02-27-2018, 11:00 AM
As usual, all resistors had drifted high, all were replaced with "dogbone" reproductions.
[Image: https://farm5.staticflickr.com/4770/3856...45e9_k.jpg]Philco 20 Chassis by Steve Davis, on Flickr
I restuffed the filter can using modern film capacitors, increasing the value of the second and third filters to 4.7uf and omitting the capacitor that is in parallel with the choke. All the capacitors were replaced in the bakelite blocks. Bakelite block number 8 contains a capacitor and a resistor. I normally reinstall the original resistors, but in this case the number 8 capacitor had shorted causing a high current to flow through resistor 8, burning it up.
[Image: https://farm5.staticflickr.com/4703/2549...64b6_k.jpg]Philco 20 by Steve Davis, on Flickr
I replaced the two missing output tubes and gave it the "smoke test". The radio came to life and played beautifully! The cabinet was going the need a lot of attention and I had some more pressing projects, so I set the chassis aside for a few months. A couple of months ago I started on the project again. I remembered Ron had posted a how to in the "Tech Section" of the Phorum, a modification to remove the B+ from the primary of the interstage transformer. Take a look here. http://www.philcoradio.com/phorum/showth...?tid=14984 Open windings in the interstage transformer is a common failure in the Philco 20. I decided to do the mod to possibly prolong the life of the transformer. In the next photos you can see where I added an extra bakelite block with a .47uf cap and a 27K, 2W dogbone resistor.
[Image: https://farm5.staticflickr.com/4671/3856...2b9d_k.jpg]Philco 20 Interstage Transformer Modification by Steve Davis, on Flickr
[Image: https://farm5.staticflickr.com/4753/3856...d7e2_k.jpg]Philco 20 Interstage Transformer Modification by Steve Davis, on Flickr
More to come
Steve
[Image: https://farm5.staticflickr.com/4770/3856...45e9_k.jpg]Philco 20 Chassis by Steve Davis, on Flickr
I restuffed the filter can using modern film capacitors, increasing the value of the second and third filters to 4.7uf and omitting the capacitor that is in parallel with the choke. All the capacitors were replaced in the bakelite blocks. Bakelite block number 8 contains a capacitor and a resistor. I normally reinstall the original resistors, but in this case the number 8 capacitor had shorted causing a high current to flow through resistor 8, burning it up.
[Image: https://farm5.staticflickr.com/4703/2549...64b6_k.jpg]Philco 20 by Steve Davis, on Flickr
I replaced the two missing output tubes and gave it the "smoke test". The radio came to life and played beautifully! The cabinet was going the need a lot of attention and I had some more pressing projects, so I set the chassis aside for a few months. A couple of months ago I started on the project again. I remembered Ron had posted a how to in the "Tech Section" of the Phorum, a modification to remove the B+ from the primary of the interstage transformer. Take a look here. http://www.philcoradio.com/phorum/showth...?tid=14984 Open windings in the interstage transformer is a common failure in the Philco 20. I decided to do the mod to possibly prolong the life of the transformer. In the next photos you can see where I added an extra bakelite block with a .47uf cap and a 27K, 2W dogbone resistor.
[Image: https://farm5.staticflickr.com/4671/3856...2b9d_k.jpg]Philco 20 Interstage Transformer Modification by Steve Davis, on Flickr
[Image: https://farm5.staticflickr.com/4753/3856...d7e2_k.jpg]Philco 20 Interstage Transformer Modification by Steve Davis, on Flickr
More to come
Steve
M R Radios C M Tubes
