Welcome Guest! Be sure you know and follow the Phorum Rules before posting. Thank you and Enjoy! (January 12) x

Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Philco #91B Shadow Tuning Question
#1

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!


Attached Files Image(s)
   

Thanks
John N3MUN
#2

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.

Thanks
John N3MUN
#3

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.

When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!

Terry
#4

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!

Thanks
John N3MUN




Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Heathkit Sweep Generator TS-4
Ron Instead of Quick reply, go to New Reply, then go to the title of the very first post, and change the title text the...morzh — 02:00 PM
6TPZ and 5TPO Radio Help
I am not a specialist in car radios. I suspect they might use vibrators to convert 12V to acceptable tube voltages, so ...morzh — 01:58 PM
6TPZ and 5TPO Radio Help
Welcome to the Phorum, R3Sons! I have very limited experience with car radios other than installing custom radios back ...GarySP — 01:08 PM
462ron
OK, I need to rename this thread to say..Eico 368 TV/FM Sweep Generator instead of the current title..Heathkit Sweep Gen...462ron — 12:49 PM
6TPZ and 5TPO Radio Help
Hello, I have a Philco (Ford) 6TPZ and a 5TPO radio.  I replaced the electrolytic capacitors in both.  I have the SAMS m...R3Sons — 12:29 PM
The list of my radio & TV collection!
Hello Peter, Thank you for posting that !! Sincerely Richardradiorich — 09:40 AM
Eico 232 VTVM Schematic
It's a microwave antenna. ?;-p?. I substituted 1N4002s off the 6.3 vac winding for the 1.5 volt battery. Uniprobe is the...RodB — 05:50 PM
Eico 232 VTVM Schematic
Try BAMA for a schematic. Looks like a replacement power source for the D battery has been installed. The tube may be...Chas — 04:51 PM
Eico 232 VTVM Schematic
Is that a battery holder?morzh — 04:43 PM
Eico 232 VTVM Schematic
A friend gave me this Eico 232 VTVM and I'm having trouble locating the schematic for it. This one seems to be a later v...RodB — 04:17 PM

[-]
Who's Online
There are currently no members online.

>