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Old 12th Nov 2019, 7:57 pm   #21
buggies
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Default Re: What is it?

No datasheet but this site says 30V

http://www.chipdocs.com/datasheets/d.../BZY95C10.html

Z2A30 30V, 1.5Wt General purpose voltage reference/regulator diode Operational temperature range from 0C to 175C.
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Old 12th Nov 2019, 7:59 pm   #22
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Arrow Re: What is it?

Data from "Radio Valve and Semiconductor Data", Illife, 8th. ed.

STC Zener diodes.
Z2A33CF = 3.3 v. Zener, 5%, 1 watt.
Format: Z2AxyCF = x.y v. Zener
And so on for others in the range, up to:
Z2A300CF = 30.0 v. Zener, 5%, 1 watt.

So on that basis, your Z2A30CF is a 3.0 v Zener diode, 5%, 1 watt

HTH,

Al.
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Old 12th Nov 2019, 8:06 pm   #23
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Default Re: What is it?

Quote:
Originally Posted by Skywave View Post
Data from "Radio Valve and Semiconductor Data", Illife, 8th. ed.

So on that basis, your Z2A30CF is a 3.0 v Zener diode, 5%, 1 watt
Agree - much more likely than the site I found - especially in battery equipment.
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Old 12th Nov 2019, 9:02 pm   #24
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Default Re: What is it?

Quote:
I wish I could find some documentation but I doubt that the calibration house that lists this device would be willing to share....
One can but ask, you never know.
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Old 12th Nov 2019, 9:11 pm   #25
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Default Re: What is it?

Thanks gents for the data on the Z2A30CF.

There is no decimal point between the 3 and 0. The maximum battery voltage is nominally 7.5v - a little over 8v in practice with fresh batteries, so the data from "Radio Valve and Semiconductor Data", Illife, 8th. ed. indicating a 3.0v zenner would seem to make more sense than the 30v on chipdocs. So another piece of the puzzle put in place.

Either way though, the measurement I got across it of 1.1248v does not seem to make sense, unless it is a voltage clamping diode. Will maybe know more when I draw out the circuit.

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
I wish I could find some documentation but I doubt that the calibration house that lists this device would be willing to share....
One can but ask, you never know.
Indeed I did ask today, but no luck unfortunately.
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Old 13th Nov 2019, 11:40 pm   #26
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Default Re: What is it?

Quote:
Originally Posted by WaveyDipole View Post
Right-clicking download next to Datasheet (jpg) and 'Save Link As' did the trick for me. That's one piece of useful information returned. Thank you for finding and posing that link.

Looks like the company that made the meter movements, Sifam, is still around, although perhaps no longer in the UK.
Sifam famously supplied the meters for the BBC-certified PPM meters that Canford sold. They used to have a good line of MC meters until the firm got sold a few years back. The PPM-spec meters got discontinued (Canford had some NOS when I last looked) and the rest of the range got moved to 'offshore' production. These new meters are sold by Canford with the disclaimer that they do not meet the IEC ballistic spec for a VU meter, I.e. they will flap around and look nice in your piece of hipster audio, but aren't accurate. From my extensive research I can say that there are probably less than a handful of firms worldwide now who are capable of making the kind of meter movements that were commonplace 50 years ago. If anyone cares to disagree with this sentiment I'd be delighted!
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Old 14th Nov 2019, 9:41 am   #27
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Default Re: What is it?

knobtwiddler, thanks for that note on the history of Sifam. In a portable unit you certainly wouldn't want the meter pointers to flap around. It seemed noteworthy that these seemed to barely move at all when the unit is picked up and moved around, so they seem to be very well damped indeed.
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Old 22nd Nov 2019, 12:36 am   #28
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Default Re: What is it?

I am working my way through reverse-engineering the circuit in this unit and have so far completed three of the four boards. While working on the third board, which is positioned above the left hand meter (current functions), I came across this (see pictures). One wonders whether the circuit worked at all. At the very least it must have behaved erratically! The joint connects pin 3, which is the non-inverted input of the LM308 op amp. There does seem to be another input via the resistor at the far left of the board that connects the white wire. This might just explain the weird behaviour of the left had side of the unit.

