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 Post subject: Eico 221
PostPosted: Oct Thu 14, 2021 1:21 am 
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Hi. I picked up an Eico 221 and re-capped it. I tested it for accuracy and the scale isn't linear. The calibration controls are labeled by hand as to what they are, so I tried calibrating it against my bench equipment. When I calibrate for 5VDC on the 5V scale, i.e. full deflection, then at 2.5VDC the meter reads about 2.8V. If I calibrate at 2.5VDC on the 5V scale, then at 5VDC the meter reads less than 5V. This non-linear behavior is consistent across all ranges and modes which makes me wonder if this could be a meter movement or tube. Any ideas? I need to go find a schematic.

Thanks!


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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 1:55 am 
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My guess is that it's the meter movement. You can check it by disconnecting it and using known good equipment.


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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 5:50 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Oswego, NY, USA
I have an Eico model 222 VTVM (which is identical to the 221, but the 222's meter is just a bit smaller); and what I ran into about 15 yrs ago was that its meter movement is anything but quality (nowhere near a typical Triplett, Simpson or Weston movement); but I couldn't tell for sure who made the mvt for Eico; but it definitely had some linearity issues. So, I also vote for a meter issue which prevents you from fine calibrating your model 221. In defense of Eico, though, their kit testers filled the requirement of getting the job done and also quite affordable for young hobbyists, on limited allowances. I used to spend some quality Sunday afternoons building Eico's or Heathkits; and they didn't "break the bank", either.


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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 7:05 am 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Melbourne, Florida
You can test the meter movement by putting it in series with a DMM set to current connected to a variable supply. You can make a simple variable supply with a battery, pot, and resistor to limit the max current to a little over the meter's full scale current, in this case 1.0 ma.

The 6SN7 can also cause non-linearity.

The schematic is on BAMA.

RRM


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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 12:28 pm 
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Location: Long Island NY
Before tearing into the meter, the first thing to check is the 6SN7. The two triode sections have to be closely balanced, otherwise the meter will be nonlinear. If you have a tube tester you can look for a 6SN7 that produces the same reading on both sections, but it's still best if you have several tubes you can try in the VTVM and keep the one that works best. Subtle amounts of gas, leakage, or grid emission--below what an ordinary tube tester can detect--can also cause lots of problems due to the high resistances in series with the tube grids.

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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 2:34 pm 
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Location: Sunbury, Ohio 43074
You can easily test the meter movement for linearity. Here’s how to do it safely

HTTPS://www.bbtvtestequipment.com/testing-a-panel-meter

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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 2:36 pm 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Melbourne, Florida
Unless it has been reworked this meter is full of old carbon comp resistors. At minimum they should be checked.

I have two 221's that have been reworked. Linearity on one is better than the other. Swapping tubes had no effect, but I have seen it make a big difference on other meters.

The 221 construction manual I have gives "about" voltages for the bridge tube: plates 85V, cathodes (bridge balanced) 3.5V. Both measured with respect to ground.

There is a construction manual for the EICO 214 on BAMA. The circuitry is identical (the 214 has a larger meter face) but he 214 manual gives 90 and 2-3 for the tube voltages.

Accuracy for this kind of meter is specified as a percentage of full scale, not a percentage of readings. This means readings can vary by the specified accuracy at any point on the scale and still be in spec. An accuracy of 3% means the meter can have an error of 0.015 volts at any point on the 5 volt scale. Calibrating at lower than full scale, say 80% or so may help offset linearity errors.

These were decent meters for their day.

RRM


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 Post subject: Re: Eico 221
PostPosted: Oct Thu 14, 2021 3:39 pm 
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Chris108 wrote:
Subtle amounts of gas, leakage, or grid emission--below what an ordinary tube tester can detect--can also cause lots of problems due to the high resistances in series with the tube grids.

It's best to age the tubes under power for a few days before calibrating a VTVM, especially if it has been sitting unpowered for years or if the tubes are new. I assume this is due to gas being absorbed by the getter, but I don't really know the mechanism. The effect is easily observed, though.

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 Post subject: Re: Eico 221
PostPosted: Oct Sat 16, 2021 6:36 pm 
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Update: I found a schematic and ran some resistance checks. All the resistors in this meter including the one in the probe were total garbage. I was able to string together some hand-picked precision resistors to replace the exotic ones. I had a spare 6SN7 that tested good and each side fairly well matched but it must have been a little gassy because the meter wouldn't hold zero with the probe connected to common. It would move around all over the place. Maybe if I let the tube run for a couple days as suggested, the getter might clean it up. Bottom line, the non-linear behavior persists and from what I could tell, the replacement 6SN7 wouldn't have solved the problem anyway. It must be the meter movement. Although the movement is free, it's requiring more current to reach full deflection than it should. The only other thing I can think of is to remove the meter and take it apart for cleaning. Maybe if I'm lucky there's a series resistor or something serviceable inside.

Thanks for the help.


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 Post subject: Re: Eico 221
PostPosted: Oct Sun 17, 2021 1:19 am 
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Joined: May Mon 25, 2020 7:37 pm
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Location: Greater Boston, MA
Not to state the obvious, but I've had unstable zero reading from a dirty zero pot that was resolved by exercising it.


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 Post subject: Re: Eico 221
PostPosted: Oct Sun 17, 2021 3:02 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 5101
Location: Melbourne, Florida
EarlyRadioMan wrote:
....... It must be the meter movement. Although the movement is free, it's requiring more current to reach full deflection than it should. .....


Unless it's way off this shouldn't matter, what you're looking for is correct proportions - half the full scale current at mid scale, etc.

RRM


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