Hi Charles, I'm quite sure you're right, and certainly if properly designed the pass transistor should be inside the feedback loop, but I've also learned the hard way over many years never to take anything for granted, especially when it's not my design and I'm trying to avoid stripping out the board to check the circuit.
I would have wanted to cut back the pins of any replacement anyway, in order to add flexible tails and hopefully avoid similar mechanical mishaps in the future, so a bit of time spent with some swiss files has achieved a very similar result, although perhaps given the choice I might not have cut the pins quite that short:-) Regards, Nigel -----Original Message----- From: Charles Steinmetz <[email protected]> To: gandalfg8 <[email protected]>; Discussion of precise voltage measurement <[email protected]> Sent: Mon, 30 Apr 2018 14:56 Subject: [offlist] Re: Cropico DO4A Digital Ohmmeter Nigel, 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, Charles 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 > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
