No trigger waveform to an SCR controlled transformer primary issue.

[Chris Wilson]

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 at

http://www.gatesgarth.com/H60003.jpg

and

http://www.gatesgarth.co/H60003.jpg

They seem too big to attach here, sorry, although I'll try making the attached ones smaller.

[floobydust]

Looking at the circuit which is fairly hairy, it must be a big beast 
It'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.

[IanMacdonald]

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.

[IanMacdonald]

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. 

[Chris Wilson]

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.

[Chris Wilson]

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.

[Chris Wilson]

I should have done voltage checks before now, but I see the following:

Across Z1 13.9V

Across Z2  9.9V

Across Z3  4.3V

Across Z5  5V

Across Z 6  0V

Across pins 7 and 14 of IC2   4.2V

Across 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....

[floobydust]

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 a RC4558 good 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.

[IanMacdonald]

Across Z1 13.9V

Across 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  4.3V

A little low, should be 5v. TTL min volts is 4.75. Probably won't matter though.

Across Z5  5V

Z5 would appear to be a reference. It's fed from a constant current source, VT3 and Z6.

Across Z 6  0V

Should 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   4.2V

Provided by Z3.

Across Pins 4 and 8 of IC1  24V   

About 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 IC1/2

Thus, 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.

[Chris Wilson]

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 job

My 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 though

The Control Board schematic is at http://www.gatesgarth.com/H60002.jpg

[IanMacdonald]

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.

[Chris Wilson]

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!

[IanMacdonald]

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. 

[Andy Watson]

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!

I'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.

[floobydust]

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.

[Chris Wilson]

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.jpg

Pass transistor block schematic: http://www.gatesgarth.com/H60005.jpg

[oldway]

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.

What was the voltage on C30, C31, C32, C33 when you had only 35 to 38V max output voltage ?

[Chris Wilson]

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!

[Armadillo]

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.

Would 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.

[oldway]

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.

[oldway]

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.

Would 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.

SCR'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.....

[Chris Wilson]

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.

[Armadillo]

Am I safe just shorting the SCR's across leaving the gates connected to see if I see full output?? Thanks.

Yes 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.

[oldway]

On the highest output voltage it will muster (about 28V)  I measure about 8.6V across the pass transistors.

 

That'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.....

[Chris Wilson]

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?