ARS Power Supply

[Jeffreyabb]

Hi I'm looking for some information on an 28V ARS RS5078 power supply. It's a well built unit, but suddenly, no power. Any info on the circuit, handbook or advice would be most welcome. I've included a couple of pics, not very good but they might help identify it.
Thanks
Jeff

[Radio Wrangler]

Should be fixable without info, if need be.

Any transformer hum?
Any resistance reading from L-N on plug
Plug fuse blown?
Any internal fuses?
What load was it driving?
What experience have you?
What test equipment do you have?

Right by where the wires from the transformer connect onto the PCB there is a cluster of four big diodes. They look like Motorola 6 amp jobs. This seems to be the main bridge rectifier and as the main reservoir capacitor is off the board, I'd expect the blue electrolytic near the diodes to be in parallel with it. For a 28v supply designed to covermains tolerances, I'd expect about 40v across it.

You need to follow the mains wires in from the barrier strip and check that mains voltages aren't exposed anywhere. Hopefully they go straight to the transformer without anything touchable along the route.

It looks nicely made.

Fuses age and fail eventually even without a fault. Occasionally resistors go high, but these are premium grade parts. Electrolytic capacitors slowly dry out. The odd semiconductor fails.

David

[Jeffreyabb]

Hi David
Thanks for the response.
The load was about 1.5-1.8A at the time. I checked the fuses first, both were OK, but not knowing their history, I changed them anyway. I couldn't see any internal fuses.
I have a fair range of test equipment. AVO's, DMM, 'scope sig gen, frequency counter & others.
My background isn't in electronics, but in the chemical industry. However I have been into radio since the 60's. I do prefer to have a circuit & operating principles if I can & take advice from others.
This PSU is more complicated than I am used to, lots of transistors and I'm guessing that it incorporates some sort of remote voltage sensing (S+ & S- terminals on the front panel), strapped to the output terminals in this case. As you say, the PSU is built to a high standard, both electrically ans mechanically.
Actually, I followed the same logic that you suggest & started at the input. I followed the wires to the transformer. There are two output windings of 30V 50mA & 32.7V 4.9A. The 30V winding was very close to 30V & the 32.7V, about 33.5V.This seemed OK to me. I looked at the PCB. No smells, no sign of burning, no swollen or leaking caps. I then looked at the 4 diodes (MR761's) in circuit, but didn't get sensible results. I also thought that the 4 diodes are a bridge rectifier, but it turns out that they two pairs forming a full wave rectifier for each of the transformer secondaries. I had a half-hearted look at the two reservoir caps but didn't get a reading. I decided to stop at this point, and ask for help
Incidentally, there are 11 BC107's, 3 other transistors and a substantial device at the bottom left on the PCB. The lettering is something like AEI CX8JX5FY. There are also 2 2N3055 transistors on pretty big heat sinks behind the PCB (pass transistors?). I hope they're OK - it looks like a serious job of dismantling to get to them.
I will get access to the back of the PCB tomorrow, lift one end & test the rectifiers properly. I'll also check the circuit to the reservoir caps & let you know what I find out.
Thanks again
Jeff

[Radio Wrangler]

Hi Jeff,

i was just being a bit cautious until I have an idea of someone's capabilities. Wouldn't want to aim a beginner straight into live mainsy bits.

The AEI part is probably a thyristor crowbar, set to short the output as a last-ditch protection for the things being powered by the supply. AEI was a significant supplier of thyristors in that period. Having a crowbar is another indication of good quality.

You can check that the 2N3055s are OK and there should be emitter current sharing resistors with them, and if the reservoirs are running at sensible voltages, then you either need someone to come up with a diagram, or it'll start to get slow. I hate tracing out PCBs! but it isn't too bad if it's something interesting.

Probably the 2N3055 are used as emitter followers with their bases driven by the beastie with the round heatsink and another emitter follower before that. there will be a differential amplifier done as a long-tailed pair buried somewhere in the BC107 farm. One thing worth tracking down is the voltage reference zener and checking the voltage across it.

I was expecting a bridge,and expected a second small one somewhere for the control circuit supply.

For maximum confusion, the same board will be used with different components loaded for all sorts of power supply voltage/current variations.

There will be some funny stuff designed to create lead and lag networks to compensate the leads and lags of the feedback loop to try to keep it stable while driving capacitive loads - this is always a difficult design task, and it's why those 3-terminal regulator chips are specific about what types as well as values of decoupling capacitors they get.

Cheers,
David

[Jeffreyabb]

Hi David
Yes a sound idea to find out about someone's capabilities & equipment.
I had a closer look at the PSU this afternoon and started tracing the wiring from the transformer. The 4 diodes are definitely a bridge and provide the power. I haven't yet followed the other circuit. Sorry I misled you, my fault. It was easier to see when I illuminated the back of the PCB with a small LCD torch. The DC outputs of the bridge go to the big blue smoothing caps. (2off in parallel, 2nd one smaller than the one in the photo). The wiring from the +ve terminal goes to a fuse & then on to the bodies of the 2N3055's. The other wiring on the 3055's goes to a terminal post on the PCB. Not investigated them yet.
I then had a look at it going from the other way. The +ve output terminal on the front panel goes to the body of the thyristor and the -ve output terminal to the long terminal on the thyristor. The short terminal of the thyristor goes to terminal post 'G' on the PCB which is about 70 ohms up from the -ve terminal. (I've not had anything to do with thyristors before.) I'm going to draw out what I've done so far and then look at the 3055's. Unless there's something wrong with them or the thyristor, it looks like it's going to be a long haul to get the circuit out, unless someone has one. It sounds horrendous from your comments.
Regards
Jeff

[Jeffreyabb]

Hi David
Quick update. After a little bit of dismantling, I checked the 2N3055's and thy look to be OK. Likewise the thyristor looks OK. Next thing is to look at the small diodes and the zeners.
Jeff

[Radio Wrangler]

That's all good
Sneaking steadily up on it should do the job.

David

[Jeffreyabb]

Hi David
I have done a bit more with this PSU. The control supply bridge is there. It's 4 x 1N4148's. They are OK. I don't know if it's typical, but the low end of this supply isn't at the same potential as the power circuit. The -ve end of the smoothing cap is at+ 17.4V to the -ve of the power circuit. The +ve output of the control circuit is at 35.3V to the -ve of the smoothing cap.
I checked the zeners. ZD1 is 6.2V, ZD2-4 are 5.1V. Actual voltages across them are 6.2, 4.8, 5.2 & 5.3. Not so sure about ZD2, but the others look OK.
I also started tracing the circuit and it looks like the smoothing cap eventually gets to the power -ve line after a long walk, lots of interconnections and a couple of resistors. It looks like I really need that circuit. I'll update you if I make any more progress.
Jeff