03-19-2008, 07:16 PM
Hi all,
Some of you probably know that I am in the process of of restoring this model 90 chassis.
[Image: http://i135.photobucket.com/albums/q136/...o90Top.jpg]
After cleaning it, I decided to do a quick check to see if I could get reception. After all, there is no sense in investing in the cabinet and other parts needed if the chassis is not restorable. With that in mind, I tacked in two 10uf caps in place of the Mershon caps and connected a field coil speaker. Since I did not have the Philco speaker, I connected a 10 inch Zenith speaker I use for a bench test unit. It has a 500 ohm field coil, so I added a 2,500 ohm power resistor in series with the field to simulate the proper Philco speaker. This speaker had a push-pull transformer, so I used the center tap and one end.
I connected an antenna and slowly powered up the chassis with the variac, watching the current draw. At 80 volts the radio started coming to life, although it was oscillating, even up to 100 volts. I figured there were some bad decoupling caps, so I tacked in a .01uf cap from the RF/IF screen supply rail to ground. The oscillation stopped and the radio was picking up many stations across the dial. Frequency calibration was not too bad either. The only problem was that the RF sensitivity seemed weak. I checked voltages in the set and all were normal except for the plate of the RF tube (#24), no voltage. It turns out the primary of the RF coil is open.
I temporarily tacked a 20K resistor across the primary to get some voltage to the plate of the RF tube. Lo and behold, the radio plays great. It has lots of volume and very good sensitivity. 
So, I guess I am going to have to do a complete recap and repair that bad RF coil. It looks to me like the primary is the bottom winding? Anybody know for sure? Maybe I can repair the winding without have to rewind it?
Anyway, having had reasonably good success with the preliminary checks on this chassis, I guess I'm going to have to stay on the lookout for a cabinet and speaker, lol!
Regards,
Ed
Some of you probably know that I am in the process of of restoring this model 90 chassis.
[Image: http://i135.photobucket.com/albums/q136/...o90Top.jpg]
After cleaning it, I decided to do a quick check to see if I could get reception. After all, there is no sense in investing in the cabinet and other parts needed if the chassis is not restorable. With that in mind, I tacked in two 10uf caps in place of the Mershon caps and connected a field coil speaker. Since I did not have the Philco speaker, I connected a 10 inch Zenith speaker I use for a bench test unit. It has a 500 ohm field coil, so I added a 2,500 ohm power resistor in series with the field to simulate the proper Philco speaker. This speaker had a push-pull transformer, so I used the center tap and one end.
I connected an antenna and slowly powered up the chassis with the variac, watching the current draw. At 80 volts the radio started coming to life, although it was oscillating, even up to 100 volts. I figured there were some bad decoupling caps, so I tacked in a .01uf cap from the RF/IF screen supply rail to ground. The oscillation stopped and the radio was picking up many stations across the dial. Frequency calibration was not too bad either. The only problem was that the RF sensitivity seemed weak. I checked voltages in the set and all were normal except for the plate of the RF tube (#24), no voltage. It turns out the primary of the RF coil is open.
I temporarily tacked a 20K resistor across the primary to get some voltage to the plate of the RF tube. Lo and behold, the radio plays great. It has lots of volume and very good sensitivity. So, I guess I am going to have to do a complete recap and repair that bad RF coil. It looks to me like the primary is the bottom winding? Anybody know for sure? Maybe I can repair the winding without have to rewind it?
Anyway, having had reasonably good success with the preliminary checks on this chassis, I guess I'm going to have to stay on the lookout for a cabinet and speaker, lol!
Regards,
Ed
