12-29-2013, 08:20 PM
I have made it an annual tradition for several years now to perform electronic restoration on one specific radio around Christmastime, and then call that my Christmas radio for the year.
This year, I chose a Philco 91 code 126 chassis.
Schematic here:
http://www.philcoradio.com/tech/images/1491.jpg
It was not the best choice, as its power transformer had obviously overheated at some time in the past. There is black tar all over the top of the chassis near the four tubes closest to it, plus a fair amount of dried wax running down some of the leads coming from the transformer on the bottom.
But I dove in anyway, and soon had all of the bakelite blocks rebuilt and all resistors replaced. The tone control was rebuilt, the off-on-band switch was cleaned with DeoxIT, and two of the set's three coils were rewound (the antenna coil measured OK for continuity).
Yesterday, I applied power for the first time. First, without the rectifier tube. The two lamp bulbs, both #46, were bad and were replaced. Then, being mindful of the transformer's meltdown, I replaced the rectifier tube and slowly brought it up on the variac. Fortunately, the power transformer appears to have survived its overheating (most likely caused by bad electrolytics in the past). It came up - but did not play.
So, tonight, I performed voltage checks, which pointed to something wrong at the det-osc tube. Guess what...I had removed part (20), a 700 pF mica cap, and part (21), a 10K resistor - but I had forgotten to replace them!
See, even those of us who have been in this hobby for many decades can make mistakes, too. (Next year will mark my 40th year in the hobby.)
Well, I installed a new 680 pF mica cap and a new 10K resistor...but the set still does not oscillate. So further troubleshooting is in order.
Oh, I almost forgot: How do you guys get the caps off the Mershon electrolytics? I tried to remove the cap from the lone Mershon in this radio. I managed to make it go up about 1/8 inch but dented it up pretty badly in the process. I finally gave up and connected new electrolytics under the chassis.
This year, I chose a Philco 91 code 126 chassis.
Schematic here:
http://www.philcoradio.com/tech/images/1491.jpg
It was not the best choice, as its power transformer had obviously overheated at some time in the past. There is black tar all over the top of the chassis near the four tubes closest to it, plus a fair amount of dried wax running down some of the leads coming from the transformer on the bottom.
But I dove in anyway, and soon had all of the bakelite blocks rebuilt and all resistors replaced. The tone control was rebuilt, the off-on-band switch was cleaned with DeoxIT, and two of the set's three coils were rewound (the antenna coil measured OK for continuity).
Yesterday, I applied power for the first time. First, without the rectifier tube. The two lamp bulbs, both #46, were bad and were replaced. Then, being mindful of the transformer's meltdown, I replaced the rectifier tube and slowly brought it up on the variac. Fortunately, the power transformer appears to have survived its overheating (most likely caused by bad electrolytics in the past). It came up - but did not play.
So, tonight, I performed voltage checks, which pointed to something wrong at the det-osc tube. Guess what...I had removed part (20), a 700 pF mica cap, and part (21), a 10K resistor - but I had forgotten to replace them!
See, even those of us who have been in this hobby for many decades can make mistakes, too. (Next year will mark my 40th year in the hobby.)
Well, I installed a new 680 pF mica cap and a new 10K resistor...but the set still does not oscillate. So further troubleshooting is in order.
Oh, I almost forgot: How do you guys get the caps off the Mershon electrolytics? I tried to remove the cap from the lone Mershon in this radio. I managed to make it go up about 1/8 inch but dented it up pretty badly in the process. I finally gave up and connected new electrolytics under the chassis.
--
Ron Ramirez
Ferdinand IN
