38-7 working great...then nothing

9/21/2007 5:52:28 PMMike

I had recapped and replace bad tubes in my Philco 38-7. It was working great then suddenly I didnt hear it playing anymore. Now I get nothing but silence. Tubes are lighting, but I found the following problems with voltages when I check them:
5Y4 (Rect): I should get 315V, I'm getting about 750V
6F6 (Output): I should get 230V&250V, I get 0 on both pins
6A8 (DET-OSC): Should get 180V&100V, I get 550V&180V
6K7 (IF): Should get 100&240, I get 180&0
6k5 (Audio): Should get 170V, I get 0
If anyone can help me out here I would really appreciate it. I have the schematic, but with all the voltage differences I don't even know where to start. Thanks,
Mike

9/21/2007 8:02:04 PMPeter G. Balazsy

Mike:
Check the field coil for continuity and all your solder joints there.

9/21/2007 8:08:37 PMPeter G. Balazsy

Mike:
Check continuity on the field coil and all your connection there.

9/21/2007 8:37:21 PMMike

I see the field coil on the schematic, but where is it in the chassis? What does it look like? Also could this problem be with the power transformer since I'm getting double voltage at the test point on the rectifier tube?
Thanks
Mike

9/21/2007 8:53:55 PMMike

Ok, I made the mistake of measuring without the speaker hooked up. I'm getting voltage at all points now, but most are still about double what they should be. Is this the transformer or could it be something else??

Mike

9/21/2007 9:06:13 PMMike

I forgot to mention, when I hooked the speaker up to test voltage again, I tried the volume, and the radio worked again. I turned it back off because the voltage readings were still very high. Could the over voltage cause overheating and be why the radio eventually quits working?
Mike

9/21/2007 10:03:20 PMPeter G. Balazsy

Mike:
The transformer has a center tap and the 350vac is measured from that CT to each plate of the rectifier. If you measure across plates you'll get double the voltage because you are measuring across both ends of the high voltage winding.
The field coil is part and parcel of the speaker.
The field coil surrounds the rear area of the speaker to provide a magnetic field to operate the speaker.
If the radio works or "plays" when the speaker is plugged in.. that means the field coil is good.
You should measure all voltages while it is playing.
Perhaps it stops because the speaker connections to the field coil is loose?

9/22/2007 11:18:29 AMMike

1st, I was measuring ACV not DCV, which was my "high voltage" problem. Secondly, It worked for several hours last night before I went to bed perfectly, so maybe it was just a loose wire on the field coil. Thanks for the help Peter,
Mike