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WP-775B power supply repair

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gray5596

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I am trying to repair a WP-775B, 0 to 30 volt power supply, made by Vector-VID. My main problem is IC2 (14 pin DIP) is missing. The circuit board layout strongly suggests it is a quad op amp, but I can’t find a schematic to identify which op amp it is. This IC feeds the analog current and voltage meters, so I’d like to be reasonably sure which op amp to use so I won’t ‘smoke’ the meters.

My 1st guess is a LM324 because 24 VDC is on pins 7 and 11 and the unit appears to be built around the 90s. Also IC1 is a LM723.

Any ideas or suggestions would be greatly appreciated.
 
I uploaded a couple of pictures. I'm also finding out that LM148s were common at this time
 

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Hy gray5596,

The Vector-VID WP 755 Power Supply Unit (PSU) manual, including schematic and parts list, is attached below.

The component nomenclature appears to be different to your PSU, but I would think that the circuit principle is similar. At a guess I would think that there are two versions of this PSU: one with an LM723 voltage controller and one with an LM324. So it could be that the missing chip on your board is intentional.

Note that this PSU has a pre regulator using a phase angle controlled (I think) Silicon Controlled Rectifier (SCR) (SCR1) to maintain a roughly constant voltage drop across the linear series pass element (FET1) and thus limit the dissipation in the serial pass element.
 

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Last edited:
Thanks!

My model is the 775B and doesn't have a SCR pre regulator. However your schematic shows a LM324 op amp and so it would be a safe guess that my model would also use the same op amp.

This information is what I was looking for, thanks again!
 
Glad to help gray. Hope you get your PSU working.:)

spec
 
The missing IC was a LM324. After installing this op amp, repairing a burn trace and a few other components, it is working.

Instead of a SCR pre regulator, this has a high and low voltage output on the transformer that is controlled by a relay. A neat idea to keep the pass transistors from dissipating too much power.

Spec, thanks again for the schematic.
 
No probs Gray.

Glad to hear that you fixed the PSU.:cool:

spec
 
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