Cobra29 Chrome,L19 and L23 won't work.

[Mark A Bradley]

Hello gentleman and DXer's,I have a Cobra with issues. When going to get my RX and TX,the frequency just sits and bounces around, and will not adjust. I have my Frequency meter on TP or R61,for my RX and TX.
I replaced T20,T7, and a blown diode,D7,the PLL has 10.695, 10.240, and I can see me adjusting L19 and L23,from PLL leg 16 or 17. Next to it,to the right,facing the rear,leg 15,I believe is where I was able to see adjusting,it was at ,26.??? mhz, and it was moving as I turned the core,but my frequency counter did not budge,and it never went passed,48.000 mhz.
I also threw in,IC2,and IC3,I replaced both of the TA7310P mixers,a 10.240 crystal,and a 3 legged 10.695 filter,I guess that's what it is called.
I did this work,and no change. Is there a area I should be looking in? At this point,I am lost and confused. Can you please,help me out,this is a Chrome radio,it belongs to my next door neighbor,who asked me to look at his CB for him.What do you tell a neighbor that live right next door,I told his sure,give me a few days,so this is day 1, and not fixed. Any ideas,the freq just sits at 48.000 mhz, it bounces when you turn the core, so 19 and 23, something must be picking up the core turning.OK, thank you ahead of time,will check back in a few hours,again,I appreciate all of your time. Mark

 

[Handy Andy]

Hmmm, if it's not locking - it needs a loop signal.

So did you have a chance to peak out L22? That's where the main loop is, it takes the 10.240 and divides it by two.

You get 5.12MHz.

That's where you start, get the PLL to clock and stabilize with 5.12MHz - then L19 and L23 fall into place.

That is where R85 is used, peaks out L22 - might need a scope to see it and R88 is "tapped" to set the Varactor Voltage - on L19. On Channel 40 and Channel 1. But do most of the initial tuning on Channel 20.

Use these threads

https://www.worldwidedx.com/threads/need-help-with-side-mic-cobra-29-ltd-classic.251271/#post-702194

https://www.worldwidedx.com/threads...e-until-it-warms-up.239208/page-2#post-659728

https://www.worldwidedx.com/threads/cobra-29-nw-problem.259823/#post-740762

 

[Mark A Bradley]

Hello.how you been, just want to Thank you for the reply on my issue. You are correct, at this time,I did not get it peaked out fully. After peaking it out to it's full extent,yes,I am able to adjust the frequency to the 27.185. Just need to remember what TP does what,I know TP3 is for the RX Frequency. I bop around TP, and see what is what. I will got back to the last question I had, you gave me quite a bit to read and remember.
I can hear the static noise, I lost my receive,but faintly hear it,it is tuning nice. I have low receive,and the radio also stopped transmitting,these issues came up last night when I was peaking L22, very low receive,and no transmit. This radio has all the old Capacitors in it. I had receive and transmit ,but they were off frequency,now they are no where. Going to check Capacitors in Audio section,and check the Caps related to the transmission area first. Thank you for all of the links, Will post update early afternoon,as I am waiting on a new Lincoln 10-12 meter radio. I put the HR2510 in a box,and put it away, I will look at that another time. OK,will update as soon as I figure it out,enjoy your day,thanks.

 

[Mark A Bradley]

Hmmm, if it's not locking - it needs a loop signal.

So did you have a chance to peak out L22? That's where the main loop is, it takes the 10.240 and divides it by two.

You get 5.12MHz.

That's where you start, get the PLL to clock and stabilize with 5.12MHz - then L19 and L23 fall into place.

That is where R85 is used, peaks out L22 - might need a scope to see it and R88 is "tapped" to set the Varactor Voltage - on L19. On Channel 40 and Channel 1. But do most of the initial tuning on Channel 20.

Use these threads

https://www.worldwidedx.com/threads/need-help-with-side-mic-cobra-29-ltd-classic.251271/#post-702194

https://www.worldwidedx.com/threads...e-until-it-warms-up.239208/page-2#post-659728

https://www.worldwidedx.com/threads/cobra-29-nw-problem.259823/#post-740762

One question,where do I find the 5.12MHz,which R. Thank you.

