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cmbrow@wm.edu 10th March 2012 05:41 AM

Reliable source for precision caps
 
I'm refurbing my Heathkit IT-28 cap tester and want to rebuild the range pot. All the present caps are out of spec, so I need to replace these with pretty close tolerance: 2% or better.

The less critical caps are not an issue, but I need the following:

.02uF (orig. Paktron mylar 100VAC 2%)
200pF (orig. molded mica 100VAC 2%)
2uF (orig. was a mylar, old replacement is a film, rated at 250VAC 2%)

I've scoured the internet for these values, bit cannot find these in precision grades Ideally I'd like 1% tolerance, but I'll settle for the original spec of 2%. Does anyone have any idea where I might find them, or, perhaps more important, any single source that could supply them. The postage from multiple vendors (assuming they exist) is killer.

Many thanks!

Chandos

RJM1 10th March 2012 08:03 AM

Not all at one place, but in stock.
200pF 1% 500V Digikey
Digi-Key - 338-2656-ND (Manufacturer - CD15FD201FO3F)

2uF 250V 1% Parts express
Dayton PMPC-2.0 2.0uF 250V Precision Audio Capacitor 027-214

.02uF 250V 2% Newark
VISHAY BC COMPONENTS|BFC241842003|CAPACITOR PP FILM 0.02UF, 250V | Newark.com

merlin el mago 10th March 2012 08:08 AM

0.02uF 400V 2% MKP
BFC241942003 Vishay/BC Components Polypropylene Film Capacitors

200pF 500V 1% silver mica
CD15FD201FO3F Cornell Dubilier Mica Capacitors

2uF 400V 3% MKP
ECW-F4205HL Panasonic Electronic Components Polypropylene Film Capacitors

RJM1 10th March 2012 08:24 AM

@merlin
0.02uF 400V 2% MKP
BFC241942003 Vishay/BC Components Polypropylene Film Capacitors
Stock=0

Enzo 10th March 2012 08:32 AM

Mouser.

.02uf/400v polypro 2% are listed though currently on order, meaning will be in stock later

200pf/500v mica 2%, 1%, even saw a 0.05%, all in stock

2uf/400v polypro 3% (oh well) in stock.


Since you report the old ones have drifted, you must be able to measure your caps. SO if you can;t find precision caps, get a handful of each value at 5% or even 10% and select from among them for value.

I didn't check Allied or Digikey or Newark, but I wouldn;t be surprised if they had them as well.

dchisholm 10th March 2012 02:35 PM

Although the values you mentioned (200 pF, 20 nF, 2.0 uF) are standard values in the EIA E24 (5%) series, capacitor values in the E12 series are usually easier to obtain, at ANY tolerance level. So consider a parallel combination of two, 100 pF; two, 10 nF; and two, 1.0 uF in place of your original components.

This assumes, of course, that you're more interested in using this instrument to make measurements, rather than creating a museum show-piece that looks identical to the original.

Dale

cmbrow@wm.edu 10th March 2012 07:51 PM

Quote:

Originally Posted by dchisholm (Post 2940660)
Although the values you mentioned (200 pF, 20 nF, 2.0 uF) are standard values in the EIA E24 (5%) series, capacitor values in the E12 series are usually easier to obtain, at ANY tolerance level. So consider a parallel combination of two, 100 pF; two, 10 nF; and two, 1.0 uF in place of your original components.

This assumes, of course, that you're more interested in using this instrument to make measurements, rather than creating a museum show-piece that looks identical to the original.

Dale

You've nailed it. I'm more interested in making measurements than I am in 'restoring' it to the original appearance. I've got other outlets for that urge (I also restore vintage wood working tools--my shop is pretty much 50s- vintage light industrial tools): Delta and Walker Turner.

In fact, I confess to you all: I'm a tool addict--now, under the benign influence of Joe Roberts, expanding into DIY electronics.

I've found all of this to be very useful. I was unsure, for instance, whether there'd be issues subbing ceramic for mylar caps. I suppose these things are more important, if they're important at all, in audio signal circuits than in the sort of circuit I have here, where the value of the caps is the leading parameter.

Again, thanks!

Chandos

dchisholm 11th March 2012 10:04 PM

Quote:

Originally Posted by cmbrow@wm.edu (Post 2940978)
. . . whether there'd be issues subbing ceramic for mylar caps. I suppose these things are more important, if they're important at all, in audio signal circuits than in the sort of circuit I have here . . .

I'm not familiar with the operational theory behind that circuit, much less the instrument itself. There are about a zillion words on this Forum alone about capacitor characteristics and performance. With no intention of re-opening old arguments, I offer this summary as my understanding of those discussions:
  • For the two lower values, polystyrene film capacitors offer the best stability over temperature and over time, and the "purest" capacitance (lowest parasitic resistance, inductance, nonlinearity, etc). Unfortunately, polystyrene capacitors have been out of general production for several years. There seem to be quite a few "new and unused, but old-stock" (NOS) units still available from individuals and small retailers. The challenge will be finding the combination of tight tolerance with voltage ratings over 100V.
  • Dipped silver mica capacitors are also a good choice for the 200 pF (or 2x100 pF) value. They are currently produced, and readily available from the usual internet distributors.
  • Ceramic capacitors with "COG" or "NPO" dielectric may be tolerable, but generally inferior to the above two choices. Don't even consider other varieties of ceramic dielectrics.
  • For the two larger values, polypropylene dielectric is considered superior to polyester (mylar). As already mentioned, the value/voltage/tolerance combinations are currently produced and available from internet distributors.
  • "Polyphenylene Sulfide"(PPS) dielectric is rated someplace between mylar and polypropylene. I think this is mostly confined to surface-mount parts rather than wire-lead packages.
Range switch assemblies from old 'scopes or 'scope plugins may be a source of close-tolerance, or ratio-matched, high-quality capacitors. You sometimes find these from hamfest/salvage/surplus sources. Finding the actual values you need is another matter . . .


Quote:

. . . I also restore vintage wood working tools--my shop is pretty much 50s- vintage light industrial tools): Delta and Walker Turner. . . . In fact, I confess to you all: I'm a tool addict . . .
There's a Unisaw sitting in my garage waiting for me to resume restoring it to full operating condition. My dad had a Walker-Turner drill press that I'd guess was WWII vintage. I don't recall when it wasn't noticeably shopworn and "experienced" in appearance (I'm now 60) but it did thousands of home- and auto-related repairs and projects, soapbox derby racers, bicycle accessories, Scout and school projects, etc.

In the course of one of those projects my dad admonished me with the old aphorism, "Only a poor craftsman blames his tools.". At that age I understood it to mean, "Take responsibility for your own work.". Later I realized it also means, "Know yourself, and what you're capable of doing with the resources available to you, and don't make promises beyond your skills.".

Still later it took on an added dimension when I learned that even well into the 20th century machinists, blacksmiths, and mechanics made many of their own tools. In fact, apprentices were typically coached and guided as they fabricated their personal tools. When the tools so created were good enough to do good quality work, the worker had the skills to use them to good advantage. That approach breaks down when I imagine making my own 'scope or software compiler, though I occasionally contemplate how that wisdom (and effective method) can be adapted to present-day technical education.

Dale


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