I ran into "K9AXN SERVICE NOTE 022h IMPORTANT CAPACITOR INFORMATION" http://www.k9axn.com/capacitor/Note22j.pdf while researching capacitors for my Hali SX-100. Interesting, as I have a backlog of equipment to electronically restore now that I'm semi-retired ...again. I've had my EICO 950B on the bench sitting there open for some time, mulling over the caps in this thing. I replaced all the electrolytic caps, and the regular wax paper caps. Kind of froze when it came to the precision caps. I see a fellow offers precision replacements for these, but they are expensive. There are three precision caps, two of the General Instrument caps that appear to be described in the article, and what I believe is a mica cap, a domino. I'm tempted to just leave those alone now.

The Eico 950B manual parts list describes the bridge capacitors as "precision" but they were ordinary production capacitors, nothing special. What was "precision" about them is that they were hand selected at the factory to have exact values (2-uF, 0.02-uF, 200-pF).

Replacements can be selected the same way if you have a digital capacitance meter. What you'll find is that most capacitor manufacturers have switched to E6 values, so you'll see 2.2-uF, 0.022-uF, and 220-pF as the readily available and inexpensive values. So what many of us do is buy half a dozen or more 1-uF, 0.01-uF, and 100-pF caps and connect two of each in parallel to make the necessary values. This gives you a better opportunity to select pairs that come in exactly on the value when you connect them together. The rest can go in the capacitor drawer for radio repairs.

Any modern film capacitors will work in these circuits but I prefer to use polypropylene film caps when I do these projects. Polypropylene is more stable with temperature changes than the more common Mylar/polyester film caps, and it has far less loss. This results in a sharper null in a bridge circuit.

The range caps in a bridge are "precision" only in the sense that they have the same digits.

For example, if a 1.0mfd cap actually reads 1.03, the others will be.0103 and .000103.
Similarly, a 1.0mfd cap that's 0.97mfd would be matched with 0.0097 and .000097mfd.

The matching values are selected during production (actually before production).

By doing this, all ranges are calibrated simultaneously by adjusting one variable resistor.

Note that precision laboratory gear (General Radio, HP, etc) don't use this method.
They calibrate each range individually.

- Leigh

Best to keep your bridge capacitors capacitors at the specified value using the method described by Chris. If you values are all off by the same amount then you will have to have your resistors off by the same amount or the dial won't calibrate.

Dave

Ok thanks for the responses, I thought I was onto something, I'll go ahead and replace those caps too. I've started going through my test equipment first, I want to make sure that's stuff is right before I go onto other stuff, the radio's and tv's.

i was thinking aout something. since the range resistors are supposed to be 1% tolerance, shouldnt the capacitors be 1% tolerance from 2mfd? i mean if i uses 2.5 mfd and .025 and.00025 the dial will be way off. that means using values anywhere from 198 to 202.


then again, it is only fairly accurate in the middle of the scale. at either end the accurancy drops off.

That's the reason pairing up caps to make exactly 2-uF, 0.02-uF and 200-pF was recommended. If you do it that way, it should be possible to have the dial calibration hold for both the capacitance and resistance ranges. If you were to put in, say, 2.2-uF, 0.022-uF, and 220-pF it would be possible to get the dial into calibration for either capacitance or resistance, but not both. You'd have to replace the bridge resistors with ones that were 10% off to fix that.