If only one is conducting this will give a low voltage.
If the rectifiers are OK it is a possibility the Electrolytic is open?
Max.

I will check those caps

The output of d 333 is -72v
The output of D332 is +69

 

If only one is conducting this will give a low voltage.
If the rectifiers are OK it is a possibility the Electrolytic is open?
Max.

Electrolytics check out, both close to 470uf.

 

The foil layout of the power supply board shows 4 red dots at the two full wave rectifiers.

The two inside contacts are marked for D333 and the two outside contacts for D332.

I am wondering what the significance is of these points, as there is no voltage stated.

I get no voltage on any of the inside contacts of either diode, only on the outer contacts of both.

I am wondering if that means there is an issue with one of them??

Voltages recorded above are measured from the outside contacts of both.

 

Do I see a resistor between the output and the line with 85 volts noted on it? This would imply that it is under load and the voltage would be lower because of the resistor.

 

Do I see a resistor between the output and the line with 85 volts noted on it? This would imply that it is under load and the voltage would be lower because of the resistor.

That is R340 and 341. I have checked them and they are spot on in terms of their value. Not sure they effect the voltages we are supposed to get at C335/336?

 

Hey Max ...
The Fluke does have a diode testing setting ...
I just checked both full wave rectifiers again, voltage drops look normal in forward position (as recorded above), no continuity the other way (when leads are reversed) on either side of each unit. I am not sure what you mean by "open".

A DMM diode check function is good for checking the forward volt drop of a diode - for reverse checking its best to use the highest Ohms range, with normal diodes any leakage is a fail.

There is an exception to that statement - Schottky-barrier diodes show some leakage as normal, they can be identified by a Vf of 0.2V or less. You probably won't find any of those in a mains frequency rectifier anyway.

 

A DMM diode check function is good for checking the forward volt drop of a diode - for reverse checking its best to use the highest Ohms range, with normal diodes any leakage is a fail.

There is an exception to that statement - Schottky-barrier diodes show some leakage as normal, they can be identified by a Vf of 0.2V or less. You probably won't find any of those in a mains frequency rectifier anyway.

OK, can do. Presume these will have to be pulled, and that cannot be done in circuit??

 

Pulled the S5151R diode (D333) and put my meter on the highest resistance setting.

There is leakage on both sides of the diode.

See photos

 

I don't think you will find they are that abnormal, I have measured stud diodes and have got inconclusive readings with the ohm's range, what does the diode range show?
Max.

 

Here is S5151 ... some leakage too (D332)

 

I don't think you will find they are that abnormal, I have measured stud diodes and have got inconclusive readings with the ohm's range, what does the diode range show?
Max.

On the diode setting both show to be good ... no continuity when the leads are reversed.

D 332 has voltages drops of 0.502 and 0.479
D333 has voltage drops of 0.495 and 0.477

The only abnormality maybe is the left side of D333 which has a reading in ohms far more than the other sides of both diodes. 12 M ohms as opposed to 1 and change.

 

The two inside contacts are marked for D333 and the two outside contacts for D332.

I am wondering what the significance is of these points, as there is no voltage stated.

I get no voltage on any of the inside contacts of either diode, only on the outer contacts of both.
I am wondering if that means there is an issue with one of them??
Voltages recorded above are measured from the outside contacts of both.

The D333 pair have their anodes connected together as the -ve rectified point, D332 have their cathodes connected for the +ve rectified point, the opposite end of each pair is an AC voltage.

You have checked the transformer output and it appears OK, so the next step is to connect the rectifiers and if necessary remove any load just a suitable voltage rated electrolytic capacitor, its not critical, from common to the cathode or anode pair.
And see if there is any anomaly, if the load is disconnected the voltage may be higher than shown on the print.
Incidentally I assume you are trouble shooting with the 10 pin socket/plug on the left removed?
Max.

 

It would be unusual for a winding to increase in voltage if there is a fault just in that winding.
If a couple of turns got shorted then I would expect the output voltage to decrease.

