Restoration Commences on 1947 RCA Victor 721TCS

[Penthode]

Originally Posted by bandersen

Here's what I found when I unrolled a cap from an RCA 630TS. Rotting aluminum and pinholes through the insulating layer.

Interesting and I have seen the same. Notice the foil is very thin and exposing the corroded outer layer you tore the foil.

My question is that if only on the outer exposed foil, the thin aluminum foil has degraded, how about inside? And why how would the minor corrosion materially affect operation if the dielectric maintains low leakage and the capacity remains unaffected?

I disconnected the input filter from the 721TS to measure leakage and capacity this morning. This is after "patient"reforming after the 73 year old capacitor which had not been used for 60 years. The leakage had actually dropped in the 45 hours use from 500uA to 100uA at 400v for the 80uF capacitor. At the high line voltage around here (123vac) the input voltage across this capacitor runs at about 330VDC. The rule of thumb absolute maximum leakage from two sources online suggest 1mA leakage for every 50uf for a 450v capacitor.

One thing I noticed is that the 60 Hz Power Factor is not as good as a new capacitor.

Anyhow here are some meter readings from this morning. The Sprague meter is on the 0.6mA full scale range and reads 0.1mA at the 400v on the VTVM.

Meanwhile try opening up that 630TS capacitor to see if the corrosion permeated the internals. Somehow I don't think it did. Otherwise, the capacitance would have been noticeably reduced.