I'm working on a 1939 Beam of Light Philharmonic Deluxe and noticed a wiring error (ok, maybe)...in looking at the schematics available, it gets worse. Refer to the 3 images attached as needed. Here we go:
In the 1937 schematic, and in most sets I've worked on, pin 8 (cathode) and pin 5 (grid #3, usually tied to the cathode) go directly to ground. Pin 5 (at ground) is used as a tie point for the "pass through" caps in the IF cans. In the 1939 schematic, it looks like Scott added a cathode bypass (400ohm/.05mfd) - BUT - the schematic still shows everything grounded (look closely at the diagram, the R/C network does nothing as drawn).
Now the puzzler - the set I'm working on is wired like the 1939 schematic, but "correctly", the cathode and pin 5 are sitting on the R/C network, NOT at ground. HOWEVER - the two very original caps passing through the IF are connected to pin 5 - therefore NOT at ground potential.
Now I'm sure the primary IF bypass (goes to B+) should be coming back to ground. And pretty sure the secondary one should too. I have two options: remove the R/C network and just ground things like the 1937 schematic, or move the pass through caps over to a ground tie point. I"m leaning toward the latter solution. Would love thoughts on this from the techs in the group!!
David C. Poland
Try it. Run the two IF caps to some other ground. Keep the new "cathode bypass network" in a functional circuit. Seem beneficial?
Seems to me Scott would introduce the modification for some good reason (adding cost of 2 additional components in each stage of the IF amp) .
That Potter cap to left be an old repair/replacement?
Nov 11, 2024
David C. Poland
Note - the combo Philly diagram Riders 14-5 shows this IF cathode network added only for the 3rd IF tube, and no longer in the 1st and 2nd IF tube.
Nov 11, 2024
Bill Liers
Kent,
Cathode bypass not at DC ground potential but still close enough to AC ground through bypass cap. No major functional difference certainly at If frequencies.
Nov 11, 2024
Kent King
I also spoke with Norman...I'm going to restore it as it was built and see what happens. On the 2nd IF, the B= cap had shorted, and that took out the 400ohm cathode resistor and not the 1K feeder (which is what usually goes). It will be a while before I can tell you all how it works - this is a remote control set and I've got quite a few hours of work ahead before powering it up.
Dave - that Potter cap is a replacement, but I'm recapping the set throughout.
Nov 12, 2024