Good afternoon folks, I would sure like to ask for your help, clarification (and direction). I have Cobra 2000 GTL that will not show the SSB offset on the counter when selecting SSB mode. (e.g. on USB CH. 20 receive I would expect the counter to read 27.2065) but it reads 27.2050 in all three modes. I've measured the carrier insertion frequency of the radio and it measures correctly at the connector to the counter input (7.8015 MHz). The AM and LSB measure correctly as well. Some research says this is correct, that the counter should read 27.2050 in all three modes while others say it should read the offset 27.2065). Could someone please clarify which condition is correct? In transmit the same is true (27.2050 in all modes) I did replace "almost" all of the capacitors on the counter module (many were open) and made measurements with a scope but don't see any obvious anomalies compared with the service manual. I sure am pulling my hair out and would greatly appreciate the expertise of the good folks here. Thank you so much for any and all help.
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Cobra 2000 GTL frequency counter not reading SSB offset
- Thread starter David Springmire
- Start date
Stop worrying, your radio sounds fine. It should show 27.205 on all modes.
Thank you Cable Guy! I sure appreciate it! Please forgive this next silly question but why does the owner's manual indicate it should read the actual receive and transmit frequency? By the way the frequency does change while adjusting the the clarifier. Thanks again! Much appreciated!
That frequency display does something you don't see very often. It has two inputs. In SSB modes, it reads the carrier crystal frequency and the 34 MHz PLL output and does the math. This compensates for the offset. In AM receive, it substitutes 7.8000 for the missing carrier input. When you transmit it subtracts the 7.800 MHz AM-carrier frequency from the 34 MHz PLL output like it would for sideband receive or transmit.
The chip at the heart of this display is the Texas Instruments TMS1000. The same family of chip used in TI calculators in the 1970s, but containing custom firmware. They had to use a calculator chip to do the math, seems like.
The mode selector feeds inputs to the counter to tell it what mode is selected. A fault in that section of the mode selector can cause quirky faults in the displayed frequency.
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The chip at the heart of this display is the Texas Instruments TMS1000. The same family of chip used in TI calculators in the 1970s, but containing custom firmware. They had to use a calculator chip to do the math, seems like.
The mode selector feeds inputs to the counter to tell it what mode is selected. A fault in that section of the mode selector can cause quirky faults in the displayed frequency.
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Nomadradio, thank you so much for the detailed explanation! So this "compensation" explains why there is no change in the counter frequency display when changing from AM to either USB or LSB in receive? Does this same compensation happen in SSB transmit? There should be no change in displayed frequency as well? I assume so since it would be radically changing with voice frequencies? If that's true then for the user the only real display reading we concern ourselves with is the display reading when we adjust the clarifier in receive? Really sorry for the questions, just want to fully grasp it to understand the "correct" behavior. Thanks again so much! -
The FC isn't reading the final output frequency, which would show as you expect under ssb modulation, and the FC would track and roll with the audio. Instead, it is reading the steady, unmodulated, underlying IF frequency and doing its own calculations automagically to show you the frequency you are on. I understand your question and it is valid. The Cobra 2K GTL FC is more sophisticated than a standard FC, if there is such a thing. There are some really great threads and discussions here on the C2K FC, and your unit is functioning as expected.
Yep, like he said. The display does respond to the setup of your clarifier, er "voice lock". The factory made it a receive-only control. Transmit frequency was set to the center of the channel by a trimpot inside. Most specimens of that radio have had the clarifier "unlocked", so that you transmit and receive on the same frequency, wherever the clarifier is adjusted. I prefer to call this setup "locked", as in transmit is locked to the receiver frequency. If it were still factory-stock, the clarifier would not change the frequency if you turn the knob while transmitting.
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