brass top coil rebuild

[Dave S]

Over the years, I have rebuilt many model t coils with good results I replaced the capacitor and tested with both hcct and ecct. I just installed new capacitors in four brass top coils and I can not get a reading or function on any of them. My ecct shows the capacitor as "poor" The side terminals test at 3300 ohms. What did I do wrong ?

[ABoer]

Dave , I had the same problem with testen the capacitors with a ECCT .
Between the points I put a piece of paper ,as I did with the HCCT .
But that paper is not thick enough with a ECCT . And Shows Poor .
Now I take a thicker isolation and it works .

Toon

[Dave S]

Toon,
Thanks for your reply. I just used one of the fiber spacers used on the brass top with same result. The capacitor is connected between the upper and the lower bridge. Are brass tops wired differently than regular coils? Thank you.

[MKossor]

Dave, the ECCT capacitor test is very sensitive to leakage so Capacitor test results can easily be skewed as a result. The most important detail is the Capacitor Value - should be 0.47uF but can vary (+/- 20%) so Capacitor Test results displaying the Value in the range of 0.33uF to 0.56uF is acceptable and does NOT warrant replacing the capacitor; even if the capacitor test result is "Poor".

You can test a coil with brand new 0.47uF capacitor of the correct type and the test result will be "Poor" for any of the following reasons:
1. Fingers are touching both point elements during the capacitor test, current traveling through your fingers is "leakage"
2. Leakage from coil winding to the iron core the vibrator spring touches when being held open - As Toon suggested, place strip of paper between the coil core and the vibrator spring so it does not touch the iron coil core when being held open for the capacitor test.
3. Conductive black paint used to coat the top of the coil; some paints contain carbon which is conductive and causes leakage between point elements

For Example: Here is the Capacitor Test results for a coil with Brand New Capacitor.

ECCT Capacitor Result with Leakage.jpg (45.01 KiB) Viewed 1905 times

The Capacitor Value of 0.47uF BUT the test Result is displayed as "Poor" because my fingers were touching both point elements.



The numeric values of the test parameters provide a better clarity:

Capacitor Leakage Test Failure.jpg (16.65 KiB) Viewed 1905 times

Note that the capacitor Value is only 8.2% high (PASS), the leakage resistance value (4.8 Meg Ohms) is lower than typical (>50 Meg Ohms) so it is flagged as excessive leakage. BUT this is still a tiny amount of leakage that will NOT adversely effect coil performance. In hindsight, the capacitor leakage test threshold should have been set much much lower (1000's of Ohms) to where it may begin to effect coil performance. There are notes in the manual on the capacitor test results but may not be clear. Apologies for the confusion.