No trigger waveform to an SCR controlled transformer primary issue. - Page 1
Forum section: electronics / repair

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 06, 2017, 06:53:48 pm
content: Once again my old Farnell 60V @ 50 Amps linear bench supply has given me trouble! This time (and just possibly this issue could be behind previous other issues...) it's the SCR control circuitry that controls the 240V 50Hz mains input to the huge transformer's primary. I am getting little or no voltage on the secondary or the primary of the mains transformer, so after much thinking what would be safe I have fond there's virtually NO rigger waveform to one of the two big SCR's. Disconnecting the gate of the one that has no trigger (measured at the control board) made no difference, nor did swapping in a known good transformer T1 one from a similar supply. I see no short to ground between the board connector strip (board removed) and the cut right by the SCR gate wire. I am now at a loss as to why I get a very healthy 50 Hz sine wave on one of T1's secondaries, the one that feeds pins 8 and 9, but nothing on pins 6 and 7. Can anyone point me to other possible issues please? When probing SCR control board terminals 8 and 9 the PSU will suddenly give some output, which immediately decays. The 150 Ohm R27 gets mad hot when it's run, even with the gate of SCR2 disconnected. Thanks. The schematics relevant are athttp://www.gatesgarth.com/H60003.jpgandhttp://www.gatesgarth.co/H60003.jpgThey seem too big to attach here, sorry, although I'll try making the attached ones smaller.

author: floobydust
author_rank: Super Contributor
timestamp: August 06, 2017, 07:49:39 pm
content: Looking at the circuit which is fairly hairy, it must be a big beastIt's difficult because the SCR's and drive transformer at floating at mains (neutral or live if SCR's off) potential so scope measurements are confusing.T1 should output high-frequency pulses to trigger the SCR's, assuming IC2 SN7413 is getting proper sync etc.I'd expect a mains-frequency square-wave with high-frequency modulation (the same signal out of both T1 secondary windings), not a sine-wave at all. Are you seeing that with a scope?I would check D7, D8, and wiring around them, and the SCR's. Never run T1 with both SCR's disconnected.VT5 may be shorted and loading down the control board power supply as just hum.I would use a dummy load like two of 1k ohms (in place of SCR gates) across 8-9, 6-7 to observe the SCR drive outputs, and also look at the signals at VT4.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 06, 2017, 09:47:14 pm
content: If R27 is getting hot this implies that VT5 is conducting all the time. This could be due to a failed VT5, or it could be a problem further back. A check on base-emitter voltages of VT5 and VT4 should give some idea of where. 0.7v for a saturated transistor.If R27 is likely to burn out, should be OK to disconnect one end whilst testing. Or just disconnect R25 from VT5 base and see if it still gets hot. If so, VT5 is faulty.I think it's likely the way this works is that both SCRs are fired together on both mains half cycles. One will be reverse biased and so will do nothing.In bipolar circuits, PNP power devices are always a prime suspect. They seem to be more fragile than NPN.BTW, SCR trigger pulses can be very short duration and easy to miss on the scope.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 07, 2017, 08:38:18 am
content: Incidentally, just trying to figure out what IC2 does:Seems to be that if pins 4 and 1 are both high it will oscillate with a low duty cycle, putting out pulses of (very approximately) 30us width. Thus, it should not in principle be possible for R27 to be hot if this is working properly. Which suggests your fault is in the area of IC2, VT4, and VT5.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 07, 2017, 09:49:29 am
content: Thanks for the replies, and thanks Ian for taking the trouble to draw out the functionality of the IC. I have replaced both socketed chips IC1 and IC2 to no avail, but will spend an hour or so on it in a bit and hopefully post some wave forms. Whilst I have the schematics for this PSU the user manual is for an earlier version with all discrete components, so the circuit descriptions and alignment instructions are not anything like 100% compatible with this supply, but are something of a guide. This supply has blown the pass transistors several times and latterly i was sure I saw the volt meter go off scale before the bang. I have replaced all the pass transistors and pass driver pair with 120V MJ15003 devices instead of the 60V 2n3055's. The new ones at least allow whatever fault is putting the unregulated output of the transformer across them without going pop, so hopefully I can fault trace in a more leisurely manner, but I am an abject beginner, so please excuse stupid comments and questions!Thanks again everyone.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 07, 2017, 10:08:11 am
content: This is the wave form capture at the connection between IC2 Pin 8 and R21, a sine wave. That seems wrong! I also remembered a while back Zener diode Z3 failed spectacularly, and I replaced it with a higher wattage one, after which the supply worked fine again until now. Where does this board get its 50Hz reference from, I was expecting to see a connection pre the centre tapped rectifier arrangement?Given the wave form at R21 could you please advise where to probe next?Thanks.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 07, 2017, 11:07:42 am
content: I should have done voltage checks before now, but I see the following:Across Z1 13.9VAcross Z2 9.9VAcross Z3 4.3VAcross Z5 5VAcross Z 6 0VAcross pins 7 and 14 of IC2 4.2VAcross Pins 4 and 8 of IC1 24V Is this excess voltage due to Z2, it seems to have a sensible 9.9V across it... Or a fault with Z1??What else could excess volts across the rails have killed? Thanks, and i know, PS voltages first....

