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 Post subject: FET VOM = VTVM ?
PostPosted: Aug Wed 14, 2013 5:46 pm 
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Hello All,

One of the neat things about being into radio is that we get to use all these arcane acronyms!

I just signed up to get a used Radio Shack model 22-220, 22 FET VOM. The attractive thing about this piece of test gear is that it reportedly has an input impedance of 10 megohms. Would this make it the equivalent of a Vacuum Tube Volt Meter for work on radio receivers? I didn't check to see how low a voltage it will measure, and I suppose it might be useful to measure some very low voltages when doing receiver alignment.

Cheers,

Brian Burns


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Wed 14, 2013 5:54 pm 
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One concern with modern meters is maximum input voltage.
Products designed for working with solid-state equipment may only go up to 200 volts or so.

Vintage radios need ranges up to at least 500 volts, with 1000 volts DC being desirable.

The other feature of a VTVM is the isolating resistor in the probe. There's usually a switch to bypass it for AC/OHMS readings.
This isolates the reactance of the meter leads from the circuit being measured.
Without it, if you try to measure voltage in a resonant circuit, the meter leads may detune the circuit so far that it stops working.

To accomplish the desired function, the isolating resistor must be at the probe tip, not inside the meter.

- Leigh

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Last edited by Leigh on Aug Wed 14, 2013 5:58 pm, edited 1 time in total.

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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Wed 14, 2013 5:56 pm 
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Yes, they are basically the "more modern" equal to a VTVM. One like that is nice in that it has current measurement which is a function not found on many common VTVM's. A down side to those FET meters is they can be unforgiving if overloaded compared to a VTVM, so watch that you start in upper ranges when measuring unknowns and that you aren't in the ohms function when measuring voltage or current.

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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Wed 14, 2013 11:05 pm 
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The overloading issue is really a moot point and very overblown when you're looking a quality instrument of shop or instrumentation grade. FET and TVM's are small, light, very low powered and lend themselves to be easily incorporated in inexpensive designs that make compromises such as low end VOM-like instruments. These can't hardly be compared to the build or function of a tradional lab or service grade VTVM. If you move away from these VOM type TVM's and compare the features of a FET analog of a service VTVM such as B&K's 290, Hickok's 370, RCA's FET Volt Ohmyst.
you see the differences.

The units are different in:

They are copiously input protected with multiple spark gaps across the inputs and from circuit ground to earth, breakers or fuses, and some form of input protection to the FET side of the differential amp. Be it diode or transitor clippers that never let more then a ceiling voltage ever reach the gate or punch through it.

They have center zero capability.

They have scales in PP and RMS voltage

They DO have isolation probes with a resistor switched on the DC side for DC voltage.

They in fact have higher input impedance in many cases then the VTVM's

They usually offer better sensitivity then VTVM's with ranges below a half of a volt full scale. Lowest scales of .05, .03, even .01 are found.

Service fet voms never got to mature properly because of the concentration of efforts on DVM's. There were few hints though where these instruments could have gone such as autopolarity, 20K and 1K VOM emulation, peak hold, increased frequency response similar or better then a VTVM, RF immunity, autoranging, self zeroing. There were some laboratory quality meters that had these features, but never received much attention or were quietly discontinuted. Some service grade meters actually did incorporate autoplarity such as Heath and Hickok. Radio Shack and Helper Instruments offed autopolarity and autoranging that were very short lived, but worked well. Progress halted in favor of DVMs

As it stands, the bulk of the better meters are in fact just as equal to a vtvm except for two areas:
RF immunity
Upper frequency limit a few hundred hertz as opposed to 1 meg or better with a VTVM.

There were some that did address these issues, but for the most part development stopped. There was too much of a temptation and incentive to build these in inexpensive and plastic cases or follow a conventional, VOM analog format with conventional leads without a formal, shielded probe and using inexpensive meter movements rather then a quality taut band movement. Many old timers as evidenced would never accept these as the real deal no matter what improvement or feature was added. Furthermore, these were more suited for solid state electronics often with low powered ohms ranges. Tube equipment was on the wane, so FET VM's quit trying to emulate VTVM's. It was hard to convince someone to buy a unit with a few transistors and 2 fets that weighs nothing compared to the heavy build of a VTVM. The early stigma of problems with bias settings, drift, matched devices and the headaches early adopters encountered was never shaken off, so few companies bothered to invest in higher spec instruments at the service level even though by the mid 70's they had all been overcome. Just like with anything else, self proclaimed experts and those with absolutely no experience will parrot the same nonsense essentially poisoning the well for further development. The industry can be blamed for rolling out early products with unrealistic expectations that bugs, current state of art, and problems doomed the future.


