I have a very early (not the best design) SMPS from the late 80's.
All but one of the caps have lost a considerable amount of capacitace, one resistor burnt out, and the transistors need replacing too. The inductor is also very sketchy looking so i will replace that if I repair the PSU.
So I will be spending $35-40 on Mouser to repair this PSU.
Alternately, i could find a more modern SMPS. Taking care of 5V is very simple, i could get a small enclosed unit or something like the PS-05-5 by Mean Well.
The problem is nobody makes anything for 18v anymore, let alone +/-18V.
So i was thinking perhaps I could get a 24v SMPS and 5v SMPS for less than the cost of parts to repair the existing PSU, but then i'd have to hit some 18v regulators with the 24v SMPS signal.
However, it won't work that easily, will it? I would need two separate 24v SMPS, since it seems impossible to find a bipolar supply for either 18v or 24v.
Provided i have space to install all of them, i could spend $30 and get two 24v SMPS and one 5v SMPS. Or I could spend $40 and simply refurbish the existing PSU with all new parts.
Besides the headache of hooking up 3 different SMPS, is there any reason really not to save about $10 and figure out how to best stuff those 3 SMPS in the case?
The one i will be either repairing or replacing is very huge for what it is - about half a 1u rackmount case - so i will have room to fit in three small SMPS if i go that route.
I am more concerned about the safety of refurbishing this old SMPS design than anything else, really.
If the SMPS has lasted from the late '80s till now it mustn't be such a poor design.
As an old design, it is probably more "repair friendly" than modern ones.
Transistors don't die "just for the giggles", so if you decide to go the repair route, take a long, hard, look at that inductor.
I remember, years ago, a SMPS in a Sony "Profeel" picture monitor, where the standard appearance of the inductor was black, which hid any sign of overheating.
The resistance of the inductor looked reasonable, so I passed it as OK, replaced the resistors & transistors, & all should have been good.
At switch on------"Bang!!".
Luckily, another Profeel came in for a separate fault, giving me a known "good" sample to compare with.
Both inductors still looked much the same resistance wise, but the inductance meter told the story----the inductance was very much less!
I then added the inductance value of the good one to the schematic so as nobody would get caught again!
It is a bummer if you don't have a good one to compare with, but I have seen suggestions for indirectly checking these things, like exciting it with a square wave, or make an oscillator using the inductor.
If the "Q" looks poor around the SMPS operating frequency, the inductor is probably faulty.