[AMRadio] Something strange


Jim Wilhite w5jo at brightok.net
Tue Nov 18 16:36:34 EST 2014


About 2-3 years back I replaced the electrolytic caps in a Globe Champion on which I was doing an  electrical restoration.  I used Sprague Atom caps in the set, the bias circuit and the low voltage circuit.  The low voltage runs about 300 volts and I used a 450 volt capacitor.  In the bias circuit I used 350 volt capacitors where the voltage is about 100 volts.

I have been using the set off and on since restoration and last week it suddenly developed a lot of hum on the carrier.  That was with the audio off so it was a puzzle.  I found one of the 350 volt filter capacitors in the bias circuit has opened.  Not only that, the one in the Low Voltage supply had opened as well.  

The two caps were in different lots and I formed them with a TO 6 before installation.  If I did a value check, they checked the proper value, but when I did the leak test they started out fine but quickly developed an open.  All voltages were in specification and replacing them fixed the problem.

I thought it strange two would go at the same time in different circuits and am a bit puzzled by it.  Oh well it gave me something to do for a few days.

Jim
W5JO


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