I've been working on an RCA 9X641 trying to chase down an intermittent static/noise problem and finally nailed it. All the capacitors except mica had been replaced so I was checking for touching parts, shorted wiring, bad tube sockets, etc. Then equally erratic, the pilot lamp would change brightness by a small amount. I took voltage and resistance readings, made visual checks, moved wiring around, and sniffed for anything burnt or over heating. I also removed the bulb, cleaned the socket but found nothing. Yet the problem continued --sometimes accompanied by static and noise. Finally I decided to try a different pilot lamp socket so de-soldered pins 2 & 3 of the 35Z5 and installed a spare. All problems immediately disappeared. This was one of the older 3/4" sockets holding a #47 and had the cardboard wrap. I very carefully opened the socket up by bending the tabs away on the bottom and removing the fiber washer where the wires pass through. Below the fiber washer, resides the spring and then a second contact washer where the center wire solders. In this area, I found a metal fragment that apparently broke away from the first washer when the socket was assembled. In the photo, the fragment is just below the red center contact disc.

Nice detective work!
It's amazing how such a little thing and cause problems. Thanks for taking the time to post this as well.
Ben

A good find! That reminds me of a ground-loop short I found, where a metal filing was embedded in the plastic insulator of a large electrolytic capacitor. The clamp band squeezed the filing into the case and made contact. I could not see the filing, but could feel it as it was the size of a splinter.
Don

Yes indeed, those hidden defects can really shake the faith in your knowledge. I've learned to expect the unexpected.

The old pilot lamp sockets are often suspect: that little "cardboard" insulator in there goes to pot over the years and there's not much clearance to a possible short. I've replaced them with a nylon washer that was luckily the same size.

Well thanks for posting that I'll keep a eye out and check those sockets

Rich

That type of socket is also found in older auto's. And in either case can be just as big of problem with the globe out.
With a loss of compression on the spring and a bit of shrunken insulation. the spring can cause, quite a perplexing short.

Marc

Well that's a useful tip. Then it seems logical to leave the bulb in it's socket even when burnt out.

Absolutely... Never leave the globe out of a powered socket of that style. I have seen it happen and as just as annoying as those who install accesories etc. in cars, & put srews through the wiring loom, even had a neighbour drill into the suspension tube of a British "Mini Minor" (That real sinking feeling: Followed by deflated ego and suspension)

Marc

You know what would have been nice when these tube radios were designed, they should have put the electrolytic capacitors in a metal can with pins that could be easily plugged in or removed just like a car radio vibrator since they frequently go bad. But what I do now is use the original chassis hole that attached the capacitor mounting ring as a mounting point for a 4-terminal strip to facilitate the addition of radial-lead replacements. You just jumper the two center lugs together and connect to B minus and the two outer lugs become B plus. If you want to add additional, you can wire units from both sides of the terminal strip. Be sure to use radial leads rather than axial since the lead spacing is just right for terminal strips of the era.

In photo below, new electrolytics were added just to see if the radio would play. The wiring is cracked and insulation falling away (RCA 66X1) and, of course, needs the wax removed. It now resides in a radio pile "maybe someday". LOL

download/file.php?id=16569

Some older professional equipment such as radio broadcast gear have electrolytics that plug into octal sockets.



Dave

Very nice and exactly what I envisioned. I bet they were expensive but very sensible.

I refurbished a Lyric 70 series ( Circa 1929) Both the transformer and the bulk of the caps, were in cans held down by two screws. The bases plug into sockets on both. An early attempt at modular serviceing.

Marc

As usual, just when you think an intermittent problem has been resolved, it starts up again. This is the case with an RCA 9X641 with a persistently returning pilot light flicker. Therefore, I've built a new tool to test for a bad tube socket. It's been my experience that cleaning does not always result in a permanent fix and so my efforts are aimed at socket replacement. But which socket is the offender? For a lack of a better name, I'm going to call my new tool the "Socket Jock-IT".

The Socket Jock-IT consists of an 8-pin tube socket with eight clip leads attached to temporary move a tube out of it's existing socket and into the test socket. To facilitate a temporary mounting, two-inch alligator clips are attached to the tube socket rivet holes with 4-40 hardware.

Parts List
(1) Tube Socket
(8) Split Lamp cord, 6" lengths
(8) Mini Clips (RS 270-1545)
(2) 2" Alligator Clips
(2) 4-40 Hardware
(8) Heat Shrink Tubing, 0.5"

Proceedure
1. Remove tube and place into temporary socket
2. Attach temporary socket to chassis via 2" alligator clips
3. Attach clip leads to socket pins under test
4. Turn radio on, see if trouble stays or goes

I started with the 35Z5 socket and under test, the pilot lamp stopped flickering. The voltage reading across pins 2-3 reads an unwavering 3.48 vac after warm-up. This is a #47 bulb with a 100-ohm resistor in parallel.

If the result remains constant for 24-hours, I will return the 35Z5 to it's original socket to see if the flickering returns. If it does, I will desolder and replace the socket.