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GM 300 off frequency?

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k7ieu

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i got a couple gm300's for cheap, i programmed 446.000 into channel 1, but it is actually transmitting centered on 445.992


question

1. is this fixable ? IE can we adjust to actually be on frequency?
 

hp8920

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If you have the radios connected to RSS, go Main Menu, Service, Alignment, Reference Oscillator Warp Adjustment. The instructions note that it may take up to 10 seconds before adjustments take effect. Be sure to program the value before you exit.

If that can't fix it, you can always precompensate the TX and RX frequencies when you program them in.
 

merlin

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This is beyond a VCO alignment, Looks like a bad memory chip.
Fixable but is it worth it.
 

petnrdx

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Disassemble the radio before you try anything else. Its easy, takes maybe 10 minutes.
Clean the pins that run between the LOGIC board and the RF board.
This often fixes it.
Then do VCO alignment.
Done hundreds of these.
If you are going to throw it away, throw it my way.
 

k7ieu

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ok i just programmed it hi in freq to compensate and live with the problem



I’ll try cleaning the pins later maybe if it goes wonky again
 

merlin

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Nonsense. VCO off frequency and a common issue with these radios.
That is true but no reference VCO will go 1Mhz off frequency.
A couple Khz is stretching it
Very obvious the PLL data is not right.
TX mixer switch diode gone bad might do it.
 

MTS2000des

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That is true but no reference VCO will go 1Mhz off frequency.
A couple Khz is stretching it
Very obvious the PLL data is not right.
TX mixer switch diode gone bad might do it.
EEPROM failures result in radio not booting up, it does a checksum validation every time you turn the radio on. If it boots up with a positive beep, it's good.

It's very obvious components have drifted over time, and having serviced hundreds of these in my career, rarely have I ever seen the EEPROMs in these fail. These aren't like Jedi or Astro radios that do constant background writes to the EEPROMs, they only are written to when you actually use RSS to write a codeplug or tune the radio, so unless that's been done a hundred thousand times or more, it ain't the problem.
 

WB5UOM

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445.992 from 446.00 is not 1mhz off (its true I was lousy in math in School but I subtracted 5 times to chevk myself) and should easily be tweaked on with RSS as mentioned.
 
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jeepsandradios

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445.992 from 446.00 is not 1mhz off (its true I was lousy in math in School but I subtracted 5 times to chevk myself) and should easily be tweaked on with RSS as mentioned.
Yup. I asked what he used to verify its off but no reply. Unless its a calibrated meter or monitor I wouldn't remotely worry about it. I have watched folks use frequency counters tell me my APX is off frequency that just came off the auto tune.
 

hp8920

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Yup. I asked what he used to verify its off but no reply. Unless its a calibrated meter or monitor I wouldn't remotely worry about it. I have watched folks use frequency counters tell me my APX is off frequency that just came off the auto tune.

At around 8 kHz off, you'll start to have PL decode issues with a normal radio. It's also not legal in the case of Part 90, you're allowed 2.5 ppm in that range, and he's 18 ppm off.
 

jeepsandradios

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At around 8 kHz off, you'll start to have PL decode issues with a normal radio. It's also not legal in the case of Part 90, you're allowed 2.5 ppm in that range, and he's 18 ppm off.
How do you know he is 18ppm off without knowing what he used to check it. His frequency listed is not part 90.
 

hp8920

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How do you know he is 18ppm off without knowing what he used to check it. His frequency listed is not part 90.
8 kHz is way off. That's more than 1 max deviation. If a reasonably modern non-Baofeng radio that hits the local repeaters fine says it's 8 kHz off, then I trust it, particularly with the known issues with this radio.

If the OP was reporting they were 1 kHz off, I'd agree with you and say to use a calibrated service monitor or frequency counter before tweaking anything, but 8 kHz is at the point where you get PL issues and audio noise.
 

prcguy

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8 kHz is way off. That's more than 1 max deviation. If a reasonably modern non-Baofeng radio that hits the local repeaters fine says it's 8 kHz off, then I trust it, particularly with the known issues with this radio.

If the OP was reporting they were 1 kHz off, I'd agree with you and say to use a calibrated service monitor or frequency counter before tweaking anything, but 8 kHz is at the point where you get PL issues and audio noise.
You will have PL and distortion issues way before being 8KHz off, more like about 2KHz off will start sounding distorted on a WB channel and 5KHz off is usually a fuzzy sounding mess.
 

iowajm780

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That is true but no reference VCO will go 1Mhz off frequency.
A couple Khz is stretching it
Very obvious the PLL data is not right.
TX mixer switch diode gone bad might do it.
MTS2000des is a radio GOD!. Whatever he says is what you should take as the answer. Learned quite a bit from his various posts.
 

merlin

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I have to concede not having much hands on with GM300s.
I have worked on multitudes of radios from early PLL frequency generation
Most TCXOs drift but still within PPM specs. prcguy is right when frequency get 2Khz +
the signal will be about useless. Worse with SSB mode.
Soft pots only problem is the limit of plus or minus 124 steps. (probably 1 Hz)
A 450 Mhz radio with 45 Mhz reference is 10 to 1 so 1Hz change is 10 Hz at the VCO.
8 Khz would be like 800 Hz change to the TCXO and I have never seen one adjust that far.
Not with soft pot's anyway.
 
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