http://www.nostalgiaair.org/Resources/712/M0015712.htm
I just finished recapping my RCA R-74. The radio didn’t work when I started. It comes on now and I can tune in station but it has a building squeal, kind of like a siren. It builds quickly and sounds like something is going to blow any second, so I turned it off. Tried it again same thing. I probably have an electrolytic in backwards, but which one? I CANNOT read schematics. All the caps are in a can with color coated wire but polarity is not listed for the electrolytics. Can anyone help? I need to know what color wire goes to what pole.

The rectifier cathode is the most positive part of the set. The transformer center tap is the most negative part. Regardless of whether the electrolytic is going across some part of the power supply or a cathode bias resistor, its positive lead should face the more positive part of the circuit, and its negative lead should face the more negative part of the circuit.

Though your problem could be due to a backwards electrolytic, it could also be due to a capacitor being connected to the wrong part of the circuit, or absence of a capacitor, or tube shields missing. If a big capacitor went to some portion of the set that was either oscillating, or was being caused to oscillate by the capacitor, the capacitor's slow charge-up rate might cause the siren effect you hear. Check the wiring.

T.

I appreciate your help but I really cant read schematics well enough to do that. I went completely by the color code and I am sure I got it right except for the polarity of the electrolytic caps. I traced each wire to its connection point to verify I had the right wire. The only thing the cap gang diagram didn’t show was polarity for the three 10mfd and the one 7mfd caps (I used 10mfd for all four).
I was just testing it, so I didnt have the tube shields on. Would it get that loud without them? It sounded like something was gonna blow.
Thanks, Brett

Wow, I didn't know tube shields could do that much. I put them all on and presto. Thanks Tom.
Brett

No prob. I just hope that you have those electrolytics connected properly. Make sure that none of them get warm, and study your schematic symbols.

T.