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Author Topic: EICO VTVM cal  (Read 4049 times)
WB4SPT
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Posts: 166




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« on: March 27, 2013, 09:59:24 AM »

I know "boatanchor" is a stretch here, but I know you guys are cool with this.  After all, the device does have two tubes. Wink  Anyway, I can't seem to located a schematic for this rather rare 235 model.  It's like a 232, except it has a larger meter, and has 5 calibration pots.  One pot does DCV + , one does DCV-, one for ACV, one for AC zero.  Can't get an effect from the 5th pot, doesn't affect ohms, just don't know??  THe unit does suffer from an ohms issue;  my 1% cal resistor looks good on the scale resulting in 2/3 deflection, but gets to a 20% error with the next scale down.  I've got this VTVM for grid DC measurements, so the ohms is not high on my need list.
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N3QE
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Posts: 2421




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« Reply #1 on: March 27, 2013, 10:10:21 AM »

Check the ohmmeter battery. Or, maybe, two batteries (some used a 9V or other oddball "transistor" battery for high ohms.) Then look for poor contact on range switch or even probe connection, as the high impedance of the VTVM on other modes will not reveal poor contacts.
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WA0ZZG
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Posts: 158




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« Reply #2 on: March 27, 2013, 12:26:50 PM »

Both the Eico 232 and 235 are the same meter.  One just has a larger meter face.  They are nice instruments and reasonably accurate.  The trick is using the correct meter probe.  Eico included a probe with a rotating head. Everything but ohms in one position, ohms in the other. Recommend, if you haven't already, that you start with fresh batteries, AND CLEAN THE BATTERY CONTACTS.  While you are at it, clean the switch contacts.  These things will run for ever.
Dave
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W9GB
Member

Posts: 2659




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« Reply #3 on: March 27, 2013, 03:16:05 PM »

EICO 235 Professional VTVM
20 ranges with 25 meg DC input.
1% multiplier resistors.
Protected 6" 200ua meter with zero-center scale.
Ranges:   p-p volts 0-4-14-42-140-1400-4200
   DC rms volts 0-1.5-5-15-50-150-500-1500 (plus 0.5v DC range)
   Ohms 0.2-1000 megs in 7 ranges
Response: 30Hz to 3MHz
Cabinet Size: (HWD) 8 1/2" X 7 1/2" X 5"
Weight: 9 lbs
$49.95 (1970)

Surplus Sales of Nebraska -- EICO 235 Manual :: $16.00
http://www.surplussales.com/Manuals/man_eico.html

EICO 232 VTVM Construction Manual (BAMA Mirror)
http://bama.edebris.com/download/eico/232(3)/Eico%20232%20assembly%20man.pdf

EICO 232 VTVM Instruction Manual (BAMA Mirror)
http://bama.edebris.com/download/eico/232(3)/Eico%20232%20user%20man.pdf

Construction Manual for the EICO 232 Peak to Peak VTVM
with the revised Schematic for the new switch PN 6006.
David R. Lane, AC7PQ
http://bama.edebris.com/download/eico/232(2)/EICO_232cm.pdf

EICO's probe is called: UNI-PROBE
The probe has a switch with TWO SETTINGS:  DC;  AC-OHMS

The UNI-PROBE has a 1 Mohm 1/2-watt resistor in-line with the Hot test lead,
and a switch to bypass that resistor.


Probe connector to EICO 232 is Switchcraft 2501F, readily available from Mouser.
Classic guitar, microphone, and PA amplifier restorers still use.

YouTube restoration of an Eico 232 VTVM (Dec. 2012)
http://www.youtube.com/watch?v=5_Z3h_OpoQw

The original EICO test probe has a set screw -- (eHam.net posting), to keep the coax cable,
from backing out of the probe body.  Over tightening that setscrew can be shocking.
http://www.eham.net/articles/7682

Sharpie probe
http://golddredgervideo.com/kc0wox/rfprobe.htm
« Last Edit: March 27, 2013, 03:52:19 PM by W9GB » Logged
WB4SPT
Member

Posts: 166




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« Reply #4 on: March 27, 2013, 04:17:56 PM »

Great links, though I'm left with a meter with a different schematic than the 232.  Looks like the $16 Nebraska deal is the only one on the table.  My ohms issue is gone after a day's on time in the cabinet.  Seems some magnetic influence messed with the linearity of the scale. Battery holder is/was perfect on this unit; went thru deoxid, and new filter cap, good to go now.  Just the one mystery pot.  The 25Mohm input and the different adjustments for + and - volts are unusual to this model.   
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KD0REQ
Member

Posts: 1043




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« Reply #5 on: March 27, 2013, 05:50:53 PM »

static on the plastic meter faces will also drive a lot of meters nuts.  old fighter pilot trick from WWII was to rub a little bit of dishwashing detergent on the plastic with a clean soft cloth until it's gone.  it still works with a good detergent like Ivory Liquid original
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WA2CWA
Member

Posts: 311


WWW

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« Reply #6 on: March 27, 2013, 06:43:07 PM »

I also have both the Eico 235 assembly and operating manuals available. Actually I have over 244 different Eico manuals.

Pete, wa2cwa
http://www.manualman.com
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WB4SPT
Member

Posts: 166




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« Reply #7 on: April 07, 2013, 01:31:57 PM »

thanks manual man.  ur manual led me to a 0.5v cal pot.  ANd the soap trick worked well also.
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W9GB
Member

Posts: 2659




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« Reply #8 on: April 07, 2013, 03:26:59 PM »

Post a photo, when you are finished with repair and restoration.
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