PSU II High Voltage at Test Point 4

[Fred Shaw]

I do not have a WAD Pre Amp so the high voltage at tp 4 of PSU II isn’t a concern other than it may indicate a problem which may eventually affect the output to Phono II - which I do have. The no load reading at tp 4 should be 110Vdc but it is 376Vdc. All other readings from the test points are within 15% of the ones given on the WAD build sheet. Is the high voltage something I should be concerned about or should I leave well alone.

Fred.

[Richard]

Hi Fred,

Check where you're putting the earth probe. 367V is about 40V higher than it should be even if ZD1 has failed - we'd expect 325v.

The heaters are raised 40V and I wonder if you have the pos probe on TP4 and neg probe on the heaters. Do a check with the meter to see what is earthed where.

If that's not the case then suspect ZD1 has failed.

[Fred Shaw]

Hi Richard, I had the neg probe on the earth post & the pos probe on tp4. I’ll re check & if the reading is high I’ll order a replacement ZD1 to see whether that cures the problem. Could the reading be higher than expected - even with a failed ZD1 - because the mains voltage was 243V?

[bob orbell]

Fred, check from earth post, with neg probe and both sides of the Zenner, ZD1, with pos probe, if the Zenner is working you should see about 110 volts DC on one side and about 370 on the other. Bob

[Richard]

Yes check as Bob says.


Your 376V instead of 325V may be due to higher mains and also to having no Pre2 load as that voltage is not regulated and will be lower with load.

[Fred Shaw]

Hi Richard & Bob, checked as above and both sides of zener give a reading of 379V. I assume this means the diode is dead & needs replacing. If that’s the case I will order one and replace the faulty one.

Fred.

[bob orbell]

Not dead, but, short circuit, if you don't use the Pre output, you may be better to do away with the Zenner and reconfigure the PSU to PSU III, it is only a couple of large resistors that need changing, not running with the Pre connected puts a lot of stress on the 100 volt Zenner. Bob

[Fred Shaw]

Thanks Bob. Can you tell me which resistors need changing to bring the PSU up to PSU III spec? Also, if I just remove the zener completely will that be OK & not affect the phono side?

[Richard]

"Hi Richard & Bob, checked as above and both sides of zener give a reading of 379V. I assume this means the diode is dead & needs replacing. If that’s the case I will order one and replace the faulty one.
Fred"

Hi Fred,
I've attached the drawing of the output section of PSU2.
We probably all know what we mean but best to pause and check,
If one probe is on the case earth I cannot see how both sides of the zener are at 376V.
One side of the zener is connected to earth and the other is connected to HT via R4.
I'm guessing the zener is actually open circuit and Fred means both sides of R4 are at 376V?

[Fred Shaw]

Hi Richard, I checked the voltage with the black probe connected to the earth point and the red probe first to one side of the zener and then to the other. I wasn’t measuring across R4. Both placements gave the same reading of 379V. I can’t explain why but that was the reading. The red probe was placed at the zener/PCB junction in both instances.