I am working on an Emerson 502 (schematic available on this site). It's blowing its dial lamps. One section of the big two-section electrolytic in it was replaced before I started, with a value of 100uF instead of rated 50uF. I replaced the 30uF side of it with a new 30uF cap as the radio was buzzing. I also replaced one of the paper caps that was drooling wax. Other paper caps "looked ok", and some others had been replaced by early plastic caps at some point in the radio's life. Radio played wonderfully, but after a few hours of playing the dial lamp blew.
I was unable to make out the voltage of the original lamp, so I replaced it with a 14V bulb. After about 1 minute of playtime, the lamp suddenly brightened, and blew.
This lamp hangs off of a 35Z5GT rectifier. Ohmmeter shows 35.7 ohms between pins 2 and 7, so I don't think the heater filament is toast. Any other suggestions as to what it might be? Did I guess wrong on lamp voltage? Something else that might cause this?
Thanks...Ben
Replacing the electrolytic caps was the right thing to do. C17 on pin #8 of the 35Z5 shouldn't be 100 mfd. This high capacity will increase surge current and can damage the rectifier and lamp.
The lamps in these radios are usually #47, 6 volt. If you are burning out 14 volt lamps the heater tap in 35Z5 may be open. Check continuity of 35Z5 from pin #3 to #2 and #7 with the tube out of your radio.
Since the radio did play for two hours the 50L6 may be drawing excessive current. Measure voltage on pin #5 in reference to pin #8 on the 50L6 tube. Pin #5 should be 5-8 volts negative. If zero or positive the tube will draw too much current, distort and cause hum. The cap connected to pin #5 is often leaky.
Norm
: Hi,
: I am working on an Emerson 502 (schematic available on this site). It's blowing its dial lamps. One section of the big two-section electrolytic in it was replaced before I started, with a value of 100uF instead of rated 50uF. I replaced the 30uF side of it with a new 30uF cap as the radio was buzzing. I also replaced one of the paper caps that was drooling wax. Other paper caps "looked ok", and some others had been replaced by early plastic caps at some point in the radio's life. Radio played wonderfully, but after a few hours of playing the dial lamp blew.
: I was unable to make out the voltage of the original lamp, so I replaced it with a 14V bulb. After about 1 minute of playtime, the lamp suddenly brightened, and blew.
: This lamp hangs off of a 35Z5GT rectifier. Ohmmeter shows 35.7 ohms between pins 2 and 7, so I don't think the heater filament is toast. Any other suggestions as to what it might be? Did I guess wrong on lamp voltage? Something else that might cause this?
: Thanks...Ben
: Replacing the electrolytic caps was the right thing to do. C17 on pin #8 of the 35Z5 shouldn't be 100 mfd. This high capacity will increase surge current and can damage the rectifier and lamp.
OK, I replaced it with a 50uF cap.
: The lamps in these radios are usually #47, 6 volt. If you are burning out 14 volt lamps the heater tap in 35Z5 may be open. Check continuity of 35Z5 from pin #3 to #2 and #7 with the tube out of your radio.
I checked this, and pins 2, 3, 7 of the 35Z5 all have continuity with each other.
: Since the radio did play for two hours the 50L6 may be drawing excessive current. Measure voltage on pin #5 in reference to pin #8 on the 50L6 tube. Pin #5 should be 5-8 volts negative. If zero or positive the tube will draw too much current, distort and cause hum. The cap connected to pin #5 is often leaky.
I measure -4.87Vdc. The cap off pin 5 has been replaced before at some point in the past (because the cap that's in there is an early plastic cap, and not one of the stock paper caps, and the solder joints are blobby, unlike the rest of the radio).
I just ran the radio for half an hour with a new 14V bulb and it didn't blow. However, there was one time shortly after turn-on when it glowed quite brightly. I think if it had been a 6V bulb, it might have blown. Also it seems to glow fairly bright, which surprises me, I'd expect a 14V lamp to be more reddish if it was really running at the right voltage...
Should I be concerned about the voltage at pin 5 of my 50L6? What should I do to make it more negative - replace the cap or the tube, or?
Thanks again...Ben
Any leakage in the coupling cap and the negative voltage will be low. The data book suggests -7.5 volts. A gassy tube can also cause this voltage to be wrong and tube to draw excessive current. A bad tube can cause this voltage to go toward zero after the radio has been on for a time.
It's not voltage but current of the bulb that's important. If you connect a 3 volt flashlight lamp it wouldn't light since it draws high current. #47 bulb is normally used with these radios. It's rated 6 volts at .15 amps.
The light should come on bright then dim. After the radio starts playing it will brighten. At what point does the lamp burn out?
