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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 30th Nov 2019, 10:04 pm   #41
Chris55000
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Default Re: What is it?

Hi!

That instrument is an accurate Multimeter Calibration Test Set – built in the mid to late 1970s it would have been designed to an accuracy and stability more than adequate for calibrating analogue scale multimeters and separate meter movements – the N/R toggle switches reverse the polarity of the test current for centre–zero instruments.

I think you'll also find there's a voltage inverter to provide up to 500/600V (see right–hand knob)!

This instrument is well worth the restoration effort as it will service any piece of gear with an analogue meter – you can set the o/p current slowly and accurately with the multi–turn dial on the 0–1.6mA range (l.h. knob) to test separate meter movements!

Chris Williams
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Old 1st Dec 2019, 5:23 pm   #42
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Originally Posted by Herald1360 View Post
Well it works in a simulation, at least. In the attached circuit, the current through the resistor is zero and the voltage across it ditto. The current through the 10R is exactly 500mA as would be expected from a 5V source fed via zero R to a 10R load.

You can think of the current source in parallel with the 100m as being equivalent to another voltage source with an internal resistance of 100m connected in series between the 5V supply and its 10R load if you like.
Chris, thanks. I copied your simulation into LT Spice and got the same results. I had a play with it by changing R2 to 20R and found that the current on i2 needed to be set to 250mA strike a balance Your last line helped me understand I think. With a regular in series current shunt measurement, the 100 milliohm resistor R1 would impose a negligible, but nevertheless a definite effect upon the measurement. However, the current source (i2) cancels out this effect by means of a current flowing in the opposite direction. The result is that the current across R2 in the circuit of V1 exactly matches that of i2. You can therefore measure the current across R2 on i2 with zero (cancelled out) resistance which should yield greater accuracy.

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Originally Posted by Chris55000 View Post
Hi!
That instrument is an accurate Multimeter Calibration Test Set – built in the mid to late 1970s it would have been designed to an accuracy and stability more than adequate for calibrating analogue scale multimeters and separate meter movements – the N/R toggle switches reverse the polarity of the test current for centre–zero instruments.

I think you'll also find there's a voltage inverter to provide up to 500/600V (see right–hand knob)!
Thank you Chris for providing that insight which helps me understand the usage of that instrument a but better. I had wondered about that 50/500 a.c.V marking on the RH knob. I couldn't see anything in circuit that would generate that kind of a voltage nor any parts that would handle it, so maybe this one doesn't have the inverter, but I will investigate further.

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Originally Posted by Chris55000 View Post
Hi!
This instrument is well worth the restoration effort as it will service any piece of gear with an analogue meter – you can set the o/p current slowly and accurately with the multi–turn dial on the 0–1.6mA range (l.h. knob) to test separate meter movements!
I had a play with this abd the S48 does indeed produce a small current on that range. The BIAS needs to be turned on. I'm not sure what the different ranges mean, but the output on 1-2 is about ten-fold of that on 0-1 and on 2-3, a bit less than double that of the 1-2 range. The output seems a bit random and does not seem to correspond with anything, although with the AVO on the 50μA range, setting the vernier to 100mA gave a reading of almost exactly 1V (i.e. 10 on the 1-30 scale) on the voltage scale which may or may not be a co-incidence.

With the LHS control on the mV ranges, the LH meter moves gradually to the right and progressively further over as you go down the ranges until it is hard over right on the lowest one. It seems unaffected by the vernier (100mV/turn) control setting. I am not sure what this all means yet and whether I need a current across the terminals for this to start making sense.
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Old 1st Dec 2019, 6:57 pm   #43
jamesinnewcastl
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Default Re: What is it?

Quote:
Originally Posted by Herald1360 View Post
Well it works in a simulation, at least. In the attached circuit, the current through the resistor is zero and the voltage across it ditto. The current through the 10R is exactly 500mA as would be expected from a 5V source fed via zero R to a 10R load.


You can think of the current source in parallel with the 100m as being equivalent to another voltage source with an internal resistance of 100m connected in series between the 5V supply and its 10R load if you like.
Hi

But it's not measuring or implying the value of R1 at all. You can put any resistor you like there. All that is happening is that the Voltage generator and the Current generator have fulfilled their task in life and each 'sees' what it has to see. The current source does not need to generate any voltage, nor does the voltage source need to change the current (which is the same as that from the current source). The current is right and the voltage is right.

In your simulation you should be able to remove R1 and get the same result.


