06-30-2016, 01:01 PM
Stand back! My head is going to explode!!!!!
Turned this set on and got a low hum and after 15sec the 80 tube's plates are starting to turn RED, their not suppose to. Quick turned it off and hooked up the voltmeter to the hv. Should be about 350v was 125v, that's why the 80 was getting an angry red. Personally I like a dead short there much easier find. Ones that have a few thousand ohms across the hv are a bit trickier to locate. Started to disconnect some of the resistors feed the hv to various stages, It's not there. Ungrounded the filter caps and bypass cap, their ok. Next I started unplugging tube all but the 80. Magically the hv jumped up to 350v.Hmmmm.. Reinserting the tube one by I got the last one, one of the 42 output tubes. It sucked the voltage down to 125v again. Replaced the 42 and all is well I though.
I wanted to go though the rf alignment to make sure it's working on all band and such. The BC band was in good alignment. Checked over the police band. Philco sez to set the upper end of the band at 4.5mc (there is no marker at 4.5mc however there is one at 4.6mc) a little odd but I can count. Did the LW and SW bands. Now the trimmer to adjust these are not laid out very logically so I may have tweaked one that I had already adjusted. Anyway went back to the bc band and now at 1700kc I'm picking up a local at 1210kc. Low end is good 600kc is still at 600. The upper end is only tuning up to 1210kc = lo is operating from 1010kc to 1670kc. It should be working up to 2160kc. Somewhere I lost 490kc. After checking connections,bandswitch, trimmer cap and coil. I'm coming to the conclusion that the coil is bad but two things bother my about the coil prognoses. Darn thing oscillates and it has a low resistance (about a quarter what the diagram shows (1ohm verses 4.2) The high end trimmer (29) has very little effect on the frequency about 1kc or less throughout it's range.
As you can see from the diagram the osc coil doesn't have a feedback winding it uses caps to couple the signal from the plate to the grid. So it's just a single winding with a tap for the low end padder. I suppose I could make a prototype to get the right amount of inductance and the tap in the proper place. The old one has a bunch of wires going to the bandswitch real a pita to get apart. Also thought about using a replacement bcb osc coil but not sure if I can get the proper range out of it.
Any thoughts??
Terry
Turned this set on and got a low hum and after 15sec the 80 tube's plates are starting to turn RED, their not suppose to. Quick turned it off and hooked up the voltmeter to the hv. Should be about 350v was 125v, that's why the 80 was getting an angry red. Personally I like a dead short there much easier find. Ones that have a few thousand ohms across the hv are a bit trickier to locate. Started to disconnect some of the resistors feed the hv to various stages, It's not there. Ungrounded the filter caps and bypass cap, their ok. Next I started unplugging tube all but the 80. Magically the hv jumped up to 350v.Hmmmm.. Reinserting the tube one by I got the last one, one of the 42 output tubes. It sucked the voltage down to 125v again. Replaced the 42 and all is well I though.
I wanted to go though the rf alignment to make sure it's working on all band and such. The BC band was in good alignment. Checked over the police band. Philco sez to set the upper end of the band at 4.5mc (there is no marker at 4.5mc however there is one at 4.6mc) a little odd but I can count. Did the LW and SW bands. Now the trimmer to adjust these are not laid out very logically so I may have tweaked one that I had already adjusted. Anyway went back to the bc band and now at 1700kc I'm picking up a local at 1210kc. Low end is good 600kc is still at 600. The upper end is only tuning up to 1210kc = lo is operating from 1010kc to 1670kc. It should be working up to 2160kc. Somewhere I lost 490kc. After checking connections,bandswitch, trimmer cap and coil. I'm coming to the conclusion that the coil is bad but two things bother my about the coil prognoses. Darn thing oscillates and it has a low resistance (about a quarter what the diagram shows (1ohm verses 4.2) The high end trimmer (29) has very little effect on the frequency about 1kc or less throughout it's range.
As you can see from the diagram the osc coil doesn't have a feedback winding it uses caps to couple the signal from the plate to the grid. So it's just a single winding with a tap for the low end padder. I suppose I could make a prototype to get the right amount of inductance and the tap in the proper place. The old one has a bunch of wires going to the bandswitch real a pita to get apart. Also thought about using a replacement bcb osc coil but not sure if I can get the proper range out of it.
Any thoughts??
Terry
