An old dinosaur bench top power supply - help needed

[Kg74]

Re: An old dinosaur bench top power supply - help needed

RJ- No voltage on the output.

I put the ground of my VOM on the heatsink and poked the probe under the board. One leg read 30V and the other gave no reading.

[R_J]

Re: An old dinosaur bench top power supply - help needed

Do you measure any voltage on the output? If you get 0 volts the regulator must be open internally

[Kg74]

Re: An old dinosaur bench top power supply - help needed

Thanks redwire and thanks again RJ.

RJ - The regulator reads about 30V on one leg and nothing on the other, regardless of where the pot is set. So the 7805 is dead.

The browned resistor is for an LED and is working.

I will look into the LM317 conversion - Redwire I assume you are talking a regulator with three legs vs this current through the board 2 leg setup. Any recommendation of wire gauge? I figure I could pull the 7805 and solder jumper wires in the existing holes and run it to the back per your suggestion.

[R_J]

Re: An old dinosaur bench top power supply - help needed

Thanks RJ - sounds like it may be worth some attention. I measured the transformer output and it is 25V a/c. Diodes seem to check out.

Any suggestions on testing the UA7805? Can it be done on board or should I remove it?

Just measure the voltages, the regulator should have around +35v input and the output should vary depending on the position of the control, (you can also check the ground pin voltage on the ic) The output of the regulator should ajust from +5v to around +30v If it does not, the regulator ic is likely bad.
The burnt resistor looks like it is the current limiter for the led?

[redwire]

Re: An old dinosaur bench top power supply - help needed

It was a trick to make a fixed voltage regulator IC adjustable by putting a pot in series with its GND pin. So you could get 5-30V depending on the transformer voltage. It has a 50V filter cap, IC is 35V in max. Similar schematic below.

For the size of this, it needs much better heatsinking something I would add to the back panel. It won't make 1A for long without heating up too much.
If the IC has failed, I would convert it to use a LM317 on a heatsink at the back.

Why is the resistor burned on the brown wire?

[Kg74]

Re: An old dinosaur bench top power supply - help needed

Thanks RJ - sounds like it may be worth some attention. I measured the transformer output and it is 25V a/c. Diodes seem to check out.

Any suggestions on testing the UA7805? Can it be done on board or should I remove it?

[R_J]

Re: An old dinosaur bench top power supply - help needed

The control is used to vary the ground of the 7805 so the output voltage will vary from +5v to something higher 12~20v
There looks like a burnt resistor above the switch? or is it just a black shadow?
Something like this: The component values are different but it shows the general idea of how it works.

[Kg74]

I have had this old bench power supply which I have decided to look into repairing. I have no idea where it came from.

Attached is a picture. I cannot find much information on it to even figure out what it is rated for. The analog display goes to 25V, but the voltage regulator chip is rated for 5V (UA7805 - KC - F 8221 - Korea). The cap is rated a 100uf 50V. There is noting on the bottom but some traces. Pretty much what is seen is what is there.

Any insights into testing? Capability?