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 Post subject: EICO 222 flying blind
PostPosted: Mar Sun 04, 2018 7:23 pm 
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Joined: Sep Tue 12, 2017 3:04 am
Posts: 95
Location: 251 Moon Lane Spring City, TN
I picked up a nice on the outside Eico 222 VTVM. Someone has badly hacked the insides. They added a solid state mess for I guess the 1.5 volt battery and removed the rectifier and used the 6AL5 tube. Either genius or disaster. Anyway the meter did not work. I cut the solid state stuff out and went to work, but found out I cannot find a schematic for the 222. So I've compared it with the 232 and the Heathkits and found it close, but ALL resistor and cap values are different from what's in my 222.

Does anyone have a schematic for a 222 that they can email me please?

jsbowman@bellsouth.net


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Sun 04, 2018 11:02 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17690
Location: Albuquerque, NM 87123
Fred Scoles was kind enough to email the manual to me; I forwarded it to you. I have been having troubles with my emails, so tell me if you didn't get it.


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Sun 04, 2018 11:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1998
Location: Oswego, NY, USA
Johnnysan and Josh, so great minds think alike????.....I just emailed a copy of the manual to Josh about the same time you did.....how about that???? So now Josh can read the manual in "stereo vision".

Fred


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Sun 04, 2018 11:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17690
Location: Albuquerque, NM 87123
Thanks Fred, for all your help over the years.


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Tue 06, 2018 3:58 am 
Member

Joined: Sep Tue 12, 2017 3:04 am
Posts: 95
Location: 251 Moon Lane Spring City, TN
Fred, thanks so much! The manual did the trick. If you look in the pictures, I replaced all the parts below the probe and cut out a hack that the PO had installed above the probe (I think the hack was a battery eliminator). I actually thought they had hacked the 6AU5 tube to be a power in rectifier and not having anything but a 232 schematic, I rewired it per the 232 schematic and put a solid state rectifier in (the meter worked fine that way). Low and behold looking at the 222 schematic, the design was to have half of the 6AU5 tube be the power rectifier and the other half be the AC signal rectifier. So tonight out came the solid state mod and back went the wiring for the original 222 design. The only difference I see in the operation is the 222's AC seems to load circuits a bit more than the 232 design. I only saw it on a voltage standard that I have. At 5 volts the meter only see's 4vac. But on the variac and with a good digital meter as the standard, the meter reads fine from 5 volts to 150 vac. BTW, the probes body is made from a piece of 1/2" CPVC pipe drilled out to 1/2". Slapped some red paint on it and filed the detents for the probe tip. Works well.
These meters are kind of fun projects, but I think I'm done after 7 of them!
Thanks again for the help.
Josh

P.S. Johnny, thanks for the thought, but I didn't get your email. Big files are tough sometimes. My server sometimes kicks them back on me.


Attachments:
eico 222 small.jpg
eico 222 small.jpg [ 184.5 KiB | Viewed 479 times ]
Parts from Eico 222 small.jpg
Parts from Eico 222 small.jpg [ 181.62 KiB | Viewed 479 times ]


Last edited by jhbowman on Mar Wed 07, 2018 12:31 pm, edited 1 time in total.
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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Tue 06, 2018 12:24 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7888
Location: Long Island
The Eico 232 has a novel full wave meter rectifier circuit so it can read peak to peak voltages on asymmetrical waveforms. The 222 was comparable to the RCA WV-77 series. Those also use half of the 6AL5 for the power supply rectifier. Current draw of a VTVM circuit is only a couple of milliamps so this works out okay in practice. In most other VTVMs, including Heathkit, B&K, RCA WV-98, etc., there's a separate power rectifier. One half of the 6AL5 is used for the diode on the AC voltage ranges. The other half of the 6AL5 is connected backwards so its contact potential cancels that of the active diode. This gives you better sensitivity on low voltage AC since it is not necessary to overcome the contact potential before you can measure anything.

The extraneous parts you removed do indeed look to be part of somebody's battery elimination project.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Wed 07, 2018 4:35 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3372
Location: Florida
Chris108 wrote:
...... The 222 was comparable to the RCA WV-77 series. Those also use half of the 6AL5 for the power supply rectifier. ......


The WV-77E has a full wave rectifier for P-P ac scales and a selenium rectifier. Don't know about other models. The RC-19 tube manual has a circuit as you described.

RRM


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Wed 07, 2018 11:42 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7888
Location: Long Island
Learn something new every day! I never had a 77E, didn’t know they changed it. The earlier versions were a little simpler.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Thu 08, 2018 5:04 pm 
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Joined: Sep Tue 12, 2017 3:04 am
Posts: 95
Location: 251 Moon Lane Spring City, TN
I have a basic question. Why on the EICO 235 and RCA 98C is the 6AL5 tube wired like 1 diode like a half rectifier and other going to gound?


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Thu 08, 2018 5:45 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7888
Location: Long Island
Deleted ... RRM gives the better answer below.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Last edited by Chris108 on Mar Fri 09, 2018 5:46 pm, edited 1 time in total.

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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Fri 09, 2018 6:32 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3372
Location: Florida
The 232 and the 98C use voltage doublers. In this type of doubler one diode (the grounded one) charges a cap that is in series with the input, on one half cycle. This voltage then adds to the voltage passed by the other diode (the series one) on the other half cycle. The total then appears on a cap from the second diode to ground.

Referring to the 98C circuit, the grounded diode charges C1 on positive going half-cycles. This voltage is then added to the negative going half-cycle voltage passed by the other diode and the total then appears across C2.

I have a Precision VTVM that uses one diode to cancel contact potential as Chris describes. The 232 and 98C do this by adding a small reverse polarity voltage through a large resistor (90 megs or so). This is the what the "AC-BAL" or "AC-ZERO" cal pots adjust.

RRM


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Fri 09, 2018 12:56 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7888
Location: Long Island
As an aside, the RCA WV-98(A) does not contain an AC zero pot. It does use a full wave peak-to-peak rectifier circuit but they left the contact potential to fend for itself. Evidently the arrangement was not satisfactory as the WV-98(B) and WV-98(C) have the additional voltage divider resistors and AC zero pots.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: EICO 222 flying blind
PostPosted: Mar Sun 11, 2018 10:58 pm 
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Joined: Dec Wed 23, 2015 12:47 am
Posts: 39
WOW,,,been looking for the same thing for years,,,schematic to the Eico 222 ,,,or manual,,,can someone send me one also,,,Thank You in advance,,,Kenjr


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