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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 3rd Jul 2022, 2:12 pm   #1
The General
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Default Advance bench PSU revived from the dead

I've split this story into two parts so it hopefully doesn't become a borefest...
This Advance bench PSU came from Glyn (Welsh Anorak) in North Wales. I thought it might be worth a go. I hadn't worked on any tranny kit for a while, it looked like a challenge & it would make a good backup bench supply. Glyn told me it had been owned by a 'dabbler' so I expected to see missing parts, actually there weren't but it did need a fair bit of forensic investigation as we will see.
Cosmetically it was very dirty & scruffy but removing the side panels & some work with a paintbrush & airline cleaned it up pretty well. Mechanically it's very well made, all the panels unbolt from the chassis, the PCB is easily removable for servicing & the chassis mounted components are laid out in a logical order.

First thing we see is that there are lots of extra capacitors tacked in. All over the place. Someone's apparently been having trouble with unwanted oscillation.
Number two is that there appear to be two separate sets of series pass devices, both using just the chassis as a heatsink. The minimal heatsinking probably indicates dual switching & linear regulation like my big Roband bench supply.
Three, the 30V panel meter is clearly a larger replacement as it is covering some of the front panel lettering.
Four, there is a switch to select a maximum output of 30V or 60V. the transformer - which looks original - supplies a secondary voltage of 37V to a bridge rectifier so there's no way it can provide a 60V output. Some of the electrolytics are only rated at 40V anyway & they also look original.
And finally #5, there's a PCB edge connector wired into the harness with nowhere to connect it & nowhere to mount an extra board.

I can't find a schematic for the model number on the front panel but I'm beginning to suspect that panel isn't original anyway. There are some unconnected terminal pins on the PCB & looking at the harness routing it became clear that the wires connected to the edge connector should actually go to those instead.
So nothing for it but trace out the circuit & draw a schematic for it. It wasn't too hard to do & it gave me an opportunity to inspect the PCB as I did so, a good job too as some PCB tracks had random holes in the copper, almost as if they were markers for drilling component leg holes. Some of them were in the middle of thin tracks, almost cutting through them. One had actually broken a track causing what was probably the original fault which took the unit out of service.
All these areas were repaired by soldering tinned copper wire across them.
Part two follows...
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Old 3rd Jul 2022, 2:29 pm   #2
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Default Re: Advance bench PSU revived from the dead

As I suspected, the PCB contains two separate circuits, a switching regulator followed by a linear one. The unconnected terminal pins were on the switching regulator side of the PCB. There's also a current transformer associated with the switching regulator, ooh, need to get that wired in the right way round... Also, it was apparent that the voltage & current controls on the front panel were the wrong way round. They can't be swapped as the multiturn voltage pot won't physically fit in the other hole. The wires were neatly dressed with them as found too... That panel must be from another Advance PSU, dammit!

Now for the fun part, determining which harness wires connect to the unconnected PCB terminal pins. Fortunately, the wires had been left in their original length when they were soldered to the edge connector, so by checking their physical length & with a bit of educated guesswork, I worked out which wire goes where. All the tacked on additional capacitors were also removed.
Prudence dictated that this part of the circuit should be tested separately & so I connected up another current limited bench supply to the switching regulator & carefully applied power. Well it worked, so throwing caution to the wind it was time to apply mains power to the unit & test it fully. Again, it worked. Good stuff. With the original component values as found in the voltage & current regulation circuits the maximum output was about 35V on the '60V' range & 18V on the '30V' range. Max current limit was 3.6A on the 18V range & 2A on the 30V range. Checking the transformer core size indicates a rating of about 150VA which equates to 4A AC output & somewhat less DC so this is somewhere in the ballpark.

The bridge rectifier was only rated at 2A so I changed it for a 6A one. The meter was sticking, it was quite easy to dismantle & I found some small fragments sticking to the magnet & blocking the coil. There was enough access room to pull a tiny sliver of cloth through the gap which nicely removed the annoying bits. The voltage range was accurate enough but the current range was wrong so a tweak to a resistor value sorted it.
A good soak test with some wirewounds cooking away on my bench was the final bit of testing along with scoping out the circuit to make sure there was no spurious unpleasantness to ruin one's day.

Actual final bit was cleaning the outer panels with a damp soapy sponge & relabelling the front panel controls. I consider this a repair rather than a restoration so the white labels are OK even though they look a bit naff.
I think it turned out OK overall for something that looked initially like a parts unit....
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Old 3rd Jul 2022, 2:39 pm   #3
Refugee
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Default Re: Advance bench PSU revived from the dead

I can see two shiny new power transistors.
They oscillate like crazy in old power supplies.
You need to put some small capacitors in the negative feedback loop as is the case with more modern power supplies.
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Old 3rd Jul 2022, 2:52 pm   #4
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Default Re: Advance bench PSU revived from the dead

Thanks for the warning. Those shiny trannies were fitted to the unit as found. If they are replacements, they were done some time ago as the solder joints look 'old'. Those two are part of the switching regulator, I did scope everything out during testing & found nothing untoward but I am mindful of the possible issues that you state.
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Old 3rd Jul 2022, 3:12 pm   #5
duncanlowe
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Default Re: Advance bench PSU revived from the dead

I think you have figured what has happened. The innards of one unit have ended up in the box for another, which is why nothing matches what it says.
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Old 4th Jul 2022, 9:56 am   #6
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Default Re: Advance bench PSU revived from the dead

I'm delighted to see the old thing running again. I passed a Advance PSU on to wishiknewmore on the forum a unit in similar condition and I wonder if it had the correct front (see picture 1).

I suspect the original owner worked for Advance or similar as there are some bare boards and what appears to be a prototype board. Unfortunately nothing matches what you have.
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Old 4th Jul 2022, 11:50 am   #7
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Default Re: Advance bench PSU revived from the dead

That's a seriously impressive rescue. Most would have given up, I think.
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Old 5th Jul 2022, 8:47 pm   #8
The General
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Default Re: Advance bench PSU revived from the dead

Thanks for the kind remarks gents, I saw it as a challenge. I can post the schematic here if anyone's interested, if so, please give me a few days to draw it out a bit better, I don't want to post an embarrassingly scribbled job!
Mark
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