# WTB VTVM

G

#### [email protected]

Jan 1, 1970
0
Hi Folks,

I'm working on an old tube radio (Zenith) and could really use a VTVM. I don't have much money at all. I'm looking for something very basic and cheap. I had an old Heathkit but the meter/dial broke.

Thanks,
Providence, RI

W

#### William Sommerwerck

Jan 1, 1970
0
You can get a cheap DVM for a couple of bucks at Harbor Freight.

T

#### tm

Jan 1, 1970
0
Hi Folks,

I'm working on an old tube radio (Zenith) and could really use a VTVM. I
don't have much money at all. I'm looking for something very basic and
cheap. I had an old Heathkit but the meter/dial broke.

Thanks,
Providence, RI

And the reason it needs to be a VTVM and not a $25 Radio Shack digital multimeter is? C #### Cydrome Leader Jan 1, 1970 0 tm said: And the reason it needs to be a VTVM and not a$25 Radio Shack digital
multimeter is?

the input impedance on throw away (and even good bench) meters can be
really really low on some ranges, like AC.

I use a Leader FET input? meter for stuff like this. These replaced VTVMs
but retain the essentially zero load on the test circuit.

One use is for takings readings with the 40kV Fluke probe. You'll get
dangerously low readings with pretty much any digital meter on AC ranges.

T

#### tm

Jan 1, 1970
0
the input impedance on throw away (and even good bench) meters can be
really really low on some ranges, like AC.

I use a Leader FET input? meter for stuff like this. These replaced VTVMs
but retain the essentially zero load on the test circuit.

One use is for takings readings with the 40kV Fluke probe. You'll get
dangerously low readings with pretty much any digital meter on AC ranges.

Good point. However you will pay an accuracy premium on the DC and Ohm
ranges. Most VTVMs will only give 3% FS vs <1% with even the cheap DVMs.

M

Jan 1, 1970
0
tm said:
And the reason it needs to be a VTVM and not a 25 Radio Shack digital multimeter is? Because a VTVM can be much better at RF frequencies. Also, your old service data gave alignment and measurement instuctions referencing a VTVM. However, by the time you add in an RF probe (missing from most VTVM's you see on eBay) on a good working VTVM, you could probably buy a decent working 100mHz 'scope instead. Back in the day, a VTVM was way cheaper - most people had no hope of buying a 100 meg 'scope. MarkZ . H #### [email protected] Jan 1, 1970 0 Because a VTVM can be much better at RF frequencies. Also, your old service data gave alignment and measurement instuctions referencing a VTVM. However, by the time you add in an RF probe (missing from most VTVM's you see on eBay) on a good working VTVM, you could probably buy a decent working 100mHz 'scope instead. Back in the day, a VTVM was way cheaper - most people had no hope of buying a 100 meg 'scope. MarkZ . The little HF LCD display multimeters seem to have pretty high input impedance, but I've never actually tried to measure it. Certainly much higher than the old Simpson multimeters. T #### tm Jan 1, 1970 0 Because a VTVM can be much better at RF frequencies. Also, your old service data gave alignment and measurement instuctions referencing a VTVM. However, by the time you add in an RF probe (missing from most VTVM's you see on eBay) on a good working VTVM, you could probably buy a decent working 100mHz 'scope instead. Back in the day, a VTVM was way cheaper - most people had no hope of buying a 100 meg 'scope. MarkZ . The little HF LCD display multimeters seem to have pretty high input impedance, but I've never actually tried to measure it. Certainly much higher than the old Simpson multimeters. +++++++++++++++++++++++++++++++ Now, the 100 meg scope would be my choice for working on the old tube radios. Should be able to pick up a good analog 100 meg scope for (way)under100.

A good Tek 475 will do 250 megs. Just set up a search on ebay and wait.

Regards,
tm

B

#### Bill Gill

Jan 1, 1970
0
the input impedance on throw away (and even good bench) meters can be
really really low on some ranges, like AC.

I use a Leader FET input? meter for stuff like this. These replaced VTVMs
but retain the essentially zero load on the test circuit.

