I presented this specific problem on the other forum, but have failed to get a response so far:
The radio has been recapped, and resistors replaced. The SW and Police band antenna coils and oscillator coils have been removed. The broadcast oscillator coil has been replaced by a Miller 70 osc. So this radio is now single band. The original schematic is below, followed by the wiring changes I made. I get strong IF oscillations at 450 KHz from antenna to speaker. The problem is RF. By using the osc. padder, trimmer, and coil slug on the Miller 70, I can adjust it so I receive RF from about 1150 to 1650, but nothing below. By re-adjusting everything, I can get 600 KHz, but only with the tuner set around 1300 on the dial, and then, of course, I lose everything higher. None of the blades on the tuner short out. The padders and trimer are both good. I assume I'm getting negative voltage on the grid of the 6A8, but whenever I touch my meter (both VTVM and digital) to the pin, the radio stops oscillating, and it shows 0 volts. Where do I go from here? Can't find another oscillator.
!st a legible schematic can be found here on pg 250: https://www.worldradiohistory.com/Archiv...37-ABC.pdf I appreciate your gallant effort in drawing a diagram of the osc but w/o the tube's elements it does little good, mostly cause I'm a lazy bum at heart. I re drew a simplified diagram of the osc circuit to make it easy to follow. Crosley must pay their draftsmen in string and used bubble gum. Schematic are the WORST!
Replacement parts need to be solder in close to where they go. Not clip leaded in, lead length is important in osc circuits. Clean ground connection are good too.
The 6A8 is operating as a pentode oscillator and the in between grids are either for inputing the signal from the ant coil or to speed up the electrons to get them up to the plate. So mostly we're focused on grids 1&2& the the very top one (suppresser).
As you know the osc's output needs to tune a range of frequencies from 1000kc to 1900kc to cover the bcb. Double check the paper work that accompanies the replacement coil to be sure that the windings are connected properly
And now for the mystery question where are the original coils???
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: 12-12-2023, 10:57 AM by Radioroslyn.)
Generally speaking, the local oscillator frequency runs higher than the desired RF frequency by a value equal to the IF value. For example, for your set with a 450KHz IF, the oscillator has to run 450KHz higher than the desired RF signal.
You state that you "get strong IF oscillations from antenna to speaker". Do you mean that you injected a 450KHz signal with 400Hz modulation from a signal generator?
Does the oscillator section of the tuning cap have the same amount / size of plates as the RF section? I am not sure what range of capacitance the Miller 70 coil is supposed to run at, but if designed to run on a 365pF (with plate closed) cap (the older standard where the osc and RF sections of the tuner and the same size and capacity) and your oscillator section of the tuning cap is smaller than the RF section (the newer style), this may be the issue. With the smaller section of the tuning cap, the oscillator freq may be too high. The reverse may also be true. (I don't know if a lower capacity padder would fix that.
You state that when measuring the grid voltage, the oscillator shuts down. Do you mean G1, the Osc grid on the 6A8, Pin 5 or do you mean the grid cap, G4 (the RF grid). It is possible that measuring the bias on G1 (The oscillator grid, Pin 5) could shut down the oscillator.
Bottom line, your oscillator is running about 700KHz higher (or lower) than the desired signal, instead of 450KHz higher, which it should do. You need to change parts (padder?) to drop this to a value that is 450KHz higher than the desired tuned value.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
"You state that you "get strong IF oscillations from antenna to speaker". Do you mean that you injected a 450KHz signal with 400Hz modulation from a signal generator?" Yes.
"Does the oscillator section of the tuning cap have the same amount / size of plates as the RF section?" Yes. They are identical.
"You state that when measuring the grid voltage, the oscillator shuts down. Do you mean G1, the Osc grid on the 6A8, Pin 5". Yes, the oscillator grid.
The only difference I can see between my schematic and Terry's, is he eliminated 17b, a .02 mfd cap.
"You state that you "get strong IF oscillations from antenna to speaker". Do you mean that you injected a 450KHz signal with 400Hz modulation from a signal generator?"
Without the generator?
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!
I think he means, is the IF stage oscillating by itself? This can happen. One of my restorations was missing a bypass cap in the IF stages, and the stage would regenerate or oscillate according to how it was adjusted.
The replacement coil may also be the issue. It's a clue that you cant get the dial even close to the received station... If you can follow the oscillator with a frequency generator or another radio, set things up to receive and find the oscillator signal +450kHz from the received station e.g. 1050, for the station on 600 kHz.
Yes that's what I was getting at is the IF stage oscillating? If so then you need to get that sorted. It's independent from any setting on the set's dial or whether the osc is working or not. Sometimes a slight detuning of the IF will stop the oscillation and still leave lots of gain for the set to preform well.
The next issue is to figure out what the oscillator is doing. The simplest way is to use a well calibrated receiver to track the signal from the 6A8. If it uses a 6A8G wrap a few turns of wire around it and connect it to the ant of the tracking receiver. If it's a metal tube hook the two ant together with cap between. At 550 the osc should be running at 1000kc more or less. At 1500kc -1950kc or so. As you turn the dial on the crosley you should be able to follow or track the osc w/the other receiver. If the osc doesn't tune 1000kc to 1950kc most of the time the the rf section won't tune the same frequency - 450kc so the set won't receive stations in the proper places on the dial and it will poor sensitivity.
So for now don't worry about what the dial sez just see how far you can get the osc to track across the 1000kc-1950kc range. That's what it need to be doing.
OBTW I don't draw the tubes upside down.
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!
