PSU II High Voltage at Test Point 4

[bikerhifinut]

It sounds like the negative side of the Zener isn't connected to earth (negative rail),and I guess if the Zener is also S/C then you'll get the full HT measured.As the others have said, if you meter from the negative rail then the tail of the Zener should read zero volts and if its working properly you should get 100V on the cathode (positive end).
I'd be checking my soldering for dry joints and with the power disconnected meter for continuity along the circuit tracks. Check and double check everything.

A.

[bikerhifinut]

Is PSU2 like PSU3 in that there's a lift resistor between the mains/case earth and signal earth? If so then measuring voltages from case earth will be inaccurate? Especially under no load conditions. I cant remember offhand what the lift resistor value is 10 or 100 ohms?
I'd be checking everything from the transformer onwards as I wonder if the boards been left "floating" rather than referenced to mains earth in the unit.
?

A

[Fred Shaw]

Hi Andy(?). Thanks for the post. I understand what you are saying & it makes sense to me. It seems my task tomorrow will be checking the solder joints and continuity.

[Fred Shaw]

Yes there is an earth lift resistor & I believe it’s 10 ohms. I’ll check it all out tomorrow.

[bob orbell]

OK, so this gets interesting, the earth lift resistor should be 100 ohms 5 watt and Andy's post corrected my post in as much that the Zener must be open circuit to 0 volts, it appears that the Zener is OK because if there is no connection from 0 volts to the cathode, then full voltage will be present on the anode and the cathode, so you must do as Richard say's and check continuity. Bob

[Richard]

The earth lift is 100R so almost no effect unless it is faulty. If the neg probe is on the earth post and Fred is getting (any) readings at the zener the earth post must be connected.
Agreed, my next move would be as Bob and Andy say to follow the PCB tracks and connections and resolder anything suspect.
Just before that though it may be worth putting the probes directly onto the PCB tags for the readings to minimise connections and hopefully home-in a bit closer.
See attached and try the 0V tags with point 4 etc.

[Richard]

Just re-reading and Andy's post 11 would be an explanation.
The zeners are cheap though so I'd probably just buy a few and change it, check and go over any suspect soldering, and try the voltages again.

[Fred Shaw]

I have removed the PCB, checked continuity on the track from the diode to the earth pin and found it to be OK. But metering from the diode leg to the earth pin gave no reading. Obviously a badly soldered joint. Metering across the diode gave the same reading regardless of which probe was on the anode, so I assume the diode has gone s/c. Not sure why I didn’t get this result when I checked the diode yesterday - but I have now ordered a couple of replacements. I’ll post further when the diodes arrive.

(I shouldn’t try posting while watching ‘Have I Got News For You’.)

[Fred Shaw]

New zener diodes arrived & installed. The voltage readings are now as they should be ( to within an acceptable 10% to 15% variation).

Thanks to everyone who contributed to this thread.