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Thurlby PL320 power supply

 
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Thurlby PL320 power supply

Post by Cathovisor » Wed Jul 15, 2015 6:14 pm

(Split away from viewtopic.php?f=13&t=11573)

When I get to the bottom of why it's playing silly sausages I'll post a thread on it. Mine's got a dead segment in the current meter on the side that's playing up as well.

I can live with that...

 
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Re: Thurlby PL320 power supply

Post by Mark Hennessy » Wed Jul 15, 2015 6:18 pm

They are pretty reliable. I did re-cap mine as part of an overhaul, but it didn't seem to make any odds. I've had more trouble with my newer one, in fairness.

The old ones can suffer from minor drift - probably caused by those expensive wire-wound pots getting intermittent with age. When I serviced mine, I found that the fixing screws on the TO3 power transistors were a bit loose. Otherwise, they're basically OK. The rats nest isn't fun, but you do get used to it.

Mine is a QMD (quad mode dual), and I did have a problem with a track on the small PCB that does the switching between isolated, parallel, series and tracking. A short section had vaporised - presumably the supply had been hit externally. But other than that, no real problems to report.

The ADD3701 chips are hard to find (them being discontinued is what forced the redesign), so if the PSU fails for some other reason, someone out there will be happy to buy them from you.

 
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Re: Thurlby PL320 power supply

Post by Cathovisor » Wed Jul 15, 2015 6:50 pm

Mark, yours is the same as mine - a PL320QMD. I did once have a drift problem with one back at my former employer and it was the current limit pot causing it, of all things!

Mine had a non-functioning fine voltage control (again, in the dead section) but a new 1k pot sorted that. I can see what I'll be doing on Sunday; anything to avoid the real job of tidying up at home... :cch

 
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Re: Thurlby PL320 power supply

Post by FIXITNOW2003 » Thu Jul 16, 2015 7:51 am

I repaired one at the last place I worked the diode across the output terminals went s/c living it in current limiting mode

 
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Re: Thurlby PL320 power supply

Post by Cathovisor » Fri Jul 17, 2015 10:40 am

FIXITNOW2003 wrote:I repaired one at the last place I worked the diode across the output terminals went s/c living it in current limiting mode

It's not that - apart from having checked the diodes, if that diode was s/c turning the output switch off would remove the short. I am beginning to think that it's the current limit pot: as it was detached from its wiring when I got the unit (it had been rotating and had torn the wires off), it may well have suffered damage and playing with another unit I have access to suggests the fault I'm seeing is a combination of a dirty output switch (see also the Farnell LT30 I've just renovated) and the set current limit potential being at zero volts because the pot is faulty.

New pots on order...

 
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Re: Thurlby PL320 power supply

Post by Cathovisor » Sat Jul 25, 2015 1:56 pm

Well, I've got to the bottom of this one and it's now in fine fettle :bba

The problem was with the master PSU (the right-hand one): more often than not it would sit flashing the decimal points in the current meter at me no matter what the position of the output switch was, and as a result you could not get an output from it. I should explain that on these PSUs the flashing decimal points indicate that the unit is in current limit mode - and in this case, limiting the current to 0A!!

The cause was the front panel 'current limit set' pot.
PL320 current limit.PNG
Block diagram of current limit circuit.

If we look at the circuit, VR1 is the aforementioned current limit set pot, and R8 is the current sense resistor. A voltage generated by Z3 and then divided down by the components VR1¦¦R21, R22 and PR3 is compared with the voltage across the 0.5Ω resistor R8 by the comparator IC2 (an LM358). When the voltage across R8 exceeds that at the wiper of VR1, the comparator changes state and puts the PSU into limit. Note that the output of IC2 also controls the decimal points.

So what had happened with mine?

Well, when I got it I did notice that VR1 turned endlessly - because the nut on the control had worked loose and the pot had torn itself away from the wires soldered to it. I refitted it and although I initially though it had survived, when in the fault condition there was 0V at the wiper of the pot in any position dmm - the top end of the track was now intermittently o/c.

Fitting a new pot - which incidentally, is a logarithmic one - fixed the problem.

Another fault was the decimal points not switching properly between the output 'off' and 'on' states - as ever, this was due to switch oxidation. Working some DeOxit down past the toggle followed by a drop of Electrolube sorted that.

The final job was to check the metering, and on the master PSU the voltage readings were quite wrong. This turned out to be a calibration error, but I did need to change a resistor: R28 in the attached diagram. It was 15kΩ, but is now 10kΩ - in the manual it's an AOT component.
PL320 metering.PNG
ADD3701 adjustment
PL320 metering.PNG (9.68 KiB) Viewed 1686 times


So there we have it - a very useful addition to my workshop.

 
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Re: Thurlby PL320 power supply

Post by Mark Hennessy » Sat Jul 25, 2015 2:40 pm

Excellent - well done :aap

They are nice units, despite their age. Good, solid design, with low noise and ripple, and no fans. Not the easiest to work on, but pretty reliable overall, so that's probably fair enough. Apart from the voltage pot and the ADD3701 chips, all parts can be replaced today. And it's good that TTi try to help with these old beasts where they can.

 
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Re: Thurlby PL320 power supply

Post by Cathovisor » Sat Jul 25, 2015 3:26 pm

I have a lot of time for TTi. I bought my first 'scope from them thirty years ago :ccf when they were agents for Hitachi - and I still have that 'scope today (a V-222). I also have had one of their hand-held PFM-1300 frequency meters which, with the march of time and the improvement of techniques, has now become the PFM-3000.

Also one of their 'scope storage adaptors...


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