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640x question - Printable Version

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640x question - troym - 01-29-2011

Hello,

I have a 640X console radio. One of the capacitor cans is a triple with values of 30uF, 15uF, and 10uF. However the schematic that I have shows those three values in the can as 2uf,2uf and 1uf (item # 68 30-2114) . I dont think the radio has been worked on before. Was there a modification somewhere or will it matter which are used.

Thanks

Troy

Re: 640x question - Ron Ramirez - 01-29-2011

Hello and welcome!

The three-section electrolytic has definitely been replaced at some time in the past. Philco did not increase the values of those electrolytics that much; in fact, I just looked through my copy of the Philco "Changes in Models" sheets, and I did not find any change to part (68).

You should replace these three electrolytics, ideally, with two 2.2 uF and one 1 uF. You could replace all three with 2.2 uF, 3.3 uF, or even 4.7 uF, 450 volts. But there is no need to go above that value. 30, 15 or 10 uF in those positions won't hurt the rectifier tube, but would be a waste of money as there is no need to have capacitors that large in those positions.

Re: 640x question - radiomikee - 01-30-2011

When the tube starts to glow red and get really hot, it means it is drawing alot more current than normal. This can happen in audio output tubes when the incorrect bias voltage is applied to the screen grid. Check the power supply voltage dividing/biasing resistor for continuity between all sections, and ground, you may need to take the wires off the terminals to be sure. One dead section in the primary of an output transformer will not cause the tube to glow red. Check all the electrolytic condensers for correct polarity of connections, particularly the one that runs from the power transformer high voltage center tap to the chassis, needs to be negative on the center tap, positive on the chassis.