Norm
: Thanks, Norm!
: : Replacing the electrolytic caps was the right thing to do. C17 on pin #8 of the 35Z5 shouldn't be 100 mfd. This high capacity will increase surge current and can damage the rectifier and lamp.
: OK, I replaced it with a 50uF cap.
: : The lamps in these radios are usually #47, 6 volt. If you are burning out 14 volt lamps the heater tap in 35Z5 may be open. Check continuity of 35Z5 from pin #3 to #2 and #7 with the tube out of your radio.
: I checked this, and pins 2, 3, 7 of the 35Z5 all have continuity with each other.
: : Since the radio did play for two hours the 50L6 may be drawing excessive current. Measure voltage on pin #5 in reference to pin #8 on the 50L6 tube. Pin #5 should be 5-8 volts negative. If zero or positive the tube will draw too much current, distort and cause hum. The cap connected to pin #5 is often leaky.
: I measure -4.87Vdc. The cap off pin 5 has been replaced before at some point in the past (because the cap that's in there is an early plastic cap, and not one of the stock paper caps, and the solder joints are blobby, unlike the rest of the radio).
: I just ran the radio for half an hour with a new 14V bulb and it didn't blow. However, there was one time shortly after turn-on when it glowed quite brightly. I think if it had been a 6V bulb, it might have blown. Also it seems to glow fairly bright, which surprises me, I'd expect a 14V lamp to be more reddish if it was really running at the right voltage...
: Should I be concerned about the voltage at pin 5 of my 50L6? What should I do to make it more negative - replace the cap or the tube, or?
: Thanks again...Ben
: Any leakage in the coupling cap and the negative voltage will be low. The data book suggests -7.5 volts. A gassy tube can also cause this voltage to be wrong and tube to draw excessive current. A bad tube can cause this voltage to go toward zero after the radio has been on for a time.
I replaced the capacitor, and it is now -6.08V on the 50L6 instead of 4.87. Should I look for a newer 50L6?
: The light should come on bright then dim. After the radio starts playing it will brighten. At what point does the lamp burn out?
It seemed to randomly just flare up and pop. The first
time was after several hours playing time. The second
time was soon after the second brightening (bright, dim, bright, pop!).
Note that it hasn't blown since I replaced the 100uF electrolytic with the 50uF that the schematic has. So maybe it's fixed, although I haven't played it for more than half an hour.
Regards,
Ben
: Hi Ben
: Any leakage in the coupling cap and the negative voltage will be low. The data book suggests -7.5 volts. A gassy tube can also cause this voltage to be wrong and tube to draw excessive current. A bad tube can cause this voltage to go toward zero after the radio has been on for a time.
: It's not voltage but current of the bulb that's important. If you connect a 3 volt flashlight lamp it wouldn't light since it draws high current. #47 bulb is normally used with these radios. It's rated 6 volts at .15 amps.
: The light should come on bright then dim. After the radio starts playing it will brighten. At what point does the lamp burn out?
: Norm
:
: : Thanks, Norm!
: : : Replacing the electrolytic caps was the right thing to do. C17 on pin #8 of the 35Z5 shouldn't be 100 mfd. This high capacity will increase surge current and can damage the rectifier and lamp.
: : OK, I replaced it with a 50uF cap.
: : : The lamps in these radios are usually #47, 6 volt. If you are burning out 14 volt lamps the heater tap in 35Z5 may be open. Check continuity of 35Z5 from pin #3 to #2 and #7 with the tube out of your radio.
: : I checked this, and pins 2, 3, 7 of the 35Z5 all have continuity with each other.
: : : Since the radio did play for two hours the 50L6 may be drawing excessive current. Measure voltage on pin #5 in reference to pin #8 on the 50L6 tube. Pin #5 should be 5-8 volts negative. If zero or positive the tube will draw too much current, distort and cause hum. The cap connected to pin #5 is often leaky.
: : I measure -4.87Vdc. The cap off pin 5 has been replaced before at some point in the past (because the cap that's in there is an early plastic cap, and not one of the stock paper caps, and the solder joints are blobby, unlike the rest of the radio).
: : I just ran the radio for half an hour with a new 14V bulb and it didn't blow. However, there was one time shortly after turn-on when it glowed quite brightly. I think if it had been a 6V bulb, it might have blown. Also it seems to glow fairly bright, which surprises me, I'd expect a 14V lamp to be more reddish if it was really running at the right voltage...
: : Should I be concerned about the voltage at pin 5 of my 50L6? What should I do to make it more negative - replace the cap or the tube, or?
: : Thanks again...Ben