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 Post subject: Eico 950b bridge caps and resistors
PostPosted: Mar Thu 08, 2018 7:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3195
Location: Waukegan, IL
1Ive been reading the threads on bridge caps and resistors for the eico.

I read where they were hand selected in the past to closely match. and today, some have suggested using 1% parts and be done with it.

are you saying to use 1% parts and not match them, or use 1% parts and still match as close as possible. my caps are 200pf, .02 and 2.02. that one is paralleled so i can still still undo the smaller cap and dial it down.

the resistors are 20.0 200.2k and 199.8k. this shoud be good enough. its not like its accurate anyway once you read th bottom third of the scale anyway

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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Thu 08, 2018 7:14 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17699
Location: Albuquerque, NM 87123
I use mylar 1uf caps then test them; usually there will be some slightly lower and slightly higher than rated value--then just put them in parallel. Use at least 250 volt caps. Same with the .02uf; use two .01 in parallel.
Most digital capacitance testers are fairly accurate today.


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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Fri 09, 2018 12:21 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3372
Location: Florida
Using either 1% caps or making them up using a cap meter should get you as close or closer to the exact values (200pf, 0.02, 2) than the originals and likewise will give the same or better accuracy. Remember that these units were supposed to find bad caps not be used as lab instruments.

RRM


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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Fri 09, 2018 5:41 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7893
Location: Long Island
There are two different things going on here. Final accuracy of the bridge depends on how closely you set the pointer on the shaft of the bridge pot. The reason for having the bridge capacitors line up in exact multiples, and the bridge resistors too if the resistance ranges are going to be used is, there's one balance pot and one pointer, and you would like them to be accurate no matter what range you are on. For example, if the 200-pF cap was actually 180-pF, the 0.02-uF cap is 0.02-uF, and the 2.0-uF cap was really 2.4-uF the ranges won't line up. Assuming you set the pointer correctly on the C2 range, everything on C-1 would read low and everything on C3 and C4 would read high. The dial scales on the front panel were drawn with the expectation that the bridge caps and resistors would be of uniform ratios.

If you use the parts values in the manual, assuring that they are in exact ratios, then all the ranges should line up fairly closely. When setting the pointer, the advice is to "split the difference" if you find a small discrepancy between two ranges. They should be checked with the dial approximately mid-scale where it is most accurate.

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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Fri 09, 2018 6:03 pm 
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Posts: 33212
Location: Maryland 20709, USA
Hi Tony,

The critical factor to making the 950B operation uniform is holding the range caps/resistors to an obvious offset.

This means if the 200pf is actually 210pf, then the .02mf should be .021, and the 2.0 be 2.1.

The same is true for values on the low side: 198pf, .0198mf, and 1.98mf.

The same applies to the resistors.

- Leigh

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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Sun 11, 2018 3:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Waukegan, IL
do the resistors and caps have to match each other , or so long as the caps are matched to themselves, and the resistors matched to themselves.

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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Sun 11, 2018 6:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4267
Location: Charleston, W.Va.
Hi Tony,
When restoring any vintage bridge-type capacitor tester, the crucial issue is in selecting the bridge caps themselves. Absolute value of these caps is not that important, the importance is that they be perfectly matched in value. Thus there is no need to use 1% precision caps, provided you can accurately measure the value of the caps you intend to use.

For choosing replacement caps for the bridge, you need a modern digital cap meter to select them. In my experience, even an average-quality stand-alone digital cap meter will be far more accurate than the capacitance function of a typical DMM. You can buy such a digital cap meter for well under $50, and although you probably won't use it much for antique radio repairs, the price is well worth it for restoring your vintage cap tester. If you have a good assortment of caps in stock you can probably find two that will match up for the bridge circuit, or if not you can add a small-value cap in parallel with one of them to make up a perfect match. (Note: As an example, Sprague actually did exactly this at their factory in manufacturing their very fine TO-series TelOhmike cap testers.) But in any case, you need the digital cap meter to measure the values.

As I have mentioned in the forum before, this is one of those cases which is like a "Catch-22" situation. You need a modern digital cap meter to restore your vintage bridge-type cap tester, after which the vintage cap tester can do things the digital cap meter cannot.

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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Sun 11, 2018 6:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11760
Location: Mpls, Minnesota
trainman wrote:
do the resistors and caps have to match each other , or so long as the caps are matched to themselves, and the resistors matched to themselves.

Yes...it is very important that the resistors and capacitors match, otherwise you will never get the dial calibrated.

Dave


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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Sun 11, 2018 6:26 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3195
Location: Waukegan, IL
i know these things arent lab grade, but do all of them start being not accurate at both lower ends of the scale?

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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Sun 11, 2018 6:42 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3372
Location: Florida
A note on precision caps: One-percent caps are no longer expensive. Mouser has Kemet brand caps for not much money. I think I bought enough to fix two bridges for $10 or so.

When I first fixed my 950B about 15 years ago I just tried parallel combinations until it read most caps close to their marked values. When I changed to 1% caps accuracy only changed significantly on one range and that was only 5% or so max. The best thing these old RC bridges do is test caps for leakage at working voltages.

EICO did not specify accuracy for the 950B. You an go nuts trying to get one to meet the accuracy of a modern cap meter.

RRM


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 Post subject: Re: Eico 950b bridge caps and resistors
PostPosted: Mar Mon 12, 2018 4:30 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4267
Location: Charleston, W.Va.
Retired Radio Man wrote:
When I first fixed my 950B about 15 years ago I just tried parallel combinations until it read most caps close to their marked values. When I changed to 1% caps accuracy only changed significantly on one range and that was only 5% or so max. The best thing these old RC bridges do is test caps for leakage at working voltages.

EICO did not specify accuracy for the 950B. You an go nuts trying to get one to meet the accuracy of a modern cap meter.

RRM

I completely agree with above comments by RRM.

Similar advice will also apply to the Sprague TO-series TelOhmikes. These are metered instruments, much higher quality and more accurate than the EICO 950B, but even so they have their limitations. Complete accuracy throughout the full dial scale on all ranges is nearly impossible to obtain. Nevertheless, they are good performers as service-grade instruments which is what they were designed to be.

As RRM says, these will never meet the accuracy of a modern digital meter in terms of measuring capacitance and resistance values.
But on the other hand, they can do many things which the modern digital cap meter cannot.
Which is why it is worthwhile to own both a vintage bridge-type capacitor tester as well as a modern digital cap meter.

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