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Old 10th October 2013, 12:27 AM   #1
jblmar is offline jblmar  United States
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Default Eico 221 VTVM Probes

Just bought an Eico 221 VTVM. The set works fine, but I plan to make up new probes. The DC probe has a 15 Meg 10% carbon comp resistor in series with the probe tip. I would like to use a 1% Precision Metal Film resistor instead. is this an improvement on the original design or unnecessary?
Need your opinions as I'm new to VTVM use.
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Old 10th October 2013, 02:11 AM   #2
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You're not far from where EICO was born! The only diff should be in the frequency response.
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Old 10th October 2013, 04:56 PM   #3
jblmar is offline jblmar  United States
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Quote:
Originally Posted by jackinnj View Post
You're not far from where EICO was born! The only diff should be in the frequency response.
Yes, the meter says, Brooklyn, 3 New York. That's the old zip code. I did some research to find the location with no luck.

Do you suggest I use the same carbon comp 10% resistor on the new probe?
Thanks for your help.
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Old 10th October 2013, 05:52 PM   #4
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there is a +DC and a -DC calibration pot, to set up your meter. I don't think it realy matters much on the tolerance of the 15megohm resistor you use. The divider resistors are 1% and the meter accuracy is +/- 3% of full scale on DC, 5% on AC. So on the 100volt scale accuracy is +/- 3 volts of the reading. if you are measuring 20VDC a readying of 17 volts to 23 volts is within the meters accuracy range. Therefore you should always try to use the proper scale for the reading you are taking and keep the meter pointer in the top 1/3 of its range. I have an old Heathkit VTVM I use for tube stuff for poking around. I don't want to blow the inputs on my DVMs or Scope. Schematics for the ECIO meter are available. I used to hate trouble shooting transistor circuits as the difference between a transistor on and off is only a few millivolts and the meter was usless at those levels. So I used relative readings. A junction that is a bit more positive is probably on and one that is a bit more negative is probably off. Have fun
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Old 10th October 2013, 07:09 PM   #5
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Carbon comp resistors have a negative resistance coefficient with respect to voltage (above 250V).
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Old 10th October 2013, 08:03 PM   #6
jblmar is offline jblmar  United States
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The VTVM held its calibration. I didn't need to touch the pots inside the meter. On the Ohms scale, setting 0 Adjust & Ohms adjust gives full range deflection.
DC volts are accurate as well. Haven't tested ACV yet. I will when I pull the Distortion Analyzer out to finalize tests on the Pioneer SA 9500 I'm working on.
My question re the 1% resistor was an attempt to increase the accuracy of DCV measurements.
I'll be using the VTVM mainly for FM alignments, so the more accurate the DC probe the better.
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Old 10th October 2013, 10:31 PM   #7
jblmar is offline jblmar  United States
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Just to clarify. There are no internal adjustments for the Ohms function. IT depends on the condition of the battery and gain of the 6SN7 tube as well as the condition of the 1%precision resistors on the Range Switch.
For fM alignments there is a Zero Center position useful for checking/adjusting the FM discriminator. Here's where I need the most accuracy of the VTVM & probe resistor.
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