Posts: 411
Threads: 42
Joined: Sep 2008
City: Ironton Ohio
I am restoring a 48-206 leatherette I could not find the schematic so I used a 49-500 which seems identical to the chassis except for a couple bypass cap sizes, anyway, all caps, o.o.t. resistors were replaced. The radio reception is great for the first few minuts but hums like a bad filter, then the hum goes away but the reception gets weak and very distorted at the same time. the original filter calls for 30/25/20 I used 33/30/22 the voltage across the filters call for 115v/105v/90v to B- mine tests at 115v/90v/75v to B-. what would cause this voltage drop after it operates for about 5 min. All resistors and caps have been checked, filter caps have been substituted, feild coil is ok, tubes check ok and have been substituted with known tubes nothing helps. I have restored many AA5s with this same or similar chassis with no problems but Im lost on this unit and would appriciate any input or suggestions.
Thanks, Todd
Posts: 91
Threads: 9
Joined: Feb 2010
City: Macon, GA
I would try using a hot air torch to heat your components one at the time until you find an area in the radio that is affected by the heat. You may be able to narrow down your search a little quicker that way. Check your solder joints and also any connections to the chassis (IF cans, tuning condensor, rivets, etc.) that may be loose or not propperly isolated, whichever the case may be.
Dave
Posts: 411
Threads: 42
Joined: Sep 2008
City: Ironton Ohio
Motorola man Wrote:I would try using a hot air torch to heat your components one at the time until you find an area in the radio that is affected by the heat. You may be able to narrow down your search a little quicker that way. Check your solder joints and also any connections to the chassis (IF cans, tuning condensor, rivets, etc.) that may be loose or not propperly isolated, whichever the case may be.
Dave
I dont think it has anything to do with heat breakdown because you can time the failer to the second and when I would sub a tube it would start the 5 or so min. breakdown time over again even after being off just long enough to replace a tube. I have another chassis I could use but I still need to figure this out for thew next time it could happen.
Posts: 2,128
Threads: 18
Joined: Oct 2008
City: Merrick, Long Island, NY
If you did a total recap, and your DC voltages do not change when the hum begins, I think you may have a heater to cathode short in the 50A5. There should be 5 VDC on the cathode (pin7) relative to B-. To test this, hook a meter to read AC volts from the cathode to B-. Then let the radio play until the hum occurs. If you find AC on the cathode the tube is no good and must be replaced. If not, you will have to work backwards through the set to find the origin.
Posts: 411
Threads: 42
Joined: Sep 2008
City: Ironton Ohio
codefox1 Wrote:If you did a total recap, and your DC voltages do not change when the hum begins, I think you may have a heater to cathode short in the 50A5. There should be 5 VDC on the cathode (pin7) relative to B-. To test this, hook a meter to read AC volts from the cathode to B-. Then let the radio play until the hum occurs. If you find AC on the cathode the tube is no good and must be replaced. If not, you will have to work backwards through the set to find the origin.
Originaly the hum started within 30 sec. of power but disipated after about 5 min. when the volume and reception faded. now I have found that the 50a5 was breaking down so I replaced it and the radio opperated ok but now my voltages across the filters are about 10 volts more than they should be. Could this be due to the larger uf caps? if so, would I be able to drop the voltages to proper with a larger resistor between 1st and 2nd filter, which is now 220 ohm? how much of a voltage raise compared to the schematic would be acceptable?
Posts: 2,128
Threads: 18
Joined: Oct 2008
City: Merrick, Long Island, NY
10 volts is not important. It is due to the generally higher voltage at your wall socket nowadays.
Posts: 411
Threads: 42
Joined: Sep 2008
City: Ironton Ohio
Well I thought I had it, but now after about 1/2 hour of play the volume fades to about half of normal and the 50A5 gets extreemly hot. I have been all through this thing and can not pinpoint the problem besides the higher than normal voltages. I am going to check the voltages on another good restored set with the same tube lineup to see if the 10 volt raise is due to higher line/house voltage or if it matches the schematic.
Posts: 7,214
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Sound like you've got another bum tube. You could hang the voltmeter across the avc buss and see if that voltage drops when the volume does. That will tell you if you are looking for an audio issue or a rf/if issue.
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 411
Threads: 42
Joined: Sep 2008
City: Ironton Ohio
Finally, I was bypassing resistors with a decade box and found a plate resistor on the 50A5 way out of spec. (even though it was a new flame proof resistor) as soon as I touched the leads across it, the volume trippled, it now plays perfect.
Thanks to everyone for ther help and suggestions.
