11-23-2008, 10:21 PM
All,
I picked up a 70 a few weeks ago. I discovered that the field coil was open, which was a big disappointment. However, I found a good speaker, and I have started replacing the electrolytic and paper capacitors. This would be the right time to admit I'm a newbie.
With new electrolytic capacitors (10 uF), I got brave and brought the unit up slowly on a variac. The PT voltages look good, and with the speaker hooked up, I was able to tune some stations! I was very excited! Here's the catch:
With the unit receiving 93VAC from the variac, the voltage drop across the field coil was 103VDC. However, the field coil specification is 3100 ohm (and actually measures 3090) rated at 40 mA. Unless I'm missing something, this would be achieved with 123.6 VDC across the field coil. I'm concerned that if I bring the unit up to operating voltage (115VAC) that I may burn out the field coil. Since the field coil was open in the original speaker, I'm thinking this is a real concern.
Any pointers? Is there an acceptable voltage drop across the field coil? The filter choke is good (149 ohms) and I replaced (out-of-tolerance) resistors at 29, 32 (250,000 ohm) and 30 (100,000 ohm) in this schematic:
http://www.philcoradio.com/tech/images/70a.jpg
Take care,
M100C
I picked up a 70 a few weeks ago. I discovered that the field coil was open, which was a big disappointment. However, I found a good speaker, and I have started replacing the electrolytic and paper capacitors. This would be the right time to admit I'm a newbie.
With new electrolytic capacitors (10 uF), I got brave and brought the unit up slowly on a variac. The PT voltages look good, and with the speaker hooked up, I was able to tune some stations! I was very excited! Here's the catch:
With the unit receiving 93VAC from the variac, the voltage drop across the field coil was 103VDC. However, the field coil specification is 3100 ohm (and actually measures 3090) rated at 40 mA. Unless I'm missing something, this would be achieved with 123.6 VDC across the field coil. I'm concerned that if I bring the unit up to operating voltage (115VAC) that I may burn out the field coil. Since the field coil was open in the original speaker, I'm thinking this is a real concern.
Any pointers? Is there an acceptable voltage drop across the field coil? The filter choke is good (149 ohms) and I replaced (out-of-tolerance) resistors at 29, 32 (250,000 ohm) and 30 (100,000 ohm) in this schematic:
http://www.philcoradio.com/tech/images/70a.jpg
Take care,
M100C