author: floobydust
author_rank: Super Contributor
timestamp: August 07, 2017, 06:41:15 pm
content: Your (zener) voltages look OK but Z3 (5.1V) is low at 4.2V so IC2 or Z4 (V?) might have some trouble there.IC1 does run off 24VDC OK, today it would be aRC4558good to 36V.It is a bit confusing where to ground your oscilloscope. I think that is why the sine-wave.You should be able to earth-ground the power supply -O/P terminal so the entire low voltage side is not floating on this PSU. Check with an ohmmeter.OP, I think you are looking for a short which caused an open.Z3 cannot fail "spectacularly" unless you had an unusual short-circuit somewhere, like metal chips or loose wires.Check T1 ohms primary to secondaries to ensure insulation did not fail there.AC reference is from VT1, VT2 and you would see the waveform on C5.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 07, 2017, 06:45:16 pm
content: Across Z1 13.9VAcross Z2 9.9V-Seems reasonable. The are the supplies to the control circuit. I would have thought these were 15v and 10.5v zeners so maybe a little low but probably OK.Across Z3 4.3VA little low, should be 5v. TTL min volts is 4.75. Probably won't matter though.Across Z5 5VZ5 would appear to be a reference. It's fed from a constant current source, VT3 and Z6.Across Z 6 0VShould be 3.3 methinks. Might be a problem here. Although, Z5 has a sensible voltage so are you sure about Z6?Across pins 7 and 14 of IC2 4.2VProvided by Z3.Across Pins 4 and 8 of IC1 24VAbout as expected. It's the sum of the + and - rails on Z1 and Z2.The NE5558 is an op amp. (Not to be confused with an NE555) Op-amps can typically handle up to 30v, so not a problem.As for the waveform on IC2/8, be aware that this whole circuit is on a floating ground, connected to pin 13 (I think, number is damaged) which is the series pass output. It's possible that no point in the DC circuit is grounded, so if your probe ground is on the case you are just picking up induced hum in a floating circuit.To check this waveform you need to use two probes set up to identical volts ranges and both on the same zero line, one on IC2 pin 7 and the other on pin 8. Then set the scope to subtract mode.Some care is needed with placement of scope grounds on such high current equipment. If you put the probe ground on the wrong place you could burn out the scope.Anyway, I would be more concerned with whether VT4 and VT5 are being turned hard on by an earlier fault, or if one of them is s/c. Disconnecting R21 or pulling IC2 would eliminate any earlier fault. If R27 is still very hot, you have your problem area. A voltmeter is all you need here to measure Vbe and Vce.Never overlook the possibility of more than one fault. However, you need to tackle things in stages. First, find out why R27 is overheating.As to how it works, IC1/1 is an error amp which compares the voltage across* the series pass unit with a reference, Z5. IC1/2 is a comparator which changes state when the voltage on C5 exceeds the output of IC1/1. C5 is charged via R13, providing a time dependent voltage. When the rectified AC goes through 0v, VT1 turns off, allowing VT2 to turn on and discharge C5, restarting the 'timer' consisting of C5 and IC1/2Thus, the voltage on C5 increases progressively through the half-cycle. When it exceeds the output of IC1/1 the comparator fires, activating IC2, which provides SCR firing pulses. By varying the comparator reference voltage, the error amp can exercise 'phase control' over the AC input to the transformer to keep the voltage across the series pass unit to a few volts, regardless of PSU output voltage.* This is why the circuit has a floating ground, because it measures a voltage (across the series pass unit) which is not referenced to ground.BTW I would check that pin 12 is not connected to some wrong place. Since it's directly on a TTL gate with no series resistor, if that got a substantial voltage on it, it would knock the hell out of that circuit. Might explain Z3 burning up.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 09, 2017, 02:14:45 pm
content: Wow, great post, great detail, I thank you!Well, some progress, but it brings more and different questions, fixing this thing up is becoming obsessional, but I need it and it's a learning curve, just so long as the more technically astute folk on here don't get too bored! I couldn't find any real issues with the board after doing all the tests suggested, so I put the SCR board in a working sister H series supply, a 60V @ 25 Amps one, and the supply seemed to work just fine! BUT, when I swapped in the control board from the troublesome supply the volts meter immediately went off scale, even with, I think, both coarse and fine volt knobs on zero. So it looks like the control board from the 50 Amp supply is really the one at fault.What I just cannot get my head around is why, on the 50 Amp machine, I see no waveform at all on one of the secondaries of T1. I may check what can be seen on the smaller working supply. Is there any way it is correct for only one secondary to show a waveform? What I will do next is look at the working supply. In the faulty one the wire wound R27 still gets mad hot, and the base of both VT4 and VT5 show around 0.7 V or more on a multimeter, but whether it's pulsing I don't know, maybe I should scope it? With more correct probing methods I DO see a saw tooth wave form on the collector of VT2, which I guess is something.On the main general schematic why does only SCR2 have a resistor and cap across it and not both? I assume it's a Zobel network? What if C25 was shorted? It's SCR2 that doesn't seem to get a signal to the gate, but there's still no apparent signal on either pins 6 or 7 of the SCR board output connectors even with the gate disconnected at the SCR2? I believe BOTH SCR's should fire, never just one?Thanks for all the help! Swapping boards is OK, but I have a real fear of ending up with two knackered PSU's by trying those tricks. The boards are not very good quality, so removing components or lifting legs usually results in a track coming off the substrate. The control board for the troublesome supply is already patched with fine insulated leads to replace some tracks that were blown when I got the thing, years ago, and it's getting worse with every de-soldering jobMy current thoughts 9forgive the pun...) are that something on the Control Board is not allowing the SCR board to "tell" it to start the SCR's conducting, so no volts to the primary of the mains transformer. just what could do that is beyond my capability to fathom out thoughThe Control Board schematic is athttp://www.gatesgarth.com/H60002.jpg

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 09, 2017, 09:58:10 pm
content: If VT4 and VT5 are showing 0.7v Vbe on a DMM that means they are being DC driven, hard on. Check of IC2 pin 8/ R21 top end. If this has near to 5v on it, IC2 is driving them hard on. If only 0.7v, likely VT4 is C-B shorted. This is your problem area that you need to concentrate on. There is definitely a fault here.When you say there is a waveform on one output of T1, what waveshape does this take, and amplitude ? If connected to the SCR you cannot measure the firing pulses (of just a few volts) because they are superimposed on top of mains AC.R99 and C25 are a snubber network, to prevent the SCRs being triggered by mains spikes. A Zobel network is to compensate for speaker inductance, nothing to do with this.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 10, 2017, 08:58:27 pm
content: Hi Ian, thanks for sticking with me!I took a risk and installed all 3 known good working boards from a sister supply. The faulty one started to work immediately! But with a strange issue. Even with both coarse and fine voltage control knobs at max it won't generate more than about 28 to 30V. At 20V and under the meter oscillates gently over a range of a few volts. Something I did when one of the pass transistors last failed was change them all, including the two similar driver ones, from the stock 2N3055's rated 60V @ 20A for a full set of MJ15003 ones which are the same package. these are rated 140V @ 20A. I was assured they would work as a drop in replacement, maybe I was told wrong? Could it be they need more "drive" than the supply was designed to produce or something?Back to the fault, by swapping back the original boards it appears the SCR board we have been analyzing is actually OK, and the R27 resistor stays mildly warm. But swapping back the previously troublesome Control Amplifiers Board brought the issue back and R27 heats up again and the situation is as before. What I did also notice is on the "Supplies Board" even whn the PS is running, albeit with reduced output voltage capability, is that VT10 and VT11 get pretty darned hot to the touch, and my el cheapo infra red remte thermometer shows about 43 degrees Celsius on the metal case. VT10 seems to run a bit hotter than VT11, measuring the output voltages shows a steady +15, 0V and -15V however.Before I start on the control amps board is there any chance changing from 2N3055's to the new MJ15003's could have caused an issue of my own making? Any thoughts on low V output? Thank you Ian!

