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Cobra 29 LTD Classic High VCO voltage. Will not adjust.

Low_Boy

Low_Boy

Sr. Member
Jan 21, 2010
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Rochester N.Y.
I have a cobra 29 LTD Classic with 4.8V on TP2 R88. L19 will not adjust the voltage at the test point or pin 19 of the PLL. Even with L19 removed I am still getting 4.8V at 19 of the PLL. I did for kicks swap out L19 with another coil and same thing. Pin 3 of IC2 is putting out the correct voltage and Pin 1 of IC3 is putting out correct voltage. I need some guidance on this one. Thanks
 

In order to make L19 even work, L22 has to "fire" - so it needs that 10.240 Xtal to help.

Do you get your 10.240 on Pin 13 - L23 sending a signal to Pin 13?

L22 runs about 10MHz AT LEAST - Pin10 on PLL for that...

At least that is where I'd start...

C101 and C102 - I usually replace these right from the get go, for if you don't know the age, and you run into a problem like this - just replace them first, to see if she'll oscillate - then you're halfway there in getting it aligned and working again.

Cheap insurance.
 
Last edited:
KEPC - wow - old board! Hmmm. gave you wrong cap numbers for another 29 from UNIDEN days earlier...sorry about that...

Ok, you get 5MHz - so you're ok...on L22 - L21 - Has any alignment been done recently?.
Cobra 29 TP2.jpg​

So your issue is with output of L19 / L18 - so do you get any RX? TX?

L19 is TX - L18 RX - if you don't get anything - DEAD, then look also at TP 3 that wire that jumpers across to the RX side from L18 - do you get a signal there?
 
No RX or TX. Light goes from green to red when keyed. I went through the synthesizer alignment. All looked good if I remember correctly. I will check the jumper when I get home.
Thanks
 
L22 not peaked? Sounds like the 15.36 MHz that should be coming out of it is missing, maybe?

Easiest way to check is pin 22 of the PLL. L22 should be peaked to get at least a Volt or two peak-to-peak feeding into pin 22.

And if it shows a very gradual peak with the slug DEAD EVEN with the rim of the hole, that coil has a failed capacitor inside it.

73
 
I can peak out L22 using pin 4 of IC2 as Sam's photofacts instructions. I can not get a reading off pin 22 of PLL. I did try another L22. I am actually getting. Anywhere between 18.xxx and 16.3745 and that will change by turning L19. But L19 will not change the voltage at TP2. By the way Sam's says reading at TP5 should be 16.270mhz. Still no RX or TX.
 
Low_Boy said:
I can not get a reading off pin 22 of PLL.
Click to expand...

And that is why the PLL is out of lock. Pin 22 is the converted-down VCO-frequency input controlling the PLL.

Pin 4 of IC2 is an input from L22. Since you see a peak there, the problem is between pin 4 and the PLL chip's input. Pin 6 of IC2 is the mixer output, the difference in frequency between the 15.36 from L22 and the 16.27 you should have on channel 1 is 910 kHz. This frequency changes channel to channel, and also when you key the mike. But the 16.27 reading is only for channel 1 receive mode.

Pin 22 on IC1 should have this 910 kHz (0.91 MHz)_difference frequency visible. It comes from pin 6 on the TA7310 IC2. If you have no signal there, IC2 is suspect. Never did see the japanese Toshiba version of this chip go bad, but it comes from third-party low bidders now. Have seen this chip go bad in Galaxy radios, but not many Cobras.

73
 
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Yes pin 22 is sure showing no peak to peak. and absolutely no frequency. I will remove the pile of wax that was poured in there and replace IC2 and update.

changing the subject a bit I often think all the wax the factory puts in the PLL area of these radios causes problems. It seems that almost all the radios that stop working end up with numerous solder joint problems always in the waxy areas. I wonder if the wax affects or seeps through the poor's in the solder and weakens the joint?
 
Low_Boy said:
Yes pin 22 is sure showing no peak to peak. and absolutely no frequency. I will remove the pile of wax that was poured in there and replace IC2 and update.

changing the subject a bit I often think all the wax the factory puts in the PLL area of these radios causes problems. It seems that almost all the radios that stop working end up with numerous solder joint problems always in the waxy areas. I wonder if the wax affects or seeps through the poor's in the solder and weakens the joint?
Click to expand...

I was always told the glue is corrosive and I've personally seen the green beast on joints exposed to it.
 

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