HP 8753C possible power supply problem

[Mrt12]

Hi,
I have a 8753C network analyzer including the test set. The unit looks almost like brand-new, and it works fine.
However, it has a strange behaviour: when it is turned on for a while (approx. 10min or so), then the message "low air flow! replace fan filter" appears. However, my unit does not have a fan filter. I checked the schematics and found out that the low air flow condition is detected by the temperature difference measured by two sensors. I checked the heatsink temperature, but it was quite good I think (around hand-warm, which is okay I guess). I thought the low air flow message could have something to do with the temperature sensors.

Further, there is another problem: if the analyzer is turned on while it is cold (room temperature), no problems at all. However, if I let it run for a while, then turn it off, and turn it back on again while it is still hot inside, it reports 'no phase lock' until PRESET is pressed. From there on, it works still fine.

I wonder where I should start looking for the solution to this strange problem. Every performance test is PASS, the supply voltages are fine, and the heat sink is not really hot. What could be the case?

[Mrt12]

I made a video of the green power supply LEDs during turn-on. See this link:

https://hb9fsx.ch/wordpress/wp-content/uploads/2018/06/20180604_162034.mp4

There's definitely something wrong with the power supply! another 8753C doesn't have this problem. I think the LEDs should be OFF, then ON after a a short time. However, since this does not happen on my unit, it loses phase lock :-(

[Bud]

if you have two of them why dont you swap the power supply and see if it resolves the problem.

[Mrt12]

because one of them does not belong to me ;-)

[Bud]

Which heatsink you refer to?

[Mrt12]

The large one in the centre.

[Bud]

The large one in the centre.

This is post regulator, the secondary one. You may need to troubleshoot the switching regulator enclosed in the metal box. Check out this repair article, where the symptoms seemed to be same as in your case, i.e. flashing LEDs on the post regulator board

http://www.makarov.ca/8753C_repair.htm   

[Mrt12]

thanks for the link. I'll check that.
Today I found out that the problem vanishes if I disconnect the test set. So it looks like the test set draws a bit of current! :-o

I connected the test set from the other analyzer, no problem with that test set.

[Bud]

Take that with a grain of salt. The troubleshooting procedure in the article involved removing the plug-in boards also in suspicion that one of the boards may draw too much current, but in the end that did not have to do with the fault which turned out to be a leaking cap in the switching power supply. A subtle difference in the load current was sufficient to make a difference between normal start and failure.