Cropico DO4A Digital Ohmmeter
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
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@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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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@gmail.com
To: GandalfG8 GandalfG8@aol.com; Discussion of precise voltage measurement volt-nuts@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@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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On 29 April 2018 at 21:57, Nigel Clarke via volt-nuts volt-nuts@febo.com
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.
Nigel, GM8PZR
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
I have a few very low resistance resistors (around 50 milliohms), four terminal of course, rated at .05% or maybe better. I built a low resistance Ohmmeter but don't use it since I got my HP3456A with very good resolution. I can measure the resistance of a screwdriver shaft with it, although there can be issues with contact potential. It has a function to eliminate that error but I am not too confident. Measuring using AC would be a good solution.
I bought some cheap Kelvin test clips but they don't hold on too well, and I don't know how to modify them for more solid clamping. (They are similar to ordinary clips but each jaw is insulated separately so you can use them for 4 terminal measurements. The DIY meter simply uses four clips.)
Bob
On Sunday, April 29, 2018, 2:18:50 PM PDT, Dr. David Kirkby drkirkby@kirkbymicrowave.co.uk wrote:
On 29 April 2018 at 21:57, Nigel Clarke via volt-nuts volt-nuts@febo.com
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.
Nigel, GM8PZR
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
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and follow the instructions there.
Fractional and even Ohm resistors-
Once, in a package from Russia, with some items totally not related to
this group discussion, the seller included 2 boxes of 1/2% 200 ohm
resistors with a note of thanks for the purchase. Each box contained
several cardboard carriers with 25 resistors lined up in idividual
precise slots.
At that time I checked them with the bench Simpson 260 and sure enough
they were 200 Ohms. Checked several and they were all 200 Ohms. On the
Simpson VOM all tested were remarkably the same, so next they were
checked on the bench Simpson 463, a 3 1/2 digit .1% (I think) digital
meter. There they were again, 200.0.
That's when I decided to put them away for a future project, aside from
the usual resistor stash (I almost said TRASH!).
Reading this thread today prompted a second look at them, this time with
updated equipment
Each sample tested read between 200.2 and 200.7 Ohms. A random sample of
10 tied in parallel read 200.0529. Interesting.
Next step was to figure out what combinations 10 or 20 of those could
created if hooked together with clip leads, in various clusters of
series/ parallel groups.
Simple calculated that from 1 to 10 in parallel would yield 10 different
resistances, same # for series. So 20 different combinations divided
over 2 groups of 10 resistors.
Next I tackled the possible combinations that utilized one set of series
plus one set of parallel possibilities, in series. This yielded 100
combinations
I didn't even begin the possible combinations in parallel.
There are some very interesting numbers on that list, which include
22.22222, 33.33333, 66.66666 266.66666 along with many the normal whole
numbers starting at 10.
This is where my research stalled (read: mind=boggled) and decided to
ask the computer experts here if they can do a spreadsheet that would
calculate all the possibilities of 20 ea. 200 Ohm resistors.
I can envision two smallish boards with two rows of 10 terminal each,
one row on each side and the resistors soldered across the terminals
ladder style or maybe criss-crossed so they would all be in series
already.
Clip leads to do the dirty work, and a spread sheet for "programming"
information.
If this or some similar would be of interest to the traveling
calibration folks, I will donate 100 resistors to the cause, enough for
5 pairs.
George Dowell
On 2018/04/29 04:35 PM, Bob Albert via volt-nuts wrote:
I have a few very low resistance resistors (around 50 milliohms), four terminal of course, rated at .05% or maybe better. I built a low resistance Ohmmeter but don't use it since I got my HP3456A with very good resolution. I can measure the resistance of a screwdriver shaft with it, although there can be issues with contact potential. It has a function to eliminate that error but I am not too confident. Measuring using AC would be a good solution.
I bought some cheap Kelvin test clips but they don't hold on too well, and I don't know how to modify them for more solid clamping. (They are similar to ordinary clips but each jaw is insulated separately so you can use them for 4 terminal measurements. The DIY meter simply uses four clips.)
Bob
On Sunday, April 29, 2018, 2:18:50 PM PDT, Dr. David Kirkby drkirkby@kirkbymicrowave.co.uk wrote:
On 29 April 2018 at 21:57, Nigel Clarke via volt-nuts volt-nuts@febo.com
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.
