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Author Topic: Calibrating old signal generator with modern transceiver  (Read 9022 times)
K3STX
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« on: September 08, 2012, 08:53:29 AM »

I am trying to calibrate my old Eico 324 signal generator so I can align some radios. But first I have to calibrate it. When I put a short wire "antenna" over my old Crosley AM radio tuned to AM630 I can get a nice signal when my signal generator dial is set to about 625 kc. OK. But when I put this SAME short wire antenna on top of my modern Kenwood TS-850S and tune it to AM630 I do not hear any of the RF coming out of the signal generator no matter how high the "gain". Is this because the Kenwood is so well shielded? How can I get the weak RF signal into the Kenwood, surely not directly hook to antenna input?

I also notice that when I tune my radio to AM630 and my signal generator to 625 kc I hear the signal. Great. But I ALSO hear a signal on the same AM630 station when the signal generator is tuned to about 750 kc. Why is that, it is not a harmonic.

paul
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AC5UP
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« Reply #1 on: September 08, 2012, 09:41:12 AM »

Is this because the Kenwood is so well shielded? How can I get the weak RF signal into the Kenwood, surely not directly hook to antenna input?

Yeah, it's because the Kenwood has a metal case and is better shielded than the Crosley. The signal generator has an output attenuator and level control which should provide an adequate level for receiver test & alignment. If the generator is still too hot at the lowest setting couple the output through a very small condenser, like 3-5 mikemike, or the highest value resistor in your junk box to the Kenwood antenna jack. Also wouldn't hurt if the generator output was terminated by 75 Ohms.

BTW: Broadcast stations are usually very close to spot-on frequency so if you have two good stations, one near the bottom of the band and the other near the top, your best accuracy should be in zero-beating the generator against those stations. Set the generator dial to exactly the same frequency then align the appropriate slug or trimmer to zero beat. If the Kenwood has an internal calibrator with 100 KC steps that's also good. Check the calibrator against WWV for the helluvit.

I also notice that when I tune my radio to AM630 and my signal generator to 625 kc I hear the signal. Great. But I ALSO hear a signal on the same AM630 station when the signal generator is tuned to about 750 kc. Why is that, it is not a harmonic.

This has the makings of a math problem. With a strong signal and a lo-tech superhet it is possible to hear a signal image 455 KC's (typical) above the actual frequency. In your case the differential is 125 KC's and that's an odd IF frequency. It's also possible your EICO needs a little TLC as a weak output tube is being overdriven and throwing spurs. Another possibility is signal overload, and that's why alignment work is always done at the lowest usable signal level. You do not not want the receiver AVC kicking in to mask a signal peak. Another thought involves an unterminated output jack being a bad idea. Your best bet is to consult the EICO user guide to refresh your signal generator-fu if you haven't done so recently.

Nostalgia Air has one here:  http://www.nostalgiaair.org/References/Manuals/EIC/EIC_10.zip
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G3RZP
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« Reply #2 on: September 08, 2012, 09:41:48 AM »

If the '750' is actually 770, (we know already the calibration is a bit off) and the Crossley has a 455 IF, then twice 770 is 1540, minus 910 (twice the Crossley IF) gives 630. So the second harmonic of the generator is on the image frequency of the Crossley.
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N4NYY
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« Reply #3 on: September 08, 2012, 09:43:42 AM »

Should have gotten a Heathkit IG-42

LOL
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AC5UP
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« Reply #4 on: September 08, 2012, 09:46:48 AM »

What do you have against K3STX ?

It's not like he swapped out the selenium rectifier for silicon and is suffering from circuit ground disorientation........   Roll Eyes
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N4NYY
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« Reply #5 on: September 08, 2012, 01:01:02 PM »

What do you have against K3STX ?

It's not like he swapped out the selenium rectifier for silicon and is suffering from circuit ground disorientation........   Roll Eyes

I think the Igggggggggy is better than the Eico. It is shielded everywhere, including the attenuator.
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G3RZP
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« Reply #6 on: September 08, 2012, 03:03:18 PM »

Dunno about the US, but over here, the test gear wasn't in Benton Harbor green....
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AC5UP
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« Reply #7 on: September 08, 2012, 04:31:56 PM »

I think the Igggggggggy is better than the Eico.

