[dconant]

"I would recommend simply installing 1M resistor across the caps (the right two holes on the picture) and not to think of it anymore."

I was reading this thread and saw the above. What is the purpose of the 1M resistor?

Dan

[radiorich]

hello everyone,
I know it has been a while since I have posted about this radio but finally got chance work on it .
I pulled the tuning capacitor So I could finish taking it apart and put in my Ultrasonic Cleaner the one thing I was wondering I  have to do  remove the mica sheets on the trimers .
Sincerely Richard
P.S.
This tuner is filthy and tons of old grease

[RodB]

Yes, you should carefully remove the mica sheets from the trimmers otherwise you risk trapping foreign material in the trimmer. You'll also have to clean the mica sheets before putting them back. Maybe leave them separately in the ultrasonic cleaner. Make sure they're dry before reinstalling.

[dconant]

I have a question about the mica sheets. I thought I saw somewhere on the Phorum where someone used thin plastic report covers in place of mica. Is this acceptable?

[morzh]

It is acceptable but not desirable.
Plastic is not an approved cap dielectric, and even though a reasonably thick piece will have a good voltage breakdown rating, it's dielectric constant's behavior over temperature range is not guaranteed to be stable.
I do not know if it is good or bad, but I never researched this. We do know that this material is not used in caps, even though some plastics (polyesther etc) are used, but then again, they are not as stable as mica or some ceramic (NP0/C0G) types. This is why the performance might not be as good. Grtanted, today's radios are not everyday use/main communication device as it used to be, so it might not be as important, but still this has to be considered.

Or, as Jesus referred to this practice of breaking rules:

"On the same day, having seen one working on the Sabbath, he said to him, O man, if indeed thou knowest what thou doest, thou art blessed; but if thou knowest not, thou art cursed, and art a transgressor of the law."

[Steve D]

Most common plastics have a dielectric constant in the 3-4 range whereas mica is around 6. This means that even if you were to use a stable high dielectric breakdown plastic such as Polyethylene or Kapton of the same thickness, the capacitance adjustment range would be about half. You could possibly measure the thickness of the mica and try to find a thinner plastic that provides the same capacitance. Capacitance varies linearly with the distance or insulation thickness between plates. Since Kapton and polyethylene have very high dielectric strengths a thinner piece could provide the required capacitance and still withstand the voltage.

Steve D

[dconant]

Thanks for the info guys. I always wondered why the use of mica. Now I know.

[radiorich]

Hello Dconant ,
they sold mica to use as a insulator between transistors and their heatsinks too.
Rob I will remove them and I thought i might use my Capacitor checker and see what value it reads before taking them apart.

Steve for sure and you still can get mica and I bet if you went snooping around MCMaster-Carr you could find something to replace it with .

Sincerely Richard
P.S. I sure like my ultrasonic cleaner it's very handy I even clean my electric razer's rotory cutters in it too.

[dconant]

Rich do you dunk the whole razor in there with it turned on to get a good mix?