Paper/wax caps don't have a polarity, one end is usually marked to designate the "outside foil" so you can attach the wire lead from that end to the lowest impedance part of the circuit. This is effective in shielding the circuit from outside interference. You'll notice the black ring around the end of the cap. That's the outside foil end. The only problem is your new cap isn't marked. There are several ways to find which end should be marked but you might not care to take the time to do this. Go ahead and plunk the new one in either way (Some purists will cringe). I use an oscilloscope attached to the cap, hold the cap in my fingers and look at the level of noise the cap displays then reverse the leads and observe the level again. The lead attached to the ground wire of the lowest level of noise is the outside foil.
I also noticed that the photo doesn't reflect what the schematic shows regarding part 21. Perhaps someone else with more experience on this set can enlighten us.
Looks like capacitor 21 is not a paper cap but actually an electrolytic in a waxed cardboard tube. Part number on the cap agrees with the number and description in the parts list. Cheap electrolytics were actually made this way in the 30's and 40's. This also goes for cap 20 which was also a cardboard electrolytic.
Parts list specifies both cap 20 and 21 as electrolytics, the confusion is the waxed cardboard case which makes it look like a typical paper cap. Polarity is noted by the black band but I am not sure if this marking indicates positive or negative. It looks like the end of cap 21 connected to the 6U7 pin 6 connects to the IF transformer primaries and a film bypass cap, which should be the negative of the electrolytic. Positive of cap 21 should connect to the B+ supply line and negative to the connections to the IF transformer primaries.
The wiring of the cap 21 to the 6U7 socket is also confusing, but it appears that the tube pin is not used on the 6U7 and the socket terminal is only being used as tie point for various wires. If you trace the circuit connections of cap 21, they should connect across the primary winding of saturable transformer 4 which is part of the tuning indicator circuit. The brightness of bulb 33 varies with tuning as the plate current of the 6L7 and 6U7 changes with AVC control voltage.
(This post was last modified: 12-19-2021, 02:55 PM by Mondial.)
Question about Condenser #21 see the attached picture
The schematic to me looks like it should be jumped across the two taps of #4 110786. It actually goes from the tap on #4 to 6U7G. If i follow this then what is the (-) and (+)
side
Negative should go to the pin 6 on the 6U7G, positive to transformer 4 terminal where the big 15K is connected.
There should be a wire from the pin 6 on the 6U7G to another terminal on transformer 4 so effectively the cap is connected across two terminals of transformer 4
Powered up the radio and here is what I have. The short wave band plays great the broadcast band is ok between 550 and 900 ish when I start to get near 1000 I get a heavy buzzing noise.
