06-09-2016, 06:12 PM
If I may...
Tube Radio, with all due respect, you are missing the point of a proper alignment in a Philco receiver.
These sets were not intended to give razor-accurate tuning across the AM band.
For the purposes of my argument, allow me to focus on the AM band only.
The proper way to align the AM band in a Philco set is to first peak the trimmers (oscillator, RF if used, and antenna) at the high end (usually 1500 kc). This is done to set the upper end at 150 on the dial, yes.
However...
The purtpose of the low end padder is not to set the dial exactly at 60 (or 580 kc as is sometimes used), but to make the oscillator properly track with the set's IF.
This is where "rocking" the tuning gang comes into play.
Once the high end has been aligned, you set your signal generator for low frequency alignment and tune the radio to wherever the 600 kc signal is coming in. It may be as much as 50 kc off (more if the radio is badly out of alignment).
Now...
You want to adjust the low frequency padder slightly in one direction or the other, then retune the radio to keep in tune with the signal from the signal generator. You need to watch your output meter or oscilloscope while doing this.
If the amplitude of the signal increases, do this again (turn the padder very slightly, in the same direction as before, then retune the radio to keep the signal in tune).
If the amplitude decreases, you want to turn the padder slightly in the opposite direction and then retune the radio to keep the signal in tune. This is the "rocking" procedure.
You continue to slightly turn the padder, and retuning the radio each time, until you reach a point where the signal does not get any stronger and in fact begins to fall off.
When that happens, you need to turn the padder slightly in the opposite direction to re-establish that peak.
Then...
Go back and realign at 1500 kc (or wherever the Philco instructions tell you to for the high end).
Then repeat at 600 kc.
Usually going through this twice on each end is sufficient, but if you're really picky and have a lot of time to spend, you can repeat this as many times as you wish.
When you are finished, your radio will be accurate at 1500 kc but will probably not be accurate at 600 kc. However, by doing your alignment in this manner, you will have set it up so it has the best possible sensitivity across the band and the oscilator will properly track with the set's intermediate frequency.
Do you now see why your set's sensitivity is lower at 600 kc? It is because you aligned for the 600 kc signal to come in exactly at 60 on the dial.
Repeat the set's alignment using the method I have outlined, and you will find that the sensitivity will be higher on the low end even if the tracking is not 100% accurate.
These radios were consumer appliances, not laboratory standards, and a high degree of accuracy simply isn't possible with these old Philcos.
Tube Radio, with all due respect, you are missing the point of a proper alignment in a Philco receiver.
These sets were not intended to give razor-accurate tuning across the AM band.
For the purposes of my argument, allow me to focus on the AM band only.
The proper way to align the AM band in a Philco set is to first peak the trimmers (oscillator, RF if used, and antenna) at the high end (usually 1500 kc). This is done to set the upper end at 150 on the dial, yes.
However...
The purtpose of the low end padder is not to set the dial exactly at 60 (or 580 kc as is sometimes used), but to make the oscillator properly track with the set's IF.
This is where "rocking" the tuning gang comes into play.
Once the high end has been aligned, you set your signal generator for low frequency alignment and tune the radio to wherever the 600 kc signal is coming in. It may be as much as 50 kc off (more if the radio is badly out of alignment).
Now...
You want to adjust the low frequency padder slightly in one direction or the other, then retune the radio to keep in tune with the signal from the signal generator. You need to watch your output meter or oscilloscope while doing this.
If the amplitude of the signal increases, do this again (turn the padder very slightly, in the same direction as before, then retune the radio to keep the signal in tune).
If the amplitude decreases, you want to turn the padder slightly in the opposite direction and then retune the radio to keep the signal in tune. This is the "rocking" procedure.
You continue to slightly turn the padder, and retuning the radio each time, until you reach a point where the signal does not get any stronger and in fact begins to fall off.
When that happens, you need to turn the padder slightly in the opposite direction to re-establish that peak.
Then...
Go back and realign at 1500 kc (or wherever the Philco instructions tell you to for the high end).
Then repeat at 600 kc.
Usually going through this twice on each end is sufficient, but if you're really picky and have a lot of time to spend, you can repeat this as many times as you wish.
When you are finished, your radio will be accurate at 1500 kc but will probably not be accurate at 600 kc. However, by doing your alignment in this manner, you will have set it up so it has the best possible sensitivity across the band and the oscilator will properly track with the set's intermediate frequency.
Do you now see why your set's sensitivity is lower at 600 kc? It is because you aligned for the 600 kc signal to come in exactly at 60 on the dial.
Repeat the set's alignment using the method I have outlined, and you will find that the sensitivity will be higher on the low end even if the tracking is not 100% accurate.
These radios were consumer appliances, not laboratory standards, and a high degree of accuracy simply isn't possible with these old Philcos.
--
Ron Ramirez
Ferdinand IN
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