XTS-60 dual sorensen regulated supply current issue

[coppercone2]

I fixed my XTS-60 Sorensen dual supply that broke and retrofitted it with some precision op amps while I was at it but I have a slight problem.

One section got fried, there was some kinda event that took out all the IC's, besides the display driver, a trace and one of the transistors, so I replaced all these carefully with the parts in the other side of the supply and it worked, however it has a discrepancy from the other side. So its still entirely usable as even a cosntant current source if I please, with a minimum current that is a little high. Of course it only has a 1 turn pot to control current so its not like you would be setting it with any fidelity, but it is a little bit of a bummer because I can't really protect signal op amps on a bread board or something simple like that which would benefit from a very low Ilimit less then 100mA.

When the left side is set to 0 current like a normal lab supply, it will shut down. However the right side does not. I thought it was still broken (did not try a load test), so I replaced every active component more complicated then a diode.. but then I decided to run it on a short circuit and it turned out it does limit current, but there is a offset, so it won't regulate CC down under something like 100mA. I tried like 3 different switch IC's so I don't think its an offset from the MUX on the Ilimit voltage from the potentiometer.

I made sure it was not the retrofitted parts, because I used the same components (LT1014 & LT1013) on the other side and it did not cause an offset error or mess with Ireg there. I did not change the switch because I am not sure if there is a good drop in replacement for that switch or if its even a good idea.. the op amps I think would at least maybe improve something, I just happened to have them in my supply and not the ones that it had originally (cheapos).

This thing is a bastard to trouble shoot because there is a giant heat sink on the side opposite of the PCB with the power transistors there. Does anyone have any familiarity with this supply that can give me a suspect? I tested all the diodes in circuit and they seem ok, so I assume some kind of voltage divider drifted. I checked the voltage of the Ilimit pot and it behaves just as the other one, infact if I switch displays, they behave the same. I am kind of scared of disturbing those power transistors, so I thought it would be best to try to fix it from one side only.. plus its alot of work to try to map it out and I am not sure if its worth it.

is the base not controlling the transistor properly? i will set the power supplies to the same voltage and measure the power darlington bases voltages at the lowest current setting to compare.

Repair #2
edit: forget about repair #2 there is something funny going on, don't even bother, it needs all capacitors replaced and all solder joints resoldered before any more debugging work. This one is driving me up a wall. I expect a explosion if I work on it further.  (single supply sorensen unit)

[coppercone2]

ok, I see the problem, I think,

I took it apart because I could not figure anything else out and there is two different transistors in there. It was a hack job repair. Maybe I did it and forgot about it or it was purchased like that.

I ordered some thermal grease and new transistors and pads

[coppercone2]

I fixed one supply completely, the dual supply is just waiting for new capacitors. I compared the side with replaced capacitors to the one that was working at full load, the side without replaced components bit the dust and it has 80V on the rail now after running for 45 seconds at full load.

There is a extreme amount of noise made by the capacitors and a very small amount of noise made by the transformer in my case.

[macboy]

I have the service manual with schematics if that would help. Might be too big to post but PM me.

[coppercone2]

Lol I fixed it with the new capacitors, the wire at the transformer broke during reassembly. I also broke the switch but it was like arcing alot even with deoxit so I just routed a toggle switch to the front.

I managed to get in there with a knife and yank a little bit out so I can solder a wire to it.

Anyway save for a little transformer humm at full 1A load, the 2x60V supply is functional. I found a destroyed/missing 4.7nF capacitor, I replaced it with a disk capacitor (I think these are traditionally high voltage?, its unlabled.. but oh yea I had it in my junk box for 500 years so its great to find a use for a disk, the little axial ones I have are lower voltage and SMD are evil etc).

Next maybe a 500MHz 4 channel scope (this baby might work but it needs some service), a 3310B HP, a different HP oscillator or the Sorensen 33Vx33A power supply