[ccomer1955]

http://www.nostalgiaair.org/pagesbymodel...013838.pdf

I finally have gotten serious about that Philco 77 (busy Summer what with trips to take and somehow four other radios got in line before it), but after restuffing the bakelite boxes and the small metal cap boxes and the large cap box, recapping the tone control, AND replacing all of the resistors with 1 watt new ones, it failed a dim bulb test.

Measuring the resistance on the power transformer, I find low resistance on the primary (about 20 or a bit more-going on memory here), VERY low resistance (0.1-0.2 ohms) on the filament secondary for the 80 (10 & 11), and on the ones for the 45s (7 & 8) and 24As & 27 (5 & 6). Under 300 ohms on the HV winding (9 & 12).

The wire-wound resistors measure as they should, and I've triple-checked the large cap box hook-up. The choke and the output transformer and the field coil all measure as they should.

I guess my question is: Should I be seeing higher numbers on the various filament windings, or are these numbers about right, AND if these numbers are about right, I should be looking for a short further down the line, right?

I have yet to try powering it up with no tubes, but I guess that is next?

UPDATE: when I tried bringing the transformer up with no tubes in it and a dim bulb (15 watts) in series with my variac, I bee a faints glow around 70 volts and a soft glow at 110 volts. I guess that qualifies as a shorted transformer?

[Steve Davis]

UPDATE: when I tried bringing the transformer up with no tubes in it and a dim bulb (15 watts) in series with my variac, I bee a faints glow around 70 volts and a soft glow at 110 volts. I guess that qualifies as a shorted transformer?

Probably not. A 15 watt lamp is too small and would probably glow with no load on the transformer. Bring it up to 115 volts and let it run for an hour or so with no load. If it doesn't get hot, it's probably a good transformer.
Steve

[ccomer1955]

Lazy measuring on my part, too. I remeasured the two multi-section wire wound resistors (long and short tubular on the schematic) and found a bad section in each one. Weird how measuring from end-to-end gave a different result from measuring each section seperately. Lesson learned there. The two-section wire wound resistor connects directly to the CTs for the filament windings for all of the tubes (transformer terminals 5 & 6, 7 & 8, and 10 & 11). The section on that resistor where the CTs connect was bad (transformer terminals 3 & 4).
Luckily, I have two 77 chassis, so I swapped out the multi-section wire wound resistors. Now I've got the required resistance in each section of both tubular resistors. Do you suppose that might have been the issue?
I'll give it a go tomorrow and see what happens after I inspect for bare-wire shorts in the light of day. Transformer issues make me nervous.

[ccomer1955]

Further update: I ran the transformer with no load for an hour or so at about 117v, and after that time, it was cool to the touch. I guess I'm tracking down a chassis short somewhere.