Old analog Power Supply needs your advice

[bohaboha]

Found this old ruined power supply that I've been tinkering with but there are three wires that I don't know their location. Your help is greatly appreciated. 2 wires coming from the bridge rectifier and the third blue one should be power carrying+ as I also don't know where to place it

[Per Hansson]

Re: Old analog Power Supply needs your advice

The transformer is missing.

[petehall347]

Re: Old analog Power Supply needs your advice

2 bridge ones go to the missing transformer

[redwire]

Re: Old analog Power Supply needs your advice

I'd guess a 20 to 25VAC transformer is missing, would connect to the purple wires.
The regulator is no good - there is no heatsink for the power transistors which will get really hot. I don't know why there are mica insulators under them. There is no short-circuit protection aside from the second fuse. The filter cap is dated 1978 so it might be no good. I would not salvage the board.
The panel meters are really nice, I used to sell Kyoritsu KM-66 by the bushel.

I would find a power transformer and put in a chinese "0-30V 0-2mA" regulator board to make something modern and get adjustable current.

[bohaboha]

Re: Old analog Power Supply needs your advice

it looks like an unfinished home project. But there is quite a lot of room for a transformer and there is already a potentiometer to adjust something. i'll see what i can do anyway it doesn't looks bad. But the top cover is missing. one question: can anyone tell what those blue components are ?

[R_J]

Re: Old analog Power Supply needs your advice

Those are axial lead electrolytic's, the values will be marked on them, I can't make quite make it out, looks like 25v.

[bohaboha]

Re: Old analog Power Supply needs your advice

yes it is 25v.I need to find a better power board, this one looks pretty bad and has a lot of old parts, or maybe I'll find a transformer and give it a try, what do you guys think?

[eccerr0r]

Re: Old analog Power Supply needs your advice

I'd vote get rid of PCB or salvage it for parts. Current design on it is awful and not going to get you very far -- no regulation at all I don't think. Adjustability, yes...

[bohaboha]

Re: Old analog Power Supply needs your advice

I'd vote get rid of PCB or salvage it for parts. Current design on it is awful and not going to get you very far -- no regulation at all I don't think. Adjustability, yes...

quite right. but I am curious about the simplicity of design that the former owner used to achieve????
because on the outside it looks like a perfect power supply but inside it is missing important parts

[redwire]

Re: Old analog Power Supply needs your advice

It's a 1970's design, the regulator board. Changing over to an IC like LM317 (1A) or LT1084 (5A), along with a big heatsink would be best. Or that chinese power supply board does OK too, it is from a 1978 electronics magazine article but good to around 28VDC out max.

Of course you need the missing power transformer, which is expensive.

[eccerr0r]

Re: Old analog Power Supply needs your advice

There's not enough components on it to have good regulation.
Looking at the connectivity on the board, I'm not sure the designer knew what he/she was doing. At most it'd be an acceptable (though cheap) fixed voltage regulator, using just those components for an adjustable would not hold regulation.

You could mod it for using an LM317 with the existing components as a pass regulator, though don't know what your tolerance for redesigning the circuit versus just buying a cheap chinese switcher guts.

[redwire]

Re: Old analog Power Supply needs your advice

Another option is to power it from an old laptop brick. These are typically 19VDC at a few amps.

[bohaboha]

Re: Old analog Power Supply needs your advice

Another option is to power it from an old laptop brick. These are typically 19VDC at a few amps.

it is an option to avoid a transformer but it is not adjustable or because there is already a potentiometer that I do not know what it is for

[stj]

Re: Old analog Power Supply needs your advice

use the 19v psu to drive a chinese regulator module

[eccerr0r]

Re: Old analog Power Supply needs your advice

You don't want to run the guts you have as-is, no matter if you figure it out or not -- if the adjustment actually works, its regulation is no good no matter what due to what was discussed prior.
Part it and redesign. Put the transistors on the case so the case can be used as a heatsink, etc. The current board design is a laughing stock IMHO.

[redwire]

Re: Old analog Power Supply needs your advice

use the 19v psu to drive a chinese regulator module

The low cost chinese 0-30V 2mA-3A power supply boards do not work if you feed them DC. They only work with a power transformer because they make a -5V rail for the op-amps from the cap+diode AC voltage inverter.

[eccerr0r]

Re: Old analog Power Supply needs your advice

dang where's the heatsink for that 0-30V 2mA-3A regulator... must need a huge heatsink...?

[redwire]

Re: Old analog Power Supply needs your advice

It's weird, the board has a 24VDC regulator 7824 for a 40mm? fan yet none of them come with a main heatsink or fan, like in the pic. There is a small heatsink for the driver transistor included.
For the $5 though, you can't beat the value.

[eccerr0r]

Re: Old analog Power Supply needs your advice

ah... that picture has the heatsink, though still seems kind of small for the design.

[redwire]

Re: Old analog Power Supply needs your advice

It's your usual chinese dream, you have to derate it a lot to make it last.
Max. is 24VAC input but under a light load transformers put out higher voltage. So say 28VAC which gives around 38VDC, add the -5V for the IC's TL081 and you see they are running off ~43VDC yet their absolute max. rating is 36-42V depending on mfgr. It's pushing the IC's too hard.
Pass transistor 2SD1047 it can make around 75W max. so an old CPU heatsink and fan is what you want if you're going to work this hard.

OP has some nice panel meters and a box. Just have to decide if he wants to invest in a transformer+new board or not.