Hi everyone.
I recently purchased a good old Radiometer Copenhagen RV 36 millivoltmeter (no way i could afford an hp one - the cheapest costs 2 times the cost of mine), to use along with my other analog test tools ( farnel signal generator and gould oscilloscope). So far i was using my trustworthy uni-t digital multimeter with very ok results.
The thing is that the millivoltmeter (which used costs about 2 times more than the millivoltmeter new) doesnt’t go below 15mv- and it’s not a wrong 0 reference.
The multimeter shows 18-20mv at the same value but it can go down to 7mv. The millivoltmeter even when the showings are below 15mv, stays there.
I don’t know if i have to adjust something to change the sensitivity (like any of the trimmers inside) since nothing like that is explicitly mentioned in the manual. Banana jacks are cleaned and the probes are in perfect shape.
Any advice would be much appreciated. I attach the manual which contains the device schematics just in case.
Thanks in advance
I recently purchased a good old Radiometer Copenhagen RV 36 millivoltmeter (no way i could afford an hp one - the cheapest costs 2 times the cost of mine), to use along with my other analog test tools ( farnel signal generator and gould oscilloscope). So far i was using my trustworthy uni-t digital multimeter with very ok results.
The thing is that the millivoltmeter (which used costs about 2 times more than the millivoltmeter new) doesnt’t go below 15mv- and it’s not a wrong 0 reference.
The multimeter shows 18-20mv at the same value but it can go down to 7mv. The millivoltmeter even when the showings are below 15mv, stays there.
I don’t know if i have to adjust something to change the sensitivity (like any of the trimmers inside) since nothing like that is explicitly mentioned in the manual. Banana jacks are cleaned and the probes are in perfect shape.
Any advice would be much appreciated. I attach the manual which contains the device schematics just in case.
Thanks in advance
Do you have a DIY voltage reference?
I would start with a battery (maybe a mercury cell) and make a resister dividing network.
I would start with a battery (maybe a mercury cell) and make a resister dividing network.
Haven’t got one (never really needed one) and don’t know of its use. Is it something that goes between meter and object under test, to improve numbers?
Could you please explain and send me a schematic-layout?
Could you please explain and send me a schematic-layout?
First thing I would do is clean the contacts of the range switch with a good contact cleaner like DeoxIT D100L or similar.
According to the date on the schematic it's 50+ years old.
According to the date on the schematic it's 50+ years old.
I will check that (pretty straight forward if the switch is reachable.
There are also 4 adjustment trimmers - without any explanation of their use, and there’s mentioning in the manual of a zero adjustment without any clarification or directions- of vourse when it’s on without anytjing attached it shows 0. But a good manual for people who don’t know analog test tools calibration procedure
I guess the problem must be something with the noise floor (10-20mv) and probe impedance. And cleaning switches wold help. Even if it’s super clean and neat inside. I mean super clean. Unbelievable for its age.
There are also 4 adjustment trimmers - without any explanation of their use, and there’s mentioning in the manual of a zero adjustment without any clarification or directions- of vourse when it’s on without anytjing attached it shows 0. But a good manual for people who don’t know analog test tools calibration procedure
I guess the problem must be something with the noise floor (10-20mv) and probe impedance. And cleaning switches wold help. Even if it’s super clean and neat inside. I mean super clean. Unbelievable for its age.
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In sequence first with power off adjust the screw on the front of the meter to zero the analog meter. The short the input terminals and confirm that at the 1V range it reads close to zero with the power on. Its possible that the input device has become noisy. Its a pretty common device - 2n3906 if the noise is there at the lowest setting. The adjustments control biasing, gains and frequency response. Hopefully you should not need to adjust any of them. Check the internal sypply voltages - +24V and -18V before dealing with anything else.
Did the switch contacts. They were fine. Nothing changed. No harm caused.
The zero adjustment was fine. The 1v short just very good.
Tried short in smaller scales. I realised that even shorted there eill always be a value of 2.7mv.
So the scale of one mv is unusable. And most of the 3mv don’t work. Too bad.
The zero adjustment was fine. The 1v short just very good.
Tried short in smaller scales. I realised that even shorted there eill always be a value of 2.7mv.
So the scale of one mv is unusable. And most of the 3mv don’t work. Too bad.
Did you short with a very short 😉 cable, just a few centimeters? A stray mains field or RF pickup could make the instrument show non-zero.
This is the manual i uploaded on my initial post. Mine actually is a slightly better copy.
I will try a shorter cable. But anyway the problem of not reading very low values is there.
I mean that’s what i need to sort out and i can’t work with very short cables (or any cables at the moment) with such values 🙂
Ps. Yes the short with really short cable worked. 0v in all scales. Actually i have a suspicion that it may need a probe like an osciloscope bnc (but a banana version) instead of 2 different multimeter probes. Does anything like that exist? Is it an rf active probe? Do i need something like that for low millivot measurement?
I will try a shorter cable. But anyway the problem of not reading very low values is there.
I mean that’s what i need to sort out and i can’t work with very short cables (or any cables at the moment) with such values 🙂
Ps. Yes the short with really short cable worked. 0v in all scales. Actually i have a suspicion that it may need a probe like an osciloscope bnc (but a banana version) instead of 2 different multimeter probes. Does anything like that exist? Is it an rf active probe? Do i need something like that for low millivot measurement?
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It seems like the connector is an old switchcraft or look alike (2501 or 5501 but they don’t look like male banana at all). I wonder if a common oscilloscope probe with bana adapter would give good results. At the end if the day i always work in the normal audio frequency field (30hz-20k)
I would replace the input socket to BNC and use a shielded cable (or oscilloscope probe) for low voltage measurements.
Not a bad idea but i feel like it’s a bigger hole and then the whole thing will be unstable and i risk permanent damage. It’s pretty old and it’s pretty expensive.
Post 6 nailed it and you need a shielded cable. The PL connector is "good enough". A short is a short and not cable ends connected together.
Thanks for that. Didn’t know it’s called PL connector.
Ps. Actually it’s a pl 259 connectir and gas sizes depended on coaxial cable (i guess the same like bnc) size used.
Ps. Actually it’s a pl 259 connectir and gas sizes depended on coaxial cable (i guess the same like bnc) size used.
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This is a basic millivoltmeter that should be good enough for most purposes.
Edit : ac type so a dc calibrator will not help.
Edit : ac type so a dc calibrator will not help.
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Then use an AC wall-wort or filament transformer (that you can verify with an AC meter) and a resistor dividing network.