Cheers
James

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Old 1st Dec 2019, 10:25 pm   #44
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I have a question about the meter movements. The left hand meter was fractionally off the zero mark when power is off so I tried to adjust it with the little plastic zeroing screw at the base of the movement, but this seemed to do nothing. I also checked the other meter and the same, but at least that one seems spot on zero. I lifted the clear perspex lid off (breaking one of the retaining clips in the process - though the cover still holds on quite well) to discover that the lug that the pin engages with appears to be seized. I dare not apply much force as if this lug is as brittle as the perspex cover, then it will break off very easily. Can anyone suggest how I might go about freeing it?
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Old 1st Dec 2019, 11:20 pm   #45
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There are several reasons why a pair of MC meters may not match in terms of the full CCW position. One could be static charge. Another could be that it was calibrated that way, in order to tension the spring so that it gives particular readings under certain conditions. I would imagine this is the last thing to worry about if I am honest. I regularly put several MC meters on a bench in front of me and observe differences in their CCW positions. What matters is accuracy in use. The only meters I can think of in production today that will match when fully CCW are from the likes of Simpson or Hoyt - well north of £100 per meter. As a rule, MC meters are not amazingly linear - there will be a far higher tolerance than with a digital device. Those such as Simpson are pretty accurate, but a cursory glance at the movement (as well as your bank statement) will tell you that you really do get what you pay for. There are tricks used in circuitry such as biased diodes that can make MC meters more accurate.
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Old 1st Dec 2019, 11:56 pm   #46
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Default Re: What is it?

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Originally Posted by jamesinnewcastl View Post
In your simulation you should be able to remove R1 and get the same result.
Yes, but the point of R1 is not that it needs to be there for the current balance bit to work but that it provides a convenient way (by looking for zero volts across it) to determine when that balance is reached. The current can then be measured with any old meter in the balance current loop without R1 affecting the current which would have been there without R1 (or a resistive meter) to upset it. R1 could in theory be any value, but practical considerations suggest a value low enough not to upset "normal" circuit operation but high enough to enable the balance point to be determined sufficiently closely as to be within the error margin of the ammeter in the balance loop.
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Old 2nd Dec 2019, 8:55 pm   #47
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Originally Posted by knobtwiddler View Post
There are several reasons why a pair of MC meters may not match in terms of the full CCW position. One could be static charge. Another could be that it was calibrated that way, in order to tension the spring so that it gives particular readings under certain conditions. I would imagine this is the last thing to worry about if I am honest. I regularly put several MC meters on a bench in front of me and observe differences in their CCW positions. What matters is accuracy in use.
Thank for this. I do agree that this is not a major consideration at present. i did have another reason for removal, and that is to get at the screws to the heatsink on which one of the circuit boards are mounted. I quickly realised that this would require complete removal of the meter but then found that I could solve the matter another way.

The meter has a white matte finish, but there are a few yellow/brown blemishes. What is the best way to remove them? Is this likely to be rust from the underside or some kind of mould? The last thing I would want to do is smudge the front of the meter with discolouration.
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Old 18th Dec 2019, 9:28 pm   #48
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Phew! Its taken me a while, but I have finally managed to get the circuit drawn up!

The right hand side meter and the dial to the bottom right seem to comprise a voltmeter that measures DC voltages in 8 ranges and AC voltages in two ranges. This looks straightforward enough with a resistor network to provide the various ranges, and a pair of small glass diodes serving as a rectifier for the two AC ranges. There did seem to be one or two open resistors so my next step will be to fully test this function and replace any faulty parts.

The left hand meter and the far left control seem to comprise an ZRA function. I am still uncertain how the mV ranges relate to current measurement but I am vaguely begining to understand how the The earlier ZRA and potentiometer discussion was helpful and I can vaguely see how the earlier ZRA and potentiometer discussion might apply to this circuit.

The centre board appears to supply bias which is adjusted using the BIAS range control and the 100mV/turn vernier. I am still unsure how the two controls relate to each other than the fact that positions 1-2 and 2-3 switch in different resistor feedback networks and the vernier control sets the output level.

At this point I have completed the drawings but have not yet had the time to fully study the circuit.
Attached Files
File Type: pdf Varipower-CS48-Voltmeter.pdf (46.7 KB, 65 views)
File Type: pdf Varipower-CS48-BIAS.pdf (43.3 KB, 31 views)
File Type: pdf Varipower-CS48-On-Off-switch.pdf (29.5 KB, 59 views)
File Type: pdf Varipower-CS48-ZRA.pdf (67.4 KB, 40 views)

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Old 2nd Jun 2020, 12:59 pm   #49
Chris55000
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Default Re: What is it?

Hi!

With the amicable agreement of Member "Wavey Dipole" to purchase this, I'll be taking up the story from hereon, and will be searching through the American Radio History archive to see if I can find something on its use – the fact it's built on Vero with LM308 opamps implies it can't be much earlier than the late 1970s at the most, and I'll be carrying out some experiments of my own with this myself – certainly I suspect one of it's uses is to provide a stable and finely adjustable low current sources to test moving–coil meter movements!

Chris Williams

PS!

Obviously anything I do find will be posted for the interest of other Members.
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Last edited by Chris55000; 2nd Jun 2020 at 1:05 pm.
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Old 3rd Jun 2020, 7:13 pm   #50
WaveyDipole
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Indeed, I am more than happy for it to be examined by someone who has more knowledge than myself. While the build is not much to look at, the unit does seem quite unique and I will certainly be interested to see whether the ARH archive reveals anything and what light Chris's investigation might shed on its intended use.
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