Nigel, GM8PZR
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
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and follow the instructions there.
You didn't mention the power rating of these resistors. Do they have a brand name? They are likely wire wound; how much inductance, self resonant frequency?
I could use them for various purposes depending on the answers to the above.
The thing that makes me curious is that your 'average' is outside the individual values; did you make a typo?
Looking in old texts, you will find a four-resistor method of making a decade box. Since your resistors are 20 Ohms, you perhaps would want to use a parallel pair for each value, resulting in a box that goes in 10 Ohm steps to 100 Ohms. If these are very accurate resistors, you will have to be careful how you connect or switch them so as to add minimal error.
How to use such an instrument remains a good question.
Bob
On Sunday, April 29, 2018, 4:10:54 PM PDT, geoelectronics@rallstech.net wrote:
Fractional and even Ohm resistors-
Once, in a package from Russia, with some items totally not related to this group discussion, the seller included 2 boxes of 1/2% 200 ohm resistors with a note of thanks for the purchase. Each box contained several cardboard carriers with 25 resistors lined up in idividual precise slots.
At that time I checked them with the bench Simpson 260 and sure enough they were 200 Ohms. Checked several and they were all 200 Ohms. On the Simpson VOM all tested were remarkably the same, so next they were checked on the bench Simpson 463, a 3 1/2 digit .1% (I think) digital meter. There they were again, 200.0.
That's when I decided to put them away for a future project, aside from the usual resistor stash (I almost said TRASH!).
Reading this thread today prompted a second look at them, this time with updated equipment
Each sample tested read between 200.2 and 200.7 Ohms. A random sample of 10 tied in parallel read 200.0529. Interesting.
Next step was to figure out what combinations 10 or 20 of those could created if hooked together with clip leads, in various clusters of series/ parallel groups.
Simple calculated that from 1 to 10 in parallel would yield 10 different resistances, same # for series. So 20 different combinations divided over 2 groups of 10 resistors.
Next I tackled the possible combinations that utilized one set of series plus one set of parallel possibilities, in series. This yielded 100 combinations
I didn't even begin the possible combinations in parallel.
There are some very interesting numbers on that list, which include 22.22222, 33.33333, 66.66666 266.66666 along with many the normal whole numbers starting at 10.
This is where my research stalled (read: mind=boggled) and decided to ask the computer experts here if they can do a spreadsheet that would calculate all the possibilities of 20 ea. 200 Ohm resistors.
I can envision two smallish boards with two rows of 10 terminal each, one row on each side and the resistors soldered across the terminals ladder style or maybe criss-crossed so they would all be in series already.
Clip leads to do the dirty work, and a spread sheet for "programming" information.
If this or some similar would be of interest to the traveling calibration folks, I will donate 100 resistors to the cause, enough for 5 pairs.
George Dowell
On 2018/04/29 04:35 PM, Bob Albert via volt-nuts wrote:
I have a few very low resistance resistors (around 50 milliohms), four terminal of course, rated at .05% or maybe better. I built a low resistance Ohmmeter but don't use it since I got my HP3456A with very good resolution. I can measure the resistance of a screwdriver shaft with it, although there can be issues with contact potential. It has a function to eliminate that error but I am not too confident. Measuring using AC would be a good solution.
I bought some cheap Kelvin test clips but they don't hold on too well, and I don't know how to modify them for more solid clamping. (They are similar to ordinary clips but each jaw is insulated separately so you can use them for 4 terminal measurements. The DIY meter simply uses four clips.)
Bob
On Sunday, April 29, 2018, 2:18:50 PM PDT, Dr. David Kirkby drkirkby@kirkbymicrowave.co.uk wrote:
On 29 April 2018 at 21:57, Nigel Clarke via volt-nuts volt-nuts@febo.com
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.
Nigel, GM8PZR
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
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and follow the instructions there.
