[Test-Equipment] Question about series/parallel capacitors

[Barry]

> I really think the use of balancing or equalizing 
> resistors is a non-issue here. These are very low leakage 
> film caps, mostly equalizing resistors are used with 
> seriesed electrolytics to prevent excessive voltage across 
> one _when they charge up_, after that the current is fairly 
> low. Film caps have such low leakage that I don't think 
> charging voltage or static voltage can cause any problems at 
> all.
> When putting capacitors in either series of parallel the 
> errors in their values are averaged, however, that assumes a 
> normal distribution of error and my not be true of a small 
> sample, especially if they are all from the same 
> manufacturing lot. You must really measure them to see at 
> least in what direction the error is. If you have a known 
> precise capacitor of the right value, no matter what its 
> voltage rating, you can measure to a very high degree of 
> accuracy by the substitution method where the cap becomes 
> the standard. The overall accuracy depends on the precision 
> of the measuring instrument, its accuracy is of no 
> importance since that is established at the desired value by 
> the comparison standard.
> I think in practice there is no significant difference 
> between the two series-parallel arrangements. As a side 
> note, it would be interesting to know what is in the 
> original GR standard cap. The catalogue shows caps up to 500 
> volts and some to 700 for much lower values of capacitance 
> than the one in the bridge. It may be that the bridge 
> standard is a specially made single stack cap or its 
> possible its made up of two seriesed catalogue standard caps 
> in a single case. Since the cap is bad you could open the 
> case to find out what is in it. The fact that its shorted 
> suggests its a single cap since its unlikely that both caps 
> of a series would short (but possible).
> The accuracy of the bridge with the replacement cap can 
> be established by measuring a known accurate cap. If the 
> standard cap is off value the error will be the same on all 
> ranges using that cap. The _precision_ of the bridge will 
> not be affected.
> 
> 
> --
> Richard Knoppow

This along the line of what I was thinking.  If I can get the 1409-X, I can ensure my home-made replacement is at least accurate.

According to Henry (the designer of the 1617A), the original cap was a mix of polyster film and mica (at least I think that's correct) the reason being that the TCs of both cancelled each other out.  I plan to open the original.  It would be nice if something internal besides the caps shorted and I could fix it but I doubt that's the case.  I assume there aren't individual caps with leads, etc., inside so repairing it is probably not feasible but I'll know more once I open it.

The replacements I had planned to user are polyester film.  While there could be some TC issues, hopefully it would be good enough for hobby work; if not, then I can rely on the 1409-X and just remember not to turn the bias up over 500V.  Yes, it would probably take it, but I'd hate to ruin a 1409 finding out...

Thanks!
Barry - N4BUQ