[volt-nuts] Cropico DO4A Digital Ohmmeter

[Didier Juges]

Or expose enough leads of the damaged parts to check what it is and find a
replacement.

If it's on the heatsink, it is probably used as a linear regulator and
there will be a large section of the datasheet that you can safely ignore.

On Sun, Apr 29, 2018, 7:02 AM Nigel Clarke via volt-nuts <volt-nuts at febo.com>
wrote:

> Perhaps somewhat off topic but just wondering if anybody here would have a
> service manual for this instrument, or even just a schematic particularly
> of the power supply section?
>
> I've just bought one that was totally dead on arrival and it looks to have
> been blessed by the idiot's touch, both from an original mechanical design
> point of view and subsequent user intervention.
>
> It turns out the the TO220 PSU power transistor is mounted on the back
> panel and rigidly soldered to the circuit board, although I still haven't
> figured out quite how they managed that in this instance, and there's
> nothing to prevent removal of the back panel, even with the covers still
> attached, which someone has obviously done and snapped off all three
> transistor legs flush with the body.
>
> There's no obvious PCB damage, so it could have been a lot worse, and I
> should be able to add some flexible tails to the PCB and then solder these
> to the replacement transistor pins to include some future stress relief,
> but this LM723 based PSU looks to be quite complex, it includes a lead acid
> battery charger with overvoltage and deep discharge protection, thermal
> protection too by the looks of it, and all involving another five ICs plus
> associated circuitry, so some adjustment is likely to be necessary, which
> is where the manufacture's instructions would be most useful.
>
> I guess plan B might be to expose enough of the leads on the existing
> transistor to reuse that if possible, whilst hoping the original adjustment
> still holds, but aside from the quite "interesting" surgery required it
> would still be nice to check the adjustments anyway.
>
> Nigel, GM8PZR
>
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