Big Farnell PSU SCR control circuit problem

[Chris Wilson]

I have just realised C12 and D10 are not present or catered for on the (older??) 50A supply SCR board, but are there on the (later) 25A supply SCR board! But to answer your question directly Dave, it can't be C12, it ain't there Thanks of course


See_Mos: Yes, I realised this difference as it first I thought one was in wrong. the 25A supply boards use BC182P and the 50A supply BC182L versions and the pin outs ARE different, thanks for the warning.

[Chris Wilson]

I thought it might be useful to show photos of the two SCR boards, I have had a more detailed look and it *SEEMS* as if the circuits are identical EXCEPT fro the addition of C12 and D10 on the 25A supply one, and a quick look at a photo of a borrowed board from another person's 50A supply which I think is much newer than mine, shows D10 and C12 are present there, too. What might C12 and D10 be for?

[dave cox]

D10 will stop VT2's be junction from getting reverse biased. This is a bad thing if it 'zeners' (at about -6v) when permanent damage will likely occur. C12 will stop any noise that could cause the controller to ‘lock out’ for fault conditions (dc fuse blows?).

Might be worth checking if SCR1 is conducting. How much current is flowing in R16 ?

dc

[See_Mos]

C12 is probaly a time delay for the overvoltage circuit, D10 prevents VT6 base becoming reverse biased too far when terminal 15 goes too far negative of treminal 13. If the overvoltage crowbar works correctly D10 isn't essential.

R32 holds the base of VT6 high so the collector is low, the SCR is off and the supply is available for IC2. When the voltage across C30 rises to a critical point the negative voltage at terminal 15 via R31 pulls the base low, the collector goes high, SCR1 turns on and kills the supply to IC2 which stops the main SCR's from working. The circuit is reset by turning off the power and waiting a short while.

If SCR1 was leaky or turned on it would kill or reduce the supply to IC2. If IC2 has no supply there will be no output at pin 8 to turn on VT4. An intermediate situation is theoretically possible but as the voltage across Z1 is so low I don't think this is the case.

D10 could indeed be leaky but that would also turn onVT6 and SCR1 which again would kill the supply to IC2.

I think VT4 or VT5 are the culprits.

[Chris Wilson]

Originally Posted by dave cox

D10 will stop VT2's be junction from getting reverse biased. This is a bad thing if it 'zeners' (at about -6v) when permanent damage will likely occur. C12 will stop any noise that could cause the controller to ‘lock out’ for fault conditions (dc fuse blows?).

Might be worth checking if SCR1 is conducting. How much current is flowing in R16 ?

Thanks for the further replies Dave. I measure 200mV across R16, so across 470 Ohms that's only 425uA I think?


Edit, with the working 25A machine SCR board fitted I see over 9V across R16 and across Z3 I see 4.9V

Across Z3 on the none working original board I see only 0.8V

[See_Mos]

Is Z1 fitted the correct way?

I just had another look at the pictures. The layout looks identical apart from the differences between components to suit the different unit except I see Z1 is the opposite way around on the non working board. Although it is fitted according to the silk screen it would not be the first time that I have seen such an error on a PCB. A quick look at the track side will confirm if it is correct or not.

Chris, I'll send you a PM

[Chris Wilson]

I think the silk screen is correct, I have a photo of that board before I changed that diode, but the stripe is far from clear. Hopefully attached is a photo of the track side of two boards from the supply, both original. The one on the LEFT is the SCR board in question. Thanks See_Mos

[dave cox]

They have polarising slots in different positions ?

[See_Mos]

That's because the board on the right is the secondary side control card and it looks to have a couple of burned out tracks.

The one on the left looks correct with the cathode of Z1 to positive of C3 & C4

[See_Mos]

oooops! I made a mistake. No volts across Z1 cannot be caused by VT5. I just noticed that VT5 collector and the primary of T1 return to the negative line via R27, not to the anode of Z1. If VT5 was short then R27 would get hot but this would not affect Z1, in fact I see anything obvious on the SCR card that could affect the voltage on on Z1 but not Z2. The only possibilites left are Z1 is faulty, the wrong way round, which would explain the 0.85 volts, or a short between PCB tracks somewhere.

[Chris Wilson]

I spent an hour checking all resistor values and found some are different between the early and later boards. I will document them later, but in checking I found R22 was low in value (about 750 Ohms). This brought my attention to the area in detail and under a good light source I found the base of VT4 shorted to the 15 positive (top) rail! Just a whisker, but definitely a short. I fixed that but R27 is still getting very hot, maybe VT4 is now dead? I will replace it and check the original out of circuit. I think progress is being made! Thanks. I'll respond to your PM in a bit,, got an errand to do, cheers See_Mos

[Chris Wilson]

I now have 14.6V across Z1! But it still won't light the test bulb across the output of the bridge. P1 measured just 98 Ohms at its set position, which sounds awfully low, I noted its physical position and twiddled it 5 turns forward and back, and then back to where it was, and it now shows 880 Ohms, I suspect it's suspect, can you say what a grossly out setting would do? I am searching for a suitable multi turn pot in my "stuff" to see if I can just replace it. I feel as if progress is being made, thanks, many many thanks BTW I think R27 getting hot is maybe a red herring, on the other board, when working, it too gets pretty hot. I see in the later board, for which the schematic values apply, they show 150 ohms for R27, and the earlier original board has 220 Ohms here. Similarly R12 has 330 Ohms in the early board, whilst the schematic for the later ones shows has this at 220 Ohms. Finally R28 and R29 which are 39 Ohms on my early board are replaced by jumper links on the later boards.

