Retro DC Bench Power Supply

[Brendon@aa]

Hi, pretty new to repair and have an OLD benchtop DC power supply with no output. There is no voltage showing on the volt meter display however the current meter is full scale at 5a when on the 2a limit setting and 1.35a when on the 500ma setting.

230V AC is making it past the fuse and into the main transformer - a yellow pair on the secondary has 30VAC on it, Brown pair 37V, there is what I think is a centre tap coming from the main transformer in green that has 15V and 20V respectively from the yellow and brown windings.
From there the power dives into what looks like a control card, there are two big caps and what I thought would be large rectifier diodes on the heatsink are actually two ZN3055 matched pair transistors.

The bottom transistor is running very hot at 51 degrees centigrade, the top one is a balmy 31 degrees (the workshop is 24 degrees today)

Seems pretty obvious there is is a short somewhere, now how to find it., what is the best was to go about investigating this?

The PS looks either home made or made from a kit, there is no makers mark of any type anywhere so not expecting to find a schematic.

A diagram about how benchtop power supplies with matched pair transistors used to work in general would be very useful

My next moves unless there are better suggestions are to:
1) check the heatsink is not live
2) remove the guts from the can so I can trace where the power flows from the main tranny and sketch out a bit of a circuit
3) google power supplies that use matched transistors to get a hint
4) check the voltages on the input and output of the ZN3055's and compare with the data sheet - something is clearly wrong there.

Any help really appreciated

[harp]

Is output shorted?
Not sure what transformer is heating, I see only one, not top and bottom.
Are you thinking at 2N3055 bottom and top?...

[Brendon@aa]

Thanks Harp, there’s nothing plugged into the outputs but it definitely feels like the output stage is shorted

Sorry, meant bottom transistor - have edited my typo

[redwire]

It looks like an electronics magazine build from the 60-70's? You would have to draw a schematic of the board and replace the really old parts, and even then you don't really know how well it worked in the first place.

The main parts look good - huge power transformer, good heatsink, panel meters.
What I would do is put in one of the $7 chinese 0-30V 2mA-3A PSU kit boards that are a copy of a 1978? British circuit. They use three op-amps and give adjustable current limit potentiometer as well as good regulation - max. is 24VAC input I think. Measure the DCV across the big filter capacitors and see if the voltage is OK or too high for the kit board, 35VDC max.

Or go on an adventure and try fix this.

[Brendon@aa]

Thanks Redwire - it was our bench PS in the lab at the office for years, so has been going great. Yes I reckon it’s a kit and your age feels about right

It’s a project more than a necessity so will attempt to create a schematic but if that’s a bust I’ll go the Chinese kit route as you suggest - thanks so much for the suggestion

[harp]

Seems pretty obvious there is is a short somewhere, now how to find it., what is the best was to go about investigating this?

So, check with multimeter in diode mode all semiconductors like transistors and diodes for short... and visualy inspect that nothing phisicaly do not shorts over wires or on pcb track.
Transistor who is warm mean that in that circuit is something shorted, or have low resistance, just follow track to inspect.
From your actual pictures I can not see much.

Some tips:
Hesitate with mains live testing, and always use light bulb current limiter
Pay attention on na fluffy wires, to not connect, crossover in air joints
Before removing pcb from can, take notes where some wires go, because wire can easely snap and lost place of connectin. Some pictures also help to document other relevant things in the future in repair process.

[redwire]

The PSU is acting like there is a low resistance or short across the output. I have seen that when the output diode for reverse-polarity protection gets damaged.
People will go to charge a battery, connect it backwards and the diode smokes and shorts. Look across the output jacks or on the PC board for a blown diode.

[Brendon@aa]

Thanks Harp and Redwire, both great suggestions, I will have some time to get into this this weekend, really appreciate your ideas, and yes I have a light bulb limiter, another of Richard's (LER) suggestions that seems like a great idea! Thanks again, I'll let you know how I get along