The PHILCO Phorum
Philco #91B Shadow Tuning Question - Printable Version

+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco #91B Shadow Tuning Question (/showthread.php?tid=24882)


Philco #91B Shadow Tuning Question - jjfritz5528 - 02-09-2024

I have seen a few discussions on this talking about rewinding the coil, but my coil is OK, 867 ohms. A little lower than those I saw mentioned in the previous discussions at about 1100-1200 ohms. My Shadow unit is Part #6497.
However, I don't have the resistor #14, 1k ohms in my set! While the Electrolytic Filter caps were changed previously, probably in the early 1950's, nothing else appears to be changed.
Could this be a Factory modification to eliminate the resistor? I didn't want to power-up and damage it, if the 1k resistor is required.
FYI - Both the Oscillator & Detector transformers had open windings that I re-wound. Not uncommon.
I included a copy of Ron's schematic. Item #14 is the resistor in question. The Parts Layout Diagram shows it connected on the Capacitor Block #56. However it is NOT shown anywhere on the Layout in Philco Service Bull. #129B, but does appear on #129.
Any thoughts are appreciated!

RE: Philco #91B Shadow Tuning Question - jjfritz5528 - 02-10-2024

Well, I did some testing, using a resistor decade box. Started with a 1k ohm resistor in series with the Shadow meter (as shown on the schematic) and reduced down to 15 ohms in steps. I got the same voltage across the meter, regardless of the resistor.  It was approx 4.5V to 6.0V.
SO I removed the resistor completely. I do get a good variation in the shadow width while tuning a station. Made some very minor positioning adjustments to the lamp.

RE: Philco #91B Shadow Tuning Question - Radioroslyn - 02-12-2024

Most of the circuits w/sm I've seen have a resistor in parallel w/the coil. This stops the disabling of the set if the sm fails. As I recall it's 2 or 3K. It also helps if there is a short in the plate or screen circuit to divide the current across the coil.

RE: Philco #91B Shadow Tuning Question - jjfritz5528 - 02-12-2024

Thanks Terry!
I didn't look at any of the other Models that have the Shadow Tuning, but of the various versions (at least 3) for the Model 91, some didn't have any resistor, and others had the 1k in series. Your theory makes good sense!