03-13-2009, 11:50 AM
Hi all,
Let me say that I would probably not make any permanent changes to my early model 70, but it would be a fun project to see if avc could be easily added to the model 70. I have noticed the similarity in the circuits of the early model 70 and the middle model 90. Other than the early 70 using a plate type of detector, they are pretty similar. Here are the schematics.
Early 70 - http://www.philcoradio.com/tech/images/70a.jpg
Middle 90 - http://www.philcoradio.com/tech/images/90b.jpg
Here are my thoughts as things necessary for a conversion to avc operation.
1 Remove original volume control and install fixed 250 ohm resistor between points G & H.
2 Tie cathodes of RF and IF tubes to point G.
3 Lift secondaries of RF coil #2 and IF coil #18 and bypass each to ground with a .05uf cap.
4 Attach a 470K resistor to each lifted secondary and tie them to a newly created avc buss.
5 Create new avc buss by using a signal diode (either a 1N34 or a 1N914) with cathode connected to the hot (top)side of the detector coil #20 secondary. Load anode with a pair of 51K resistors in series (or a 100K resistor) to ground.
6 Install a new 250K (or 500K) volume control pot in place of resistor 32 at grid of 47 tube.
That's it. What do you model 70 and 90 mavens think? All opinions will be appreciated.
Thanks,
Ed
Let me say that I would probably not make any permanent changes to my early model 70, but it would be a fun project to see if avc could be easily added to the model 70. I have noticed the similarity in the circuits of the early model 70 and the middle model 90. Other than the early 70 using a plate type of detector, they are pretty similar. Here are the schematics.
Early 70 - http://www.philcoradio.com/tech/images/70a.jpg
Middle 90 - http://www.philcoradio.com/tech/images/90b.jpg
Here are my thoughts as things necessary for a conversion to avc operation.
1 Remove original volume control and install fixed 250 ohm resistor between points G & H.
2 Tie cathodes of RF and IF tubes to point G.
3 Lift secondaries of RF coil #2 and IF coil #18 and bypass each to ground with a .05uf cap.
4 Attach a 470K resistor to each lifted secondary and tie them to a newly created avc buss.
5 Create new avc buss by using a signal diode (either a 1N34 or a 1N914) with cathode connected to the hot (top)side of the detector coil #20 secondary. Load anode with a pair of 51K resistors in series (or a 100K resistor) to ground.
6 Install a new 250K (or 500K) volume control pot in place of resistor 32 at grid of 47 tube.
That's it. What do you model 70 and 90 mavens think? All opinions will be appreciated.
Thanks,
Ed
