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

Charles Steinmetz csteinmetz at yandex.com
Mon Apr 30 09:56:41 EDT 2018



It is vanishingly unlikely that the parameters of any individual 
transistor (in particular, Vbe and hFE) would change the calibration of 
the instrument or the operation of the PSU protection circuits.

A heatsunk TO-220 is clearly the pass element, not a voltage-determining 
component, of the PSU.  It will be inside the error amplifier's feedback 
loop, so any difference between individual transistors will have no 
effect on the regulated voltage.  Also, consider that the LM723 has a 
fair amount of temperature and aging drift -- orders of magnitude 
greater than any possible effect of changing the pass element.

To reiterate, replacing the transistor with a different sample (or a 
different TO-220 part of the same gender and configuration, for that 
matter) will have no effect on the calibration of the meter.

Of course, after repair it will be only as accurate as it was before the 
damage, so you are still left with the problem of calibration.  But you 
can be secure in the knowledge that replacing the transistor will not 
affect the meter calibration or the PSU protection circuits.

Best regards,


On 4/29/2018 4:57 PM, Nigel Clarke via volt-nuts wrote:
> 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

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