I would disconnect the transformer from the circuit and check it for resistance in each winding and compare the two 60VAC windings.
You could then power up the transformer and read off the various voltages to see if they are correct.
You really need to decide if it is dual secondary or centre tapped secondaries before throwing the transformer out if you think the two 60VAX windings are shorted.

 

You really need to decide if it is dual secondary or centre tapped secondaries before throwing the transformer out.

The print show a single secondary (apart from the lamp pair) with 5 taps for four supplies.
IF there was some defect in the one of the secondaries and the others otherwise performed OK, it should be possible to stick in another small single secondary transformer just to take care of that winding without trying to replace the specialized transformer.
The OP measured the AC out and it appears OK on all windings.
Al.

 

The D333 pair have their anodes connected together as the -ve rectified point, D332 have their cathodes connected for the +ve rectified point, the opposite end of each pair is an AC voltage.

You have checked the transformer output and it appears OK, so the next step is to connect the rectifiers and if necessary remove any load just a suitable voltage rated electrolytic capacitor, its not critical, from common to the cathode or anode pair.
And see if there is any anomaly, if the load is disconnected the voltage may be higher than shown on the print.
Incidentally I assume you are trouble shooting with the 10 pin socket/plug on the left removed?
Max.

Hey Max,
I have floated the entire power supply board with wires and alligator clips.
And yes the 10 pin socket is empty BUT for a ground connection to pin 4 back to the the center tap of the transformer.

As for the rest, not sure how to go about removing a load from the rectifiers and putting an electrolytic in there instead.

 

It would be unusual for a winding to increase in voltage if there is a fault just in that winding.
If a couple of turns got shorted then I would expect the output voltage to decrease.

I would disconnect the transformer from the circuit and check it for resistance in each winding and compare the two 60VAC windings.
You could then power up the transformer and read off the various voltages to see if they are correct.
You really need to decide if it is dual secondary or centre tapped secondaries before throwing the transformer out if you think the two 60VAX windings are shorted.

I have disconnected the transformer from the circuit, and it remains so.

When measuring AC using the center tap as reference, both windings APPEAR to be low.


Ground>red 45.1 Vac
Ground>orange 45.2 Vac
NOMINAL 60

Ground>green 66.7Vac
Ground>yellow 66.6 Vac
NOMINAL 85

These "nominal" reading are off the printed circuit board and can be seen in a ph0to on post #4. There are no numbers on the schematic straight out of the transformer.

So the readings with reference to the center tap are lower than printed on the board.

If one takes the readings straight across each tap, (without reference to the center tap) then they double and appear to be higher than what's printed on the board.

While the output of each winding appears to be low when using the center tap as reference, they are also both almost perfectly symmetrical.
Would this not be odd if they were damaged?

I can measure the resistance of each secondary winding, both in reference to the center tap and straight across each output. Not sure what that will tell us.

 

Don't forget the resultant smoothed DC is 1.4 x the AC.
I doubt they would be that symmetrical if damaged.
Max.

 

Here are the resistance measurements for each winding:

yellow/green (80v) 4.5 ohms
yellow>center 2.5
green>center 2.5

red/orange (60v) 0.8 ohms
red>center 0.5
orange>center 0.5

EDIT: remeasured, omitting the ground lead to the power supply board I had left hooked up (and measured from) These measurements are straight to the black wire from the center tap.

Happy to measure any other combinations if anything can be divined from the readings.

 

Nothing jumps out as being wrong.
Max.

 

Nothing jumps out as being wrong.
Max.

Given the full rectifier diodes appear to be good and that the voltages at C335/336 are low, I am wondering if there is anything in circuit that could be dropping these voltages right out of the diodes?

Is it possible that the transformer is no longer putting out high enough voltage?

I am ready for any next step to track this problem down, however it will have to be very detailed, as my electronic knowledge is pretty basic