Hey guys, now that I have some questions answered, I wanted to ask about the coils that are in the radio. This is a 1932 cathedral Philco 71B (single speaker)
There is a R.F. coil
Det. coil
R.F choke
OSC. coil
First I.F. coil
Could any of you guys please tell me the range of Ohms' each of these coils and choke should be at in the circuit when I check them. I need to test each and have a feeling that one of them is bad.
I also need to do a full recap, someone put in a more recent electrolytic, and on a controlled initial power up I didn't pick up a single station with an antenna connected. The audio section is good, I did a in depth check on it.
Thanks for you're help
Jon
When you had the radio powered up, did you do any voltage checks on the RF, osc/det or IF tubes? If you have plate voltage ( around 200V-250V) on the plate socket pin of each of these tubes, then you can assume that the primary windings of their respective coils are ok. The plate voltage is fed through the coil, so if it is open there will be no plate voltage on that tube.
Typically the coil resistances will be on the order of a few ohms up to 50 or so for an IF transformer coil. Anything much higher than that would be suspect.
The det/osc circuit is a bit complicated because a single tube is used as both a mixer and oscillator, called an Autodyne circuit. The plate circuit contains both an oscillator coil and an IF coil. If either has an open winding you will get no reception at all. This is where I would first look.
http://www.nostalgiaair.org/PagesByModel...013837.pdf
No I did not do any voltage checks, where would I start.
Look up the pin-outs of the tubes and attach a voltmeter, set to DC volts at the highest range (unless it is autoranging,) on the plate of each tube. Attach the "common" wire to the chassis. If you read high DC voltage on your meter with the radio on and warmed up, then the PRIMARIES of IF coils are not open.
You could also determine this by tracing out the wiring, and with the radio OFF, measure the Ohms from the tube plates to the component or connection on the other side of the primaries.
To measure continuity of the secondaries, measure Ohms with the radio OFF from the grid of the tubes to the resistor on the other side of the secondary (or to the chassis.) If you get any reading at all, the secondaries are not open.
Since neither voltage or resistance of the coils are on the schematic you provided I'll have to rely on others to tell you what the readings should be. Write down your readings so when they pipe in with their readings you won't need to re-measure.
I'm not sure where you can get pin-outs of the tubes. Normally I'd go to Nostalgia Air, but their tube search seems to be off line at the moment.
Do you have the service data? What code is the 71B (121, 123, 125)? I would recommend checking out Joe's restoration of a 630B. Lots of good advice for a beginner:
http://www.youtube.com/watch?v=IelT-Qp13po
- Geoff
The link I posted above from Nostalgia air has both a voltage reading chart, tube pinout diagram and schematic. The chart shows voltages measured with the negative of the voltmeter to the cathode of the tube, but you will get close enough readings with the negative to chassis. The cathode to filament voltage column measures the voltage across the bias resistor for the output tube (15 to 20 V), so not really significant for the RF section.
It is most important to measure the plate and screen grid voltages of the RF, det/osc and IF tubes. Should be around 200V-250V on the plates and around 90V on the screens. If you don't have close to these voltages, you will get poor or no reception.
Here is the link to the info again:
http://www.nostalgiaair.org/PagesByModel...013837.pdf
Thanks guys for your help and many thanks for the information for testing the voltages, my first time.
Good news, Not only did the the R.F., Det. Osc., and I.F. have voltage, I was able to pick up stations off the dial from 700kHz to 1400kHz, including my SSTRAN at 980kHz (with good fidelity on the SSTRAN and a clear station at 1400kHz).
The Voltages for all three coils, These voltages that tested on the Plate were plus or minus a bit.
R.F--------- Plate Volts P to K 245 (Tested 303 volts) Screen Grid SG to K 90 (Tested 100)
Det. Osc.--- Plate Volts P to K 235 (Tested 277 volts) Screen Grid SG to K 90 (Tested 83)
I.F----------Plate Volts P to K 255 (Tested 277 volts) Screen Grid SG to K 90 (Tested 95)
Now I did not test the Plate voltages for the Det. Rect., Audio, Output, and rectifier. But since I received audio loud and clear and power from the Rectifier, I would figure these are all good.
I would think the coils on this radio are good from my findings, I'm only a amateur at this so please give me your thoughts.
Thanks
Jon