Don’t Trust Everything You Read (On Old Radio Schematics)

Electrical drafting is primarily done on a computer today, with software such as EAGLE or KiCAD. This wasn’t the case back when tube radios ruled the airwaves, though – schematics were drawn up by engineering draftsmen by hand. And as with any process with a human element, they didn’t always get it right.

I’m working on a 1934 Philco 66. It came to me in excellent original condition with little evidence of having been service, and throughout the process, I’d been relying on the schematics to guide me in the right direction. Unfortunately, along with a laundry list of other issues, my reliance on the schematic to be “the truth” led me around in circles longer than I needed to be to resolve a power supply problem.

Below is a schematic snippet of the power supply and audio sections of the 1934 Philco 66, with the RF chain to the left of the #75 Detector/1st Amplifier tube hidden for simplicity’s sake.

In green, I’ve highlighted the path B+ (high voltage) is supposed to flow from the rectifier cathode to the plate of the first audio amplifier. It’s a very straightforward path…if the draftsman had indicated that tube was supposed to be connected to the power supply. In red, I’ve indicated a missing connection symbol. Without it, there was no power being supplied to the first tube in the audio amplifier stage and the audio signal was being killed at that point before it could make it to the final output amplifier. Using an alligator clip, I restored that connection to test, and the radio sprang to life making noise on the next power-up.

The second filter capacitor should have been connected to both B+ and to the plate path for the #75 tube, rather than just the plate path. (Incidentally, the two capacitors are both at the same potential, so under the correct connection scheme could have been replaced with a single capacitor of a larger value.)

It’s not done yet, but I’m inclined to believe the final wiring issue has been corrected, and it’s on to performance.

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This entry was posted in Commentary, Projects, Radios and Tubes, Vintage and tagged , , , , , , , , , , , , , , , . Bookmark the permalink.

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