I have yet to map the various wired connections between boards and various panel controls. That will be the next step after I complete reverse engineering the fourth board. I will then post further details. In the meantime, does anyone know how I might be able to find more information regarding the vernier control? It is marked as follows:

Colvern 2402/13 SKOJ 7825

There seems to be no resistance indicated and resistance measurements with a DMM don't seem to make sense. I get about 3.1k across two of the pins and a bit more than 2k across the other two combinations.

With regards to the right hand side of the unit, since there was no output and not obvious fault, I figured that it must require an input. It turns out that when a small voltage is fed to the two terminals at the top right, I do get a reasonably close voltage reading on the right hand meter. I checked several DC ranges at different voltages and there all seemed to correlate. I haven't checked the AC ranges yet and am not sure what the vernier dial is for, or the bias settings. There are also the two 'sense' jacks to consider. I had rather hoped that the unit would produce reference voltages but that seems not to be the case.
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Old 22nd Nov 2019, 9:54 am   #29
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Default Re: What is it?

Most, if not all, of the Colvern pots I've come across have been wire-wound. Does this one feel a little notchy as you sweep it around? I wonder if its unductance could upset some DMMs? Forgive me if you have already realised this!
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Old 22nd Nov 2019, 10:20 am   #30
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Default Re: What is it?

Colvern 2402/13 are 500 Ohm pots and could be either 3 turn or 5 turn - source of info Wireless World August 1975 a48 (page 100) Electronic Brokers Ltd advert.
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Old 22nd Nov 2019, 11:42 am   #31
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Default Re: What is it?

Quote:
Originally Posted by Skywave View Post
.

I've met ZRA= Zero Resistance Ammeter, but only in technical writing, never in reality.

Al.
I guess a Hall effect or fluxgate magnetometer dc current transformer type would get pretty close......
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Old 22nd Nov 2019, 7:29 pm   #32
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Default Re: What is it?

Quote:
Originally Posted by knobtwiddler View Post
Most, if not all, of the Colvern pots I've come across have been wire-wound. Does this one feel a little notchy as you sweep it around? I wonder if its unductance could upset some DMMs? Forgive me if you have already realised this!
No, it turns quite smoothly throughout, except that there is a slight click when you get past the 360deg point, but that I think is the counter movement rather than the pot itself. I must admit that I hadn't really considered the possible effect of inductance on a DMM.

Quote:
Originally Posted by Terry_VK5TM View Post
Colvern 2402/13 are 500 Ohm pots and could be either 3 turn or 5 turn - source of info Wireless World August 1975 a48 (page 100) Electronic Brokers Ltd advert.
Thanks for that information. I found an online copy of the magazine and found the reference in question at the bottom of the page. I have attached a couple of photos of the part that I have.

The readings I was getting were rather high for a 500ohm pot. I rotated it back and forch a handful of times and tried measuring again. It now tests as a 5k pot, however at the fully clockwise end, I do get a variable reading up to about 15k between what I think is the wiper and the track. This settles if I turn it back a bit, say to 9.5, so it may be that the wiper is making bad contact at that end. At the fully anti-clockwise end I get about 5ohms between wiper and end contact. Is there any way to clean these? They don't seem to disassemble like a conventional pot. The other difference is that the vernier has ten turns although that might be down to the way it is geared. I can't tell without further disassembly.

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by Skywave View Post
.

I've met ZRA= Zero Resistance Ammeter, but only in technical writing, never in reality.

Al.
I guess a Hall effect or fluxgate magnetometer dc current transformer type would get pretty close......
Yes, indeed the only way you could get close is to use a non-contact method, such as the ones you mention. This particular Varipower is perhaps more comparable to something like this:

http://www.acminstruments.com/produc...stance-ammeter

Naturally anything inserted into the current flow is going to affect that flow, so I'm not sure how the term "Zero Resistance Ampmeter" applies here. The Varipower CS48 circuitry is relatively simple, consisting of a handful of op amps, a couple of references and the a few passives, including a shunt for the high current range. Being op amp based, it can probably measure current using a shunt of a much lower resistance, so perhaps compared to an analogue multimeter of the period, it might seem to offer 'zero resistance' but strictly speaking that is obviously not the case and the term does perhaps seem to be something of a mis-nomer.
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Old 23rd Nov 2019, 2:44 pm   #33
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Default Re: What is it?

If you pass an unknown current through a resistor, you will get a voltage drop.

If you simultaneously pass a reference current through the same resistor in the opposite direction, and adjust that reference current until you get zero voltage drop across the resistor, the reference current will exactly match the unknown current. You can now measure the reference current, and get the value of the unknown current without introducing any voltage drop in the unknown current path - ie. a Zero Resistance Ammeter.

