Very old home brew bench PSU

[Refugee]

I bought this about 30 years ago cheap and found parts to make a much better one weeks later.
The volt meter is missing its cover and is also open circuit this fault being in the moving coil itself.
I have a replacement that reads the other way round so another scale will have to be made so that it can be mounted with the coil at the top.
It is crude and will and there is a burn on the PCB close to the rectifier.
Currant limit is by a heavy pot that shorts out the currant limit pins on the 723 regulator when set to maximum and it only works from 1 amp to 2 amps and the meter only goes up to 1 amp so it is not a lot of good really as it is.
The mains transformer has been bodged a bit too but I have no idea why it was done.
The good thing it that it has circuit breakers both sides of the transformer.

[AlanBeckett]

I think 'Rough' would about cover it

Looking on the bright side I suspect it could be the basis of quite a decent PSU. There looks to be a good Transformer, a useful Case, holes for Meters and Terminals etc, so why not start again?
Al (skywave) has a Thread somewhere about building a Low Voltage PSU. You could use his Circuits as the starting point.

In a fit of enthusiasm last year I bought a small Valve-based PSU at Golborne. It's clearly home-made, and meant to supply small Valve Projects. It doesn't work properly and I've been pondering on whether to work out the Circuit and mend it or smply recycle the bits. One of these days .......

Alan

[pwdrive]

Those overload cutouts look similar to the ones that Thorn used in there CTV's, the transformer looks similar to some of the ones that Thorn used ?

Lawrence.

[Refugee]

I have got a choice of two NOS boards that I can use for it.
I have reverse engineered it and have a circuit diagram.
The other is rated at 2.5 amps.
Also I would prefer to have the meters recessed so that the fronts don't get scratched so easily.

[Refugee]

It may well have been built with Thorn bits.
I will need to fix the funny mod where for some strange reason the builder has reversed the ends of the secondary winding that lead to the tags on the coil former.
I will have a quick look at getting it to go all the way to zero volts. This involves creating an extra regulated negative supply rail to reference the voltage sensor to. This only needs to source 100ma or so just like the adjust pin on an LM317 chip.
C1 and C2 will go to the newly created power ground and the extra supply will have its own decoupling capacitors on it.
I have found a free CAD package and got it going in an evening.
Now I have got the reverse engineering looking professional.

[AlanBeckett]

That certainly looks the biz.
Which Package did you use?

Alan

[Refugee]

It is the native one that comes with the flavor of Linux that I have installed.
I just added it while I was doing all the updates to deal with that security threat we had.

[AlanBeckett]

Ah, so. There looks to be a Windows version so I may give it a go.
Thanks

Alan

[Refugee]

Now it is time to look at what on earth is going on with the transformer.
It has been bodged and I have undone the bodge and measured some voltages.
It looks like 2X 15 volt windings but with no tags for the second pair of wires.
It looks like I have a choice of 0-12 volts at 2 amps or 0-24 volts at 1 amp.
I have a 1 amp board or a 2.5 amp board to hand. I am tempted to alter the currant limit resistor to bring it down to 2 amps as the transformer looks like it is only rated at about 50VA.

[pwdrive]

Do you know the purpose of the slot in the laminations of that transformer?

Lawrence.