What kind of capacitors are these?

[oPossum]

These are in a Heathkit IN-27 decade capacitance box. They have a waxy feel. Looks like a single layer dielectric - not wound. SRC may be the manufacturer - Societe de Composants Record.

What is the likely dielectric and plate material?

[RJFreeman]

My guess would be Silver Mica Capacitors

[+Mikie]

My guess would be paper capacitors. If you're interested in replacing them use either ceramic, MKT or MKP.

[Z80]

I'd say they were mica caps, they look like old micas.  Being used in a substitution box they need to be accurate and stable.  I seem to remember most of the paper caps were cylindrical and as reliable as a 2 week weather forcast.

[vk6zgo]

My guess would be Silver Mica Capacitors

Mine, too!
Australian mica caps usually were coated with hard Bakelite, but those from the USA & UK often had that waxy encapsulation.

[HB9EVI]

silver mica!

[David Hess]

The 1% tolerance, size, and form factor are enough to know that those are silver mica capacitors however in case there is any doubt, they were made by SRC (Stability Radio Components).

[oPossum]

Wow. No doubt that is what they are. I have never seen this style of old cap before, and all the silver mica caps I have seen are the modern epoxy encapsulated style. Thanks to all who responded.

Testing with a DE-5000 suggest they are still within spec.

Code: [Select]

103.7  +3.70%
203.0  +1.50%
302.9  +0.96%
403.0  +0.75%
503.7  +0.74%
604.3  +0.72%
703.4  +0.49%
804.1  +0.51%
904.7  +0.52%
1004.7 +0.47%

1.003  +0.30%
2.009  +0.45%
3.017  +0.57%
4.006  +0.15%
5.007  +0.14%
6.015  +0.25%
7.020  +0.29%
8.021  +0.26%
9.028  +0.31%
10.026 +0.20%

10.07  +0.70%
20.12  +0.60%
29.85  -0.50%
39.75  -0.63%
49.83  -0.34%
59.87  -0.22%
69.57  -0.61%
79.69  -0.39%
89.73  -0.30%
99.79  -0.21%


[Gyro]

Testing with a DE-5000 suggest they are still within spec.

Don't damage that soft wax coating (yes, it is wax) that keeps the moisture out and they should stay that way for a long time. Stability is Silver Mica's forte.

[+Mikie]

I'd say they were mica caps, they look like old micas.  Being used in a substitution box they need to be accurate and stable.  I seem to remember most of the paper caps were cylindrical and as reliable as a 2 week weather forcast.

Yeah, my bad. Paper ones were always cilindrical, you could see the layers from the sides. It was fun unrolling them, that's about all they were good for

[David Hess]

I'd say they were mica caps, they look like old micas.  Being used in a substitution box they need to be accurate and stable.  I seem to remember most of the paper caps were cylindrical and as reliable as a 2 week weather forcast.

Yeah, my bad. Paper ones were always cilindrical, you could see the layers from the sides. It was fun unrolling them, that's about all they were good for

They did make rectangular paper capacitors as shown at the first post here for M1 and M2:

https://www.radiomuseum.org/forum/replacing_old_capacitors.html

My ESI impedance bridge uses mica capacitors are shown for M4 at the above link with the colored dots for the oscillator and filter.