You could set the reference current either with a feedback loop, or manually set with the multi turn pot.

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Old 28th Nov 2019, 4:55 pm   #34
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Default Re: What is it?

I have been trying to get my head around that since you posted it, but can't seem to figure out how one might go about passing a current across a resistor in both directions simultaneously? You could not have common ground on both sides of the resistor simultaneously? On the other hand if the grounds were not common (i.e. they were separate floating flows), then there would be no common reference voltage? Is there a practical circuit to demonstrate this?

As a means of adjustment, a feedback loop (e.g using an appropriately configured op amp), or multi-turn pot would certainly do the trick. I have now reverse engineered all four circuits and will now proceed to mapping out the connections to the various panel controls and connectors. Tracing all of the wires is going to take some time, particularly working out the rotary switches, but if I can at least work out the input and output jacks, meters and pots, and the simpler switches, then that should go some way towards making sense of each circuit.

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Old 28th Nov 2019, 5:25 pm   #35
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Default Re: What is it?

That looks like a 10-turn pot and a turns-counting dial.

Colvern made some multi-turn pots in paxolin tubes.

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Old 28th Nov 2019, 6:09 pm   #36
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Default Re: What is it?

Once you’ve adjusted the reference current to give zero volts across the resistor, there is of course zero current in the resistor.

Think of two current loops, one carrying the unknown current, the other carrying the reference current. Then break both loops and reconnect them via a shared resistor, to make a circuit with two conjoined loops.

Now consider the unknown current flowing towards the resistor, but getting diverted through the reference current loop. Because the reference current is adjusted until there is zero volts across the resistor, you’ve effectively added a zero resistance ammeter in the path to the unknown current.

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Old 28th Nov 2019, 6:34 pm   #37
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Default Re: What is it?

Yes, it's a version of the Wheatstone Bridge - where you use a locally-generated current of opposite-polarity to 'balance' the current flowing in the circuit-under-test so there is zero-volts showing across the test resistor.

Then you decouple the instrument from the circuit-under-test, and measure the current now flowing in your instrument - which will be equal to [but of opposite sign to] the current that had been flowing in the circuit-under-test.

Something vaguely-similar is used in the world of explosives-and-detonators to measure the continuity of the 'firing loop', where you can't just go at it with a conventional ohmmeter for reasons which should be self-evident.
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Old 28th Nov 2019, 9:29 pm   #38
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Default Re: What is it?

Quote:
Originally Posted by WaveyDipole View Post
I have been trying to get my head around that since you posted it, but can't seem to figure out how one might go about passing a current across a resistor in both directions simultaneously? You could not have common ground on both sides of the resistor simultaneously? On the other hand if the grounds were not common (i.e. they were separate floating flows), then there would be no common reference voltage? Is there a practical circuit to demonstrate this?
If your balance current source is floating it doesn't matter how the current you want to balance is ground referenced (or not). When the unknown and balance currents are equal and opposite there will be no voltage across the sense resistor. All you need is a null detector on the sense resistor and an accurate ammeter in the balance current loop. Both floating, of course as are battery powered DMMs and analog multimeters.

The whole arrangement is effectively the current dual of a potentiometer arrangement for infinite load resistance voltage measurement.

Ever wondered why a potentiometer is so called?
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Old 30th Nov 2019, 3:04 pm   #39
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Default Re: What is it?

Hi

What an interesting conundrum this zero voltage - but with currents thing.

But it can't happen as it would mean an EMF forcing current one way against a 'flow' of current the other from another EMF. Also either electron motion in the resistor would generate heat and that isn't going to cancel out unless you have generated 'negative' heat.

I can't imagine connecting two hose pipes together and the water forming orderly flows up and down inside the junction!

The answer must be that the currents flow in the other CC generator and NOT in the resistor, the two generators simply do their job pushing their own current through the zero resistance of the other. This requires current flow with no voltage - which is what you get when you short out a CC source with a zero value resistor.

Since a CC generator has no internal resistance you should be able to short CCs of unequal currents and get no voltage across the terminals - connect as many resistors as you like! Connect a TV set, same result.

So you can't make a measurement like that in theory?

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Old 30th Nov 2019, 9:30 pm   #40
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Default Re: What is it?

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.
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