Diode rectifier question - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 06, 2012, 10:11:04 pm
content: I acquired a huge and heavy KSM (old British make) SCT 35-20 linear power supply that gives 0 to 40 volts at up to 20 amps. When I first got it it worked fine, but when I tried to use it last week it powered up with the fan running, but no output at all. I had a cursory glance inside tonight and at least two rectifier diodes are dead. They are 40HFR40 , see :http://docs-europe.electrocomponents.com/webdocs/0026/0900766b800266fb.pdfThe other two rectifier diodes look similar but each have one extra terminal, and are marked 31RCS20 72 47 I can't find any info on these at all, and am not familiar with what the extra terminal is, can anyone advise what they may cross reference to, and if there's any particular test for them, or do I treat body and main terminal just like a normal rectifier diode to check for open or short circuit? One of them is showing open circuit either way across the large terminal and body.Photo attached. Cheers!

author: David_AVD
author_rank: Super Contributor
timestamp: August 06, 2012, 10:40:44 pm
content: Looks like an SCR in a stud mount package.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 06, 2012, 11:15:16 pm
content: Thanks, that makes perfect sense as this is the only info I have. It is one a family of three PSU's of up to 100 amps output. I am told it has the following features:These are superb PSU's, with thyristor pre-regs to reduce dissipation in the pass transistors. Pass transistors, main rectifiers and thyristors are mounted on an aluminium tunnel which is force cooled by a fan at one end, avionics style.My problem is i have no schematic for them, so am working blind here.Thanks for the reply.

author: David_AVD
author_rank: Super Contributor
timestamp: August 07, 2012, 02:03:09 am
content: Did you test the diodes with the diode test function on your meter, or just the Ohms range?I'd guess that the PSU has another secondary (or at least another rectifier) that powers the control circuitry. If the 40HFR40 diodes are still ok, maybe look for bad components / joints on the control board?

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 07, 2012, 06:33:41 am
content: I tested the diodes with both my multimeter on the diode setting, and with my Peak DCA55 semiconductor tester. The two diode rectifiers on the main heatsink, removed from circuits, showed one AOK (and looks much newer than the other...) and the second showed open circuit unless the loose middle terminal is pushed hard in, so mechanically it's very suspect. I was expecting 4 diode rectifiers in a full bridge though. I have some new lower amperage stud mount diode rectifiers for an automotive starter / charger I have not yet used, I wonder if I can fit one temporarily to see if it cures the issue before buying two correct new ones? The voltage rating should be adequate, and I can test on a minimal amperage load. The main electrolytics fpr rectifier smoothing tested AOK. Whn this one is back on line again I have its big brother to sort outm which has a scorched patch on one of the control boards, and I suspect will be much trickier to sort without a schematic.Is there a way to pick a suitable pair of SCR's without the schematic, or parts list? I fancy replacing those, too, whilst inside, access isn't good, might as well do them all. I have Googled the number stenciled on the SCR's to no avail, it says : 31RCS20 72 47Thanks for your help David.

author: David_AVD
author_rank: Super Contributor
timestamp: August 07, 2012, 08:57:22 am
content: I'd probably replace that pair of power diodes first in that case and see if that changes the symptoms.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 08:43:50 am
content: Hi David, thanks for the reply. The pair of rectifier diodes arrived this morning, but after fitting them the symptoms remain the same. A sensible voltage out from the transformer, but nothing across the smoothing caps for the rectifier bridge, or at the output terminals bar about 0.5 volts, un-variable. I am wondering if I can test for a signal at the small terminals on the two SCR's? What sort of switching voltage should be on there, and can I use my `scope to check for their presence? The control circuit boards look complex, and I do not have a schematic or manual for this thing. I can test diodes, resistors and capacitors OK, but there are some multi lead semiconductor devices that are possibly voltage regulators? and I doubt I can test those. I would like to make a valiant attempt at getting this running again though, cheers.

author: David_AVD
author_rank: Super Contributor
timestamp: August 08, 2012, 09:10:48 am
content: It sounds like the SCR's are either dead (possible) or not being driven from the control circuitry (more likely).Did you find a second transformer secondary and/or bridge rectifier? If the power rail for the control circuitry is bad, that could prevent the main rail being activated (via the SCRs).