[Chris Wilson]

I found tapping / flexing the SCR PCB would make the thing fire the SCR's, sometimes it would blow the fuse, and just once it lit the test bulb. Eventually I fond the (new) Zener Z3 was iffy, and not always giving measurable DC, instead just noise with the multimeter jumping about as digital ones do. I replaced it and now the test bulb lights and is controlled by volts from the separate bench supply! But in comparison with the SCR board from the 25 Amp supply, as the voltage is increased and decreased, the bulb just doesn't light or go out, there's a point on raising and lowering the voltage where the bulb's intensity rises either just before turning off, or just after turning on. As the other board doesn't do this i am not sure if it's a potential issue and what could be causing it. But at least some proper progress has been made, for which I am hugely grateful to all who have contributed!

[See_Mos]

When everything is up and running P1 sets the maximum voltage drop across the bank of output transistors in order to keep the heat dissipation down.

If you now remove the variable supply, move the lamp to the output terminals and connect everything back up the voltage across C30 should be a few volts higher than the the voltage at the output terminals and should change as the output voltage is varied.

[Chris Wilson]

Thanks See_Mos, did you get my PM?? I think there's also an issue with the original control amps board, I will test it later. On the SCR board what does P2 adjust please, can you tell? The manual I have is for a supply with all discrete component boards and the pots and adjustments are different, so some care is going to be needed.

[Chris Wilson]

OK, great progress. Wired the pass transistors back up and now have, using the control amps board from the 25A supply, an output and the V drop across the pass transistors is controllable by P1 on the SCR board It will only give 25V out, with about 34V into the pass transistors. But of course it's a board for a different supply with some different component values.

When I fit the correct board I get an immediate 90 odd V into the pass transistors, and off 60V meter scale out, even with both the coarse and fine V adjust front panel knobs on zero volts. This makes investigating the issue dangerous I think, even though I have uprated the pass and pass drivers to 140V devices.


Is there a way to test this Control Amplifiers board in a sensible way please?

Schematic for Control Amps board should be attached, with general schematic to save ferreting in the big thread. Thanks.

Better (bigger) images at http://www.gatesgarth.com/H60001.jpg

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

[See_Mos]

PM received OK.

It's not clear what the function of P1 is but as it adds a little bias to IC1B it probably sets a minimum firing angle / voltage across C30

If you get 90 volts across C30 there is still a fault with the SCR card, the overvoltage circuit should shut the thing down to protect C30...C33.

I can't see an easy way to test the control amplifier but as you have the luxury of a 30 volt unit I would do it in that one, alternatively on the 60 volt unit move one of the bridge rectifier connections to the centre tap on the transformer secondary until you have the control card working.

Start with the good board in the 30 volt unit, set the output voltage to 20 volts. With the light bulb as a load measure from the 0v rail at the junction of C1 & C2 to pin 6 of the op-amps then do the same with the faulty card to see if we can get any clues from there.

What about the copper track that appear to be blown out on the photo?

[Chris Wilson]

Hi See_Mos, even better new this AM, it's now working, although needs setting u. And it's working with all its original boards! The issue with uncontrolled V out when inserting the Control Amps board was that VT2 a 2N3053 was shorted. I had some spares so replaced it and the board was back to life. I can control the volts drop across the pass transistors with SCR board P1. We are not sure what P2 does.

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.

Bigger / clearer version of the Control Amps board schematic at http://www.gatesgarth.com/H60002.jpg

Schematics for earlier boards, to which the manual refers are at http://www.gatesgarth.com/farnell_h6...schematics.pdf

[See_Mos]

On the front panel, which pots are voltage and which are current?

On the control amplifier P3 and P4 are offset trim for the op-amps. I suspect that they are adjusted by dissabling the SCR drive by removing the SCR card and setting them for zero volts at the op-amp outputs relative to the junction of C1 and C2 but you can confirm this by checking against the good board, assuming that one is the later version.

Not sure about 5 and 6 yet.

[Chris Wilson]

This general schematic shows the front controls and I annotated it to show which are which, is that what you need? There's a slight chance somewhere local to me has a correct manual, I am driving over tomorrow when I have some work in the same area, so don't get too involved in case I have the bumf on it soon. But so far the right manual has eluded me! Thanks See_Mos