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Eico 221 VTVM Probes

jblmar

jblmar

JBL & marantz
Hi,
I need to make a new pair of probes for my Eico 221 VTVM. I changed the original jacks to Banana Jacks. The manual calls for shielded cable for the DC probe. Which cable should I get?
In addition, the probe has a 15 meg ohm resistor soldered to the probe tip. I would like to solder this resistor to the DC Jack so I can use one set of probes instead of making a dedicated probe. Does this present a problem? Sorry, I'm new to VTVMs.:o
Looking forward to your opinions.
 
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Not a good ideal to put the resistor at the jack. It is in the probe for a reason=isolation of the cable. The DC cable needs to be shielded, also. For best results, put it all back the way it was.
Rick
 
Rick,
Thanks for replying.
Not a good idea to change the jacks to Banana?
What type of cable do you suggest for the DC probe?
Something like RG 58?
 
If you were going to change the connector to anything, I'd go BNC. I think those used whatever the connector is with the button in the middle, most commonly seen on microphones.

Cable type isn't overly important, but stranded RG58 would probably give you reasonable flexibility. That might actually be easier to use the original connector though if you don't have the crimp tool for BNC.


Not quite related, but my RCA VTVM does not use a shielded cable. I found a site where a guy goes over restoring one, and he suggested (and converted his own) installing a BNC specifically for the shielding. I have noticed with my RCA's unshielded leads that you can throw it off by moving the wires around. When I got it, someone had put the resistor inside of the unit so it didn't need the special probe. For most things it didn't matter, but it would mess up RF circuitry. I was trying to use it to peak up an oscillator and connecting the leads would shut the circuit down. I worked around it by sticking a 1 meg resistor at the end of the probe to decouple the lead, basically like the original design had, and it worked. I have since converted it back to stock design.
 
gadget,
Banana jacks are a perfect fit for the cutout holes on the front panel. I would need to drill out the cutout for BNC even though I prefer BNC. The original probes work, but they are rusted and need to be replaced.

I'll be using the VTVM mainly for RF circuits so I'll take Rick's & your advice and solder the resistor as specified in the manual. To the probe tip. My thought was to solder the resistor to the DC jack so I would have 1 probe for both AV & DC measurements.
 
jblmar said:
Rick,
Thanks for replying.
Not a good idea to change the jacks to Banana?
What type of cable do you suggest for the DC probe?
Something like RG 58?
Click to expand...

You need a two conductor jack so you can connect the cable shield. A banana jack is only one conductor. The original 1/4" phone jack would work fine and it's easier to wire up a cable to a 1/4" phone plug than to a BNC.
Any shielded cable will do. It is not critical. The only caution is maximum cable voltage rating. If you are going to measure 1,000 volts then make sure your shielded cable is rated for that.
Rick
 
Rick,
Thanks. I may put the original jacks back. The original probe jackets are cracking exposing the shield.
I'll get the cable that matches the VTVM rating. No need to compromise.
 
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