No trigger waveform to an SCR controlled transformer primary issue.

[Armadillo]

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?

Across Z1 I see 15.4V and across Z2 I see 10.0V

Could 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?

To 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.

[Chris Wilson]

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.

[oldway]

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 !

[Armadillo]

but I will try and test

Need 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].

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]

If above proofed OK then;

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.

Please use DC coupled, the voltages are important.

[Chris Wilson]

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.

[oldway]

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.

I 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.

[Chris Wilson]

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 up Thanks oldway and everyone!

[oldway]

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 ?

[floobydust]

I'm not suggesting to try anything as others are helping out and this is too much fun 

Something 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 in Post #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.

[oldway]

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.

[Chris Wilson]

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

[oldway]

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.

Z1 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.

[Chris Wilson]

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 40V

Across 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.

[floobydust]

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.

[oldway]

With just the original Auxiliary Supplies Board fitted.

Why ? 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.

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.

Try 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.

[Chris Wilson]

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.

[floobydust]

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?

[Chris Wilson]

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.

[floobydust]

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?

[Chris Wilson]

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

[Chris Wilson]

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 up

Mains voltage input variation

Separation voltage stability across the pass transistors

and 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 at http://www.gatesgarth.com/H60002.jpg


Schematics for earlier boards, to which the manual refers are at http://www.gatesgarth.com/farnell_h60_50_early_schematics.pdf

Manual for earlier PSU with all discrete board components is at http://www.gatesgarth.com/farnell_h60_50_0-60v_0-50a_power_supply_early_manual.pdf

Thanks to everyone for getting me to this excellent stage!!

[floobydust]

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°C
P5 - Ammeter cal
P12 - Voltmeter cal

Control 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 knee
P6 - Short circuit/SOA protection 60V knee at <1A

[Chris Wilson]

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

[floobydust]

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.