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 Post subject: EICO 235 VTVM
PostPosted: Aug Fri 05, 2011 4:36 pm 
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Does anyone out there, possibly an owner of a Eico 235, know what is inside the so-called Uni-Probe? Although I have the Operating Manual and Schematic, nowhere inside the manual or detailed within the schematic is there any clue as to what's inside the Uni-Probe.

On the other hand, Heathkit Co. VTVM's describe their probe and show what's inside on the schematic. They show a 1 meg resistor in play when the switch on the probe is in the DC position. In the AC/ohms position, the device under test goes directly into the VTVM. I am guessing that all of the VTVM's, back in the day, worked their switchable probes this way by utilizing a 1 meg resistor in DC measuring mode.

I can only guess that Eico's Uni-Probe works the same way, but it would be nice to know for sure what's inside!! I would like to calibrate my unit correctly. So, if anyone can shed some light on this, please do. Thanks


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 Post subject: Re: EICO 235 VTVM
PostPosted: Aug Fri 05, 2011 6:05 pm 
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Quote:
I am guessing that all of the VTVM's, back in the day, worked their switchable probes this way by utilizing a 1 meg resistor in DC measuring mode.
Not all vtvms, some simply used a second jack. The Eico uniprobe held the 1 meg. resistor and a rotary switch to select.


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 Post subject: Re: EICO 235 VTVM
PostPosted: Aug Fri 05, 2011 7:26 pm 
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1 meg about 1 watt resistor inside and the probe barrel twisted to select DC or AC/OHMS function

Image

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 Post subject: Re: EICO 235 VTVM
PostPosted: Aug Fri 05, 2011 8:06 pm 
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Here are pics from the assembly manual -


Attachments:
UniProbe2.jpg
UniProbe2.jpg [ 43.79 KiB | Viewed 1293 times ]
UniProbe1.jpg
UniProbe1.jpg [ 76.96 KiB | Viewed 1293 times ]

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Everyone is ignorant...
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 Post subject: Re: EICO 235 VTVM
PostPosted: Aug Fri 05, 2011 9:34 pm 
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Thanks to everyone for taking the time to respond. It's great to have the actual facts about something instead of guessing about it yourself. You are all the best!!


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 Post subject: Re: EICO 235 VTVM
PostPosted: Aug Sat 06, 2011 2:08 am 
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Kudos to all. Now those are what I call responsive and helpful posts. Not a single one said "You don't need to..."
Clear Ether


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 Post subject: Re: EICO 235 VTVM
PostPosted: Dec Thu 07, 2017 3:07 am 
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Joined: Nov Sat 07, 2015 6:32 pm
Posts: 14
This post was very helpful for me as well.

I have what must be a very early Model 232 VTVM with no markings at all on the uni-probe, and I had no idea how it works.

The plastic nose rotates, and the two operating positions are with the white pin on the probe barrel set in one or the other of the two notches opposite each other. In the diagrams, contact K1 is asymmetrical, and contacts either contact K2 or contact K3 which are opposite each other. With the white pin in one position the resistance measured between the probe tip and the center contact of the cable should be zero. That's the AC/Ohms position. With the pin rotated to the opposite position the resistance should be high but not open circuit. That's the DC position.

According to the assembly manual I have for the 232 contacts K2 and K3 are phosphor-bronze and the resistor R1 is 1 Meg Ohm, 1/2 W.


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 Post subject: Re: EICO 235 VTVM
PostPosted: Dec Thu 07, 2017 1:05 pm 
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And what I have done in the past when I had a bad (or non-existent) probe for either a Heathkit or EICO was to just make two probes. A red one was straight through for Ohms and AC and a black on which contained the 1 meg resistor. Yes, you do have to change probes when going from and AC circuit to a DC circuit. But do you really do that so frequently that it is excessively inconvenient?

For me, it just works...

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 Post subject: Re: EICO 235 VTVM
PostPosted: Dec Thu 07, 2017 8:27 pm 
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^^ a good suggestion.

I found when I was shopping for my VTVM that they very often don't have the probe, if there is one you don't know if it works in both positions, and the replacements I saw were rare and expensive. I was very relieved to find that the probe on my EICO VTVM actually works. I am still figuring out if the VTVM itself works.


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