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Heathkit IM-38 VTVM
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 |
#2
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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. |
#3
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"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 |
#4
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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 |
#5
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"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 |
#6
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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 |
#7
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"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) |
#8
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"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. |
#9
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"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 |
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