One use is for takings readings with the 40kV Fluke probe. You'll get
dangerously low readings with pretty much any digital meter on AC ranges.
I worked on tube type radios for many years and never needed
a VTVM. A normal multimeter worked just fine.

Bill

T

#### tm

Jan 1, 1970
0
Bill Gill said:
I worked on tube type radios for many years and never needed
a VTVM. A normal multimeter worked just fine.

Same here. That plus a good scope (2465B). Meets 90% of the needs. A useful
tool that helps save time is a good ESR meter to do a fast check of the
filter caps.

M

#### Michael Black

Jan 1, 1970
0
Because a VTVM can be much better at RF frequencies. Also, your old service
data gave alignment and measurement instuctions referencing a VTVM.
That was mixed. One minute it would be a high impedance point, like the
agc bus in a receiver, and the schematic would warn about using a VTVM.
But in other cases, the schematic would indicate that the the measurements
were made with a Simpsons 260 VOM. The high impedance point needed the
high impedance meter, but the VOM points expected the load of the VOM, so
using a VTVM meant the readings would be high. I thought the schematics
with the VOM measrued voltages were because the serviceman was more likely
to have VOM, so might as well list what they'd see.

VTVM weren't so great at RF. Generally you needed an external RF
detector. Yes, the 410B has a wonderful RF probe, but that's ane
exception. The average VTVM had an "AC probe" but had the diode in the
case, so there was that long cable to it, not making it suitable for RF,
even if the diode was okay for it.

The same with DMMs, lousy for RF if for no other reason than the diode
isn't at the probe. One could build an RF probe, plug it into the DC jack
on the DMM, that used to happen a lot and it makes no difference whether
the meter is a VTVM, a DMM or a VOM (except for the loading of the last).

The problem with DMMs is that the readout is digital. Not good for
peaking or watching treands. Once upon a time, decades ago, one company
had a DMM with a small analog meter for peaking purposes, but that never
was a trend. Instead, the best we get is a bargraph on an LCD readout, but
the one I have is so slow that it's not useful for peaking.

One might as well dig out a decent size meter (or even a tuning meter off
an old stereo) add a jfet input op-amp, and build a relative DC meter for
peaking. Don't fuss about calibration, the DMM is for absolute voltage.
You don't even need an stepped attenuator with precision resistors, just a
few divider resistors or even a high value pot. Since it's all relative,
you just need something to ensure the input doesn't deflect the meter too
much. And then use it when you need to peak.

As for finding a VTVM, a couple of years ago I was walking along a street
I'd not been on for a long time, lots car traffic though less foot
traffic. I look down, there's a cardboard box on the sidewalk, and inside
is an RCA Voltohmyst VTVM. Seems intact, I've never gotten around to
plugging it in. That was all, no other neat stuff no other garbage, and
no obvious store where it might have come from.

It becomes all a more interesting story since I probably was the only one
walking along there who'd recognize what it was and bring it home. If I'd
not gone that way for some extraordinary reason, the meter would have gone
to the garbage dump.

Michael

T

#### tm

Jan 1, 1970
0
Michael A. Terrell said:
The 1 Meg resistor in the probe for DC measurements in the RF
sections. It prevents detuning by isolating the cable capacitance from
the circuit you're testing.

Yes, that is why I sometimes wrap a resistor on the tip of the probe to make
the measurement.
Of course with a scope probe, that's not a problem.

Still, there is much one can do with a Harbor freight \$10 DMM.

I wonder, if you could only have one instrument, what would you choose? If
cost were the main consideration?

M

#### Mark Zacharias

Jan 1, 1970
0
Michael Black said:
That was mixed. One minute it would be a high impedance point, like the
agc bus in a receiver, and the schematic would warn about using a VTVM.
But in other cases, the schematic would indicate that the the measurements
were made with a Simpsons 260 VOM. The high impedance point needed the
high impedance meter, but the VOM points expected the load of the VOM, so
using a VTVM meant the readings would be high. I thought the schematics
with the VOM measrued voltages were because the serviceman was more likely
to have VOM, so might as well list what they'd see.

VTVM weren't so great at RF. Generally you needed an external RF
detector. Yes, the 410B has a wonderful RF probe, but that's ane
exception. The average VTVM had an "AC probe" but had the diode in the
case, so there was that long cable to it, not making it suitable for RF,
even if the diode was okay for it.