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 10, 2017, 09:39:16 pm
content: Unlikely. The series pass transistors have a two-stage current amp before them, so even if they were slightly lower gain it shouldn't affect current output. The only consequence of low gain would be VT31 getting hot if it's inadequately rated or heatsinked.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 10, 2017, 10:38:29 pm
content: Quote from: Chris Wilson on August 10, 2017, 08:58:27 pmI've not followed the whole story, but here's a thought.Consider the possibility that changing the 2N3055s to MJ's has simply masked the true fault - which might be that the pre-regulator (SCRs and control board) is stuck in the "full on" state - thereby creating excessive power dissipation in the pass-transistors.I would un-couple the drive to the SCRs and concentrate on obtaining the correct drive waveform at the input of T1 - which should be some form of pulse; NOT a sinewave.

author: floobydust
author_rank: Super Contributor
timestamp: August 11, 2017, 12:00:36 am
content: Have you changed any of the electrolytic capacitors on these boards?I wondered if they have those crappy Philips blue axial ones, the rubber cracks and they dry out. Got any pictures.I would "shot gun it" which means blindly replace all the PC board caps, given we are looking at a 20-40 year old PSU.Or you can do component-level troubleshooting, it just takes a lot of time.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 10:19:30 am
content: Thanks for the further replies. I have changed 9a long while a go, a few years...) all electrrolytics although none tested bad out of circuit, and last night I isolated each huge rectifier cap and cecked them and all were fine.I have now tried putting 3 known good boards in and whist it now works the max V out is still about 35 to 38V. I have wracked my brains to think what else I unsoldered or fiddled with changing the pass transistors and suddenly I remembered a stud diode on a small heat sink that holds the two MFJ driver transistors. It is supposed to be in parallel with the main pass transistors, I will check I haven't screwed up with that, but can you tell me what its function is please?It is shown on the overall schematic as D23, but doesn't appear on the pass transistor, pass driver transistor schematic. it *IS* present in the PS though, but conveivably i may have wired it back up incorrectly somehow. I have to now assume that although functional with three different boards in it that the low output voltage is not related to the boards but something wrong in the main body of the supply. Needless to say the fact that re-inserting the original Control Amplifiers Board causes issues, so that will need addressing, but for now, with a full compliment of three known good boards, all seems well *EXCEPT* the low output voltage attainable. Thanks everyone yet again.Overall schematic;http://www.gatesgarth.com/H60001.jpgPass transistor block schematic:http://www.gatesgarth.com/H60005.jpg

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 10:54:46 am
content: QuoteWhat was the voltage on C30, C31, C32, C33 when you had only 35 to 38V max output voltage ?

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 11:06:28 am
content: Hi oldway, thanks for joining in! I have to pop into the workshop in a bit, and the PSU still has the good boards in it, so I'll grab the measurements, rather than go by my somewhat haphazard memory these days!

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 11:27:12 am
content: Quote from: Chris Wilson on August 11, 2017, 10:19:30 amWould it be time now to verify the transformer MT1 available VA?1) Isolate the series pass transistor board, jumper a wire across the SCR, then measure the voltage across C30, What voltage can you get? > 56Volts? [best with a DC load bank or a LOAD]2) Also take out the SCR and verify its performance.

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 11:34:33 am
content: I already repaired a lot of such power supplies with triac or dual scr's pre-regulator, mostly Sorensen, and they are quite easy to repair.I thought you could fix it quickly and that more help was unnecessary.I don't understand why do you have so much troubles with this one.....If voltage on the C30, 31, ... is too low even with known good working boards, look for a faulty diode D19, D20, D21 or D22 (interrupted diode)Disconnect the diodes and measure each one.

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 11:44:12 am
content: Quote from: Armadillo on August 11, 2017, 11:27:12 amSCR's are quite reliable....If they fails, they go generally short circuit and you have a high voltage on C30.Sometimes, old scr's can loose gate sensitivity, but then, only one of the two SCR's is triggering, and primary voltage is asymetrical, the transformer saturate and there is a very high primary current....the fuse burns immediately.The problem has nothing to do with scr's.....

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 12:21:10 pm
content: I have had a hour with it, and this is what i have found. D23 is wired correctly and tests out of circuit fine. All main rectifier diodes are connected correctly and test fine out of circuit.When I wind the V knobs fully up and see 25V DC on the PSU output terminals I see a steady 35DC on the smoothing caps. But if I look at the output with the V knobs wound down to say 12V out the ouput pi=ulses and I can see pulses with an analogue AVO meter from the secondary of the main transformer. There's no pusing on the other supply, nor was there on this. The pulses are circa two a second.On the highest output voltage it will muster (about 28V) I measure about 8.6V across the pass transistors. The secondary of the mains transformer shows about 6.8V ACon a good digi meter and about 2.5 to 2.8V pulsing on the AVO AC but I suspect that it's not a good measurement as it's being switched?Am I safe just shorting the SCR's across leaving the gates connected to see if I see full output?? Thanks.

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 12:36:01 pm
content: Quote from: Chris Wilson on August 11, 2017, 12:21:10 pmYes but unless the SCR is defective,so since you are going to verify the SCRs out of circuit, take it out and place a jumper to be rest double sure.Edit: I would just jumper it if I am repairing the equipment.

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 01:09:08 pm
content: QuoteThat's a very important information: there is enough input voltage to increase output voltage, but it does not....That's mean that the problem is NOT coming from the pre-regulator but from the pass transistor control.....