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 12:02 am 
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The big complaint with FET VOMs, when they first appeared in the early 1970s, was that they could not tolerate an occasional "zap." An inadvertent static discharge, accidental contact with high voltage, or even a big spark nearby would blow the FET(s) and the meter would be on its way back to factory service. After that happened a couple of times, most technicians simply went back to their trusty--and comparatively indestructible--VTVMs. By 1975, FETs with protected gates were available, and some of the last FET VOMs used them. But by then, FET VOMs had a bad reputation for being too fussy and delicate, and a British company named Sinclair had just put the first small low cost (under $250.00) LED DMM on the market.

The Radio Shack 22-220 is probably one of the most popular FET VOMs ever made. It worked well and was reasonably priced. About the only negative thing I ever head about them was that they were not very rugged. One tumble off the bench and it's all over, 'cept for the crying! Radio Shack went the opposite direction of B&K on this. Instead of building a solid state version of a VTVM that ran on 9-volt battery, they tried to make the 22-220 like a traditional VOM, but with a lot more sensitivity on DC volts thanks to the transistors.

The good news and the bad news is that this meter is intended to work with ordinary VOM probes with banana plug ends. Good because they are readily available and replaceable. Bad because there's no provision to attach a shielded cable for low level DC measurements or RF. Maximum voltage across the probes is 300 VAC/VDC unless you move the positive probe to the DC 1-kV or AC 1-kV jacks. But be aware that the housing is only rated for 500 volts, so in order to measure higher voltages you have to put the meter on an insulated surface, connect it to the de-energized circuit with alligator clips, then turn the power on from a safe distance!

So no, it's not exactly the same as a tube VTVM. But it will work as well as a modern DMM for antique radio troubleshooting, and its analog display gives it an advantage for the kinds of troubleshooting where you want to note trends rather than absolute numbers.

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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 12:27 am 
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Hello All,

Many thanks for all the excellent background on the FET VOM!

It's coming to me for nickles and dimes, so if the only thing I ever use it for is measuring current, it will be worth it.

So I suppose that I will be in the market for a VTVM at some point. I had a couple of old HP VTVM's given to me a few years back and they were great while they lasted. Both eventually went south, and were above my pay grade to repair, so I traded them off to guys on the HP test equipment forum who would restore them.

Thanks again for your help!

Cheers,

Brian


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 2:03 am 
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i got this one from the Shack when they had a closeout on them, 22-216 Auto Range, Auto Polarity FET.
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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 3:15 am 
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Neat Lou.

I followed a link from a discussion to Utube and seen one of these for the first time in action. My only previous knowledge of a service grade auto polarity was the Heath IM-5228, Helper VA502 and the Hickok 370. Only the Helper and this meter I've ever see do autoranging too. Maybe there were others or those that never made it stateside. I never knew that RS sold such a meter and rarely go in one anymore.


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 4:09 am 
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Lou deGonzague wrote:
i got this one from the Shack when they had a closeout on them, 22-216 Auto Range FET.

Amazing! I never even knew that there were any auto-ranging mechanical meters made, or that such a device could even be built. That shows how much I was paying attention, I guess.

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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 4:43 am 
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This is the Helper Instruments Voltadder VA502 first generation. It not only is autoranging and auto polarity, it will display both AC and DC voltages simultaneously, peak hold, and a scope output on the rear. It's also portable and can be manually ranged. There's gell cells on the inside that are trickle charged during normal use so it's AC/DC . The second gen had some other features added to it had different cosmetics. Very few were made of either generation compared to the thousands of their other pieces of gear they made for the communications industry. This was made during the 80's, well into DMM autoranging meter manufacturing to compensate for the lack of high impedance analog movement meters short of buying a HP-427, cheap VOM-like FET meter or DMM with a tiny relative analog meter that in most cases was too small to catch tiny dip or peak.

Short of a used market, there simply wasn't anything new for corprate comm shops with a big equipment budget to source from a catalog outside of HP at the time. Moto did work with Triplett on several variations of their telephone test sets that could act as a solid state VOM and do double duty as a twisted pair test set using their model 3 and 4 Telco TVM's as the base of the design. Several of the manufacturers of radio test sets incorporated some form of analog reading DC meter into their sets be it a very responsive bar graph on a CRT or actual meter movement. Some early ones were accessories that fit inside the lids of test sets from Motorola and IFR to compensate for the lack of sudden unavailablity of analog meters. Nothing has really changed since.


