08-02-2019, 11:46 AM
Joe
I'm giving a good try. I'll have to see if I can find some silver paste.
Did cln the rotor forks and their mating surfaces, added a ground connection to the third mounting screw, clnd the oscillator trimmers on tuning cap, chased all threaded connected with mounting parts for a good cln surface, check cap for resistance showed open at 20megs. Measured resistance from rotor to ground and show short on 1 ohm scale.
Didn't see any corrosion on the plates did carefully dust them with a brush and air pressure.
On the circuit side of things I replace the plate resistor for the 76. Color code sez 25K measured 46K. read the service notes that the early runs had a 50K there and was lower to 25K on later runs for oscillator stability. Did raise the volt from abt 45 to 70vdc which seem close to what it should be. Replaced the 700mmfd that couples the plate energy from 76 the to the feedback winding on the oscillator coils.
Best I can tell there is no (separate form the one on the tuning cap) trimmer(s) or fixed caps in the in the tuned circuit for bands 4/5. For me the circuit is hard to follow as it is broken down into 4 half's of the two sections of the bands switch. It's a little blurry so finger and rotor corrections are a little hard to separate out.
The oscillator doesn't act like it has a short in the tuning cap as the places it fails are in two different places in it tuning range. It's a bit less capacitance on the band 5 than band 4. It also appears to be lower in amplitude as you get closer the frequency that it fails.
If I was a betting man it could be not enough feedback. Was thinking that maybe baking the coil to mitigate any moisture out of the form. Moisture can be a Q killer. Another idea I was pondering was making an adapter for the 76 socket and replacing it with a higher gain tube. Something like a 6C4 or 12AT7. This would be for testing purposes to help the troubleshooting.
Have run in to this problem before too on a few 30's communications receivers but this one seems be exceedingly difficult to sort.
Am open to other suggestions.
I'm giving a good try. I'll have to see if I can find some silver paste.
Did cln the rotor forks and their mating surfaces, added a ground connection to the third mounting screw, clnd the oscillator trimmers on tuning cap, chased all threaded connected with mounting parts for a good cln surface, check cap for resistance showed open at 20megs. Measured resistance from rotor to ground and show short on 1 ohm scale.
Didn't see any corrosion on the plates did carefully dust them with a brush and air pressure.
On the circuit side of things I replace the plate resistor for the 76. Color code sez 25K measured 46K. read the service notes that the early runs had a 50K there and was lower to 25K on later runs for oscillator stability. Did raise the volt from abt 45 to 70vdc which seem close to what it should be. Replaced the 700mmfd that couples the plate energy from 76 the to the feedback winding on the oscillator coils.
Best I can tell there is no (separate form the one on the tuning cap) trimmer(s) or fixed caps in the in the tuned circuit for bands 4/5. For me the circuit is hard to follow as it is broken down into 4 half's of the two sections of the bands switch. It's a little blurry so finger and rotor corrections are a little hard to separate out.
The oscillator doesn't act like it has a short in the tuning cap as the places it fails are in two different places in it tuning range. It's a bit less capacitance on the band 5 than band 4. It also appears to be lower in amplitude as you get closer the frequency that it fails.
If I was a betting man it could be not enough feedback. Was thinking that maybe baking the coil to mitigate any moisture out of the form. Moisture can be a Q killer. Another idea I was pondering was making an adapter for the 76 socket and replacing it with a higher gain tube. Something like a 6C4 or 12AT7. This would be for testing purposes to help the troubleshooting.
Have run in to this problem before too on a few 30's communications receivers but this one seems be exceedingly difficult to sort.
Am open to other suggestions.
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