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 01:26:00 pm
content: Armadillo, thanks, I have checked MT2 outputs and they appear fine, as do the outputs from the "Supp;ies Board" after rectification.oldway: Thanks again, what is odd is that the Control Amplifiers Board (schematic linked below) works fine in a sister supply, so I assume i have either done something stupid connecting the new pass transistors or there's a fault external to the board elsewhere? If I get a 9V battery and say a 1k pot, or using a different small variable bench supply, could I see if I can externally control the pass transistor block by applying a variable voltage to (when disconnected from the Control Amplifiers Board) pins 6 and 10 which go to the pass transistors driver pair?

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 01:36:11 pm
content: QuoteThat's mean that this board (control of the pass transistors) is also NOT the cause of the problem....Look for the connections ...(bad contact, loose or wrong connection)On schematics, it show that there are different connections possible for the voltage and current adjust.....there is probably something wrong with this...

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 02:39:55 pm
content: Quote from: Chris Wilson on August 11, 2017, 01:26:00 pmThe transformer I wrote is Transformer MT1, not MT2.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 03:16:00 pm
content: Armadillo: Oops, sorry, I can't find any issues with it, it's never been got hot, and never been got wet, but I will bypass the SCR's and isolate the pass transistors and check it can create the goods, thanks

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 03:25:32 pm
content: Quote from: oldway on August 11, 2017, 01:36:11 pmThe selection is made by eyelet ended jumper wires, they have NEVER been swapped since I got the supply and from when it has worked fine, and I have (several times) checked continuity between whichever letter terminals they are supposed to bridge in what the manual calls "normal operation". But I'll check again, I am obviously missing *SOMETHING*! There is a diode shown near them, but I think it's irrelevant in "normal operation" but will double check. Any idea what D23 is actually for across the pass transistors? It seems fine but I am curious as to what it does. Thanks.I am seeing what to me are odd voltages measuring between the base of the first driver transistor for the pass block, and its collector. Please clarify where I should measure voltages at max and min volts for:A: The first and second driver transistors (which I have changed from 2N3055 to MJ15003)B: The pass transistor block itself where bases, collectors and emitters are all effectively strapped together.I want to be sure I am measuring voltages getting to there correctly. Thanks!

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 03:34:54 pm
content: Quote from: Chris Wilson on August 11, 2017, 03:16:00 pm10,000 is to 1, you never know. But worth a try.If the transformer is able to supply 64Vrms [*loaded], then Do check the SCR also then check firing at wrong angles etc.

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 03:51:41 pm
content: Quote from: Chris Wilson on August 11, 2017, 03:25:32 pmCan you picture your installation and soldering of the series pass transistors board particularly the mica insulation and spacers etc... More eyes easy to spot for you.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 04:46:51 pm
content: Mechanically it's a big job gaining access to the pass transistors, but I have some photos of the tunnel heat sink when I had it apart ages ago. The only thing I can think of that might be suspect in the soldering / mounting is if the white heat sink compound has got on the two pins and bridged them, but i don't think it's conductive is it? I am hoping not to have to go in there again, but of course will if needs must. The bases, emitters and collectors are all strapped together with hefty brass strips, and if I put my Peak DCA55 semi conductor tester on the three strips it says its a transistor and all looks well. I think if I have any doubts it might be the two external driver transistors, I need to triple check they are wired up 100% soundly and correctly. I will post voltages I see on the bases later, when I am sure if I need to measure base to emitter or base to collector. I am thinking base to collector.I think it's just because it's a huge brute of a thing, but 6 times out of ten when I turn it on it trips the mains RCD, and sometimes it will when I turn it off, but a little bell says the SCR's are on the mains input side.... The smaller (25 Amp instead of 50 Amp @ 60V) sister supply never does that though.... Thanks Armadillo.

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 05:08:13 pm
content: Quote from: Armadillo on August 11, 2017, 03:34:54 pmOops! Sorry Chris, I missed your first post where you have swop the T1 transformer. Thanks for your patience.

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 05:17:32 pm
content: The first thing we would succeed to do is to let this power supply work correctly with the 3 known good working boards ....Later, we will worry with the failure of the other boards.One thing we don't know is if these boards are exactly the same or not and if the components have the same values....so long I know, those boards had a lot of modifications, that's possible that output limit to 28V would be only a problem of values of components on the control board of the pass transistors.D23 is only a protection of pass transistors against reverse polarities as it could happen if you connect a battery to the power supply switched off.I would like to know if below 28V, the power supply is working properly with the boards known as good.Does the current limitation work?Is the voltage well regulated?Does the voltage drop on the pass transistors vary greatly when the voltage is set at 5, 10, 15V and with different loads?In short, is the 28V max output voltage the only fault?

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 06:01:35 pm
content: OK I took the time to study the circuit.I agree with Ian, VT4 and VT5 should be checked.You need a digital logic probe or a oscilloscope.Please confirm as per the diagram attached.You need to answer "Is there pulses, is it alive?".Vbe voltage is not enough to answer the question. thanks.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 06:29:36 pm
content: Quote from: Armadillo on August 11, 2017, 06:01:35 pmWith the three known good boards and it set to it's max of 25 to 28V out, no load, I took these scop shots using a X10 probe. One is pin 7 of IC2 another is on the collector of VT4, and finally on the collector of VT5. All used pin 13 of the SCR board as the scope ground.Let me know if more scoping is needed, thanks you all

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 06:42:30 pm
content: So there should be pulsing across the T1 at pin 6, 7 and 9 and 10.Edit: Additional scope points attached. If you adjust P2, the duty cycle increase and the rectified voltage should follow to increase.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 06:53:23 pm
content: Quote from: oldway on August 11, 2017, 05:17:32 pmThe control amplifier board that seems to be the one that is faulty is the same part number as the one I replaced it with from the smaller 25 Amp supply. The schematic shows no alternative component values. Similarly the "Supplies Board" is identical. The SCR control board is borrowed from another 50 Amp supply and I cannot see any different component values on it, but the SCR and the Control Amplifiers boards do have some preset adjustment pots. The one on the Control Amplifiers board that should raise the max voltage level is already nearly on full, but that board in the 25 A supply it's from allows over 60V. I agree this is a bit hit and miss. The other thing is if I turn the supply on with the 3 good boards in it, and the volts on max, there's a pause of three or more seconds where a very low voltage shows on the V meter, then it suddenly jumps up to its max of about 25V.I have just checked the supply with two off 1 Ohm 100W metal clad resistors in series as the load. Things are *NOT* normal. This is what occurs:V and A knobs fully counter clockwise, no V or A show on the meters. I advance the V coarse know clockwise, leaving the A knobs at zero. No volts show. Leaving the V knob or knobs advanced a little, I rotate the coarse A knob clockwise. Current shows and so does V on the meters, however current follows the V knob, the A knob has no effect. If I rotate the A knob fully counter clockwise the supply should not output and V, but it still does and the current showing on the meter does not reduce.Volts across the terminals 6 and 10 on the Control Amp Board change with the V control knob in so far as they reduce as V is increased. At the max attainable of about 25V I see about 6.5V, and as I reduce the V knob back to zero the V across 6 and 10 rises to about 40V and then decays, fairly fast at first, but slowing unti after about 30 seconds or so I see a resting V across pins 6 and 10 of about 17.2V That seems very odd... I am hoping it means more to you than IThank you!

