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.
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.
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.......
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.
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!
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.
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.
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.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
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?
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
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..
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.
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. ;-)
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