[Marsupial]

today I got a VTVM from a co-worker. Its a heathkit that his dad used to diagnose and repairs radios in the 70s.

[Image: http://i146.photobucket.com/albums/r253/...9383-1.jpg]

Can this be disaligned? Its a 2-tube circuit.
I read nothing at all in ohms.

in VCC I cannot decipher the readings... if I place it to 1.5V and read a battery that my digital voltmeter reads at 1.2V, it show something that varies between 7 and 9.

Maybe I need to do a recap job?

[Sean]

That's similar to the Heathkit VTVM I have. Yours looks like a later model. There probably are not many capacitors in it that would need to be replaced. Maybe just an electrolytic or two and maybe a weak selenium rectifier. For resistance it most likely needs a battery, usually a C cell. For measuring voltage from a battery you want to have it set for DC+ and the 1.5V range. Connect your two leads together and adjust the Zero Adjust pot for a zero reading. Connect the leads to the battery and read the measurement from the 0-15 scale. Mine has the original probe that has a switch that selects if you are measuring DC or AC voltage. In the DC position a 1Meg resistor is switched in.

[Marsupial]

Thanks Sean!

I didn't think there was a battery inside, being that we plug the VTVM on the mains...

I have the original probe with switch.

When placed in DC at 1.5, the reading for a 1.2 battery was at around 8 on the meter. How do we compute the value? 10 would correspond to the multiplier I selected? i.e. 1.5V?

I never had a meter with analogic/needle display like that before.

From what I read online, this IM-5218 VTVM is a "modernized look, same circuit" of the previous IM-18.
Are those VTVM desirable for radio repairs? I have a digital multimeter already, do I gain anything by using a VTVM, other than the coolness factor?

[Carl Travis]

The schematic for an IM-18 can be found here:
http://www.vintage-radio.info/heathkit/index.htm

[Sean]

With the meter set for the 1.5V range, you would read the 0 to 15 on the meter as 0 to 1.5V. So 8 on the meter would mean you are measuring 0.8V. You might have some dirty contacts on the switches or some resistors that have drifted in value.

Having an analog meter is nice when you are aligning a radio. Since you are typically looking for a peak in output during alignment it is easier to distinguish by watching the needle move as opposed to a DVM. DVMs usually are somewhat slow in updating.

I have two VTVMs that I have swapped back and forth on my bench over the years. The Heathkit is a somewhat compact model that is good for general purpose work. I also have a Hewlett Packard 410B that is capable of measuring AC with a flat response up to about 700Mc. That one is a little cumbersome with the way the leads are attached to the meter and the large shielded AC probe.

[exray]

If you have a strong broadcast station in the area (like I do) a VTVM can act very funny.

[Marsupial]

sean, you're right. There was an empty socket for a C battery inside. Now I can manage to read resistors!

I still have bad readings on voltage, however.

[Radioroslyn]

How bad? Did you set the meter at zero with the zero adj pot?
Terry

[Marsupial]

A battery that reads 1.2V on my digital multimeter reads around 0.75V on the VTVM - and seems to slowly climbs to 0.8V. The value is eve less coherent if I change to the 15V scale.
I can adjust the zero, but it won't give me that kind of difference. Can such a difference be caused by the current pull of the VTVM compared to the DVM? Maybe the battery really reads less when connected to the VTVM.... I don't really have lots of test subjects to prove the values of the VTVM, to be honest.

Also, can it be caused by one of the tubes? What's the usage of the tubes in those voltmeters?

I would have liked to be able to easily tune it in place... I already have a voltmeter. The VTVM matches my "brand new" heatkit scope, but otherwise might not be what I use the most, so I don't really feel like doing maintenance on it; at least until I have the radio fully restored. (maybe I need vacations, to keep up with all the "projects" I have?)

[Radioroslyn]

When you have it open I believe that there is a pot that adjusts the 3 volt scale. Double check the manual as it been a long since I've had one apart.
Terry

[Marsupial]

Being that it was a kit, I assume there would be tons of pots to adjust... but it didn't slip my mind to check for that. Thanks for the suggestion!

I'll dig the manuals and try it out!

[scottericsonon]

if you don't have the manual I have a copy of the service manual for a V-7A I'd be happy to send you if it helps.....

calibration is'nt hard on these.....

basically set the DC for + and the range for 1.5V ....use the "zero adjust" to set the needle on zero and hook your test leads to a fresh "C" battery ..........adjust the DC calibrate till the needle shows 1.5VDC (full scale)

to adjust OHMS set the meter on "OHMS" and tweak the OHMS adjust knob on the front for full scale (infinity) making sure the test leads arent touching anything

then touch test leads (AC/OHMS and COMMON) together and meter should drop to zero

for AC remove the AC/OHMS test lead and tweak AC BAL so that need has minimal movement when cycling switch through AC/DC+/DC-

plug your AC test lead back in and set range to 150V with the other knob set to AC

connect your AC/OHMS probe and common to a regular AC outlet (I like to use the surge protector thats already on my bench and I'll stick a small "pocket" screwdriver with a plastic handle into one side of the outlet to clip my "common" onto)

adjust AC CAL untill meter shows 117V

if your meter is the same face as mine that will be just past 1/2 way between 11 and 12 on the DCV/ACV scale

to read the scales

on the 1.5V range:
for voltage...use the 0-15 scale and move the decimal one place to the left....
(9 on the scale is .9 ....15 is 1.5 etc)

5V range:
for voltage use the 0-50 and move the decimal one place to the left....
(25 on the scale is 2.5 etc)

15V range...use 0-15V scale and no correction needed 7v shown is 7v

50V range...use 0-50V scale and no correction needed 25v shown is 25v

150v range use the 0-15 scale and add a zero 11V becomes 110V etc

500V range use 0-50 scal and add a zero...25V shown is 250v

1500v range use the 0-15 scale and add two zeros 11V becomes 1100V etc

this works the same way whether you are using AC OR DC

to use for ohms just keep adding zeros

RX1 needs no correction but above that it's all multiples of 10...

RX10= resistance TIMES 10 = 20 shown =200 OHMS.

RX100= resistance TIMES 100 = 20 shown =2000 OHMS.

and so forth...make sense?

[Carl Travis]

I believe that there are only 3 adjustment pots inside - AC & DC calibration and AC bal.

[Marsupial]

Thanks!

that's going to be very helpful.

With all this being said, I doubt I'll need a manual after all.

[Marsupial]

Thanks a lot for that calibration procedure, scottericsonon. I now have a VTVM that gives me very similar readings to my DVM, with less digitals in the reading

It is somewhat tricky to do, but once done it makes this a very valuable device.

the probe I have is switchable between DC and ohm/AC. Could I make replacement probes? What's the switch doing?