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 07:11:42 pm
content: Quote from: Chris Wilson on August 11, 2017, 06:53:23 pmThe duty cycle determine the firing of the SCRs and hence the rectified voltage hence the duty cycle is important. If there is no P2 pot on the SCR board, then it is not the same board and those bias setting will be different also.it would be better to put back all the original boards and troubleshoot from there then to mixed and match with wrong boards causing confusion.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 07:16:43 pm
content: Armadillo:Scope shots attached, Pin5 IC1 is the same point as you wanted to see if the saw tooth waveform is.Note I change V scaling between captures.The files are too big for the forum to accept as all attachments so I have zipped them up and put them on my server ashttp://www.gatesgarth.com/armadillo.zipI think the file names should be descriptive enough, thanks. I will now adjust the control and see what hapens

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 07:28:38 pm
content: Quote from: Armadillo on August 11, 2017, 06:42:30 pmAdjusting P2 appears to make no difference to the rectified voltage at any given V out level.

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 07:40:30 pm
content: Check VT2, your sawtooth swing is very weak. And if your reference point is Pin 13, why does it go negative?I cant see very clearly but how many volts is the swing. Is this the original board.?Measure voltage of C4 positive with respect to Pin 13 and C4 negative with respect to Pin 13. (Z1 and Z2 voltages)Edit; The voltage at the ramp comparator is important, can you probe again using DC coupling.Edit: Take out C5 [0.47uF], measure ESR. [Ramp Generator]

author: oldway
author_rank: Super Contributor
timestamp: August 11, 2017, 07:47:55 pm
content: Quote from: Armadillo on August 11, 2017, 07:11:42 pmOk, I agree, boards seems not to be compatibles from one power supply to another.From the start:I am getting little or no voltage on the secondary or the primary of the mains transformer,that's indeed a problem of the scr's control board.Let's remember how it works: it control only the voltage drop on pass transistors....It must stay between 5 and 12V ...So, if the voltage drop on the pass transistors is withing this values, the pre-regulator is working fine.Let's now check the pulses at the same test points you have measured on the good working scr's control board.P2 is an ajust of the max firing angle of the scr's.P1 is an ajust of the voltage drop on the pass transistors.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 08:33:30 pm
content: Quote from: Armadillo on August 11, 2017, 07:40:30 pmHi again, you are being very kind spending so much time with this! C4 + to pin 13 SCR board shows 15.76V and C4 - to pin 13 SCR board shows 30VI hopefully attach the revised scope shot in DC coupling mode and 5V per division scaling. The scaling automatically compensates for the X10 probe settingI lifted a leg of C5 and with my PeakESR70 ESR meter it shows "open circuit or low capacitance" I lifted the leg of the same capacitor on my borrowed SCR board from the same 50A supply and that one measures o.56uF with zero Ohms ESRWhat type of cap can I replace it with, I can see what I have, and what voltage rating please? I have 470nF poly film ones rated at 250V DC?

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 08:49:10 pm
content: Quote from: Chris Wilson on August 11, 2017, 08:33:30 pmFirstly C4- and pin 13 why you get -30V... it should be 10V? Check Z2.I checked with the resistance in series to the capacitor. it will charge only at 5.4v at 100Hz. So the ramp is actually correct in your DC coupled diagram.The circuit will not care whether its positive cycle or negative cycle, it will pulse both SCR together. We need another scope as belowMake sure above voltage is correct then;Scope 1: Pin 6 and Pin 7 Channel A, Trigger at Channel A, Pin 5 IC1 with respect to Pin 13 Channel B.Next;Scope 2: Pin 8 and Pin 9 Channel A, Trigger at Channel A, Pin 5 IC1 with respect to Pin 13 Channel B.We need to know when the SCRs are triggered after zero crossing.Edit: Adjust P2, and you should see the trigger point follow [the wave shifted]Edit Measure across Z2. what is the voltage?

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 08:56:06 pm
content: C5 fell apart when messing with it, so I have fitted a 470nF 250V DC working polyester film cap. It has made little difference, nothing measurable, to the saw tooth wave form. I returned to looking at the secondaries of T1 on the original SCR board I just renewed C5 on. I am sure things are not correct here. With my probe set X10 and a vertical scope scaling of 5V per division I see the attached. But when I probe pin 8, on the other secondary, even a 50V per division scaling isn't enough. I had to add an automotive attenuator to the input socket of the USB scope, a device I use for looking at injector waveforms and coil on plug waveforms on engines. With X10 probe and 20:1 attenuator, and a vertical scaling of 10V per division I get the other waveform hopefully attached. I suspect this latter one is NOT correct and may point to a faulty SCR??

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 09:06:53 pm
content: So you found it.But you better probe in pair like 6 as common and 7, and then 8 and 9 as common. since there are isolated.You need to probe properly using correct common before can make the call.So the Z2 voltage is 10 or 30?

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 11, 2017, 09:37:58 pm
content: C5 (0.47uF) charges through 47k from 15v. The initial charge current will be 0.3mA (300uA). 1uA will charge 1uF at 1v/s, so so the cap will charge at a rate of approx 600v/s. In 10ms (a half cycle) it will charge by 6v. The actual value would be a bit less because of diminishing current as the cap charges, and you'd need integral calculus to get a precise answer. (Too late at night for that sort of thing!) This is good enough for our purpose anyway. Thus the sawtooth should be about 5v amplitude as a ballpark.The problem with measuring the SCR firing pulses is that you can't measure a pulse of a volt or two superimposed on AC ripple from the off-load transformer primary, which will be ~700v p-p and too much even for a x10 probe on a 5v/div range.* If you want to check the pulse transformer T1 outputs, disconnect the SCRs and use a 33R (or thereabouts) test load on each pulse trans secondary.(* Similarly I keep telling the climate change afficonados that youcan't possiblymeasure a fraction of a degree change superimposed on 50C day/night and summer/winter changes, in which case the 'ripple' isn't even a sinewave but highly random. A similar measurement problem to this, and I'd call the IPCC's bluff over the notion that they could do so accurately.)

