I guess that you didn't pay attention ; that "hard shiney (sic) plastic probe" could have micro stress cracks and with perspiration could become conductive in the outside and give you a more than healthy "tingle". Play it safe and replace it with a modern one.



Yeah, I guess that there is a bit of OCD at work here too.

As some else stated, if you want to DISPLAY it with old test leads, by all means do so; but if you want to SAFELY use it, get new probes for it.

Kind of like getting a 1950s car, then getting old 1950s tires and taking if for a drive in the freeway; it might look "period" but likely you'll blow one.

I still have [and use!] the HK IM-28 VTVM I built around 1973 and used in my work bench while I worked at one of the Heathkit service centers. However, I never use it for anything higher than about 100 V DC out of respect for the old wiring in the probe.

The reason why I modified my VTVM's is to make their usage more like a modern multimeter, with normal test leads and being able to take floating measurements, separated from ground level.

I think that is a good modification HG. One aspect I don't like with the HP 410B VTVM is that the common lead is at ground. I know it's a personal preference thing, but I much prefer the leads to be floated on a VTVM as they are with my Simpson 303. It might be in my mind, but it gives me an added feel of safety.

Modern vinyl insulated test leads are considerably safer than old rubber insulated ones. As a general rule, don't use 40+ year old rubber insulated test leads, its just asking for a needless accident.

-Mark-

If you don't mind spending a little more, you can buy new test probes from the larger industrial electronics suppliers (Allied Electronics, DigiKey, Mouser, and Newark--plus many smaller companies) that look and feel exactly like the old ones. The designs have not changed all that much in the last 60 years!

As for modifying a VTVM to make it work more like a VOM, I think it is safe to say that a lot of clever engineers worked on these products in many different manufacturing companies over the years, and they looked at every aspect of the design. Somebody must have investigated the possibilities of isolating the circuitry from the case. It would be a neat selling feature--maybe a whole new product line--if it offered some real benefit and did not detract from performance in other ways. If you want a VOM that contains one tube so it can be used as a DC VTVM, the Triplett 631 is a neat gadget. I believe Simpson had something very similar. The Triplett 631 is housed in a bakelite case same as the famous 630 VOM and runs on batteries, so there are no concerns about line isolation.

Hi,
Well...I got my working HK V-7A and after letting it warm up and zeroing,it told me a 1.5 volt battery was really 4 volts and a 47 ohm 1/2 watt resister was really 22,000 ohm? Whats funny is my little POS chi-com VOM from HF hits it on the money! I admit to radio electronic ignorance but I am known to be able to read a VOM,any ideas?

Did you by any chance doused the range switch with contact cleaner?, as in really soaking it?

Hi,
No,the VTVM is suppose to be good to go. Just wondering if it's me or the meter???

Does your DC Probe have a 1 Meg resistor inline?
If not, you'll probably get an incorrect reading, as it's designed for 11 Meg total - 10 inSIDE the VTVM, plus 1 more Meg out on the probe.

See the Probe Assembly page (PDF page #6)
http://www.jogis-roehrenbude.de/Roehren ... -A/V7a.pdf

AC & Ohms are both straight through, however, dunno if it's been calibrated, or has a good '1.5 Volt' battery in the Battery connector, etc.

Mike Y
KM5Z

What exactly do you mean by "supposed to be good to go"?

Typically, any VTVM you get on Ebay or similar needs to be calibrated, even those claimed to be "in working condition". It may have been a while since it was calibrated, and these do tend to drift over time. Also, once calibrated, let it run for 48 hours and check again.

Also, as noted previously, be sure to check that your probe has the 1M resistor in it for DC measurements.

I would get DC voltages working first. Check against a couple of known reference voltages across different ranges.

Once you get that working, then try AC volts and ohms. Personally, I found the ohms measurement to be the least useful of VTVM functions. A cheapie DMM will give you just as good readings in most cases. If the ohms scale is not reading accurately, the first thing I would do is change the battery.

I'll take it this is the meter purchased from member SmoothOscillator in the classifieds? If so, I would simply message him and find out either what might be going on with the meter or what you might be doing wrong. From my experiences here the respectable members will stand behind what they sell.

-Mark-

Hi,
Yes it is from Smooth Oscillator and he offered to take it back but that is not my intention. I want the VTVM but I'm sort of perplexed why it won't test a 1.5 watt battery correctly. It has the resister in the probe. I'll keep playing with it as I assume it's operator error on my part. What I really need is a mentor to get me goimg but don't know any? Thanks!

Heh - takes some getting used to...

Make sure the DC probe is in the DC socket as well, and Ground probe to the Ground socket. Sorry; just checking(!) - sound dumb to ask, but I've made that mistake a dozen times.
Like: Measure Twice, Cut Once. Belt + Suspenders, etc.

Function switch to DC+. Range switch to 1.5V.
Should see the meter zoom a little past the 1.5 mark and stop on or near the little red tick just to the right of 1.5V.
They presume a 'fresh, 1.5V dry cell' is 1.55 Volts (approx) and that's where the tick mark is.

Adjust the DC Cal pot (in back, with the cover off, CAREFULLY - there be high voltage dragons there) 'til it does.
Use a non-metallic screwdriver if you have one - that just minimizes the chance of shorting something.

If it's still way off, might be something in the DC Cal pot, or possibly (but not likely) the Range Deck (the one with 7 Meg, 2 Meg, 700K, 200K, etc...). If the range or function switch is soaked with 'Deoxit', that could play havoc with it...

Mike Y
KM5Z

Hi Mike,
Thanks for the help. Now when I do a voltage test AC or DC+ the needle just pegs all the way to the right! I have another HK V-7a that I bought for $5 and it works as well as the $40 one,go figure! I'll have to find someone that can help me out as it seems to get worse as I go along! Thanks again!