Hameg HM7042-5 no current readings

[salomonander]

Hey there,
maybe someone here is smarter than me. I bought a used hameg hm7042-5 for next to nothing. But off course it has issues. I get no current reading on none of the channels. Everything else works great. I can also limit the current using the corresponding pot, but the reading on the display is always zero. Maybe someone more experienced can point me to where i should start my search?

Thanks
Jakob

Service Manual:
http://www.frankshospitalworkshop.com/equipment/documents/workshop_equipment/manuals/Hameg%20HM-7042%20Power%20Supply%20-%20Service%20manual.pdf

[Doggone]

Hello Jacob,
I am not familiar with the 7042-5 I think you model is a long way on from mine but I will have a look at the manual you have attached and if I can give you some pointers that may help- still battling the fault in my 203-5!
Regards nick

[salomonander]

That is too kind. Thanks!

[Zenith]

From what you say, and from a brief look at the schematics in the service manual, it looks like a problem with the IC which controls the four 7 segment displays showing current on one channel. I assume that if you use that channel with an external ammeter, you have no problem.

You have two other channels to compare the defective one with. It might be that the IC receives no input for current, or that you have a cracked circuit track or bad soldered joint. A lot of faults with electronic equipment are caused by bad connections. It's a matter of working methodically to isolate the problem.

If it is caused by a defective IC controlling the four LEDs, I'm not sure whether it's a generally available item, or a special IC made for this piece of equipment. If it's special it may be available from Hameg (R&S these days).

[salomonander]

Thanks!
Maybe i wasnt clear, but this problem is the same on every channel. No current showing on the display on any of them. Controls do work.

[Zenith]

Then this is pretty much the same as any repair. You have to work through the schematics and understand how it works, and then deduce what's stopping it working as it should, and fix it. Check voltage levels are what you expect and if you have an oscilloscope you can examine signals. There may be independent faults affecting all three current displays, but it seems more likely it's a single fault causing all current displays not to work.

Start with a visual examination, looking for bulging capacitors, anything burnt, or previous repairs. It may be something like a bad soldered joint. Look with a magnifying glass. Unmake and remake connectors. Bad connections are a common source of problems, especially with equipment which has been kept somewhere damp. such as a garage.  Then go on to check that the voltage supplies to ICs are the correct values and free of ripple. You just have to work at it systematically.

 

[salomonander]

Thanks,
I opened it up today. Someone has done a repair on the 5v channel as three opamps were replaced by hand.
But what really caught my attention is the white residue around the small electrolytics. One of the bigger caps has this as well.
Maybe these have leak badly? Ill test them tomorrow. But i have never seen that before, and i recapped lots of units.
At first i thought that maybe they have glued them on, but why do that on the small caps only…

[salomonander]

Oh, and i was wrong about ch2. I can actually set the current on this on. But there is a short on this channel between the two banana plugs. I only realized this when i had the output running (nothing connected) and the set voltage could not be reached. It didnt go past 1,8v and some part on that channel started getting hot. I could smell it too. The banana plugs also look slightly damaged from the outside. And i measure 0 ohms between the plugs. That protection diode (D40) is certainly gone. But i guess the problem goes further. Hence the repair work on the pcb

[Zenith]

Bear in mind I have no experience of this particular power supply. This is the first time I've heard of them. I've had a quick look at the cct diagram.

What exactly is the problem? I gather that you can set the voltage on each channel. There's a problem with current limiting and the current display on each channel. Is it just the display - that is, does it behave as expected with a suitable load and a DMM to display the current?

What's the nature of the short on Ch2? There appear to be electrolytic capacitors across the outputs. They could be a dead short.

The four small electrolytics look odd. The stuff at their bases looks like residue rather than glue used to secure them. I'd remove them, clean the deposit, check that it hasn't damaged circuit tracks, and replace them preferably with quality substitutes, such as Panasonic or Nichicon.

The repair work replacing the three op amps looks messy. There could easily be solder splashes or other damaged components in that area. I wonder why it was necessary to replace them. It may have been guesswork or possibly a fault which damaged the original ones and unless cleared, has damaged the replacements. It looks as if you have three similar separate circuits, one for each channel. I assume the same four capacitors for the middle channel are just out of the photograph. The large capacitor you have highlighted looks to have leaked and the leakage could have damaged other parts of the PCB. It looks a likely candidate for replacement.

There seems to be some sort of deposit around the crystals. It doesn't look like printing. That's odd. Does this PSU show signs of being stored somewhere damp, such as a garage?

Searching the WWW for this PSU shows lots of manual downloads but no links to repairing them. I can't help much with this apart from asking questions and giving general advice.