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Cobra 2000 GTL issue

Crambone

Well-Known Member
Jan 22, 2019
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ok, I have a frequency issue going on. The frequency counter didn’t work so I swapped out the frequency counter board inside (enclosed in metal box). Now it seems like my channel selector is off 2 channels from the frequency counter. Example channel 20 on 2 digit reading says 20 but the counter is at 27.175.
Before I go much further the radio has variable power that is controlled by the TONE knob and it has extra and lower channels that are controlled by the NB/OFF and CB/PA switches.
So my worries are if I try to align it (yes it will be the first one I try) do I go by the channel selector or frequency counter? The frequency counter is actually reading correctly when I hook the radio to an external counter. It also receives on the same channels as the frequency counter so it tells me the channel selector (2 digit) is wrong.

I looked at the alignment procedure and step one 1. input of frequency counter to TP13 (test point 13) IC1 pin 8. radio on ch.19 am. clarifier in center pos. NB and ANL off. check for 10.240mhz.
I checked and I do have the 10.240mhz on pin 8.

Step 2 input of RF voltmeter to TP10 radio on ch. 19am. clarifier centered. adjust L21 for maximum RF out.
I tried this on both the channel selector 19 and the counter 27.185mhz. I’m getting a very sloppy look on the scope.

Any idea we’re i should go from here?
 

What happens if you turn the coarse and fine clarifiers all the way to the right?

This sounds like a clarifier mod gone wrong, or a sloppy mod that has come loose.

For that matter, what do you see with both fine and coarse turned all the way to the left?

I have seen the clarifier wired in reverse on that model more than once.

73
 
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What happens if you turn the coarse and fine clarifiers all the way to the right?

This sounds like a clarifier mod gone wrong, or a sloppy mod that has come loose.

For that matter, what do you see with both fine and coarse turned all the way to the left?

I have seen the clarifier wired in reverse on that model more than once.

73
When I move the fine or corse clarifier knobs I’m not seeing any movement. I will trace out the wires tomorrow and look at the schematic to see if they are wired correctly
 
The clarifier control feeds a control voltage to a tuneable crystal oscillator.

If this voltage is lost altogether, the radio should be roughly two channels low, sometimes only a channel and a half low.

Restoring clarifier voltage should fix the problem.

73
 
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The clarifier control feeds a control voltage to a tuneable crystal oscillator.

If this voltage is lost altogether, the radio should be roughly two channels low, sometimes only a channel and a half low.

Restoring clarifier voltage should fix the problem.

73
You were correct no power to clarifier! Going to office max to have a schematic printed out it’s too hard to see it on my iPad!
 
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B8D66350-A237-42C3-8101-FD0D1CBC778D.png I’m looking at the schematic and not sure we’re to start looking for the lost voltage on the fine or course voice lock?
 
Seems to me there's a text file for this on the 'air-gapped' computer at our shop. Nothing helpful on file here at home.

I'll get it copied to a more-modern computer with internet and upload it here. No pictures, but better than nothing.

73
 
Seems to me there's a text file for this on the 'air-gapped' computer at our shop. Nothing helpful on file here at home.

I'll get it copied to a more-modern computer with internet and upload it here. No pictures, but better than nothing.

73
Nomad any luck finding that file?
 
The fine voice lock (vr407) will receive 8.16volts from pin 6 of IC4. That voltage is supplied to the red wire on your clarifier. You also have that same 8.16volts from pin 6 of IC4 going to one side of R424 (1k ohm) in the circuit. Look at your schematic to trace the path. I took some measurements from one of my 2000's for you...
RX Mode: AM with both fine and course voice lock (clarifier) set to center (12 O'clock)
yellow wire on clarifier 6.11v rx
orange wire on clarifier 4.44v rx
brown wire on clarifier 7.23v rx
blue wire on clarifier 7.50v rx
red wire on clarifier 8,12v rx (from pin 6 of IC4)

On the fine clarifier side you have 8.16volts on the red wire. the blue wire is to the wiper and the brown wire goes through R425 (4700) to ground. On the course side the orange is to the wiper and the yellow goes through R175 (2200) to ground, This should get you started.
-75
 
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