Philco surgery

The repair parts came in on Friday, so Saturday I got the soldering iron out and proceeded to get those bad parts out of there. First were the capacitors across the line. One was already blowed up, so out came the second one.

As you can see, I pulled the one still there away from the terminal strip to show the wax it left behind. That cap was not long for this world. Here's the replacement.

Here are the original "blowed up" cap, the new replacement, and the other cap about ready to go. You can see printed on the new cap all sorts of regulatory agency approval markings from UL, CSA, VDE and many others. These caps are specifically designed for being placed across the AC line and if they fail will fail safely (not go BOOM). Now the new safety caps are in.


You can see another cap that was replaced, the white braided thick "wire" is actually a power resistor, and you can see what it did to the old capacitor. Nice burn marks.

And now you can see a recurring headache that popped up throughout the re-cap process. The new parts are so much smaller than the originals that in many cases the leads aren't long enough for the parts to go in the same positions.

Now that I had the radio relatively safe, I wanted to check the bias voltages on the output tubes. I suspect that the open bias resistor I found earlier would cause a problem, and it did. Instead of -20V bias on the 7B5 grids I had 0. So instead of a plate voltage of 295, the tubes were drawing much more current than they should have and the plates were down to 220. Very surprised something didn't melt in there.

The next step was to replace every resistor and capacitor in the output stage, as the resistors that hadn't opened were drifting way up in value.

After all that was done, the bias was checked again and it was almost dead on -20V. Plate voltages were up around 310V, a little high but OK, those tubes took quite a beating and are probably very weak now.

That was enough for Saturday, by this time the radio was playing again. Sunday the rest of the capacitors got replaced. These were a little deeper in the chassis, and some surprises were lurking as well.

A "resistor to nowhere". Finally everything was done.

I did not reinstall the wiring to the phono socket because it's too crowded in that area and I never foresee using a phonograph with this.

Now playing on the bench. There's still a couple of minor issues to take care of. The tuning capacitor is dirty so there's much noise when tuning the shortwave bands, the calibration is also off by a bit. But the AM section sounds OK.

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.