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Old 1st Apr 2018, 12:04 am   #21
trobbins
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Default Re: Thorn Automation 40v 50A PSU fault

Have you also checked for resistance from gate to cathode on the scr's - the cathode may be electrically connected to the heatsink, depending on device. Also worth checking that resistance from back within the pre-reg board to confirm the drive signal will make it through to the scrs.

Do you have -15V from the D22 zener regulated rail ?
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Old 1st Apr 2018, 11:50 am   #22
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Default Re: Thorn Automation 40v 50A PSU fault

Turning the voltage pot what is the range of voltage at B1 if the pre-reg board?

What is the voltage at P?
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Old 1st Apr 2018, 1:12 pm   #23
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Default Re: Thorn Automation 40v 50A PSU fault

Hi See Mos,

Voltage at P varies between -0.5v and around 8v (I had thought that perhaps the fault protector circuit was holding the thyristors off.) By disconnecting the P connection, I can see that this voltage is being generated on the Pre Reg board.

B1 is interesting - this voltage stays positive and steady around 2.6v.

I have tried disconnecting the B1 line, and injecting a small -ve voltage onto the connection to the series reg board, no more than about 0.5v, and this then wakes the pre regulator up, and allows the pre regulator supply to be varied between 0 and 50+V
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Old 1st Apr 2018, 2:14 pm   #24
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Default Re: Thorn Automation 40v 50A PSU fault

I worked in the test department for Thorn Automation for 12 years until 1984, having done my apprenticeship there. I vaguely remember testing a few 'fifty-forties' as they were known. Unfortunately my present knowledge level is around zero. What I can say is that they were built for tough, reliable industrial and military use and many, many were sold worldwide. Thorn Automation morphed through several name changes and ownerships and the same company recently moved to a new site barely half a mile from the original one in Rugeley. They are now Ultra Electronics, Rugeley. If you contact them you may be able to speak to a knowledgeable test or repair engineer and/or request some service data. I know for a fact that there are older test/repair engineers still there who will have worked on 50-40s. Ask for Steve Bill or Gavin Share. Good luck!
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Old 1st Apr 2018, 4:09 pm   #25
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Default Re: Thorn Automation 40v 50A PSU fault

Thanks Steve, I have dropped them an email, hopefully they may have a manual, that is more complete than the one I have
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Old 1st Apr 2018, 9:51 pm   #26
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Default Re: Thorn Automation 40v 50A PSU fault

Ok, so a little more progress today.

The thyristors are firing, and I can see the pulse length change when I increase, or reduce the required voltage, when the power supply is under load.

However, a more accurate description is that the supply is not making full voltage - under a 10A load, the maximum voltage output is only around 22v - the series regulator is dropping no voltage at all (probably due to the pre reg not supplying enough DC voltage)

There are firing pulses on both Thyristor gates, I have also checked that the other two diodes in the bridge are functional - they both test OK.

I have scoped the AC waveform of the output of the transformer - it looks like half wave rectified DC, with a wonky section that looks to be the time where the Thyristor is firing.

Allowing for the voltage drop of the series regulator stage, it would seem to me that I am only getting 50% of the output voltage needed to satisfy the DC side of the PSU.

Any suggestions where I should be going from here?
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Old 1st Apr 2018, 11:01 pm   #27
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Default Re: Thorn Automation 40v 50A PSU fault

the output of the controlled bridge should be 100hz symmetric chopped ripple. each thyristor half should be contributing the same shape ripple pulse as the other, with negligible 50hz (if you had a spectrum analyser on that bridge output). your description doesn't quite say that.
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Old 2nd Apr 2018, 6:55 am   #28
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Default Re: Thorn Automation 40v 50A PSU fault

I am beginning to wonder if one of the two thyristors is faulty - this would explain the low voltage I am seeing I think.

I will take the rectifier module out and separate the common point then look to see if I am getting an output from both devices.

When I substituted two diodes for the Thyristors, I had 73V DC as expected from a bridge rectifier, so the problem must be in that area.
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Old 2nd Apr 2018, 8:10 am   #29
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Default Re: Thorn Automation 40v 50A PSU fault

Thorn Automation used thyristors in lots of their equipments (mainly DC motor control drives) and our field service engineers always went out with plenty of spares to any job. So to an extent, they were prone to failure. But often they were worked hard.
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Old 2nd Apr 2018, 8:40 am   #30
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Default Re: Thorn Automation 40v 50A PSU fault

I think I can rule the thyristors out now - I have disconnected the gate drive from each of them in turn, to see if the operating conditions change - I get the same result with either thyristor connected, so I guess I am back into the drive circuit again, trying to understand why they are not being driven harder into conduction.

I will wait now to see if I can get any more information - there really cannot be much wrong with it now - everything is working to a point, just not pushing hard enough.
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Old 2nd Apr 2018, 8:44 am   #31
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Default Re: Thorn Automation 40v 50A PSU fault

I've not read all of the posts, but is a current limit circuit coming in early?
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Old 2nd Apr 2018, 8:54 am   #32
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Default Re: Thorn Automation 40v 50A PSU fault

I still think it is well worth while using a scope to confirm the waveform across C4-8 is 100Hz symmetric pulsed ripple - that would be the first test of any service tech IMHO.

