Good point Ed I hadn't thought it thru completely about the osc being on the low side to produce the signal for the IF. All this time I've been reading the osc with my counter set to subtract the 460kc. Time to play some more...
Going to make some large copies to sort the osc and rf coil, caps, and circuits
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!
Steve,
From what I can gather on the two mystery trimmers (#6&7) 6 is part of a broad banded tuned circuit that covers bands 3,4,and5. The same for #7 but it's for bands 1&2. There probably won't be a sharp peak at any particular frequency with these two. I suppose you could try to see if there is any increase if they are adjusted at the center frequency of the two groups of frequencies. #7 around 1500kc and #6 around 10mc.If they did "drift" as you mentioned it's such a broad range it probably will be pretty negligible. Would use a good generator with a calibrated output in microvolts so you can track your progress.
GL
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: 07-26-2019, 07:55 PM by Radioroslyn.)
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!
Yesterday I jumped in with both feet and removed the tuning cap. I was quite surprised how dirty it was even though I "cleaned" it while it was in the set. Lots of q tips later it was reasonably much cleaned. Didn't do much with the two back sections just cleaned around as much as I could without monkeying up the the trimmers. Did drill out the rivets that hold the rotor forks and removed them for a clean. They had this hardened black crud on them that was a little difficult to remove. The two oscillator trimmer came apart which was part of this operation to see if I could get bands 4/5 work properly. Was using MEK for this job and was still a bit of a job. Replaced the rivets with small screws and solder on the pigtails to reconnect the cap to the set.
Still had the problem with 2nd gear on the dual spd dial drive. 1st gear was ok when I got it with a new o ring. The small o ring someone tried to "fix" it by slipping a larger o ring over the bad original one. That had to go. I found a little box of o rings and found one the fit just dandy, but it wasn't tall enough to meet the drive shaft. Hmmm... The only other ones I had that were close were these red garden hose washers. Too thick and too tall. It met Mr Dermel. Was able to narrow the inside lip to fit the hub and then mounted it and spun it on my drill up against a piece of sandpaper to decrease the height. Now both speeds are working swell!
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!
Good idea. Used to use them all the time at work. Small touch up work, matching colors an cleaning my mouth atomizer. Will have to walk over to the drug store and see if they have em.
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!
Jury is still out but it's not looking good. It's back working but still seem to be dead on the lower section of bands 4 and 5. Haven't gone thru to realign yet. Still have a few ideas but am getting a little annoyed with this beast. On the bright side it's nice not having use the slow ratio dial setting.
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!
No, On band 4 it dies out around 7.8mc on the dial and on band 5 the oscillator conks out @ 16mc on the dial.
On the lower bands it was working 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: 08-02-2019, 10:29 AM by Radioroslyn.)
Kudos on getting the rotor forks cleaned up! This shows you are determined to get this unit working right. I have seen this type of problem on many radios, including some manufactured much more recently. Did you also clean the mating surfaces of the rotor too? I expect you did. I have had a few stubborn cases where I had to put a small amount of silver paste between the rotor forks and the rotor surface they mate with in order to stop drop-outs at particular points of rotation of the tuning capacitor. I used some silver conductive heat sink compound made for CPU mounting. I applied it using an Xacto knife blade with the capacitor reassembled, carefully working the compound into the space between the forks and the rotor shaft.
Another possible trouble is when one of the capacitor vanes comes so close to the vanes it meshes with and shorts out at a particular spot. Those can be difficult to spot. Bright light and a magnifying glass can help spot the offending section and vane(s). Repeated careful moving of the vanes with the Xacto knife can usually clear the short. Watch for corrosion on the vanes. Some radios are known problem units in that regard.
Good luck!
Joe
(This post was last modified: 08-02-2019, 06:31 AM by Joeztech.)
The "black crud" you mentioned is something I have seen on most of these tuners. Is it possible they used grease mixed with graphite to improve conductivity? I was going to experiment with that sometime, but haven't yet. The silver paste sounds like a more modern solution.
(This post was last modified: 08-02-2019, 11:03 AM by rfeenstra.)
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.
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!
Moving ahead I constructed an adapter from the 76 tube to a 6C4. That helped some and I gained a couple of hundred kc's lower now. To track it I've been using a Kenwood TS-440's receiver. Did note that the amplitude of the oscillator lowers before it dies, can watch on the receivers S meter.
Lowered the value of the 76's plate resistor down to 12K to see if I could get a little more gain and it did but just a little. I need more dissipation!
Next I measure the heater voltage at the socket and it was 5.6vac. Kinda low. Hooked up my variac and got the voltage up to 6.2v. This helped too.
Measured the cathode voltage and it seemed to be reasonable at abt 4v. Now the 77 mixer's cathode is connected thru a 1250mmfd cap to the 76's cathode. This how the oscillator signal is coupled from the oscillator to the mixer. The mixer cathode is 5k above ground so the voltage there should be somewhat higher. It wasn't it was fairly close to the voltage at the 76. Disconnected the 1250mmf cap and replaced it. Not much of a change.
Did note that when I was measuring the voltage at the 76 cathode the oscillator seemed to perk up and almost acted as it should. I'm thinking abt adding a gimmick cap to see if this will correct the original.
There is a light at the end of the tunnel and no train is coming at me.
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!
On the RCA console I restored, I got shortwave oscillator drop out because the AM coil was open... the AM coil absorbed enough energy because of self resonance with the SW coil. This could happen in a set if any unused oscillator coils on the same former are not shorted out properly by the bandswitch.
Just a thought,
