Thurlby 30V-2A Dual, output voltage difference

[madbaz82]

Hello everyone,

Recently, I have got my mitts on a power supply I want to give a new life to. I believe it is a quite early model of Thurlby 30V-2A Dual PL320 made for and sold by RS.
It looks like it still has some years of good service left in it, and it might be a great tool for someone who has just started gathering their first bits of the kit. But I am in a bit of a pickle here. The right channel delivers voltage with 1% accuracy between the dial and DVM. Not perfect, but I could live with that. However, the left channel is more than 10% off, and that is a bit too much for my OCD to handle.
As a last resort, I could be adjusting the voltage to DVM every single time I need to use the left one, but if there is a quick and simple way of rectifying that, I would greatly appreciate if I could get some advice on how to do the calibration.
Please, have a look at the photo with the PCB layout. I have a feeling, that reaching for the the top left corner would be my best bet, but I am afraid, if I start poking around and fiddle with wrong pots, I will just make things way worse.

Many Thanks in advance

[bd139]

It’s a Thurlby PL320QMD. Go and look for the service manual for it on line. Adjustment procedure is in that. Be aware there are multiple versions each identifiable by the metering chip. Yours is a fairly early but not too early one as it has the NS DVM ICs in it. Later ones they got swapped out for a PIC.

Also beware that your DMM might be wrong too.

They are excellent power supplies generally.

Edit: correct service manuals here: https://www.vintage-radio.net/forum/attachment.php?attachmentid=119903&d=1455135495

Read very carefully!

[Andy Watson]

Before tweaking anything make sure that the sense wires are connected and then check that the output switch is functioning correctly - i.e. that it is passing the sense voltage back from the terminals to the PCB (I think it's the positive terminal that is switched.)

[madbaz82]

Thank you so much! It worked like a charm!

I finally got few minutes to play with the pots today and managed to end up with accuracy <0.15%. I am very pleased with my first-time result.
But during the calibration, I have discovered another issue. The master won't go down below 200mV with both VR2 and VR3 set to minimum.
In theory, it is not a biggie, I've never needed less than 1.2V so far, and I still have the other channel, but... I am not going to lie, it does bother me.

Can I please have a bit more help with this?

[bd139]

Glad to see it's mostly working.

It would bother me too! Not had that one before. Step one is check the power supply's power supplies to make sure they are working. So check voltage at following places:

IC1 pin 3 -> should be 5.6V approx

Z1 anode -> should be around -5.6V approx

I imagine it'll have some offset issues if the -5.6V rail is duff.

[madbaz82]

Sorry for the delay, I don't get much time in the week.

on the voltage regulator I had only 5.03V
on the diode it showed -5.3V

Is that too far off?

[bd139]

Should be close enough. If you turn the output off and turn both coarse and fine knobs as far as possible anti clockwise, does it still read 200mV approx?

[madbaz82]

Yes, with output on or off it always shows 0.19V. Of course when I turn down the current limit to minimum, all goes down to 0, but as soon as I move VR1, all pops back up to 0.19V

[bd139]

Very interesting. What happens if you short the output?

[madbaz82]

Voltage drops to 0.01V, current jumps up to whatever limit I set and all the decimal points start flashing on the current display.

[bd139]

Ok so measurement is working fine then if it hits 0.01V 

Out of ideas myself. Not sure what that is then to be fair without having it in hand. 

[madbaz82]

Just for the closure.

I didn't put too much effort into it, so I kept all connected to the circuit. All I did was, compare the readings of VRs on the working side with the other pair.

Good VR3: Max. 863Ω - Min. 3Ω
Not-so-good VR3: Max. 1003Ω - Min. 903Ω

So, the fine 1k voltage regulator is shot. I will try to find another one and replace it.

Thank you so much for the service manual and all the help, I had no idea where to start.

[bd139]

Good find