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Old 15th Jun 2021, 7:02 pm   #1
GrimJosef
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Default Kingshill 500X1 500V 1A supply - traced circuits and fault query

A few weeks ago I asked in this thread https://www.vintage-radio.net/forum/...d.php?t=180270 whether anyone had any info on the Kingshill 500X1 power supply. I have one of these and it's misbehaving, as described in the other thread, when any significant output current is called for. Sadly no-one was able to help me, and my query to Telford Electronics didn't bear fruit either. So there was nothing for it but to trace the circuit out ...

The attached pictures show what I think it is, but please, if you spot any 'that can't possibly be right' errors, do let me know. (I haven't bothered with the primary side of the mains transformer as it's more-or-less trivial.) I think I understand roughly how it works and I was able to use it to locate the fault. But I'm not really sure how the fault might have arisen, so I'd be grateful for any thoughts you might have before I start replacing blown components. You might save me from blowing them again !

The first picture (the Layout) shows the three 0-167V supplies stacked on top of one another. It looks better if you rotate it right by 90 degrees. Once you've done that, on the extreme left in each case is the relevant PCB full of low-power electronics which handle the voltage sensing and control. For my own benefit I've called the supplies Lower, Middle and Upper. Each PCB has numbered contact pins which are connected, via the wiring loom, to the higher power components in the Layout diagram. The active ones in each supply are a single 2N5240 acting as a driver for 18 paralleled 2N5240s which make up the pass element.

The Lower and Middle regulation PCBs (second pic) are identical, but the Upper one (third pic) is more complex. It turns out that it, and only it, works with the voltage and current controls to determine the Upper supply's overall output voltage. The Middle supply is then voltage-slaved, via circuitry fed by a flying lead from the Upper one. And the Lower supply, in turn, is slaved by a second flying lead from the Middle one.

As well as the voltage-slaving, the Middle and Lower supplies are also controlled by a simple current-limit circuit which is designed to prevent them passing much more than the rated maximum of 1A under fault conditions. It turns out that this circuit is what's switching the Middle supply off when the output current rises above a few tens of milliamps (the Upper supply appears to work normally, and the Lower one only misbehaves because it's slaved to the Middle one).

The signal which indicates over-current, and causes the supply to shut down, is delivered via PCB pins 7 and 9. It's Vbe for the 18 pass transistors plus whatever is dropped across their 22R current-balancing resistors. It turns out that in the Middle supply those resistors now measure very high in value. I haven't been able to reach every single one - they're somewhat buried, alongside the transistors and between the fairly closely-spaced heatsink plates - but I've measured at least half and the best I've found so far reads over 200R with the worst up close to a megohm. I suspect all 18 have failed, so a large over-current signal is generated almost as soon as the supply tries to go beyond whatever current its drive transistor can supply.

I wonder what can have caused this ? The 22Rs in the Upper and Lower supplies (again I've checked quite a few) measure spot-on. They're all a bit tiddly, but if 18 of them are sharing 1A then each should only be dissipating 70mW. When the supply is asked for very low current it can deliver the full 500V, and this is evenly shared between Lower, Middle and Upper. So I can't see how some component can be breaking down at high voltage and repeatedly stressing the 22Rs. Could it be something in the drive circuitry cooking them ? I'd be grateful for any ideas about the cause (perhaps they are simply too deeply buried and too close to the hot transistors ?) before I set about replacing them all. And should I do the ones in the Lower and Upper supplies while I'm about it ?

When I start looking at a bust piece of hard-working kit I always assume that a capacitor will have failed somewhere. Probably an electrolytic. It'll be a turn-up if it's 18 resistors.

Cheers,

GJ
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Last edited by GrimJosef; 15th Jun 2021 at 7:09 pm.
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Old 16th Jun 2021, 5:57 pm   #2
woodchips
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Default Re: Kingshill 500X1 500V 1A supply - traced circuits and fault query

I have several of the Roband Varex PSU's that allow stacking to gain higher voltage, but also that the current limit will work over the stacked supplies.

Wanting to use them to power valves during testing the current limit seemed a good idea, so I tried it and it did work. The lowest limit of the supplies set the total limit.

Whilst not of obvious use, possibly the service manual might reveal how this magic is done.
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Old 16th Jun 2021, 6:52 pm   #3
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Default Re: Kingshill 500X1 500V 1A supply - traced circuits and fault query

Quote:
Originally Posted by woodchips View Post
The lowest limit of the supplies set the total limit.
From which I understand that the PSU which had the lowest current limit determined the current limit of the series cascaded PSUs.
Now I'm not at all familiar with these units, but the following, based on fundamental principles, might explain that behaviour.