It also sounds like you haven't replaced C4-8? They must be long in the tooth by now unless they have had a swap-out at some time. They are likely to be high ripple current rated parts with low ESR, and can cook if not properly selected and then checked for RMS current with a tong meter.
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Old 2nd Apr 2018, 8:59 am   #33
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Default Re: Thorn Automation 40v 50A PSU fault

You've tested the ordinary rectifiers, but at what current?

I'm wondering if a rectifier diode or an SCR is faulty, and you only get half-wave operation at high current.

Can the transformer still source the full current? There may be a bad connection or filter component that starts to have an effect ar full current... Bad mains impedance at a socket?

Just things that need checking for completeness before digging into more complex stuff

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Old 2nd Apr 2018, 9:13 am   #34
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Default Re: Thorn Automation 40v 50A PSU fault

I admit the smoothing caps are certainly getting on a bit - some of them were replaced in the mid 1980s from the date codes - I will pop the scope across them when I have a load attached.

I have subbed the two SCRs for normal stud diodes - this gives me an off load voltage of around 72V.

I have a rotary converter here that draws 10A at no load, so have been using that to provide a load - with my two diodes in place of the SCR all is well (other than the series regulator dropping about 40v on load and getting quite warm - at 400+watts dissipation it should be!)

With the two diodes fitted in place of the SCR I can also get a full range of voltage adjustment, from 0v to over 40v o/p

Mains connection wise, the power supply has a dedicated feed from the distribution board in my shack - the only connector is on the power supply - even on load the mains is stable at it's usual 242v. The transformer output voltage is a rock steady 52v no matter what the loading is.
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Old 2nd Apr 2018, 10:01 am   #35
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Default Re: Thorn Automation 40v 50A PSU fault

I have checked the ripple voltage across the 5x 15000uf caps - this is measured at 1.5v (100hz) with a 10A load. The on load voltage was 21v - the supply can manage no more.

The ripple across the output capacitor (1x15000uf) was measured to be 500mv
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Old 2nd Apr 2018, 10:45 am   #36
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Default Re: Thorn Automation 40v 50A PSU fault

A diode bridge with 75mF and 7 ohm loading (10A) would give about 1.3Vpp ripple on 68Vdc using PSUD2 with 54Vac secondary. Are you saying you have 21Vdc and 1.5V (?pk-pk? or rms?) on the 75mF cap?
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Old 2nd Apr 2018, 11:50 am   #37
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Default Re: Thorn Automation 40v 50A PSU fault

Yes, this is what I am finding
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Old 2nd Apr 2018, 12:26 pm   #38
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Default Re: Thorn Automation 40v 50A PSU fault

Quite a lot of activity on here while I have been busy.

It sounds as if the supply is operating in current limit mode. Try replacing the load with a 60 or 100 watt filament lamp.

The voltage at B1 should vary and keep the voltage into the series regulator about ten volts higher than the required output. According to the simulation B1 goes more negative to give a higher output.

Terminal P is interesting, it could be a current limiter. I still haven't read the technical info.

I put the important bits of the pre-regulator circuit into Labcenter Proteus simulator and these are the results the real thing may produce slightly different results but you get the idea.

Yellow trace 50Hz at junction of D8, D9
Blue trace ramp at junction of R16, R17
Pink trace junction of RV2, R20
Blue trace emitter of TR10
Terminal P minus 15 volts disables the SCR drive, zero or positive enables SCR drive
The left picture is towards minimum output when the voltage at TR11 base = 5.5v, SCR gates disconnected
The middle picture is towards maximum output when the voltage at TR11 base = 4.2v, SCR gates disconnected
The third is minimum output with the SCR gates connected. The output is now a 4.5 volt pulse followed by a 1 volt high where the SCR’s are conducting.
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Old 2nd Apr 2018, 1:10 pm   #39
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Default Re: Thorn Automation 40v 50A PSU fault

Terminal P, after extensive probing and testing forms part of the over voltage protection. When the overvoltage comparator has an output, it triggers the or gate (a BC107) and then switches the fault protector circuit on - this then does two things, 1, it sends a heavily negative voltage to the pre reg card, and 2, the thyristor on the fault protector is triggered, this then energises the shunt coil on the input breaker, tripping this breaker and disconnecting the mains.

The schematics are confusing in many ways due to there being no overall complete diagram, this results in lots of sheet hopping - the dual rail nature of the regulator cards also confuses my poor brain - some of this means that I do not always measure the voltages with respect to to the correct 0v rail....

I will hook the scope up again and post pictures of my findings.
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Old 2nd Apr 2018, 1:44 pm   #40
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Default Re: Thorn Automation 40v 50A PSU fault

Operating conditions:

15V op
11r load resistor on output terminals
Mode of operation is constant voltage.

Measurements:

P -1.4v wrt 0v line on pre reg board
B1 -9.31v wrt 0v line on pre reg board

Pics in order of your list above
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Last edited by Sean Williams; 2nd Apr 2018 at 1:51 pm.
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