WD PRE3 problems, assistance needed.

[bearded fiddler]

For the second time this year my Pre3 is playing up. The problems I had six months ago could be solved by resoldering all the valve holder pins, but now it's more serious. Yesterday at switch on the right channel went dead and the left suddenly was much louder than usual and also produced a loud crackle. Replacing the valves made no difference. I duly resoldered all the valve pins (this has become my favourite course of action when I am clueless), but no luck this time. All the voltages are ok, so I guess it's a failed resistor or capacitor. The question is, where to look first?

Jan

[John Caswell]

Hi Jan,
Interesting that the level has gone up on the remaining channel.
However do not go glibly soldering joints because it "cured" it last time, so....assuming that you do not have any test eqpt try this "idiots guide" to try to locate the fault area.
Turn pre and main amp off remove interconnects from pre end of pre to main, then turn main amp back on and after warmup touch the centre of each left and right phono cables to main amp. If it hums then amp OK, if not faulty amp or l/speakers
The turn off all, reconnect interconnects, remove pre cover, main amp on, pre on, no inputs of any sort but any input except mute selected, volume at 12o/c.
With a small screwdriver touch the centre pin of the vol control. You should get a click and hum (and NO you won't get a shock)
If you get a click and a hum on both channels then the preamplifier circuitry OK. If not there is a problem in the pre circuitry that requires looking at
If OK select any input and with vol control still at 12o/c touch the centre of the phono socket you have selected, you should get a hum in both channels. If not then either the input relay area is faulty or the wiring is suspect.
Please report back here

John

[bearded fiddler]

Thanks John. Thanks to forum member David I could locate the problem: one of the shunt resistors on the volume pot had failed.
Your instructions for fault finding without test equipment are very interesting and will no doubt come in handy in the future.

Jan

[John Caswell]

Hi Jan,
Glad to see it is now sorted out.
The technique I laid out I used to use when I were a lad as a budding Radio and TV service erk. Bob, the service manager, beat it into me that it was perfectly possible to get an amp/radio/TV working with just a multimeter (Avo7 or 8) a finger and some careful thought. I am not saying that the unit would be working to peak perfection, that required, even then, various signal generators etc. but I has stood me in good stead ever since.

John