Resistance measurements

DMMs and VOMs give different readings in certain circumstances (see Ralph's post). In GB (at least) for a long time the service manuals for decent gear would provide expected voltage readings for 20k OpV analogue meters, the AV0 model 8 specifically IIRC, which saved the technician the extra bother of scratching his head when checking hi-z parts of a circuit.

Reply to
Cursitor Doom
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** The fact you are totally clueless ?

Semiconductors are not resistors and there is no right value to measure.

DMMs are designed NOT to cause didoes or BJT junctions to conduct when using the ohms ranges.

Also, DMMs ohm ranges are very sensitive to any residual DC or AC voltage on a component while analogue meters are much less so.

Interesting fact: you can measure the resistance of a loudspeaker voice coil with either type, but not if the room is full of loud bass noise.

Think about it.

...... Phil

Reply to
Phil Allison

Yes, I have seen some schematics specify the voltage was measured with a certain ohms/volt meter. It does take some guess work or head scratching out of the servicing. Makes a big difference when measuring the screen votltage that is fed with a high value resistor and you use a vtvm or a 20 k ohms/volt or a 5 k ohms /volt meter.

I don't think I have done any trouble shooting on a tube circuit in 20 or 30 years. Last thing I recall even looking at with tubes was my Heathkit sb101 that was many years ago. Only other thing I have with tubes are a couple of RF amplifiers and they use voltages that I don't work on with any power on them.

Reply to
Ralph Mowery

Sounds to me like you need to spray some contact cleaner in your test meter switches, and clean the plug contacts too.

Reply to
oldschool

Yep, I've seen several of them models. I just never owned any of them. I generally used one of my VTVMs (Heathkit or Eico) for most in circuit testing on tube gear. I have always been pretty satisfied with my older Radio Shack VOMs. I blew up a few of them many years ago, but I learned what NOT to do, and to pay attention and make sure I dont have the meter on the ohms scale when I measure voltages.

I also have a GB Instruments model GMT-19A VOM sitting right next to me, that I use for darn near everything, and it's held up well for many years.

I am not real fond of digital meters. I find them confusing, since they tend to pick up stray voltages and they also take awhile to "settle" on a reading. I have some of them, and I use them for some things, but my analog meter are usually the first ones I grab.

Seeing those Simpson meters makes me want to look for one to buy though. Just seems there should be one on my bench.......

Reply to
oldschool

I have some older DMMs. I get odd resistance readings the first time I set it to Ohms scale. By wiggling the plugs on the probes at the meter end, and occasionally working the range selector dial around a few times, I get a stable Ohms reading of about 0.4 Ohms with the probes shorted. Then, I get more sensible readings on circuits. So, these meters get poor contact on the range selector switch and the bannana jacks. So, that is one thing to check for.

Second, most DMMs use very low voltage to meke Ohms measurements. Analog meters often used 9 or even 22 V batteries for the Ohms range, to push enough current to move the needle on high resistance circuits.

If there are any seminconductors in your circuit, a DMM likely will not give enough voltage to forward bias any junctions, while an analog meter will.

Jon

Reply to
Jon Elson

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** A little WD40 or similar will fix that in seconds.

A short squirt under the edge of the switch followed by a few complete rotations is all you do. Clean up any excess.

Wet a cloth to do the 4mm plugs and a cotton bud to do the sockets.

..... Phil

Reply to
Phil Allison

For amusement I fetched the Avo Model 7 that I inherited from my father (goodness knows when he got it!). The plate on the back specifies 1.5V cell for < 1M, 2x4.5 for 1M, and external voltage sources for 10m and

40M ranges.

It also weighs 5lb (2+kg). I guess in those days servicemen were MEN!

Mike.

Reply to
Mike Coon

Yes, and built like a tank! I have a Model 7, too, IIRC it was made in

1943 and is still going strong. I'll wager when it was first produced, techs of the day marvelled at how light and portable it was. ;)
Reply to
Cursitor Doom

This is one I've never seen. I have seen Simpson and Triplett vom's but not this one.

I dont think I'd want any of these that need these special batteries. I have looked at some of the specs for the older Simpson 260 meters and some of the early models had oddball batteries too. The series 5 has D and AA batteries, and the series 6 an above have D and 9V.

I do have to ask, why these meters used BOTH the D cells and other batteries as well. (In other words, more than one kind of battery). Why didn't they just use one battery or one set of batteries for the whole device?

My Radio Shack and GB meters only have 2 AA cells, and work fine.

Reply to
oldschool

My 1920s v/i meter weighs a small fraction of that. Avos were high impedance accurate bench meters.

NT

Reply to
tabbypurr

"High impedance" back then, yeah. :)

Reply to
Cursitor Doom

Dunno. I'm sure someone here will, though. Fortunately, although 15V batteries are largely unobtainium these days,

10x1.5V AAA cells will get you there and the battery compartment is capacious enough (AVOs are anyway) to house them all comfortably.
Reply to
Cursitor Doom

They were, you could always get lower resistance meters for a lot less. Why it took so long for analogue meters to get FETs I don't know. My 1920s met er is moving iron, so the resistance is dreadful & it's nonlinear. But its worst shortcoming is that the case is the -ve electrode, you hold it in you r hand and it's bare metal. AND it's low resistance, so measuring radio/TV HT was always a fun experience. Maybe they figured if they killed their cus tomers there wouldn't be any requests for refund.

NT

Reply to
tabbypurr

A 1.5v cell is high capacity, high current & cheap. 15v batteries are low capacity, low current & never cheap.

NT

Reply to
tabbypurr

The Rx10K scale needs high voltage and low current to measure large resistors. The Rx1 scale needs low voltage and high current to measure the small resistors. One could use the high voltage 15v battery for measuring low resistances, but you would be replacing 15v batteries at an a alarming rate. Be thankful that the ancient designers of the VOM did not include a different battery for each resistance scale.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Not high enough. If you want to measure really high resistances, such as insulation leakage, you need a Megger (which is actually the name of the company that makes them but has become somewhat of a generic term for high voltage resistance testers): If you want to see if you really have water in the coax cables, you need one of these insulation testers.

I have an old and ugly meter, which has a hand crank generator. It produces enough voltage to have given me a rather nasty shock. It's fairly difficult to electrocute oneself while operating the crank, but I managed. Some modern Megger models still have such hand cranks generators: These small testers will deliver 1000V in order to measure up to 2000 Mohms. Now, does anyone still want to complain about 15v batteries?

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Come on Phil, lighten up. The guy didn't know and asked the question. We aren't all born full of knowledge; it's acquired by experience and asking questions.

Remember when you asked your mother why you have no friends, and she told you that you're obnoxious, pig headed and your feet smell like a fetid swamp? Same thing..

Reply to
John-Del

Yeah, I have one. They can still be used perfectly servicably if they're within spec. The people at Megger tell me that every so often, an old hand-crank version from the 50s or 60s will come in for re-calibration! The one I have is the 250V model which is relatively unusual here in the UK as almost all of them here are 500V. We would typically use the 500V version for testing our 240V domestic wiring. I guess the 250V model was intended for export to countries which use 110/120V.

The current range of Meggers are quite expensive, IRO $1200 but at least you don't have to crank them any more.

Reply to
Cursitor Doom

It's no problem. I plonked Phil some time ago on the advice of other posters here so I rarely get to see any of his unfortunate, socially- embarrassing outbursts. ;-)

LOL!!! Most amusing. >:*D

Reply to
Chris

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