05-13-2009, 11:41 PM
I started tinkering with electronics about two years ago to learn (in a practical way) all of the things is should learned and have forgotten in college. Rather than continue breadboarding solid-state projects (I did build an cool pH controller for my aquarium) I decided that an antique radio would be proper furniture in my house since it was built in the late 30's. The first two radios I worked on (Zenith 5S319 and a Philco 38-9) were in fairly good shape. All that was needed electronically was recapping the paper condensers and the electrolytics. The project I'm working now is leap. I couldn't pass up the 40-201 I found in an antique mall for $80. The cabinet and case were in excellent condition so I decided it was time to jump in and find out how deep the water is.
The first thing I did (other than a good dusting and vacuuming) was recapping. The radio had prior service so I replaced the electrolytics with what I found in the radio and did all of the other replacements according to the schematic. I powered it up gradually using a light bulb in series and found that the radio is functioning but not working well. For a short time I had weak, scratchy AM but all I have left now is noise. The volume control varies the loudness of the noise and the tone control seem to work. I inverted the radio and measured the voltage at the electrolytics and they are 30V above the drawing (330V and 225V spec, reading 360V and 259V, respectively) . Here's what I see on the plate on the tubes:
RF spec 182V, reading 213V
Det-Osc spec 210V, reading 254V
IF spec 225V, reading 262V
1st Audio spec 100V?, reading 165V
Phase inverter spec, 140V, reading 151V
2 push-pull 2nd audios spec, 225V, reading 191V and 157V
2 outputs spec 225V and 215V, reading 249V and 242V
I'm concerned about the high voltage and I'm wondering if I should go back to the spec electrolytic of 18 uf and 25 uf rather than the 20 uf and 40 uf replacements that were in the radio for previous repair or servicing. I've been reading Elements of Radio Servicing by Marcus and Levy to help but there's a lot there to digest. I've got a signal generator and oscilloscope but I have a hunch that may be premature. I'm a little cautious about doing anything until I get the voltage issue resolved.
If any one has any thoughts regarding the high voltage I'm seeing or where to start troubleshooting, your advice would be greatly appreciated.
The first thing I did (other than a good dusting and vacuuming) was recapping. The radio had prior service so I replaced the electrolytics with what I found in the radio and did all of the other replacements according to the schematic. I powered it up gradually using a light bulb in series and found that the radio is functioning but not working well. For a short time I had weak, scratchy AM but all I have left now is noise. The volume control varies the loudness of the noise and the tone control seem to work. I inverted the radio and measured the voltage at the electrolytics and they are 30V above the drawing (330V and 225V spec, reading 360V and 259V, respectively) . Here's what I see on the plate on the tubes:
RF spec 182V, reading 213V
Det-Osc spec 210V, reading 254V
IF spec 225V, reading 262V
1st Audio spec 100V?, reading 165V
Phase inverter spec, 140V, reading 151V
2 push-pull 2nd audios spec, 225V, reading 191V and 157V
2 outputs spec 225V and 215V, reading 249V and 242V
I'm concerned about the high voltage and I'm wondering if I should go back to the spec electrolytic of 18 uf and 25 uf rather than the 20 uf and 40 uf replacements that were in the radio for previous repair or servicing. I've been reading Elements of Radio Servicing by Marcus and Levy to help but there's a lot there to digest. I've got a signal generator and oscilloscope but I have a hunch that may be premature. I'm a little cautious about doing anything until I get the voltage issue resolved.
If any one has any thoughts regarding the high voltage I'm seeing or where to start troubleshooting, your advice would be greatly appreciated.
