Cropico DO4A Digital Ohmmeter
[offlist]
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
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 csteinmetz@yandex.com
To: gandalfg8 gandalfg8@aol.com; Discussion of precise voltage measurement volt-nuts@febo.com
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
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
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@rallstech.net
To: gandalfg8 gandalfg8@aol.com; Discussion of precise voltage measurement volt-nuts@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
Good strategy Nigel.
Wet slug tantalum- yuk.
I've opened sealed packaging around new but in extended storage Canberra
NIM 5kV power supplies, had he unit smoke first time hooked power to it.
The rest of them, I just went through and removed all the tantalum
bypass caps before first test....
A lot of older nucleonic equipment uses 6V lead-acid batteries, not just
for backup but as a regulator for the main power supply. Removing the
battery and plugging in the mains = instant smoke due to over Voltage.
Basically they are battery operated instruments that happen be float
charging 24/7.
George Dowell