[Guest]

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

[etech]

The most common reason for the failure of a field coil is the corrosion of the copper wire due to the acids in the paper insulation inside the field coil combined with humidity. It causes a green type of corrosion which will eventually cause the coil to become open.

Other reasons for the failure of a field coil in the Philco 70 would be a leaky or bad capacitor at location 5 or 28. Also, if one of the 24 tubes developed (unlikely, but possible) a plate to screen or screen to grid short, the field coil could have failed.

And yes, if your 3100 ohm field is rated at 40ma, then it should be able to handle about 124 volts. I have a Philco 70 and and a couple Philco 90 chassis here and I never bothered to check the voltage across the field or current drawn. I just made sure to replace all the tar block caps and all caps in the metal boxes and the electrolytics. I also replaced a few out of tolerance resistors. The only other problem I had was that two of the 90's had open RF coils and had to be rewound.

Ed

[Guest]

All,
Thank you for all the good input. I wanted to provide additional data for your expert evaluation. My goal here is to have you evaluate my data and
conclusions, and let me know if I've missed something, as this is a learning exercise for me.

Again, everything is referenced to this schematic:

http://www.philcoradio.com/tech/images/70a.jpg

Work Performed
--------------
1) I have replaced both electrolytic (2 x 10uF) and all condenser block capacitors (exact uF values). One exception is the tone control cap (48). Just
realized I missed it.
2) I replaced the following resistors, which were out: 29, 30, 32 and 24, with 250K, 50K, 250K and 50K, respectively. All others were in tolerance.

Resistance Measurements
-----------------------
Schematic Actual
--------- ------
(36) C->D (Field Coil) 3100 3085
(37) D->E 1060 1126
(37) E->F 2300 2480
(37) F->G (Volume Control) 250 226
(37) G -> Chassis 70 76
(37) Chassis -> H 240 250


Voltage to chassis (with 115VAC from variac)
--------------------------------------------

Point Actual
----- ------
C 277
D 147 (... 130 drop through field coil, or about 42mA)
E 103
F 11.5
G 3.1
Chassis 0.0 (I know ... just being thorough ... :-)
H -19


Conclusions
-----------
1) All resistance measurements look good to me.
2) Voltage (277) at C seems high. I see this voltage at the plate for the following tubes (left to right on schematic): first three 24A (RF, D, IF) and 47.
Max plate voltage should be 250 according to specifications for 24A and 47 tubes. I am using replacement 24As; the originals in these positions tested low,
and the bottoms of the plates were a little blued. I'm not sure why I'm not seeing some drop through the OPT primary for the 47.
3) The field coil is rated for 40mA through 3100 ohms, or a voltage drop of 123.6. I'm seeing 130. After 5 min. of play, the field coil is barely warm to
the touch.
4) Voltage on the plate of the 27 is 66. Not sure what to make of this, but seems reasonable since the voltage at E (103) goes through a 13K resistor to get
there.
5) Voltage on the plate of the other 24A (2nd D) is 225. Interestingly, the original 24A in this position tested good with no blueing. If I'm seeing this
right, the voltage from C would be dropped through resistors 30 and 29 (50K and 250K) ... again, not sure what to make of this.

Performance and Thoughts
------------------------
- With a 12 ft. wire on the antenna, in my basement, I get local AM stations just fine (590 and 1560), and they are aligned on the dial.
- I can drop the line voltage to about 110VAC with no noticeable loss of performance ... just a little volume. I am tempted to bleed voltage off C to
chassis through a resistor to drop the voltage; especially since the set will actually see about 120VAC when plugged into the wall.
- The set has a lot of volume. At the lowest setting there is just a very small amount of speaker white noise ... no hum at all.
- I can receive local AM, and they sound very good, but the volume fades in and out over the course of about one minute. When out, you can barely hear the
station, then it starts coming back and returns to sounding very good.
- All of this is without the ground clip hooked to anything. If I plug the variac directly into the wall, the chassis of the variac is grounded. The
voltage between the variac chassis and radio chassis (again, with 115VAC input) is 56.6VAC ... again, not sure what to make of this. If I move the red lead
of my DMM (Fluke 73III) to the 300mA plug, and set to measure AC amps, I get 0.61(uA?) ... maybe this is acceptable?

I welcome any and all input ... thanks again for helping me learn,
M100C

[exray]

I can drop the line voltage to about 110VAC with no noticeable loss of performance ... just a little volume. I am tempted to bleed voltage off C to chassis through a resistor to drop the voltage; especially since the set will actually see about 120VAC when plugged into the wall.

I agree with the notion to drop the voltage but that method will increase the load on the transformer. The better method would be to drop the incoming AC using a bucking transformer or even a dropping resistor.

- I can receive local AM, and they sound very good, but the volume fades in and out over the course of about one minute. When out, you can barely hear the station, then it starts coming back and returns to sounding very good.

If its a sudden drop/return bad solder joints are bad for this as are tube cap grid connections as well as the tubes themselves. If its gradual it suggests that a tube grid is floating somehow. Maybe an open grid connection, that type of thing. See above

If I plug the variac directly into the wall, the chassis of the variac is grounded. The
voltage between the variac chassis and radio chassis (again, with 115VAC input) is 56.6VAC

Thats typically the voltage across the line bypass capacitor. Caps do pass AC, ya know! Just like a resistor across a DC source. You won't see it with a low-impedance meter but it will show with a DMM.

[radiomikee]

Just because the field coil is rated at 40 mA does not mean that is how much current is actually flowing through it. To measure the current flowing through the field coil, put the VTVM in series with it and measure the current flowing. The power being consumed in the field coil will be the current times the voltage drop across it, on the order of 2-3 watts maximum. This power is used to generate the magent field for the speaker, and a small amount of heat. Given the big metal shaft through the center of the field coil and the air in there, you will not feel it as heat. For interest, go back and check the power draw for the entire radio. It's about 80 watts to my recall, not having the radio in front of me. The rated total plate current for all the tubes is 35 mA. On an average plate voltage of 250 Volts, thats about 10 watts. If you study the plate current draw per tube, 80% of the power is used by the audio output tube 47. You could also figure out the amount of power used to heat the filaments of the tubes, that's about 5-8 watts per tube depending on which one. Most power in the set is consumed heating filaments and as audio power, with small amounts consumed in the speaker field coil, and some wasted as heat passing through dropping resistors, the main choke out of the 80 rectifier ahead of the field coil, and in the dial lamp.

[DCASAZZA]

Just wanted to share something I did with my 650. After getting it all restored electronicaly I installed a thermistor in series with the AC mains on a terminal strip under the chassis. This component when cold has a resistance of about 30-40 ohms. When the set is turned on the current draw warms it up and the resistance goes down to a few ohms. This allows a "softer start" to in-rush surge and drops the AC volts about 5 or 6 volts while operating with no reduction of performance (in my 650).
These were used most commonly in color TV degausing circuits in CRT sets.
I know some may think modifying the radio this way is not kosher, but I've repaired enough equipment over the years to know surge and heat is a killer. This helps with both. I also took the wisdom of the phorum and installed a line fuse on the same strip.