--Original Message-----
From: Dr. David Kirkby drkirkby@kirkbymicrowave.co.uk
To: gandalfg8 gandalfg8@aol.com; Discussion of precise voltage measurement volt-nuts@febo.com
Sent: Sun, 29 Apr 2018 22:18
Subject: Re: [volt-nuts] Cropico DO4A Digital Ohmmeter
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
Well, the Cropico MTS2 test standard suggested in the calibration instructions actually has a specified best accuracy of 0.01%, although with a 3999 display and variable decimal point the DO4A is going to be pushed at times to display to its own spec limits anyway.
BTW the data sheet resolution chart has some errors, the manual is correct.
I certainly won't be buying an MTS2, unless as another Ebay special of course:-), and a set of 6 x 0.01% reesistors from Farnell wouldn't be exactly cheap, but still around half the cost of 0.005%.
So far I've just run it with an external PSU across the battery input and checked it against some Cropico RS3 standards. The 1K standard for example has a specified accuracy of 0.005%, albeit without a current cal certificate, and that did indicate 1.000K on the DO4A, which at least is some encouragement:-)
Nigel, GM8PZR
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-----Original Message-----
From: Dr. David Kirkby drkirkby@kirkbymicrowave.co.uk
To: gandalfg8 gandalfg8@aol.com; Discussion of precise voltage measurement volt-nuts@febo.com
Sent: Sun, 29 Apr 2018 22:18
Subject: Re: [volt-nuts] Cropico DO4A Digital Ohmmeter
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
Whoops, whilst checking the calibration is one thing, actual self calibration on the DO4A is perhaps not looking quite so straightforward, it's an automated calibration process that requires six set resistor values of 4mohm, 40 mohm, etc up to 4Kohm, and a quick check online doesn't show high tolerance versions of these values exactly falling out of the woodwork!
Nigel, GM8PZR
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On 30 April 2018 at 10:45, Nigel Clarke via volt-nuts volt-nuts@febo.com
wrote:
-----Original Message-----
From: Dr. David Kirkby drkirkby@kirkbymicrowave.co.uk
To: gandalfg8 gandalfg8@aol.com; Discussion of precise voltage
measurement volt-nuts@febo.com
Sent: Sun, 29 Apr 2018 22:18
Subject: Re: [volt-nuts] Cropico DO4A Digital Ohmmeter
I see this is a low-resistance (10 u ohm resolution) ohmmeter. Have you any
idea how you are going to check the calibration? I just bought a Simpson
444, which has 1 u ohm resolution. I have not got it yet, but are wondering
how I am going to check the calibration.
0.005% resistors are available from Farnell in a fairly limited number of
values, but certainty not in the range of values needed for a milli/micro
ohm meter.
Dave
Whoops, whilst checking the calibration is one thing, actual self
calibration on the DO4A is perhaps not looking quite so straightforward,
it's an automated calibration process that requires six set resistor values
of 4mohm, 40 mohm, etc up to 4Kohm, and a quick check online doesn't show
high tolerance versions of these values exactly falling out of the woodwork!
Nigel, GM8PZR
They are annoying values! Often one see 100 u ohm, 1 m ohm, 10 m ohm, but
most I have seen have resistances starting with the number 1.
The Simpson 444 I bought can only read up to a maximum of 2 ohms, which is
a bit limiting it must be said. The lack of GPIB, (or any other computer
interface), is another major inconvenience too.
I feel the low resistance meters that I have seen, all have some annoying
limitations. I've contemplated whether it is possible to make something
better, which outputs 0-10 V to be read by a normal multi-meter. Issues
seem to be
- Some are AC only, which makes them useless for inductive components.
- Some are DC only, which makes them suspeptable to thermal EMF
- Some use high currents, which can damage sensitive components
- Some can use fairly high voltages, which can break down oxides. (The
Simpson 444 does not output more than 100 uV, which seems quite unique in
this way) - Some don't have GPIB
- Some like my Simpson 444, has a very limited maximum resistance - in this
case just 2 ohms. - The Keysight 34420A has an impressive 100 n ohm resolution, but achieves
this by having a 1 ohm range with a 7.5 digit meter.
A combination of issues leaves me wondering if something better could not
be designed. But the only sensible way I can see would be an add-on for a
normal bench multimeter.
Calibration seems a non-trivual task, even for a commerical unit like
yours, so would be quite tricky for a home-brew unit, unless one spent a
fortune on calibration standards.
Dave