I build and repair ignition systems for vintage race cars. I've had a couple of units partially fail(they run but not well). The components are a diode, a SCR, a bridge rectifier and a 2.0mfd 600v capacitor. Since the components are at least 40 years old I am wondering if the cap has degraded to the point of not being what it was when it was younger. Any thoughts?

 

Caps & points do wear out in 40 yrs. If you do not have a capacitance or ohm meter, you can replace them with any metal film plastic type (PE, PP PTFE, PC) https://www.tedss.com/PC-2-1000-10

Some distributors charge $25 with brackets others with just wire leads $2 min., but some charge $100 (lol)

 

I build and repair ignition systems for vintage race cars. I've had a couple of units partially fail(they run but not well). The components are a diode, a SCR, a bridge rectifier and a 2.0mfd 600v capacitor. Since the components are at least 40 years old I am wondering if the cap has degraded to the point of not being what it was when it was younger. Any thoughts?

Given a diode, a SCR, a bridge rectifier and a 2.0mfd 600v capacitor in a CDI (Capacitive Discharge Ignititon) I would expect the three solid state devices to either work as new or fail completely. That leaves the capacitor as the likely culprit if indeed there is a failure. Being that the capacitor is likely a metal film plastic dielectric type there can be gradual degradation due to localized dielectric breakdown that vaporizes a portion of the metallization. This is called "clearing" and is mentioned in the

It would be nice to replace the capacitor with one having the exact footprint and lead spacing.

You can locate a suitable capacitor at Digikey. Searching under capacitor then Film Capacitors you check off the various options. Checking off

In stock
Active
Capacitance, 2.0 uF
Voltage Rating - DC, 600V to 1000V
Dielectric Material, Polypropylene (PP) & Polypropylene (PP) Metallized
Package / Case, Axial if your cap is a cylinder with axial leads, Radial otherwise
Mounting Type, Through Hole (if mounted on a PCB)
Operating Temperature, (any but 85 degree C)

By now we are left with one Lead Spacing option, 27.50 mm. If this does not fit your application go back and select the correct lead spacing.

Using the search parameters above we are left with 17 part numbers to choose from with all rated for Automotive applications. I clicked Features, High Temperature and that leaves us with three choices which are the Panasonic ECWFG series. The ECW-FG60205J is $1.98.


CDI module showing two plastic film capacitors (yellow).

Failure Modes of High Voltage Film Capacitors https://www.psma.com/sites/default/...ott Franco, Cornell Dubilier Electronics).pdf

CDI app note https://www.st.com/resource/en/appl...ive-discharge-ignition-stmicroelectronics.pdf

CDI article https://en.wikipedia.org/wiki/Capacitor_discharge_ignition

CDI article 2 https://www.elprocus.com/capacitor-discharge-ignition-cdi-system-working/

 

Thanks for the great reply. I think that part of your first paragraph is in the ether somewhere. I would like to read the
remainder of it. Also thanks for the links and part suggestions. They really help! Lee

 

Usually the probability of a component failing is related to the stress level of it's operation. The capacitor in a CDI system where it is charged every stroke by the magneto on the flywheel certainly is a higher stress operation, with a quite high peak current and a quite high voltage level. Accepting and then delivering of the electrical charge is work and work produces heat, and heat is certainly a form of stress.