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The Fine Things are Always Hand Made

Philharmonic IF wiring - schematic errors

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!!

Kent

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Comment by Kent King on November 13, 2024 at 2:03pm

Thanks to everyone (Scott - just saw your note...I concur, I checked one of my other chassis as well).

After 14 hours of work, I have finished replacing caps and checking all the wiring. I still have to do some cleaning and work on the remote control units (the remote control volume control is still out in this picture). 

Comment by Scott Seickel on November 13, 2024 at 1:54pm

Kent, this is how the cathode is wired on several sets I have.  This is correct. 

Comment by Norman S Braithwaite on November 12, 2024 at 12:18pm

I suspect the cathode return introduced a small amount of negative feedback to the IF amplifier to improve IF amplifier stability.  In every AW23 on which I have worked, one tuned circuit is shunted by a 500k resistor not shown in the schematic for the purpose of reducing Q and gain to prevent oscillation even though excessive shielding was employed.

Norman

Comment by Kent King on November 12, 2024 at 8:51am

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.

Comment by Bill Liers on November 11, 2024 at 7:03pm

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.

Comment by David C. Poland on November 11, 2024 at 12:29pm

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. 

Comment by David C. Poland on November 11, 2024 at 12:14pm

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?

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