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PSU repair, found strange cap, need guidance


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I recently stopped by HSC (a local surplus place) and found a big old lab power supply that came from the Lockheed Martin labs in Sacramento. Standard 4-knob controls (coarse/fine on both voltage and current), analog meters. 0-40 VDC @ 0-10 amps! Only problem was, it didn't work. So I managed to talk them down to $25 (from the initial $50 price tag), and took it home to the bench.
Lucky for me, this thing is 80% discretes, plus some common transistors and two very standard IC's. 100% replacable!
There were three components inside which showed heat stress: a 12V zener diode, a 1.5k resistor, and this strange electrolytic capacitor with 4 leads. I went back to HSC and bought higher-wattage replacements for the resistor and the zener, but the capacitor stumped me. They didn't have a single capacitor like it. It's a metal-can type, with glossy plastic ends and transparent shrinkwrap over it. There are two leads coming out of each end; each end has a + and - lead. It's marked "CDE" "UFT 570-75", rated at 570MFD, 75VDC, and 105.oC. There is obvious heat stress around the points where the leads enter the body. and the ends are bulged slightly. Here's where I'm stumped... I brought the cap to one of the guys who ran the store, and he said it was probably two caps in one can, as was a popular strategy a while back. However, when I got back home, both my multimeters and an old-fashioned lightbulb continuity tester indicated that the leads on each end were continuous with each other (meter read 0.00), making it appear to be a single radial cap with the leads going in one end and out the other, perhaps for better power/heat handling. Or perhaps it truly was one of those old-fashioned duplex capacitors, and somehow it has failed in a way that by some twist of fate has caused both ends to become connected?
I would appreciate any help identifying this cap, and designing a suitable setup to take its place. As a last resort, I might just slice this baby open over the acid tub, and see what it looks like inside.

Thanks in advance.

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Look at the traces. Are the leads going to different places or are they combined on the board, such as ground? A four leaded capacitor does not strike a chord with me.


All four leads go different places. However, because this is an old-fashioned single-sided board (hand-routed traces, etc.), it is possible that they just used this particular capacitor instead of the two-lead variety because it doubled as a convenient set of high-power jumpers. I will take the board out again and look more closely at where all the traces go.
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With shorted leads, aren't you shorting the two traces together. If they are supposed to be shorted, then why aren't they. They normally don't short two traces together using a component.

It's possible it had something to to with the component density and the fact that this is a single-layer hand-routed board (before the days when computers figured it out for you!). From the way the traces are laid out, perhaps they just thought it would be convenient. I am pretty convinced that this is a single cap, though, based on the following:

Found this quote on the Power Electronics trade journal website:

A better and more cost-effective alternative is to mount a second lead wire to each end of the end terminations. This increases the current carrying capabilities of the capacitor and also helps to reduce its ESR and self-inductance. A four-leaded capacitor is more resistant to failure from vibration due to the second lead wire on each end adding strength to the solder connections as well as restricting capacitor movement.


Personally I think it'd be pretty darn unlikely that two seperate capacitors in a can would somehow fuse together in such a way that there was perfect continuity between the two. In addition, I have seen double capacitors, and they usually have the leads on the same side, with a common ground. I'm going with the single-capacitor-with-extra-leads theory. What do you all think?
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I think a short is possible, but not on both sides as you have said. Maybe both sides have a connection to ground. Could you ground a can. Inspect the traces for ground connections.


Tore off some of the heat shrink and tested between the leads and the can. None of the leads have continuity to the can. I stand with my position that this is a single cap.
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That is one strange device I must say. It would be interesting to see the circuit. One capacitor with four leads that go to wherever. It could be a parallel path. Have you thought of that? It's like the diode with three leads. They make them that way to realize a certain function. It is actually two diodes that are arranged as pullups to a 5volt line. I think they are inverted diodes. You see them on DIO lines.

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The SG3524 chip is rated 8 to 40 Volts and you have 65VDC on the inputs. How is the voltage reduced and is the internal 5V ref used by the circuit? A schematic would be very helpful.


I traced the VIN lead and it brought me straight back to the zener/reisitor circuit I mentioned earlier. As it turns out, this is actually a crude voltage regulator. The resistor is connected between the positive supply voltage and the cathode of the diode. The anode is grounded. This is a 12V zener diode, so the potential between the anode and cathode is, of course, 12 volts. This is what is used to power the '3524, as well as the voltage source to drive one of the two large can-type output transistors. No wonder the resistor gets hot... it has to drop over 50 volts non-stop!
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Long story short, I set up the back terminal strip according to the strapping diagram for local sensing and control.
It miraculously worked!...
For a short period of time.

Everything was working fine, so to test the current output I put a small wire across the output. I started easing the current up, and it did fine up to about 3 amps, then suddenly the needles dropped to zero. I think I may have heard a small click, but I'm not sure. This is a 10-amp power supply -- What happened??


Now it's in a truly wierd state. When I turn it on, the voltage slowly rises, charging the output capacitor, until it hits the OVP cutoff point (set by a small potentiometer; I have it at about 46 volts) at which point the relay shuts down the output stage. This happens regardless of the voltage settings, though a higher setting seems to make it happen more quickly. Obviously the system is no longer regulating properly.

Currently I am running the manufacturer's calibration procedure to see if that might catch the problem.

By the way, there is a full schematic in the pdf I linked to.
Any suggestions?

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Greg,

It looks like U9 / U1 is a switching pre-regulator, the setup maintains the voltagedrop over Q9 to minimize the heat. Do this part work allright? C5 has an unusual symbol but I think you can replace it with a standard but low ESR cap, if it

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