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Peeling plates on tuning capacitor Model 70 - Printable Version

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Peeling plates on tuning capacitor Model 70 - Tubesforme - 02-12-2017

I purchased a Model 70 that was supposedly completely restored. However I was told that when tuning stations there was a scratching sound. There sure is when the plates are close to fully meshed. I checked the tuning capacitor and am positive that there is an area where there is what appears to be a super thin metal coating that is flaking off the plates. I have been in the business for over 50 years and have never burned a tuning capacitor. I know that it must be totally disconnected from the circuits and I once read you could use the HV from the chassis to burn this coating off. Has anyone ever attempted this and if so would you please instruct me in the correct method. I do not have a separate DC supply and I doubt if you would use AC. many thanks to all of the professional Philco help on this site.

RE: Peeling plates on tuning capacitor Model 70 - morzh - 02-12-2017

I never attempted this, didn't have the necessity, but to do it you need to dsconnect only one side - in this case the stator ( I think) , that is the side that is not screwed to the chassis. All wires of the sections you want to burn. Better all of them, no matter burned or not.

Then get a few different electrolytics, from 100uF 100V to 1000uF.

Charge to about 50V and discharge through the section. If the arc is not enough, increase the capacitor. Then voltage if still not enough.
I wouldn't go above 50V at 100uF for the start.

RE: Peeling plates on tuning capacitor Model 70 - Tubesforme - 02-14-2017

Thanks, I will try that. I was really surprised to see the peeling. Thought I was seeing things. This tuning capacitor is insulated from the chassis. I agree, do them all.

RE: Peeling plates on tuning capacitor Model 70 - morzh - 02-14-2017

Well, it is in fact connected to the chassis, but I think it is the rotor that is grounded and the stator going through the grommets with metal inserts is insulated.