Hi, Vacuum-landers,
I just acquired an old Heathkit VTVM, model IM-38. It seems to work
but has significant error when compared directly with my workshop
Fluke 111. Also, the ranges read differently, e.g. 100 VAC on the 300
VAC range (1/3 up scale) is not exactly full scale on the 100VAC
range, etc.. The pot at the back affects all ranges so I can't see how
to calibrate each range.

Any ideas? Does anyone have a schematic they would be prepared to
copy for me, or a manual to sell?

Thanks and cheers,

Roger

Engineer wrote:
Hi, Vacuum-landers,
I just acquired an old Heathkit VTVM, model IM-38. It seems to work
but has significant error when compared directly with my workshop
Fluke 111. Also, the ranges read differently, e.g. 100 VAC on the 300
VAC range (1/3 up scale) is not exactly full scale on the 100VAC
range, etc.. The pot at the back affects all ranges so I can't see how
to calibrate each range.

Any ideas? Does anyone have a schematic they would be prepared to
copy for me, or a manual to sell?

Thanks and cheers,

Roger


On many of those old meters, the probe has a resistor in it that is part
of the voltage divider. Are you using the original probe? Check the
resistor in it. Usually on dc it has 1 Megohm if I remember correctly.
I don't remember about ac.

"Bob" wrote in message
...
Engineer wrote:
Hi, Vacuum-landers,
I just acquired an old Heathkit VTVM, model IM-38. It seems to work
but has significant error when compared directly with my workshop
Fluke 111. Also, the ranges read differently, e.g. 100 VAC on the 300
VAC range (1/3 up scale) is not exactly full scale on the 100VAC
range, etc.. The pot at the back affects all ranges so I can't see how
to calibrate each range.

Any ideas? Does anyone have a schematic they would be prepared to
copy for me, or a manual to sell?

Thanks and cheers,

Roger


On many of those old meters, the probe has a resistor in it that is part
of the voltage divider. Are you using the original probe? Check the
resistor in it. Usually on dc it has 1 Megohm if I remember correctly.
I don't remember about ac.

For AC, the 1-meg resistor is shorted by the probe switch.. The switch
could be bad or dirty.

Pete

On Sat, 18 Dec 2004 12:49:23 -0500, "Engineer"
wrote:

Hi, Vacuum-landers,
I just acquired an old Heathkit VTVM, model IM-38. It seems to work
but has significant error when compared directly with my workshop
Fluke 111. Also, the ranges read differently, e.g. 100 VAC on the 300
VAC range (1/3 up scale) is not exactly full scale on the 100VAC
range, etc.. The pot at the back affects all ranges so I can't see how
to calibrate each range.

Any ideas? Does anyone have a schematic they would be prepared to
copy for me, or a manual to sell?

Thanks and cheers,

IM-38 isn't here, but you should find the input divider and cal are
similar in the IM-18. Later changes are often cosmetic.

http://www.one-electron.com/FC_TestEquipment.html


RL

"legg" wrote in message
...
On Sat, 18 Dec 2004 12:49:23 -0500, "Engineer"
wrote:

Hi, Vacuum-landers,
I just acquired an old Heathkit VTVM, model IM-38.

(snip)

IM-38 isn't here, but you should find the input divider and cal are
similar in the IM-18. Later changes are often cosmetic.

http://www.one-electron.com/FC_TestEquipment.html


RL

Thanks, downloaded to study.
Cheers,
Roger

Isn't the IM-38 the AC only/Audio meter? I know most of the standard
ac/dc/ohms part numbers, but this one doesn't ring a bell.

Thanks in advance.

"Engineer" wrote in message
...
Hi, Vacuum-landers,
I just acquired an old Heathkit VTVM, model IM-38. It seems to work but
has significant error when compared directly with my workshop Fluke 111.
Also, the ranges read differently, e.g. 100 VAC on the 300 VAC range (1/3
up scale) is not exactly full scale on the 100VAC range, etc.. The pot at
the back affects all ranges so I can't see how to calibrate each range.

Any ideas? Does anyone have a schematic they would be prepared to copy
for me, or a manual to sell?

Thanks and cheers,

Roger

"BFoelsch" wrote in message
...
Isn't the IM-38 the AC only/Audio meter? I know most of the standard
ac/dc/ohms part numbers, but this one doesn't ring a bell.

Thanks in advance.

Yes, it is an AC voltmeter only. It says "AC" and "VTVM", either side
of the switch knob. I should have made that clear. Has audio dB scale
on the meter as well as volts.
Cheers,
Roger

(snip)

"Engineer" wrote in message
...
"BFoelsch" wrote in message
...
Isn't the IM-38 the AC only/Audio meter? I know most of the standard
ac/dc/ohms part numbers, but this one doesn't ring a bell.

Thanks in advance.

Yes, it is an AC voltmeter only. It says "AC" and "VTVM", either side of
the switch knob. I should have made that clear. Has audio dB scale on the
meter as well as volts.
Cheers,
Roger

Ok, know I know what we are talking about. The IM-38 is a cosmetic variation
of the IM-21, the schematic of which I have posted over on ABSE. This unit
does not have a probe, so that is not an issue. If the individual ranges do
not read correctly, the divider resistors are damaged, there are no
calibration adjustments for the individual ranges in this unit.

Having said that, be sure that you are not expecting too much. The accuracy
of this unit is specified as 5% of full scale. This means that if we
calibrate the unit to read 100 volts perfectly on the 100 volt scale, the
readings on the 300 volt scale may be off by 5% of 300 volts, or 15 volts.
Therefore, if you put 100 volts in and read it on the 300 volt scale, the
reading may be anywhere from 85 to 115 volts and still be within spec. In
actuality, my experience is that the average unit is better than that.
However, these things are getting old, and resistors may have drifted.

I like these units, built mine in 1963 and still have it. Definitely worth
fixing up.

"BFoelsch" wrote in message
...

"Engineer" wrote in message
...
"BFoelsch" wrote in message
...
Isn't the IM-38 the AC only/Audio meter? I know most of the
standard ac/dc/ohms part numbers, but this one doesn't ring a
bell.

Thanks in advance.

Yes, it is an AC voltmeter only. It says "AC" and "VTVM", either
side of the switch knob. I should have made that clear. Has audio
dB scale on the meter as well as volts.
Cheers,
Roger

Ok, know I know what we are talking about. The IM-38 is a cosmetic
variation of the IM-21, the schematic of which I have posted over on
ABSE. This unit does not have a probe, so that is not an issue. If
the individual ranges do not read correctly, the divider resistors
are damaged, there are no calibration adjustments for the individual
ranges in this unit.

Having said that, be sure that you are not expecting too much. The
accuracy of this unit is specified as 5% of full scale. This means
that if we calibrate the unit to read 100 volts perfectly on the 100
volt scale, the readings on the 300 volt scale may be off by 5% of
300 volts, or 15 volts. Therefore, if you put 100 volts in and read
it on the 300 volt scale, the reading may be anywhere from 85 to 115
volts and still be within spec. In actuality, my experience is that
the average unit is better than that. However, these things are
getting old, and resistors may have drifted.

I like these units, built mine in 1963 and still have it. Definitely
worth fixing up.
BF, thanks so much for this. Yes, that's it! I recognized circuit
without having to go down two floors to the workshop - picture saved
for reference. You are right about the 5%. I left the unit on for
about 6 hours over the W/E then checked it again and the cross scale
errors were slightly less than that. I've not done another check
against the Fluke yet but will do so, of course. Nice little unit, as
you say.
Cheers,
Roger