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 2:34 pm 
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Here is an interesting wide ranged (100 uV to 1 kV scales) lab type FET-VOM from Millivac that complimented the looks of their RF voltmeters. Apparently they didn't sell many of these at their $695.00 asking price in 1979, as I have not seen another one for sale since I got this one for $7.95 on E-bay a couple of years ago. This is the most accurate analog DC multimeter I have encountered.

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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Thu 15, 2013 5:08 pm 
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I never knew millivac made those and DC/ohms only too which makes a lot of sense considering most multimeters wee desinged with an AC companion meter. For a minute, I thought I was looking at my shop's 828's after a long diet. Nah, no pusbuttons on the side. If this one behaves from my experience with the RF millivoltmeters, it's going to be long and trouble free. I bought mine used during the early 80's,retired from Moto's Plantation facility from a local surplus dealer when they closed it down at the time. I've never had a moments problem with either of them and dependable as the day's long. Yours makes me think a little about Hickok's two solid state 410C clones. I guess it's the meter movement.


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Sun 18, 2013 1:33 pm 
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I'll add to the pile here. I use this Hickok FET more than the VTVM on my bench. The auto polarity is great for when I'm doing a little probing in unknown areas. This one looks like it's the same as the Hickok model 370.

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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Mon 19, 2013 1:29 am 
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Great info! I wondered about these myself, as DMMs were on the rise when I first went to school for electronics.
Chris108 wrote:
...and its analog display gives it an advantage for the kinds of troubleshooting where you want to note trends rather than absolute numbers.

I read this a lot. Am I the only guy out there with a decent bar graph on my DMMs? :mrgreen:
The response is way faster than any analog movement I've ever used.


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Mon 19, 2013 7:52 am 
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Quote:
I read this a lot. Am I the only guy out there with a decent bar graph on my DMMs?
The response is way faster than any analog movement I've ever used.


I guess if you read further on the subject we've posted here that it's not the speed at issue, it's the resolution of the bar graph displays. Even with the bar displays that allow you to expand a bar(s) to further resolution don't cut it sometimes for a trend. A multiplier stage for example will show a tiny, if almost imperceptable dip in the previous stage as it reaches resonance before there's output in it's own stage stage, especially if there are several tuning elements in it or very hi Q situation like several helical resonators. This wouldn't be uncommon in a FM set at VHF multiplier chain. When a bar graph display emulates that resolution at a reasonable price, I'm ready to fold my analog inventory. So far, my experience hasn't seen that. Most stuff is broadband nowadays and probably not even tunable save for some software control over varactors, but in older equipment or restorative work, you betcha it's necessary.

Steve, I'm envious !


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Mon 19, 2013 9:01 am 
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The best I found .
Philips PM2404 Linear scales for all including Ohms .


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Mon 19, 2013 10:28 am 
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Very clean, pleasant on the eyes, and uncluttered ! Unfortunately we never seen much of the euro equipment in the states during that time. Anything overseas came from Japan or Taiwan by contract, very little other arrived. Marconi, Rhode & Schwartz and many others had some nice products seen on the net. Not here. If there was any further analog development, it was probably for the military. What we're posting here was as good as it go beyond the HP-427. Philips, Marconi, Seimens, and others did have a presence here, but not most of their equipment beyond scopes,industry specific specialized products and biomedical equipment. The parent company of AVO is here too and never bothered to import their VOMs. If they did, they made a secret of it. Any tech that would have handled an AVO would have instantly fell in love with them and maybe the 260/630 fetishization would have finally broken.


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 Post subject: Re: FET VOM = VTVM ?
PostPosted: Aug Tue 20, 2013 11:35 am 
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Dawn wrote:
Quote:
I read this a lot. Am I the only guy out there with a decent bar graph on my DMMs?
The response is way faster than any analog movement I've ever used.


I guess if you read further on the subject we've posted here that it's not the speed at issue, it's the resolution of the bar graph displays...


I see. That makes sense, but on the 6 volt range, my resolution is .1875 volts per segment. I figured that was good enough considering I use it to get close, and then read the digits as I slowly zero in on the maximum output.

Don't get me wrong, I still use an analog VOM and a VTVM, but manage fine with the DMM.


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