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 10:09:45 pm
content: Quote from: Armadillo on August 11, 2017, 09:06:53 pmAcross Z1 I see 15.4V and across Z2 I see 10.0VCould I ask you how EXACTLY to probe the T1 secondaries properly to be sure it's something up with an SCR please? I did resistance tests of them both, gate to cathode and saw both were about 35 ohms, if I can get the darned things out (another nightmare job by the looks of it...) is there any definitive test on the bench? Peak make an SCR tester, but I don't have one, so would need to rig something up. Thanks again everyone. I wonder if this could explain it so often tripping the mains panel RCB when either turned on or turned off?

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 11, 2017, 10:20:33 pm
content: SCRs are easy enough to test. The gate-to-cathode resistance (of a high power type) is a few tens of ohms as measured on a DMM, and it appears like a resistor rather than a diode. Anode to cathode should be O/C both ways.As a test circuit, use a 12 DC anode supply with a lamp in series as a current limiter. Applying 50mA or so to the gate should fire it. (say 220R from a 12v supply) After which it will conduct until the power is cut. If it passes this test it is unlikely to be faulty.

author: Armadillo
author_rank: Super Contributor
timestamp: August 11, 2017, 10:24:50 pm
content: Quote from: Chris Wilson on August 11, 2017, 10:09:45 pmTo be sure, you need to isolate the T1 from the SCRs [take out the wires 6,7,8,9)Then install temporary resistor across the T1 as shown in the attached. Some value say 100 ohms will do. Then probe across the windings as shown in attached.Edit: Since there are the resistors there already, you can short 6 to 7. and short 8 to 9. then measure across the existing resistor. R28 and R29.Either T1 is defective or the SCR or the MT1.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 11, 2017, 10:41:19 pm
content: I have to go to bed now as i have an early start with work, but I will try and test as you suggest before I go out, as the suspense will be painful wondering whether we have finally found it. The schematic says the SCR's are a seemingly obsolete CR30 /503A and I have found a pdf showing their spec. Is there a bolt in modern equivalent should I need to replace one (if one is gone I will probably replace both as getting to them looks painful). Everyone has been incerdibly helpful with this, I hope I am near the end of fixing it. Could an issue with one of the SCR's have caused a sudden uncontrolled voltage that sometimes took out a pass transistor or two? Can they have intermittent issues? A very good night and many many thanks everyone!! Fingers crossed.

author: oldway
author_rank: Super Contributor
timestamp: August 12, 2017, 06:49:59 am
content: Why do you care so much about the scr's?They are good, there is no problem with them, it is proven since with the borrowed card, they work (28V output voltage)In addition, the cathode gate resistance of 35R is correct.No need to probe the secondary of the pulse transformer.Probe collector of VT4 with common at pin13 instead.You did it with borrowed board, we will compare the results.NB: if one scr is "gone" and the other start conducting, you will immediately blow the fuse.I never seen a scr to fail open ! they fail short circuit or loose gate sensitivity.And what about R27 ?R27 of the borrowed board was getting also too hot ?Is it still too hot now?It became too hot because you don't have right pulses or VT5 is faulty...Not to loose much time, I would replace IC2 (SN7413), VT4 (BC182L) and VT5 (2N4037)I would use a BC337 for VT4 and a BD140 for VT5 but beware, they are not pin to pin compatible !

author: Armadillo
author_rank: Super Contributor
timestamp: August 12, 2017, 08:59:59 am
content: Quote from: Chris Wilson on August 11, 2017, 10:41:19 pmNeed to isolate the wires 6,7,8,9 from the Gates. [Take out the wires]Need to ensure the T1 can supply at least rectified pulses > Vgt @ Igt.Do check Diode D7 and D8 at the secondary.Do note the value of the Resistor R28 and R29 for calculation works.if T1 proved OK then then ;You demonstrate P2 working before you jumper the SCR exercise below because aside from P2 there is the P1 pot as well. P2 is responsible to set the 60V capability of the output as per the manual. You need to demonstrate you are able to shift the firing point first by; [Edit: Just Scope 1 enough, since you already proofed T1 is OK above].Quote from: Armadillo on August 11, 2017, 08:49:10 pmIf above proofed OK then;Quote from: Armadillo on August 11, 2017, 11:27:12 amPlease use DC coupled, the voltages are important.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 12, 2017, 09:11:03 am
content: Renewed VT4 and VT5 with same part numbers BC182L and 2N4037, no difference, no more than 25V out or so.When output pulses at less than max V the scope waveforms widen and narrow in tune.To be clear, both original board and known good board do the same. original board in sister machine works fine, same as its own SCR board.Do you still think the SCR's are OK?Thanks oldway.

author: oldway
author_rank: Super Contributor
timestamp: August 12, 2017, 10:06:03 am
content: Quote from: Chris Wilson on August 12, 2017, 09:11:03 amI am thinking because it became very mysterious, almost impossible.....1)To be clear, both original board and known good board do the same. original board in sister machine works fine, same as its own SCR board., it should mean that there is no problem with the original SCR's control board.2) the borrowed scr's control board work in the faulty power supply (but I think you had replaced all boards, not only the scr's control board) even you had only 28V output voltage.....If you had an output voltage, it proove the scr's are triggering and that they are good.3) the original scr's control board in the faulty power supply does not work, no output voltage.That's right ?The only logical explanation would be that the faulty board is not the scr's control circuit board, but the auxiliary power supply board.The fact that there are trigger pulses does not imply that there is an output voltage: everything depends on the phase angle of this pulse train.It would be necessary to be able to compare the phase of these pulses with the syncronization reference 50 or 60 hz.I always work with a 2 channel oscilloscope, chopp mode, syncro on line and time base slow enough.This makes possible to see the phase angle.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 12, 2017, 12:06:28 pm
content: Both SCR boards when tried in the good 25 Amp supply work and that supply works correctly. Both Supllies Boards work in the 25 Amp machine. Only the 25 Amp Control Amps board works in either machine. the one from the 50 Amp machine does odd things like give uncontrolled V out. I put all the correct boards in the 25 Amp supply and scoping the pins to the gates and cathodes of the SCR's doesn't show the huge voltage needing a scope input 20:1 attenuator to see them. I am not sure if my USB scope does chop mode, and I am getting a bit out of my comfort zone with such scope work, but I will look into that. I'm getting a bit bewildered by what i am seeing and I can only hope the info I am providing isn't false in some way, but I'm trying to do things logically and as asked, but I am a novice and working on the SCR's is a bit fraught as I don't want to blow the scope upThanks oldway and everyone!