Suppose you need 150 v.d.c. for some application. If each PSU can produce 50 v.d.c. (with a margin for a bit more) and you wire them in series, you will obtain the required 150 v. The load current flowing through the 3 series-connected PSUs will be the same for each PSU. Let's say that current is 300 mA. If the 3 PSUs are electrically identical and each, therefore, has the same current-sensing circuitry with same current-limiting designed in, then each PSU current-limiter needs to be set at exactly the same current threshold of, say, 350 mA. If they haven't been set up like that - so the current thresholds are different and one is less than 300 mA - the maximum output current available will be that of the lowest current limit.
If the current-limiting control has a calibrated user control, then the solution is obvious. Otherwise it will be necessary, by trial-and-error, to determine that current threshold for each PSU separately (using a dummy load) and to adjust it accordingly - in the case above, for 350 mA - or something sensible - above 300 mA. Then wire the 3 PSUs in series for the required 150 v. @ 300 mA (required), with just under 350 mA as max. current.

Al.
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Old 16th Jun 2021, 10:49 pm   #4
GrimJosef
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Default Re: Kingshill 500X1 500V 1A supply - traced circuits and fault query

Quote:
Originally Posted by woodchips View Post
I have several of the Roband Varex PSU's that allow stacking to gain higher voltage, but also that the current limit will work over the stacked supplies ... Whilst not of obvious use, possibly the service manual might reveal how this magic is done.
I don't have the manual for my supply, but there are manuals for other Kingshill supplies available online and in one of those they explain that the current limit works by adjusting the output voltage. So if I were to set the voltage to 300V and the maximum current to 100mA and then connect a 2kohm load the control circuitry would reduce the output voltage to 200V, limiting the current to 100mA despite what I'd set the voltage knob to.

In fact in my 3-stacked-supplies case the automatic voltage-slaving system should lock the Middle and Lower supply voltages to whatever the Upper one is. So the current limit only needs to act directly on the Upper supply.

The fault (cooked resistors) that I had didn't actually limit the supply's output via the normal current limit circuit. Instead it acted through a different current limiter - one designed to keep the current in the Middle and Lower supplies below 1A. There's no user control for this feature and I wasn't even aware of its existence until I traced out the circuit.

In other news I've discovered a mistake in my Layout circuit (the first picture in my earlier post). When I drew it I couldn't see the point of the extra three 8V secondary windings on the mains transformer, which I've shown wired in series with the two 80V ones in the Upper supply. There seems to be no good reason for that supply having a different 'raw' HT voltage from the other two. I've since measured the raw HT voltages and discovered that, even more strangely, one of the 8V windings is connected in phase with the two 80V ones, but the other two are connected out of phase. So the total AC voltage is 80V+80V-8V+8V-8V = 152V ! It seems to me that two of the 8V windings just contribute extra ohmic resistance to the total, thereby heating the transformer more than is necessary. But maybe I'm missing something ...

Anyway, the corrected figure is attached.

I should also say that I have managed to replace 9 of the 18 damaged resistors with in-spec ones and the supply is now operating normally, at least as far as I have tested it, which was to 480V and 250mA (two 60W incandescent lamps in series).

Cheers,

GJ
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Old 17th Jun 2021, 8:37 am   #5
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Default Re: Kingshill 500X1 500V 1A supply - traced circuits and fault query

Could the extra HT on the "top" supply be a tweak to give that supply a bit more voltage compliance for ripple rejection at the expense of slightly(?) higher series pass dissipation?

I'm following this thread with a somewhat proprietorial interest!
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Old 17th Jun 2021, 3:17 pm   #6
GrimJosef
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Default Re: Kingshill 500X1 500V 1A supply - traced circuits and fault query

One anomaly I should perhaps have mentioned in the first post relates to the circuit of the Middle and Lower regulation PCBs - the second attached picture there. Just to the right of centre is a 1N823 voltage reference fed by the BCY70 which is acting as a constant current source. But this doesn't seem to be doing anything, other than draining current from the upper rail. On the Upper regulation PCB (third picture) the voltage reference connects into the voltage control op amp. But in both the Middle and the Lower PCBs the copper track making this connection has been cut by hand, leaving the reference isolated. Instead the voltage control comes via the flying lead from the PCB above.

Cheers,

GJ
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