I am nearly finished restoring a 1936 Philco 60, the last version before
the model 37-60. I'll leave the details of the restoration at the end of
this post. The problem is the oscillator does not work and the radio is
mostly quiet after applying power, however, I can "jump start" the
oscillator and the radio comes to life. When not oscillating, the screen
grid on tubes 6A7, 75, and 78 are very low about 10 to 15 volts and
resistor #14 (32k ohm) gets very hot with a voltage drop of 260 volts
which is near its 2 watt limit. When it is oscillating and working
properly, the screen grids are about 60 to 70 volts and resistor #14 is
about 150 volts. On a weak station, the radio will play for a little
while then the oscillator will cut out. But, a "jump start" will bring
the radio back.
What is weird is that I can "jump start" the oscillator. To do this, I
have unsoldered the lead to capacitor #13 and the output of the
oscillator coil and reconnected it with a jumper cable. This was done to
see if I wound the output coil backwards. I noticed when disconnecting
then reconnecting the jumper would break the silence on the radio. I
think that bit of static from reconnecting the jumper would kick start
the oscillator. I can then power-off the radio briefly, and turn back on
(within the time the filter caps have not fully discharged). Does
anyone know why this is happening? I can't figure out a fix for this.
The attached schematic will show a red oval around where the jumper is
applied. I can provide a video on Youtube if anyone wants to see this.
In the restoration, the radio was re-capped. The two 8 MFD filter caps
were replaced with 10MFD caps. The bakelite condensor blocks were all
re-built with new caps. The multi caps in the metal can were replaced by
a terminal strip with appropriate caps. The shortwave portion of the
antenna coil (approx 0.7Ω) was re-wound, and the 3.4Ω portion of the
output oscillator coil was also re-wound thanks to the information from
this Phorum and other Philco sites. Initially, the 8MFD filter caps were
replaced with 16MFD caps, and thinking that possible the oscillator
needs some slight ripple to start, we have since replaced the 8MFD
filter caps with 10MFD. There was no change to the oscillator. I've
since went back to the 16MFD filter caps.
Sure sounds like the osc coil has a rotting feedback winding on it. That's why your measurement across the 25K resistor is so high, the tube isn't drawing any current. Bet you rewind it and all will be well.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 02-21-2017, 10:43 AM by Radioroslyn.)
Radioroslyn, I see the feedback circuit now. I did rewind it with 18 turns of 38 gauge wire but I have my doubts whether I did this one properly. During the build, I removed all of the crumbling cellulose and neutralized the windings underneath with ammonia/water. I replaced the cellulose with a piece of acetate and wound the coil around that. I think I need to pull the coil out and check the direction of the winding.
Is this coil suppose to be 18 turns or 17? One symptom I noticed is that 640 kc is near the middle of the dial and 1260 kc is near the end. I only got as far as tweaking the three IF trimmers before I ran into this problem.
Hello and welcome. Not so sure you should have used any water on the coil. Moisture is one reason for the coils giving trouble. There are recommendations that the coils should be baked to get moisture out of them.
Not saying this is your problem, just wanted to mention it.
Not enough turns on the feedback winding osc dies towards the low end of the band.
Too many turns a turn or two extra I don't think will make much of a difference. But a bunch extra will cause distortion in the osc output and harmonics.
Wound backwards (out of phase) osc won't work or will only oscillate at the high end of the band.
The feedback winding is there to couple some on the output signal back to the input. In this capacity (ha ha) it has very little bearing on the operating frequency. That is determined by the secondary and it's associated capacitors.
Humidity or water in the coil form greatly reduces the efficiency (or Q). Could cause the oscillator not to work.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
(02-21-2017, 03:10 PM)whirligig Wrote: Radioroslyn, I see the feedback circuit now. I did rewind it with 18 turns of 38 gauge wire but I have my doubts whether I did this one properly. During the build, I removed all of the crumbling cellulose and neutralized the windings underneath with ammonia/water. I replaced the cellulose with a piece of acetate and wound the coil around that. I think I need to pull the coil out and check the direction of the winding.
Is this coil suppose to be 18 turns or 17? One symptom I noticed is that 640 kc is near the middle of the dial and 1260 kc is near the end. I only got as far as tweaking the three IF trimmers before I ran into this problem.
I'm not sure if I'm understanding what you are doing but if you are trying to set the dial frequency by adjusting the IF transformers DON'T. They need to be adjusted to peak at the IF frequency of 460kc.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Sorry, I wasn't clear. Weeks ago when this radio came to life after much of the restoration, I started the alignment process. I only got as far as aligning the three IF trimmers after breaking a spacer backing out the screw on the HF Oscillator trimmer on the tuner trying to align the dial to 1400 kc. After repairing it, I couldn't get the radio to work until I got to this point.
Broke it off aye. Well if it was from tightening it this may be the problem. As you tighten it you are increasing the capacitance of the trimmer. This lowers the oscillator frequency which in turn lowers the signal shown on dial. If it is still flatten down or the celluloid insulator is damaged this could cause the oscillator not to tune up to the top of the band.
Wouldn't hurt to bake the coil. Needs to been a fairly low heat like 150-175deg.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Yep, the phenolic spacer that presses down on the plates crumbled. It since have been replaced with a plastic one I scrounged up. Maybe I can replace the celluloid with mica. It should have a higher dielectric constant. What do you think?
I will have a look at the coil. If direction and #turns look OK, I'll dry it out with a heat gun set on low. There's no oven in the shop.
On # 12 & 6 the voltage is very low like 10v or less. Something simple like some thin plastic packaging from a bakery idem or water bottle ect...
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Sorry for the long delay but I took another look at this radio last week. I took the re-worked coil and counted the windings. I had 18 turns instead of 17 so I removed the extra turn and then solder it back up. No luck here. It didn't respond to "jump starting" either. So the owner had me look at cannibalizing a coil from another Philco he had laying around. This coil passed the continuity check and so I removed it with its C13 and R14 and looked at it more closely. I peeled back the friction tape covering the feedback windings and counted 17 sloppy turns of 33 gauge wire around a friction tape barrier. This re-work was ancient. I made the swap then turned on the radio and IT WORKED!! I was able to pick up stations loud and clear across the entire dial. I'm going in tomorrow to align it.
The owner and I began to theorize what went wrong with my re-worked coil. Could it be the high impedance from the 38 gauge wire? Although it was neatly wound and the direction was right, the windings were at a slight angle to those underneath. We guessed that the impedance is so high or off that the oscillation would not start. Regardless after removing the extra turn I just could not get it to oscillate at all even with a "jump start". We're planning to re-work this coil with thicker wire like 33 gauge and then install it in the other Philco when it comes time to re-cap that.
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