- December 30, 2018 at 6:30 pm #10918
I have my tube tester up and running and started checking the tubes in my Philco 49-504 for my first project! The tubes were caked in black tar like goo on top and I had a difficult time identifying them but I finally hit on carefully scraping the tops with a razor blade and wiping them with fast orange hand cleaner. That worked real well as I could read them and didn’t erase the numbers. According to my tester, tube 50A5 is shot. It doesn’t heat up or register on the meter. 7A8,14A7,14B6 and 35Z5 all test in the Green zone (Good). Symptoms: Radio played well when I bought it and the dial indicator light first went bright, then settled down. I have since read that that is normal. Now, nothing works, no light,no tubes warming up, just dead. If I read the schematic correctly, the 50A5 tube is the power output tube for the speaker. If it fails, shouldn’t the radio work (light up) with no sound or a hum? Santa didn’t buy me a Fluke meter so I ordered one and still waiting for it to arrive, so I can’t really check anything yet. There is a local place that has the 50A5 tube and barring that, I found one online for $4.90 U.S. seems cheap!
December 31, 2018 at 6:10 am #10922
- This topic was modified 2 months, 2 weeks ago by garykuster. Reason: updated info
Alistair ThomsonCVRS Member
If no tubes are heating up, yet some show OK on the tester, then you seem to have lost the power to the heater chain. It looks like there is no power transformer in this radio, and the chassis is live. Be VERY careful when poking around inside, testing voltages!
The heaters are in series, so if one heater burns out, nothing works. You have identified that the audio output tube, 50A5, has a dead heater. That will be why the radio stays cold. If the other heaters are all OK, then when you replace the 50A5 you will have the radio lit up again.
It’s good that you will soon have a multimeter, because troubleshooting any electronics is virtually impossible without one!December 31, 2018 at 6:51 am #10923
Thanks once again Alistair! I should have spotted that the heater circuit was in series! To be certain I understand you correctly, Is the ‘arrow’ (forgive the hillbilly photoshop) pointing to where the break would be? Reminds me of old Christmas tree lights. I know I was getting ahead of myself again and will wait until the meter arrives before I continue. I’m learning the color codes for the resistors and locating them in the chassis with the diagram! What fun! The caps are less fun with ones that are clearly original, not necessarily jiving with the schematics.
Attachments:December 31, 2018 at 9:22 am #10925
Alistair ThomsonCVRS Member
Yep. That’s the one. If you remove the 50A5 and put your meter, set to read low ohms, across pins 1 and 4, it will be open circuit with infinite resistance.
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