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 09:19:53 am
content: Yes, there's another mains transformer with two 25V and one 16V outputs. the bridge rectifiers for those must be on the two control boards, will see if i can trace them and see if they are running. The 2 SCR's show 32V DC and 27V AC each, relative to their studs, measured on the control pins with a multimeter. Can I scope the pins and expect to see any sort of waveform if they are being controlled, but dead? Thanks David.

author: David_AVD
author_rank: Super Contributor
timestamp: August 08, 2012, 09:42:11 am
content: I guess you could carefully scope the gate pins of the SCRs. Just be careful of what's referenced to ground as your scope is most likely grounded too.The gate waveform may look somewhat like PWM. Can't say I've thought too much about it. The presence (or not) of any signal there would be a clue either way.Definitely check out those other power rails for the control side. One of the golden rules when servicing is always check basic power first.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 11:11:12 am
content: I scoped the two SCR gate pins relative to their studded bodies, and both gave similar traces. Note I was running x10 attenuation, image attached. The voltage and current control knobs gave no change in the pattern, not sure what to make of it

author: David_AVD
author_rank: Super Contributor
timestamp: August 08, 2012, 12:18:19 pm
content: Hard to say about the waveforms. Maybe do a differential measurement from gate to cathode and see what that looks like.I'd still be looking for obvious control board issues (no power etc) before replacing the SCRs.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 12:41:32 pm
content: I found a web page showing how to test SCR with a load across them,http://www.darrahelectric.com/uploads/techLib/IB1152-Troubleshooting-Silicon-Controlled-Rectifiers%28SCR%27s%29__633821542922955648.pdfand they are testing AOK, so on to the dreaded control boards next. There's a diode bridge rectifier on one board, rectifying one of the secondaries of the small transformer. It's giving 30V DC across its electrolytic, but showing 0.7 V AC, which sounds like a lot of ripple to me. This board feeds the gates of the two SCR's so I'll start there. I'll photograph and note the board connections and pull it out for a proper look see. There was no noticeable waveform gate to cathode on either SCR. I am pretty sure now it's a control problem, as you say. Thanks you David!

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 03:27:52 pm
content: Sorted!Took one of the two boards out and first lifted one leg of all the diodes, but they tested fine. then I lifted a leg of all the electrolytics and one gave a very bad reading. I changed it, I only had a radial leaded one, but it fits fine. All is now working!I just have one final question, there was a cap trade marked as a FILMCAP that measured well out of range, it may be a polypropylene cap, I am not sure. What confuses me is the silk screen on the PCB shows it as polarised... I'd like to change that and the two oblong ones of the same FILMCAP makers name. I have indicated them on a photo I have attached, can anyone tell me what's the best to replace them with please. The bigger blue electrolytic will be changed, too, but I will have to buy one, I don't have any of the right value and voltage rating. Thanks for all your help.

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2012, 03:43:17 pm
content: Polyester capacitors, change then to ones with same value and 250V ratings, which will probably be the same size anyhow.

author: T4P
author_rank: Super Contributor
timestamp: August 08, 2012, 03:47:48 pm
content: Of course you replace them with film caps ... more than 10% will be enough , it was 20% originallyIn fact any 63/100v film cap will be more than enoughhttp://uk.farnell.com/vishay-bc-components/bfc246816105/capacitor-1uf-250v/dp/1166124?Ntt=BFC246816105http://uk.farnell.com/epcos/b32562j3225k/capacitor-2-2uf-250v/dp/9752803

author: SeanB
author_rank: Super Contributor
timestamp: August 08, 2012, 03:50:59 pm
content: Nice seeing a 723 regulator, they are very nice within their limitations.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 06:35:19 pm
content: Quote from: David_AVD on August 08, 2012, 12:18:19 pmJust to add a finishing touch if anyone ever wonders if the waveform changed once the thing was fixed. Here's the SCR gate waveform when it was mended and working correctly.Thanks again David, Sean and everyone!

author: David_AVD
author_rank: Super Contributor
timestamp: August 08, 2012, 09:31:09 pm
content: Well done Chris. I bet it was satisfying to get that sucker working again.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 08, 2012, 09:56:42 pm
content: I know such a repair is trivial to you more experienced guys, but to me it gave me an immense sense of satisfaction. I am in my fifties, so learning "new tricks" is not as easy as it was during my youth. I like the hands on way of learning though, and I have spent a few bob on test gear, as I see amateur radio and electronics as a long term hobby as I get older. I have learned quite a lot from this repair, and am very appreciative of people's patience and willingness to share hard earned experience with people like myself. Cheers David.