 

[Handy Andy]

You don't see it directly...

Probe Pins on the PLL Pin 10 or C87 to see it - you just need to "tap" and see if the loop in working thru a counter - 5.11 to 5.13MHz is all you need to see.

L22 Peaks that using R88/R85 - you'll need to try scoping both for if one can't show, you won't get varactor voltage on the other either outputs at L19 and use TP3, it's better as being a buffered tapping point so it doesn't load down the system as you look for it..

I had to do a quick run to get some chores finished...

To complete this post, the R88 is the "varactor" side - you won't see much there except a DC voltage possibly a "ramp" (Stairstep) if you tune L19 and even L23 and it seems to want to try and lock - else when the loop is detected and can run, PLL Pin 19 settles down and "sees" what R85 is doing.

So...R85 and L22 to Loop...

And? L23 - if any tweaking needed - handles the PLL's Pin 12 and Pin 13. That the 2816's input for the 10.240. In many radios it's Xtal 10.240 output (the Crystals pins) is run directly to it (PLL main Clock), but in these radios L23 is "tanked" to get the Receive IF Image needed for 455kHz out of it too. So it needs a little bit of power to push out enough signal to make the system work.

If L23 cannot develop power then Yes it CAN prevent L22 from running because it can't get a signal STRONG ENOUGH to the PLL ; for the PLL to even send out to L22 in the 5.12MHZ range to even clock against for you to see Scoped' on R85.

 

[Mark A Bradley]

Hey,there is a Sants Clause, it worked,I have receive. BUT, I let it sit and receive for around an hour,finished the Lincoln 11 in the truck,came back in the garage,and barely any sound or static coming from the radio. I did a little reading,and read if low receive,install a
Fet 2, 2SK192A, had to steal from a parts radio, actually, where do I get these,somewhere where I can get Fets and Varactor from.
Well,I got it going,now little sound,and lost the voltage on the VFO, around .83,was 8.35v,had it at 3.2v. I installed the Fet in Fet 2, now are we to turn them around,opposite way? I did it both ways, did not change,the Fet is at I=.99ma@Vg=.69v,...I=.99ma@Vg=.69. This is what's in the Cobra now,these I took from a spare radio. Going to do a Diode check now. Talk soon.

 

[Handy Andy]

TP3 is a Cap and Resistor - make sure the Resistor R17 is a 3.3K (Orange Orange Red) and is not snapped in two - it can give you readings like what you experience. The Cap C16 is a 0.01uF (103 Disc) but may have a shunt cap installed on the board or bottom to shunt away signal so it doesn't BLOW UP the FET from too much RF applied by the PLL - so that is usually a 100pF (101 Disc) from Junction of C16 and R17 TO the FET side - shunts to ground. Acts like an RF splitter - takes away from the IF signal strength - so it doesn't overload the FET.

 

[Mark A Bradley]

Hey,I am back,long story,but my Father got Covid,and he is 84. He had some trouble,but he is out of intensive care and doing slot better. I have not had time to work on the radio,as I just got home, I stayed at his house,and did absolutely nothing.
I had no tools,no Oscilloscope,I could have loaded everything on the truck, but the Boss ( Wife ) said NO.
So, possibly tomorrow, I am looking forward to get back on it,benn a week plus,wow,. And I get to sleep in my own bed Tonite. I read your last post and I will look into it tomorrow,get out of the house. For now,have a great end of the weekend,check in tomorrow afternoon Adios.

 

[Slowmover]

Hey,I am back,long story,but my Father got Covid,and he is 84. He had some trouble,but he is out of intensive care and doing slot better. I have not had time to work on the radio,as I just got home, I stayed at his house,and did absolutely nothing.
I had no tools,no Oscilloscope,I could have loaded everything on the truck, but the Boss ( Wife ) said NO.
So, possibly tomorrow, I am looking forward to get back on it,benn a week plus,wow,. And I get to sleep in my own bed Tonite. I read your last post and I will look into it tomorrow,get out of the house. For now,have a great end of the weekend,check in tomorrow afternoon Adios.

Here’s a little prayer the abstract problem-solving of radio proves a balm.

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