author: oldway
author_rank: Super Contributor
timestamp: August 12, 2017, 04:59:05 pm
content: Back to the first post....everything as it was then....originals boards, check all the connections and wires and switch the power on......What happen ? Output voltage ?Voltage on C30 ?

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2017, 08:59:59 pm
content: I'm not suggesting to try anything as others are helping out and this is too much funSomething about troubleshooting and repair is that it's best to be organised and not bounce around too much.If you have another unit and swap boards, it can get very confusing and not make sense: if there is more than one problem or the problem is not with a board but rather the chassis (SCRs, fuse, potentiometers etc.)I wonder if this thing has a couple quirks:With no SCR (oscillator) pulses, if the SCR's still get triggered once/cycle due to the zero-cross one edge, per the scope traces inPost #39.If the SCR oscillator is off (command from IC1), I think VT4/VT5 might just be on steady and getting toasty.If one SCR is shorted or open A-K then growling from transformer because of 1/2 wave rectified DC and I expect very weak output from the transformer at C30 and probably MT1 mains input fuse blowing (uh where is it?).Just some thoughts here.

author: oldway
author_rank: Super Contributor
timestamp: August 13, 2017, 06:06:44 am
content: The problem of this repair is that Chris Wilson has indicated from the first post that the culprit of the problem is the pre-regulator , but, in my opinion, it's wrong.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 14, 2017, 12:31:09 pm
content: OK, I have gone back to the original boards except the conttol amps one which isn't working at all, so it has the one from the 25A supply in it. VT4 and VT5 on the SCR board tested fine but as they were out i renewed them. Z1 (the 15V one) gets mad hot and shows mV across it, where should i look for a short? i have also disconnected the SCR's to bypass any issue there. thanks, please don't think i am disagreeing with anyone's diagnosis, i am not knowledgeable enough for tricks like that1

author: oldway
author_rank: Super Contributor
timestamp: August 14, 2017, 02:25:57 pm
content: QuoteZ1 can't get "mad hot" if there are only mV across it: nonsense. You should measure about 15V across it.Check your wiring.Please be more precise when you measure something: There are only mV, that's not a technical language....you must specify how much mV ....10, 100, 600 mV ?The scr's MUST be (correctly) connected to check the pre-regulator.With 0V output , you must have about 8V on C30.This voltage can be ajusted with P1 if everything is ok with the pre-regulator.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 14, 2017, 02:35:38 pm
content: OK, I am going back to basics and looking at one thing at a time.With just the original Auxiliary Supplies Board fitted. Pins 17 and 18 in relation to 0V pin 15 all from centre tapped transformer MT2 show 40VAcross Z4 I see 5.32V. There's a chance the cases of VT9 and VT10 were touching, so I have made sure they are well spaced now, but they still feel pretty warm, as they did before. 15V output pins 2 and 8 in relation to 0V out pin 5 show 16V each. Any issues here? This is with JUST this one board fitted.As an aside the thing drives me mad as nearly every time it's turned on or off by its main power switch the 30mA RCD in the consumer unit trips. Am I right in saying old PSU's with input filter caps across live and neutral, and their centre to ground can trip RCD's or is there a basic fault here as well? the other supply has never done this, AFAIK? Thanks, I'll try and be more precise, apologies.Thanks.

author: floobydust
author_rank: Super Contributor
timestamp: August 14, 2017, 04:55:33 pm
content: Making a few measurements with it running, well you need readings from the in, the out and the middle to diagnose. Seems you have +/-16VDC output from the Aux. board so unless it's weak, it seems OK.Aux. Supply board VT9, VT10, VT11, Z4 are a constant-current source for the regulator diodes Z5, Z6 to make the +/-15VDC rails for the control board. I estimate 73mA so 1.4W of heat is normal there. VT9, VT10 will get quite warm. If they (case) touched it's OK the collectors are tied together.It can be easier to pull a board and test semiconductors using multimeter diode-test and ohms. Perhaps you'd like to try that.The RCD tripping, there is an input EMI filter with Y-capacitors C21, C22 (H60001) that could be causing the nuisance tripping on power up. I'm not sure of their capacitance.If there is leakage current from transformers MT1, T1, T2 it would aggravate this. I would ignore it until things are running.

author: oldway
author_rank: Super Contributor
timestamp: August 15, 2017, 02:50:13 pm
content: QuoteWhy ? The first thing to do is to check if pre-regulator is working correctly.You must have more or less 8V on C30 with 0V output voltage and SCR control board fitted and both scr's connected.With P1, you must been able to ajust this voltage.QuoteTry to disconnect C21 et C22.We will look for problems with auxiliary power supplies and control later, when we are sure that the pre-regulator is OK.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 17, 2017, 10:51:01 am
content: Hi oldway, thanks for the above. Yesterday I fitted ONLY the original SCR board, disconnected the +ve output of the bridge rectifier, and put a 60W 240V housebulb across the bridge output. I then used a separate bench supply to inject 0 to 10V DC across pins 5 and 13. It wouldn't work. I fitted the SCR board from the sister 25 Amp supply and with some hysteresis the change between about 7.5V and 9V would turn the bulb on and off. I see about 63V across the bridge output with bulb on, and about 125mV with it off, and to be clear, the smoothing caps are NOT across the bridge as I wasn't sure what volts that might see, and they are only rated to 75V.On the original, faulty SCR board I measured V across Z1 and it's 910mV (0.91V). I suspect something's shorted on the original board. VT5 or C8 perhaps? I already changed VT5 and the old one tested OK.

author: floobydust
author_rank: Super Contributor
timestamp: August 17, 2017, 05:34:11 pm
content: Chris, this sounds like a better (smaller) portion to test.The SCR control board has say 100 nodes and you measure one and there is no way we have psychic powers to deduce a fault that way.We need more measurements otherwise this is a waste of time.The alternative.... are you comfortable taking the board out and multimeter-testing parts to find the short/open?

