermal expansion - just about possible?

Let's hope the problem stays away.