Hameg 7042 PSU , troubleshooting / repair

[Ivanovitch_k]

Hi,

I'm experiencing strange behaviors with my Hameg 7042-3 bench PSU. It still works but I'm concerned it's on the way out...

It started by blowing the AC fuse at "cold" power on (in the morning after being switched off for the night).

Did that a few times in a few week span, replacing the fuse fixed it. Cycling power while "hot" doesn't seem to be an issue.

After replacing the fuse 4 times, I opened it for inspection, apart a few dust spots which I cleaned, nothing unusual, no puffy caps, no magic smell, no nothing...

Now, the center rail (2.7 - 5.5V) exhibits a very strange behavior: when powering the supply on (cold or hot) there is a very slow ramp up (10 seconds+) time to the set voltage.
Other channels almost immediately reach their values. This is with the output off.

An hint, tweaking the voltage adjustment knob "speed up" and seems to help the channel attain it's set point faster.

I'd like to repair it if possible, because other than that it's a very nice PSU, it's an old design but & it's still sold under Rhode & Schwarz name.

Couldn't find the service manual for it, if anyone has it...

Given those symptoms, any idea of where I should start probing / looking into ?

Thanks !

[aqibi2000]

One would assume this is related to capacitors. Blowing fuses which not actually fault due to inrush current?

[kripton2035]

this is for the 7042-5, but there are all the schematics. hope this helps.
https://elektrotanya.com/hameg_hm7042-5_power_supply_sm.pdf/download.html

[JKKDev]

I would also vote for a faulty cap somewhere on that line.

[xavier60]

Page 17 shows an inrush limiting resistor and bypass relay. Would be worth having a close look at.

[WirelessESD]

I would check VDR1 as well I had a varistor intermittently fail closed on a power supply once.