Alignment Issues for Philco 70 - Printable Version
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Alignment Issues for Philco 70 - Guest - 12-06-2008
All,
I am ready to align my Philco 70. I am getting nearby stations pretty good, but I want to properly align the receiver.
I have abbreviated the alignment instructions I got from the Riders manual below:
1) Disconnect the grid clip to the 1st detector. Connect RF signal generator clip to grid of 1st detector and the ground clip to the chassis. Adjust RF signal generator to 260 kc, and dial position on radio to 55. Adjust trimmers 1, 2 and 3, in order, to achieve maximum output signal.
2) Reconnect grid clip on 1st detector. Connect RF signal generator clip to ANT and the ground clip to chassis. Adjust generator to 1400 kc, and dial position to 140. Adjust trimmer 4 to achieve maximum output signal. There are similar instructions for trimmers 5, 6, 7, then 4 again, but that is not the issue right now.
Here's what I've done so far:
- I have an RF signal generator, and I've checked it against a digital receiver (from 530 to 1710 on the AM dial) and I am clearly getting the 400 Hz
internal tone. As I adjust the frequency on the generator, I get the maximum volume from the receiver when pointer is right on the desired frequency (at several points in the AM range), so the generator is working properly and appears to be accurate and graduated correctly.
- To measure output signal, I will hookup my DMM in parallel with the output transformer secondaries and measure VAC. That way, I can hear the speaker and see the voltage.
Here's the problem:
When I do step 1 above, I get nothing from the radio speaker at all (except static). Because the radio works, I am reluctant to give trimmer 1 too many turns ... thus far, I've been careful to keep track of the original settings. Since I cannot tune my digital receiver down to 260 kc, I can only presume that the RF signal generator is working at that frequency; however, it is on a different frequency band. The other frequency bands are working.
When I try step 2 above, I get the generator tone near 140 on the dial ... I should be able to adjust trimmers 4 - 7 without too much trouble.
Here are my questions:
a) Is there a way to ensure that the signal generator is working at 260 kHz, without a frequency counter.
b) If we assume that the signal generator is working properly, I assume that I would adjust trimmer 1 until I hear the generator tone from the speaker. How much adjustment might the trimmers need? I'm not sure if I should be seeing something within a half-turn either direction ... just not sure what to expect. Will the trimmer stop at the end of its travel?
Thanks,
M100C
Re: Alignment Issues for Philco 70 - etech - 12-06-2008
Quote: Is there a way to ensure that the signal generator is working at 260 kHz, without a frequency counter.
You could try listening for the third harmonic which should be at 780kc.
Quote: If we assume that the signal generator is working properly, I assume that I would adjust trimmer 1 until I hear the generator tone from the speaker. How much adjustment might the trimmers need? I'm not sure if I should be seeing something within a half-turn either direction ... just not sure what to expect. Will the trimmer stop at the end of its travel?
If your signal generator is working properly, you should be able to hear it even if the I.F.'s are somewhat off-tune. Typically, you should not have to adjust them more than a half-turn in either direction, since the radio is working.
One thing you might try is to tune the signal generator until you hear the signal through the I.F.'s in the radio and then see what the frequency on the signal generator reads.
Really, unless you have a digital signal generator, an inexpensive frequency counter to measure the accuracy of the signal generator would be the way to go.
Regards,
Ed
Re: Alignment Issues for Philco 70 - Texasrocker - 12-07-2008
In a pinch, a good freq counter could also be a portable type radio receiver with a digital ( crystal-controlled) tuning window readout. They come in handy for accurate "flyin by the seat of your pants" alignments using harmonics generated by questionable RF generators signals if needed.Rat Shack used to sell these type handheld portable radios on-sale sometimes a few yrs back. These portables also help for aligning " by ear", harmonics on the FM & SW bands as well if the radio receives all 3 bands.
Re: Alignment Issues for Philco 70 - DCASAZZA - 12-07-2008
Here's where the rubber meets the road. You already know your connection points.
1 Tune the IF cans for maximum sound via the speaker with a off the air signal.
2 Tune the Front end Ant RF adjustment around 1600 for maximun sound at the speaker with an off the air signal.
3 Go back and put a meter on the speaker, if you wish, and tune your signal generator to 260 kc and using the minimum acceptable signal, touch up the IF cans. At this point your not concerned about what frequency the dial shows on the generator, You know the radio's right. The IF cans dont change the radios recieve frequency. When we adjust their "resonance" we are just allowing them to pass through the desired passband at the maximum amplitude possible to the next stage.
4 Tune for max on the Ant RF adjustment as well.
5 Use the Signal generator to set the oscilator tuning and osc trimmer at the radio (1600 and 600 kc). Use the generator to find what the radio is set for, not the other way around. When finished use off the air station signals to correctly calibrate the osc adjustment and trimmer.
6 Touch up the Ant RF again at the high end of the dial.
7 Do this for each additional band for the radio, but leave the IF's alone after this first run through.
Remember to always use the minimum acceptable signal from the generator (not the radio's volume control) as too much will overload the front end of the radio and make finding that sweet spot a lot harder to discern during your alignment.
Now, sit back and enjoy your radio.