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PHILCO PHORUM

I once restored a dual-45-output (first version) Model 90 chassis for a client which, upon initial power-up after a complete recap, had no reception.

The coils were checked and double-checked; they all had good continuity. (You may recall that I have written in the past about older Philco sets being prone to having bad coil windings.)

Thus began a general check. The set would pass an IF signal by feeding IF to the grid cap of the IF amplifier tube, and then in turn, the first detector tube. It would even pass IF through the RF amplifier tube.

O.K., it sounds like oscillator trouble. The coil (44) is good; what about the trimmer condensers?

I discovered a dead short across trimmer (42). There is a resistor and a mica capacitor connected in parallel with this trimmer.

In order to find which of the three components was shorted, one leg of resistor (43) and one terminal of mica condenser (41) had to be removed from one side of trimmer condenser (42).

Neither the trimmer (42) nor the mica condenser (42) showed any signs of a short. However, resistor (43), which is normally 50,000 ohms, was reading a dead short!

Resistor (43) was then replaced with a new 51K resistor. However, that was not the end of the trouble - the set still refused to play.

Recalling a Model 90 set I had repaired years before that had a bad trimmer, I proceeded to replace mica trimmer (42) with Philco part 04000-A, a trimmer salvaged from a slightly newer Philco chassis. Mica capacitor (41) was also replaced with a new 680 pF unit for good measure. After replacement of these parts, the set finally came to life. Following a complete alignment, the Model 90 was ready to go.

Partial schematic showing trouble spot in early Model 90

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© 1997-2006, Ron Ramirez. All rights reserved. Unauthorized duplication prohibited.