[volt-nuts] [offlist] Re: Cropico DO4A Digital Ohmmeter

gandalfg8 at aol.com gandalfg8 at aol.com
Tue May 1 11:04:04 EDT 2018

 Hi George,

Many thanks for the suggestions but unfortunately it isn't quite that straightforward.

Repairing and/or replacing the pass transistor is easy enough and the regulator is LM723 based so probably that part's fairly standard.
As long as there's nothing else amiss then I will as you suggest be able to note the output from the regulator and determine which pot controls that, but this then feeds quite a complex charging circuit with several more ICs and the other two pots presumably adjust the depth of discharge and over-charge limits, with a temperature sensor falling into the mix somewhere too.

I don't have any criteria for adjusting the other two pots and don't know what feed voltage those stages are expecting so can't even be sure that what's coming out of the regulator is correct, but for now will just have to take that for granted. It seems reasonable to expect that during the charge cycle there's likely to be somewhere between say 6.7 and 7.7 Volts across the 6 Volt lead acid battery depending on the charger design but again I don't know how long the charge cycle is expected to be so can't judge too much from that either. What I probably can assume from the existence of the over-charging protection is that it's going to be at higher than float charge levels.

I have found a couple of test points marked +5 and -5 Volts, indicating that somewhere down the line there must be some more regulation/inversion plus a futher degree of isolation between the charging circuits and the rest of the instrument, so for now I'm just going to replace the transistor and keep my fingers crossed.

If all else fails I can always find use for a nice box with a mains transformer, and at least there's no exploding tantalum caps to worry about:-)

Nigel, GM8PZR



-----Original Message-----
From: geoelectronics <geoelectronics at rallstech.net>
To: gandalfg8 <gandalfg8 at aol.com>; Discussion of precise voltage measurement <volt-nuts at febo.com>
Sent: Mon, 30 Apr 2018 23:05
Subject: Re: [volt-nuts] [offlist] Re: Cropico DO4A Digital Ohmmeter

Maybe if you temporarily repair he connections to the broken transistor, the working status can be verified and readings taken, perhaps even mark those pot's positions, and when its working, twiddle them back and forth around that marked position while noting any changes in out put etc If it is actually working, you now have a baseline to which to aspire with the new part.

George Dowell

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