Capacitor Codes!

Learn something new every day, they say. I have been stumped by a largish Murata ceramic capacitor that does not seem to be working. It's from a Stax SRD 7 electrostatic headphone energizer, c. 1975. Its mate is a charred mess so I have not been able to test either.

Here is the legend on the cap: PTH BD 4R7M

It's apparent that this is a standard value 47 (something) but where is the decimal point and what is the voltage?

Transtronics' wiki comes to the rescue on capacitor codes.

The 'BD 4R7M' is evidently a military style code. The R substitutes for the decimal point. There is not a number following the R so there is no multiplier. So, it seems that this cap has a value of 4.7 pico farads, about as small as is measurable. The B indicates that a capacitance drift is not specified. The D is the voltage rating, 500 volts. The M indicates the cap has a temperature range of -55 to 70 °C .

UPDATE: a good question has been raised in the comments. The number 7 is evidently never used as a multiplier as indicated in Table 1. The multiplier format would be something like 474 which would indicate a value of 47 times a multiplier of 10,000, or 470,000 picofarads which is the equivalent of .47 microfarads. It seems to me that one would either have a R for a decimal point or a multiplier but not both at the same time. I guess neither would be necessary for a 10 picofarad cap which could be represented with just a 10, but that actually is a 10_ with the (blank) implying a multiplyer of 1. Whew.

Despite its rather significant size this is a pretty low spec cap.

Now, what about the 'PTH'? A bit of googling reveals that 'PTH' is a registered trademark of Murata and indicates the cap is actually a PTC Thermistor... What???

A thermistor is not a cap, it is a sort of current limiter whose resistance varies with temperature. PTC stands for 'positive temperature coefficient' which means the resistance goes up as the temperature goes up. But the codes on the item conformed to a capacitor!

So, it appears that today I have learned that I am confused. I wonder if in 1975 the PTH was used as a different indicator for Murata and it is indeed a 4.7 pico farad capacitor, or perhaps those codes do double duty and are used as indicators for the various qualities of a thermistor. The charred condition of the second 'cap' might indicate it is actually a thermistor that gave its life to protect the SRD 7. Hmmm.

Back to the Transtronics wiki page on resistor codes...

If a thermistor is coded like a resistor it would seem that this device has been coded via the BS 1852(British Standard 1852) and has a resistance of 4.7 ohms with a tolerance of 20% (M). The 'B' and the 'D' are not addressed. Therefore I would guess that it is not a coincidence that the device in question does, in fact, measure a resistance of 5 ohms.

Obviously, more research is needed.