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Proper replacement capacitor

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Post by John Demarrais on Thu Sep 17, 2020 5:32 pm

I have little experience working on old tube radios. I have a non-working factory radio in my 1962 Ford Falcon and want to fix it. Motorola 04MD. It does not power on, i.e. it does nothing. I got a schematic for it along with a parts list. I want to order some new capacitors for it.

My question is about what a proper replacement is for the one electrolytic cap it has. It is listed as a 200-400 MF electrolytic cap, rated at 16 volts. I do not know why the broad range rating for the capacitance. I am wondering if I get a 400 MFD cap at anything over 16 volts it will be okay?

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Post by jerryhawthorne on Thu Sep 17, 2020 7:17 pm

Well, it certainly will work fine, higher voltage fine and anywhere withing the range given will be fine. I hope it works.

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Post by Dale H. Cook on Mon Sep 28, 2020 1:29 pm

John Demarrais wrote:I am wondering if I get a 400 MFD cap at anything over 16 volts it will be okay?
It will if the working voltage of the replacement is not too high. Electrolytics need to operate at a certain percentage of their rated voltage. I would suggest nothing above a 25 volt replacement. Note also that 400 ufd is no longer a standard value, but 470 ufd should be fine.
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Post by Resistance is Futile on Tue Oct 06, 2020 4:05 pm

That is a hybrid radio both tubes and transistors.

It is a 2 Electrolytic's  That has a (C14A) 200MF and a (C14B) 400MF
It filters the 14Vdc (B+). I would use same voltage as Cook suggests of at least a 20-25 Volts on the Caps voltage minimum. It may be a dual cap. But you can install 2 separate caps.

Check R15 (470ohm) it could be open. Also the 2 spark plates may have some issue. I don't see a fuse, however it has to have a fuse. If the voltage was hooked up backwards then your Driver and pwr amp may be shorted or open.

Bietmans vol 20, page 76
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Post by Ed in W. PA on Tue Oct 06, 2020 4:51 pm

Dale, could you expand on the working voltage information? I have been searching the internet looking for some kind of information on this subject. What kind of problems are caused when using a cap with too high of voltage rating? Can anyone direct me to a website that can help? Regards, Ed

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Post by Dale H. Cook on Tue Oct 06, 2020 10:57 pm

Manufacturers' technical information for their aluminum electrolytics tell the tale. Some types of caps have a recommendation that they should not be used below a certain percentage of their rated working voltage. Others seem to be fine at any voltage below the working voltage. Unless you have solid data from the manufacturer showing that very low voltages have no adverse effect it is wise to operate at no less than about 2/3 of the rated voltage.
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