Don't make me bring out an empty chair and pretend you're sitting in it........................
The EICO was made in Flushing (Queens), NY and and that's almost like New Jersey.    But with a GED.     Tongue

UPDATE: Holy Toto !!! Just heard the news a tornado touched down near Rockaway (Queens), NY this afternoon.
I'm visualizing a young girl and her little dog swept up by the tornado. They end up in New Jersey. Where they follow a yellow brick road to a crack house...
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W8JI
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WWW

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« Reply #8 on: September 08, 2012, 06:37:29 PM »

I am trying to calibrate my old Eico 324 signal generator so I can align some radios. But first I have to calibrate it. When I put a short wire "antenna" over my old Crosley AM radio tuned to AM630 I can get a nice signal when my signal generator dial is set to about 625 kc. OK. But when I put this SAME short wire antenna on top of my modern Kenwood TS-850S and tune it to AM630 I do not hear any of the RF coming out of the signal generator no matter how high the "gain". Is this because the Kenwood is so well shielded? How can I get the weak RF signal into the Kenwood, surely not directly hook to antenna input?

I also notice that when I tune my radio to AM630 and my signal generator to 625 kc I hear the signal. Great. But I ALSO hear a signal on the same AM630 station when the signal generator is tuned to about 750 kc. Why is that, it is not a harmonic.

paul

Paul,

Modern radios are shielded pretty well inside, not so much the cabinets. The RF and IF sections are very immune to all sorts of leakage issues, partly because they are on a PC board with groundplane and also because of board level shielding.

It would not hurt a thing to couple directly into the RF port of the radio.

RZP answered most logically about the 750 signal. The 455 IF image is 910 above the tune frequency. So the radio's image would be 630+910 = 1540 and you have the generator on half of 1540.

You can confirm an image this way. If you reduce the generator level and get a beat tone on the AM radio station, the pitch you hear from an image will change as you tune the radio without tuning the generator. 

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K3STX
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« Reply #9 on: September 08, 2012, 08:45:00 PM »

Thanks to all. Lots to think about. My Eico "antenna" is alligator clips to a loop of wire, perhaps I should use shielded cable instead of jumpers. I will work on identifying the real signal from the many images. I think it will be a LONG time before I use this calibrator to align my recently fixed HQ-170A (that is working perfectly fine, could even hear NH8S tonight on a dipole).

Paul
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W2RKJ
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« Reply #10 on: September 09, 2012, 07:20:08 AM »

Hey AC5UP, did somebody from New Jersey pimp slap you or did you have a ex-wife from the Garden State? If it's the ex-wife, I can understand the thoughts.
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N4NYY
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« Reply #11 on: September 09, 2012, 07:33:15 AM »

Hey AC5UP, did somebody from New Jersey pimp slap you or did you have a ex-wife from the Garden State? If it's the ex-wife, I can understand the thoughts.

He lived here!
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AC5UP
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« Reply #12 on: September 09, 2012, 07:38:08 AM »

A few years back I caught Triumph The Insult Comic Dog on one of the NBC late night shows and he was doing interviews at some music festival or similar... Interviewee says he's from Long Island, and Triumph says "Oh, yeah, Long Island. That's like New Jersey... Except with a GED". That was my first clue that New Jersey might be good for a cheap laugh. Then I met N4NYY in this forum. Suspicions confirmed....................   Grin

http://triumphtheinsultcomicdog.com/
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W2RKJ
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« Reply #13 on: September 09, 2012, 08:15:00 AM »

I didn't catch that but I wish I had. I love a good line especially when that door opens and a good one slides in there.  Thank you.
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G3RZP
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« Reply #14 on: September 09, 2012, 09:03:27 AM »

A guy I knew from North Carolina very unkindly called New Jersey the 'Garbage State'.
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