[volt-nuts] Cropico DO4A Digital Ohmmeter

[gandalfg8 at aol.com]

Sorry, I realise now I could have explained better, I do know what transistor it is but that's not the problem, this looks to be quite a complex power suply/charger circuit, with at least three unmarked adjustment pots so if I change the device, even for the same part number, it's quite possible it will need readjustment and that's what I don't have any information on.

An additional concern is that the whole instrument is built onto a single PCB without any obvious way of isolating the supply, so there's not too much room for error.

I think for now, assuming the existing transistor checks out ok, I'll try my plan B option and see if I can remove enough plastic from around the broken legs to attach some flexible leads and hope the original settings hold up.

Nigel, GM8PZR



 

 

 

-----Original Message-----
From: Didier Juges <shalimr9 at gmail.com>
To: GandalfG8 <GandalfG8 at aol.com>; Discussion of precise voltage measurement <volt-nuts at febo.com>
Sent: Sun, 29 Apr 2018 19:35
Subject: Re: [volt-nuts] Cropico DO4A Digital Ohmmeter



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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