Philco Experts: Model 87 Grid Condenser?

[Winky Dink]

And I promise this is absolutely the last question I'll ask about the Philco 87 (unless another problem pops up).

The volume pot runs 0-10K ohms as it should, but even with no antenna strong local stations have substantial volume at maximum resistance. I tried some other potentiometers (25K, 100K, 2M) without success. What works best is attaching an antenna and touching a finger to the chassis, there's still an unacceptable minimum volume. Is there a way to achieve a better outcome (without holding my finger on the chassis)?

By the way, for anyone who's been reading this thread, the offending component was the 70K resistor even though it still reads 66K on the bench.

Tip of the Day: Don't flick solder on your foot when you're just wearing socks.

[old_coot88]

Hey good job getting the old gal going. On the volume issue, if you're located in a very strong signal area, the radio is actually working normally. In the early days, stations were generally weaker and more distant, and reception was on a long wire outside antenna. Many designers put the volume control right at the antenna input.
But today with nearby powerhouse stations, the signal gets into the set's wiring and RF tubes 'downstream', bypassing the front end vol.control. That's why you can't get the volume down to zero. The set's owner should see this as part of the charm of using such a vintage radio in today's signal environment.
To "fix" the problem would entail some judicious RF shielding of the radio.

For your own info, there's some good tutorials on neutrodyne sets if you google "neutralizing the neutrodyne".

[Winky Dink]

Never mind. I figured it out. The volume control stifles the signal coming through the antenna. The rest of the wires and stuff also are receiving the broadcast signal which doesn't go through the volume control. As I was thinking about what needed to be shielded from extrinsic electromagnetic radiation I remembered that I had a heavy galvanized plate that bolts to the bottom of the chassis. When I first took the plate off I wondered what it was for. Now I know. I'm done talking to myself for a while. Thanks for listening.

- Winky

[old_coot88]

Never mind, looks like you alreddy nailed the "problem".
Just out of curiosity, how effective is that bottom plate shield? The tubes and stuff topside still appear unshielded.

[Winky Dink]

The bottom shield was very effective. I delivered the radio last week. After putting it back in the cabinet there was a significant hum. I asked for a screwdriver, felt around for the hum adjuster on top of the chassis, and a half-turn solved the problem.
The owner was delighted with the restoration. And so was I.