circuit heating up

AgX · Apr 21, 2015

I just got an old enlarger meter/timer.

Wallner MC 505 (from 1985, based on microprocessors)

After switching it on the LED panel-illumination works, but any other function is dead.

Inspection of the circuit showed that the mains-transformer (sec. 2x 4W) is getting very warm, both major capacitators are getting warm too and one of the the two bridge-rectifiers (B80C800) is getting too hot to touch. On the mounting side even droplets of tin formed at its legs!

I guess exchange of the two rectifiers would restore the functions of the apparatus.

Or am I overlooking something?

May the microprocessors be harmed too?

AgX · Apr 21, 2015

The seller used it, then stored it for about a decade, so I already thought of a decay issue. And I guess that is more likely for an Elco.
Thank you for that hint.

paul_c5x4 · Apr 21, 2015

Chances are other components have ceased to function and are contributing to the short. Can you identify a voltage regulator, and is this also getting hot ?

AgX · Apr 21, 2015

Maybe best would be to cut off the apparatus' internal voltage supply and to hook it up on a lab power supply. To check for proper functioning.
Without looking at the circuit I would have thought of doing so behind those two capacitators.

I did not think of such regulator...
I have to look for that regulator. But I felt no warmth or heat at other locations than as indicated above.

ic-racer · Apr 21, 2015

If it is a small filtered DC power supply, I'd check or just replace the filter capacitors. The diodes in the rectifier can easily be checked with a multimeter. Pictures?

AgX · Apr 21, 2015

No digital camera...

Except for those droplets of tin at the legs of the hot rectifier everything looks perfect.

shutterfinger · Apr 21, 2015

Typical power supply layout: power cord to switch to primary of step up or step down transformer. There may be a fuse in the line also. Switch closed an ohm meter will read the resistance of the transformer primary usually low ohms.
Transformer Secondary to a bridge rectifier to capacitor/resistor or inductor filter to circuit(s) to be powered, also known as load. Resistance to ground should be 10k ohms or higher at the capacitor + to ground. Be sure the power cord is disconnected from the wall socket and the capacitors are discharged before measuring the resistance. Expect capacitor action of very low resistance that builds up to 10k + ohms within 2 to 3 minutes or faster.

I suspect the capacitors have dried out and are causing the problem. The bridge rectifier may have been weakened from over heating. It will self destruct if left on with wall power supplied. An electrolytic capacitor used in this application should charge to infinity on an ohm meter when disconnected from the circuit and may not fail until close to normal voltage. Most leaking capacitors only charge up to several hundred k ohms to a few meg ohms with a ohm meter.

AgX · Apr 21, 2015

Thank you all. That sounds encouraging.

That meter was a come-across-by-accident thing. When I got some time at hand I shall get out the circuit board, take out the capacitators and the check the functions of the meter on an external supply. If positive, I shall order the capacitors and rectifiers.

(No more electronics shops anywhere...)

mihalich · Jul 12, 2016

AgX said:
Thank you all. That sounds encouraging.

That meter was a come-across-by-accident thing. When I got some time at hand I shall get out the circuit board, take out the capacitators and the check the functions of the meter on an external supply. If positive, I shall order the capacitors and rectifiers.

(No more electronics shops anywhere...)

Any success with repairing the Wallner MC-505 Are you using it?

AgX · Jul 12, 2016

I stored it. Another project on my to-do list...

paul ron · Jul 12, 2016

AGX, here in teh USA we ahve plenty of online electronics supply now. Radio shack pulled out on us last year... our last on the street shop.

Do you know what the voltage is to the circuit after the rectifier? You can by pass the power supply with a wall wart if you trace out the power supply feed? Ive done that a few times with old equipment, even run car radios in the house.

AgX · Jul 12, 2016

I guess I expected the voltage not to have changed after the rectifier stage, thus likely I wanted to measure it there (or the AC directly behind the transformer), cut that stage off and set that voltage on an external supply.

mihalich · Jul 12, 2016

AgX said:
I guess I expected the voltage not to have changed after the rectifier stage, thus likely I wanted to measure it there (or the AC directly behind the transformer), cut that stage off and set that voltage on an external supply.

I have 505 unit. it is working. If you want I can do measurements to help you restore the one you have.

AgX · Jul 12, 2016

If you could meter the voltage behind the rectifier stage, then I'm on the safe side for the external voltage.

circuit heating up

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