HP 53131A - dead power supply

[fchk]

Hi,

I've got an HP 53131A Universal Counter with a dead power supply. Does anybody has schematics for the power supply? The service Manual only has schematics for the mainboard. Are there common failures known?

There are two fuses, but both are ok. Capacitors look ok at first glance, but a a whole re-cap would be my first guess.

Can I run the power supply without load?

Any recommendations?

fchk

[George Edmonds]

Hi

The power supply is a third party item and there are no power supply schematics in the CLIP.

Be very careful as it is always on to power the OCXO with a soft power switch for the counter itself.

It is of the "flyback" type and will readily self destruct, it may need a small minimum load.

George G6HIG

[Qw3rtzuiop]

Someone from the forum reverse engineered the ps here:
https://www.eevblog.com/forum/repair/agilent-53132a-repair-(and-possible-upgrade)/msg841547/#msg841547

[DeepLink]

Use some resistors as load ~10% of rated load is sufficient to have proper regulation

Look at the start up resistor R2, can be open

And do exchange all the caps (mine was leaking badly - even though the PSU was working)
Use proper SMPS caps - low esr, high ripple current

As the PSU is always on (and the fan is always running), it is possible to make a mod that shut down the PSU/Fan

The fan can be upgrade to a low noise, as the original is quite loud

[Decee1]

Hello,

I've just finished repairing my power supply and i thought i would leave some information about the fault.
Fault description:
No voltage on secoundary side, cannot measure any pulses on IC1 pin 6(out) but IC1 pin 4(RT/CT) is available and running with sawtooth wave.

Fault conclusion:
Q22 is acting like a short but is still fully functional. In my case it measured 1V on collector and emitter and 0.5V on base.

Measurements:
if pulses is not on IC1's output, RT/CT is active and there are no voltage on comp pin IC1 does as expected and therefor Q24 pulls IC1 pin 1 (comp) down to GND, then measure base of Q23 or R20, then it is measuring around 0.7V or above which turns on Q23, If Q22 measures the same voltage across it, the Q22 is faulty and should be replaced

Fault explained:
Q23 is 'on' which causes Q24 to 'turn on', pulls down the internal current source of IC1 pin 1(comp) and therefore 0V on IC1 pin 1(COMP) pin and no switching pulses on IC1 pin 6(out)

Hope my debugging can be useful for somebody else.