The same with DMMs, lousy for RF if for no other reason than the diode
isn't at the probe. One could build an RF probe, plug it into the DC jack
on the DMM, that used to happen a lot and it makes no difference whether
the meter is a VTVM, a DMM or a VOM (except for the loading of the last).

The problem with DMMs is that the readout is digital. Not good for
peaking or watching treands. Once upon a time, decades ago, one company
had a DMM with a small analog meter for peaking purposes, but that never
was a trend. Instead, the best we get is a bargraph on an LCD readout, but
the one I have is so slow that it's not useful for peaking.

One might as well dig out a decent size meter (or even a tuning meter off
an old stereo) add a jfet input op-amp, and build a relative DC meter for
peaking. Don't fuss about calibration, the DMM is for absolute voltage.
You don't even need an stepped attenuator with precision resistors, just a
few divider resistors or even a high value pot. Since it's all relative,
you just need something to ensure the input doesn't deflect the meter too
much. And then use it when you need to peak.

As for finding a VTVM, a couple of years ago I was walking along a street
I'd not been on for a long time, lots car traffic though less foot
traffic. I look down, there's a cardboard box on the sidewalk, and inside
is an RCA Voltohmyst VTVM. Seems intact, I've never gotten around to
plugging it in. That was all, no other neat stuff no other garbage, and
no obvious store where it might have come from.

It becomes all a more interesting story since I probably was the only one
walking along there who'd recognize what it was and bring it home. If I'd
not gone that way for some extraordinary reason, the meter would have gone
to the garbage dump.

Michael

The voice of experience! Thanks!

Mark Z.

W

#### William Sommerwerck

Jan 1, 1970
0
If you could only have one instrument, what would you choose?

Easy. One of those super-whammy digital LCD scopes that can measure just about
everything.

If cost were the main consideration?

A budget model of one of those.

J

#### Jamie

Jan 1, 1970
0
William said:
Easy. One of those super-whammy digital LCD scopes that can measure just

A budget model of one of those.

Yeah

A Hantek DSO 8060

Jamie

M

#### Mike Cook

Jan 1, 1970
0
the input impedance on throw away (and even good bench) meters can be
really really low on some ranges, like AC.

I use a Leader FET input? meter for stuff like this. These replaced VTVMs
but retain the essentially zero load on the test circuit.

One use is for takings readings with the 40kV Fluke probe. You'll get
dangerously low readings with pretty much any digital meter on AC ranges.

Would you explain please? Is the FET input impedance so much higher that it
loads less than even a high-quality DMM?

Thanks.

C

#### cjt

Jan 1, 1970
0
On 03/22/2013 09:43 PM, [email protected] wrote:
The little HF LCD display multimeters seem to have pretty high input
impedance, but I've never actually tried to measure it. Certainly
much higher than the old Simpson multimeters.
The "Simpson multimeters" were generally VOMs rather than VTVMs, no?)

J

#### Jamie

Jan 1, 1970
0
Mike said:
Would you explain please? Is the FET input impedance so much higher that it
loads less than even a high-quality DMM?

Thanks.

My Fluke 289 will measure correctly with the 40k probe doing
AC or DC. So won't a couple of older flukes I have.

THe only problem with AC and a HV-probe is you need to keep
the frequency down, otherwise, the small amount of cap present
on the input of the meter is going to attenuate it some.

It's very possible cheap meters on AC mode may not behave correctly
with a HV-PROBE and show great losses in readings.

I also have a scope meter that works very well with the HV-probe.
I just need to remember to scale the input properly.

Jamie

K

#### klem kedidelhopper

Jan 1, 1970
0
I think those were the most common.  In my lab I have one Simpson VOM
and two Simpson VTVMs.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot nethttp://electrooptical.net

But there is another factor here that no one has considered. The OP
may be used to working with an old tube type VTVM of the kind he no
longer has and would be very comfortable with a similar replacement. I
still have and use my RCA Senior Voltohmyst that I built in 1962. And
it still keeps good time. Lenny

C
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