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 Post subject: Strange roblem with Hallicrafters SR-400 transceiver
PostPosted: Jan Wed 25, 2023 4:27 am 
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Joined: Nov Mon 02, 2009 6:01 am
Posts: 6184
Location: Lincoln City, OR. 97367
Greetings to the Forum:

I swear that this radio is cursed. I found it in very poor condition at a swap meet and am trying to bring it all the way back. I'm getting close; so far, I have found and fixed 25 separate problems. However, the latest one has me completely baffled.

The crystal calibrator appears to generate two separate frequencies at once. When the calibrator is on, one can clearly hear two tones, perhaps as much a 1 KHz apart. This problem is only associated with the crystal calibrator; my service monitor has a very clean signal with no second beat tone. Naturally, I assumed that the calibrator has some sort of spur associated with it. So, I got out my spectrum analyzer and looked at the calibrator output. It is clean; no spurious signals, just a single carrier at 100 KHz and of course, when looking at the point where the calibrator signal is injected into the antenna, a nice set of harmonics, 100 KHz apart with no visible spurs.

This behavior occurs on all bands, every 100 KHz. It is only present on the calibrator signal; it is not present on an externally supplied signal from my service monitor.

The point I am looking at with my spectrum analyzer is pretty low level. I am using the internal RF pre-amplifier to get a reasonable signal level. Even so, the calibrator signals are only about 30 dB above the noise. One of the two signals heard in the receiver audio seems louder than the other; I measured it however as only 1 dB difference. This may not be a valid measurement since it is necessary to zero beat one tone to measure the other and I have to switch sidebands to measure both; the receiver gain may not be uniform under these circumstances. However, it would seem to indicate that any spur present would have to be much higher in level than thirty dB down from the actual signal, and therefore should be visible on the spectrum analyzer. It is not, even with 100 Hz bandwidth selected.

I am at a loss to explain this behavior. It is not possible to accurately calibrate the VFO with this condition present; I can't sell the radio or even use it myself until this problem is solved.... but I have no idea where to go from here.

Any thoughts?

Thanks,

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Jim T.
KB6GM
Palus delenda est.


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 Post subject: Re: Strange roblem with Hallicrafters SR-400 transceiver
PostPosted: Jan Wed 25, 2023 1:01 pm 
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Joined: Jun Sun 19, 2011 1:31 pm
Posts: 8657
Location: Minier IL
Jim,

Maybe a decoupling problem and your calibrator is also getting directly into the first variable IF in addition to the harmonic that is going through the RF amplifier?

Does the spacing between the two tones vary by band? It would if the scenario I suggested is true because the hetrodyne crystals won't be perfectly on frequency for each band causing a bit of variance between the converted and direct to IF path.

Rodger WQ9E


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 Post subject: Re: Strange roblem with Hallicrafters SR-400 transceiver
PostPosted: Jan Wed 25, 2023 4:53 pm 
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Joined: Nov Mon 02, 2009 6:01 am
Posts: 6184
Location: Lincoln City, OR. 97367
Greetings to Rodger and the Forum:

Having slept on it, I think you are most likely correct. The calibrator signal is getting into the IF somewhere and some sort of mix is occurring. I haven't figured out the arithmetic yet that could do this, but I definitely need to take a closer look at the frequency separation by band as you suggest. I believe that it is quite large on the upper part of 10 meters, allowing for two calibration spots several KHz apart, but I wasn't thinking in those terms when I made the initial measurements and so I am like Dr. Watson, when Sherlock Holmes told him "You see, but you do not observe."

More data to follow.

Thanks for your input,

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Jim T.
KB6GM
Palus delenda est.


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