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 17, 2017, 08:18:52 pm
content: Perfectly happy to test components, but I am reluctant to un-solder pins from the PCB as, for whatever reason, the tracks are very liable to lift, even using as little temp as is consistent with melting the solder. Thanks.

author: floobydust
author_rank: Super Contributor
timestamp: August 17, 2017, 10:01:57 pm
content: Simply take some readings with your multimeter on diode-test, both ways (polarity) around semi's and also ohms across resistors etc. Leave the parts soldered on the PCB.You are looking for trouble, so to speak. Sometimes adjacent parts mess up the readings, but just look for an obvious shorted component or open resistor.You can combine both troubleshooting methods- make a bunch of voltage measurements, then multimeter test suspected components.Or the other way around, multimeter measurements, then powered-up voltage measurements.Either way can lead to the failed part(s). Also possible to compare measurements with the working card.I think Z1 having low voltage 0.91V instead of 15V imbalances the circuit, and something is moving the voltage on terminal 13 (artifical ground) up.I would ohmmeter the power resistors R12, R27 and zeners Z1 Z2. I think IC1 would cook if it was shorted.What I do is quickly check each diode and transistor with diode-test, that usually reveals the trouble. Takes 5 minutes.Are the IC's in sockets?

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 18, 2017, 08:58:12 am
content: Thanks floobydust, I have had a poke about on the board with the meter on diode and resistance test and compared it with the working board, but failed so far to see any major discrepancies, but I will go over it again. Yes, the 2 IC's are socketed, removing both makes no odds to the V across Z1. Thanks, will report back

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 22, 2017, 01:51:36 pm
content: Now working!! Found a whisker across a track which was shorting the Zener and pulling down the 15V supply. Fitted a new Zener as a precaution. Now had control of the SCR's and assumed the SCR board was now good. Fitted Supplies Board and Control Amplifiers Board. Output V immediately went off the scale. Luckily the new uprated pass transistors survived, the old 2N3055's probably wouldn't have. Investigated the Control Amps board and finally found that the pass transistor driver VT2 (a 2n3253) was shorted, maybe because of the pass transistor blow up previously. I replaced it and now the supply seems to work with all original boards in it.I am left with how to decide what the various set up pre set pots do, as the manual I have is for an earlier version with all discrete components, and the machine has IC's in some parts. I have schematics for both types, but the only manual I have and can find is for the early type boards machine.On the Supplies board I know the pots are just volts and amps meter trim pots, and the set up pot for the over temp thermistor circuit.I now just need to translate from the correct schematics and the incorrect (for an earlier, all discrete component supply version) manual, what the pots on the Control Amps board do.So far I have, on the Control Amps board:P1 sets max V output of the supply.P2 sets max Amps output for the supply.P3 ?P4?P5?P6?From the manual the older board had pots to also alter:Short circuit protection set upMains voltage input variationSeparation voltage stability across the pass transistorsand other stuff. More pots than the 2 on the SCR board, more on the Control Amps board. The final thing is to try and translate waht the pots do and how to set them on these later boards... Can you tell from the schematic for the Control Amps board what any other than P1 and P2 might do please?I have further annotated the Control Amps schematic to hopefully make it easier to follow. See attached please. Can't believe I finally have it working again after nearly giving up on it! Thank you.Control Amps board schematic for the board in the PSU athttp://www.gatesgarth.com/H60002.jpgSchematics for earlier boards, to which the manual refers are athttp://www.gatesgarth.com/farnell_h60_50_early_schematics.pdfManual for earlier PSU with all discrete board components is athttp://www.gatesgarth.com/farnell_h60_50_0-60v_0-50a_power_supply_early_manual.pdfThanks to everyone for getting me to this excellent stage!!

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2017, 07:41:13 pm
content: OMG this weighs 79kg/174 lbs! Taking a wild guess at the trimpots after looking at the schematics:SCR Control Circuit Board:P1- Pre-regulator output voltage ('separation" voltage); sets pre-regulator to output above setpoint (~8.5V on old unit).P2- Zero-cross phase delay; sets dead-time from zero cross for SCR drive pulses.Aux. Supplies Board:P6- Overheat trip temp. 100°CP5 - Ammeter calP12 - Voltmeter calControl Board:P1 - Max. Vout Adjust; set with front panel Vout set to max.P2 - Max. Iout Adjust; set with front panel Iout set to max. (shorted load, full output current; risky)P3 - Vout offset adjust; set IC1 pin 6 for zero volts? with front panel Vout set to min. ?P4 - Iout offset adjust; set IC2 pin 6 for zero volts? with front panel Iout set to min. ?P5 - Short circuit/SOA protection 20V kneeP6 - Short circuit/SOA protection 60V knee at <1A

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 24, 2017, 08:04:25 pm
content: Thanks, and I think you are pretty much spot on as yesterday I fib=nally sourced the correct manual for this (heavy, you're quite right) beast. Thanks for the efforts in converting from old manual and old schematics to schematics for this unit.Final issues to overcome, and hopefully this excerpt from the manual will help, are:Control Amps board P4 will only set a slightly negative output voltage, about minus 4mV, it won't adjust to a small positive value.The manual doesn't cover setting of P2 on the SCR board and I have no idea how this should be approached. So nearly there, so much invaluable help from here, I am *very* appreciative everyone

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2017, 12:23:58 am
content: Chris, I'm not sure about the procedure to set P2 on the SCR control board.On the old (discrete) model, P2 is a different circuit and the doc's say it's to set the "minimum mark limit" of the variable mark/space ratio.It's a mains-synchonous PWM and that (P2) sets the min. PW, I think so the pre-regulator idles nicely with no load. But not too much PW or it would overvoltage the pass-transistors and trip the protection circuit.P4 offset adj. I would say is fine near zero volts (4mV); perhaps the trimpot is there to match P3 so the control circuit so (IC1 V, IC2 A) has the same offset.