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Old 12th March 2014, 01:40 PM   #1
jblmar is offline jblmar  United States
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Join Date: Dec 2005
Location: Brooklyn, New York
Default Eico 221 VTVM

I'll be making new probes for the 221. The manual specifies shielded cable for the DC Probe. Is this really necessary? I would like to use a single pair for AC/DC & Ground. What cable should I use?
50 ohm?
75 ohm?
RCA cable?
Other?
Does it matter? Sorry, I'm new to VTVMs.

In addition, I would like to solder the 15 mega ohm resistor to the DC Jack of the VTVM not at the tip of the DC probe not to make a separate probe. Any downside to doing this?
I'll change the original jacks to Banana Jacks as well.
Your opinions welcome.

Last edited by jblmar; 12th March 2014 at 01:43 PM.
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Old 12th March 2014, 04:37 PM   #2
akimmet is offline akimmet  United States
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Join Date: Jul 2011
Location: NW Ohio
Since the meter is high impedance, a small amount of capacitance or a em field can wildly influence your measurement. Yes, the probe needs to be shielded cable. The external resistor needs to be as close to the probe tip as possible as well. I would recommend either re-using the stock connector, or at least using a connector that ensures the shield on the cable is connected to the ground probe when plugged in.

I have a RCA WV77E with unshielded probes, and it is nearly useless. The meter needle has a mind of it's own, touching the probe cable will peg the meter on most settings. I haven't gotten around to making an appropriate probe for it, because I have a nice heathkit VTVM that works perfectly.

Last edited by akimmet; 12th March 2014 at 04:47 PM. Reason: added comment on bananna jacks
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Old 12th March 2014, 05:49 PM   #3
jblmar is offline jblmar  United States
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Join Date: Dec 2005
Location: Brooklyn, New York
Thanks akimmet for your reply. I can use the original probe with a new resistor and shielded cable. The original jack had a sloppy connection to the probe connector, so I'll go with Banana jack & connector.
I'll try the resistor both in the probe and then soldered to the jack to measure it's performance to see what the differences are.
Your opinions are welcome. Keep them coming.
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