How to Repair 0-100V DC Power Supply? Need Help

Start with schematic.

gb

Best guess would have to be the series pass transistor ( probably fairly large and on a heatsink ).

Confirm with a voltmeter that there is a highish voltage on its input - probably 120v or so and may be on the collector or emitter depending on whether it's an NPN or a PNP type. If input volts are present, check for a voltage on the base connection, which varies up to around 100v with rotation of the set voltage pot.

If there is no input voltage, problem is back in the raw stage ie rectifier / smoothing / maybe surge limiter resistor.

If the variable voltage is missing, disconnect, base lead and check again. If it's still missing, problem is back in control loop. If adjustable voltage is present, and input voltage is present, check to make sure that there are no shorts across the output of the transistor ( decoupling cap, protection diode, overvolt zener ) then replace transistor.

Obviously, this is a fairly simplistic approach, and assumes that it is a ' conventional ' design linear supply, but should catch most common problems for this type of fault. If problem does prove to be in the control loop, then things will get rather more complicated, but suspicion should fall first on any semiconductor devices ( transistors, control chip, diodes, zeners ).

Arfa

HP wouldn't provide a schematic of a light switch.

Since you say it died when someone hooked up something incorrectly, start by checking the semiconductors for shorts.

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Check for a shorted diode across the output terminals.

I'm not sure what good a schematic would be on a linear power supply, normally they're not very complex, you can follow the circuit visually.

Remind me again why I bother ...

Arfa

I checked the big transistor that was on the heat sink and it looks fine. I took it out to test it, and also I checked in-circuit and there are 130V across B-C and E-C while running (so I guess it's an npn). There is also a capacitor with 20V on it and a capacitor with

Arfa Daily wrote:

Post a link to pictures of the unit. Check for an open (power) resistor somewhere in the feedback loop.

Assuming then that it is NPN, if you have 130v on the collector, you would expect the emitter to be the output. To check the control voltage, you need to measure volts between ground and the transistor base, whilst turning the set volts pot up and down. You should see a variable voltage between 0 and

But before doing this, switch your meter to ohms, and just check the resistance between the emitter and ground. There just might be a short across the output from a diode, as I, and another poster suggested. If you don't see a short, go ahead and check the base volts, as detailed, and let us know.

BTW, try not to top-post, as it makes the thread difficult to follow for people ;-)

Arfa

Thanks for all the advice so far. Here is the update: There are no shorts across the output. Also the emitter, base, and collector of the big power transistor are not shorted to ground. But here is something that seems wrong: there is no variable voltage on the base of the big power transistor when the set-voltage is varied. So there is some problem before the big power transistor. This is all I have so far. Any advice?

That would be the reason that there is no output. What you now need to do, is follow the base lead back into the circuitry, and see where it comes from. There are several ways that the adjust voltage may be produced, from a simple pot on the base of a driver transistor with a reference from a zener diode in the emitter circuit, to more elaborate schemes using a chip, often a uA723 ( or any '723 equivalent ). You might try also checking that there is some kind of varying voltage on the wiper tag of the set volts pot, when it is rotated. This voltage may actually be quite small - of the order of a few volts. Would it be possible for you to take a couple of photos of the control pcb and post them up somewhere ? Might give the experienced eye a clue as to how the regulator is designed. If you don't have anywhere to post them, you could always send them to me direct, and I'll see if I can help that way.

Arfa

Thanks. I ordered the manual on ebay so I'll post again when I get it.

Arfa Daily wrote:

Thanks for the suggestions. The power supply is now fixed. The problem was one of the transistors controlling the big series power transistor. The faulty transistor was part of a feedback loop comparing the output voltage to some set reference.

The suggestion that helped the most was to order the manual on ebay.

Noam wrote:

Glad you sorted it, and we were able to be of some help to you

Arfa

You're right about HP, the computer supplier, but Agilent is in the process of collecting and making availible scanned HP manuals of older products, as well as software and user's manuals.

HP a computer company? I thought they are an ink cartridge company?

Psssst... you are not wearing your smily!!!!!

On Sun, 19 Mar 2006 19:04:05 GMT, "Michael A. Terrell" put finger to keyboard and composed:

Do you mean to say that a company like HP goes to the trouble of producing a 500 page technical manual and then doesn't bother to keep an electronic copy of it? What does that say about the value of their IP?

- Franc Zabkar