Philco 53-960

It's been a long time since I've been able to visit the bench, it's just been too busy. But instead of one of my eBay finds, one radio that I've had for a long time is starting to sound poorly so it takes a spot on the bench.

The Philco 53-960 is a 9-band (AM - 8 SW) table model receiver with 8 tubes. I like it mostly because of its large 10" speaker with a decent 7 watt push-pull audio stage. Saturday nighs I'll play oldies through it with my SSTran AM transmitter. Somehow, Sinatra sounds better through something like this.

A couple of weeks ago, it started to sound very distorted as it warmed up, so last weekend I brought it down to the cluttered workbench and took off the back.


I let it play on the bench for a while with an o'scope probe connected to the speaker. It was starting to sound a little distorted, but with the scope connected I tried to see just what kind of output power I could get, so I cranked the volume. BANG!! The radio kept playing, but I pulled the plug but quick. The chassis had to come out to assess the damage.

The capacitor in pieces and the other one still in one piece are connected from the AC line to the chassis. The unfortunate thing is that my connecting the scope probe ground to the speaker probably brought about this capacitor's demise (alarm bells should have gone off in my head when I got a spark connecting the scope ground), though any of these type capacitors (called bumblebees or 'Black Beauties') should always be replaced on sight as their chance of being good after 50 years are near zero. A look at the rest of the chassis shows a few more of these "bumblebees" (7, I think).

But this failure probably had nothing to do with the distorted sound, so with a just-obtained Sams schematic in hand I made some resistance measurements to the tube sockets. How about that, a problem in the output stage.

The arrow points to a nearly-open resistor that helps set the bias for the two output tubes. That's probably a good a reason as any for the distortion. Most every other reading seemed to be within reasonable limits.

The next step is to order the replacement parts. All of the 'bumblebee' and paper/wax capacitors will be replaced as a matter of course, since they can all be thought of as "time bombs" now. Instead of the 'bumblebee paper' capacitors across the AC line, special "across the line" rated capacitors (with so many regulatory agency approvals there's barely enough room on the part to list them all) are available which are designed to fail safely - like not go boom. Not that it really matters, since there's plenty of room under this chassis, but the modern replacements are much smaller and longer-lived than the originals.