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Cobra 29 LTD Classic FL2 Filter

Checked voltages on IC3 all good except pin 7, its dead. Followed trace back and it shows the 2v is being supplied to it.
 
PLL is out of lock. Pin15 of the PLL, if it is High, enables the TX Mixer (IC3) through D14, and the bias for the TX Buffer amp transistor through R62, I sometimes call this the Pre-Driver. If Pin15 is low, this disables (pulls down the voltage) of both of the above.

C29 PLL Lock Detect.jpg
 
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I checked pin 15 (i believe i have the right one) and it shows 4.77v, is that considered low?
 
Ok, thanks, i had it right pin @4.77v. Looked more like a 1S to me.
D14 and R62 have about the same voltage 2.?v ide have to check again.

Edit When key up the green rx light will turn red to indicate tx.
 
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Handy Andy said:
That's a good sign, so now as long as the TX light is red, double check voltages in the TX section to the readings you should get.

You're getting there!
Click to expand...
The only voltages that are close are the driver and final, the rest are off, not even close. Makes no sense. You dont think #7 dead pin on ic3 is an issue?
 
What voltages are you getting on TR16 when in TX ? The only thing that "could" pull down Pin7 of IC3 to 2v (also only active in TX) while the LD is a high condition (4.7v??) , could be a shorted Base on TR16. The Emitter should be under a volt, the Base should be .7v above Emitter voltage. Collector should be around +8v, again, only in TX.
 
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Ok, i checked TR16 again in TX

B=104.8mv (keeps climbing up)
C=7.67v
E=.798v

I did a test with a resistor across the ex speaker and pa and the "talkback" works fine. First time ive heard anything out of it.
Switched out TR16 c941 with a c945, made no difference
 
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C29 PLL Lock Detect.jpg

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C29 LD Path Enable-Disable TX.jpg

TR16 - E is the Left pin, C is Center pin, B is the Right. B should be about 0.6v to 0.7v above E voltage.

Red trace is from the LD pin (15), to JP2 under the PLL IC, then continues towards the back through R102 (3.3k), to D14 (low LD state disables TX Mixer) and to R62 (2.2k) which provides Bias to TR16. If LD High state, should enable the TX Buffer (or sometimes called TX Pre-Driver), and a LD Low state disables TX.

Try this: Remove R102 (3.3k), set aside to put back later (already pre-bent to correct shape and length), and put another 3.3k from the left trace where D14 or R62 are on that trace, to either JP31 (next to L20), or JP32 (feeds +8v TX to Collector of TR16 through L17), or to JP29, next to L17.

This disables the LD lock-out of the TX. See where (if at all) it transmits on what frequency.
 
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Take the 3.3k you pulled, solder a wire to extend it, I didn't know if you had a 3.3k laying around or in a drawer, or loose. Now try. You can always rebend it back in shape to restore it later.
 
NZ8N said:
Take the 3.3k you pulled, solder a wire to extend it, I didn't know if you had a 3.3k laying around or in a drawer, or loose. Now try. You can always rebend it back in shape to restore it later.
Click to expand...
Thats what i did. Soldered to D14 and JP31, unless i did it wrong it made no difference at all, went through all the channels with frequency counter and no tx on any channel, still no carrier at all. Thank you for the help, much appreciated.

thumbnail.jpg​
Please dont judge my soldering, i know its a temperary thing, didnt do an outstanding job on it.
On the upside i can listen to "the bowl" through the dummy load.
 
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