Diagnosing a bad crystal? Need ideas on helping me and others.

[Mark A Bradley]

As I hooked up my Frequency counter to another radio from a Buddy of mine, the radios frequency is bouncing all over the place. Radio has no receive and no transmit, of course, it is off freq. I am not sure about diagnosing this radio, as I cannot get this to lock on Transmit freq, bouncing from 8.000 to 50.000 Megahertz, and will not stop when adjusting the pots that has the 2 half settings, this one wont stop, just jumps all over the place.
What would be the proper way to diagnose a Crystal on this Cobra 29 LTD Classic, yes another Cobra 29 with a issue, seems to be my nightmare all over again, as it makes me loose sleep trying to figure out what next to do to figure this radio out. What is with these Cobras and this frequency problems, bouncing all over, and the pots for the adjustment does not do anything at all. The only idea I have is to replace them and see what the deal is. I will go back to a few previous posts and see if there is anything I can use for me to diagnose another headache.
OK,off I go, is there a procedure on checking the Crystals without a Crystal checker, as I have have problems with those $13.00 Crystal checkers, throws me off and then I am stuck again at the bottom of the Barrel looking for a way out. I think I am getting ready for a change of pace, as the radios I work on have the hardest and strangest problems I have ever seen, being new at this, trying to learn, and I get all the cluster -----, but as long as I have some guidance, I am able to learn and not loose any sleep over this issue. New at this, nevewr had a frequency go wacky like this. OK, can I please get some help with how to start tp diagnose this cluster, cause I don't want to be out in the shack till 4.00AM any more with these headache radios I get stuck with. And I don't want to tell my Buddy to take it to a repair shop, as he is like us all, no money, and on a budget, not wise to spend money on a issue with this only being a hobby, and not a necessity. OK,I am out of here, lets get this done and off my bench so I can enjoy working on these radios, instead of a major issue.
I may start with replacing the Crystal, and the 10.695 Crystal is a 3 pin, so I guess it is considered a filter as it has 3 legs, lets get this done, and this may be a good reference for any one who has the same issue down the line. OK,done rambling, lets get to work my friends. Update when I get a bit of guidance,but for now, I shall do some reading in the CB repair books.

 

[sp5it]

Where do you connect freq meter?

 

[Mark A Bradley]

I connect tp TP24,TP5,,and TP3,,depending on what you are doing,, mostly to TP3 and TP5,for the TX frequency, then TP2 for the 10.240. I think that should do it. There is a TP4 which is R6,it is marked on somer29's. Still going goofy, I checked all the Diodes, and I tried a receive alignment, seemed to get some receive, but there is a buzzing noise, it seems to be coming out of L7 and L8,,you can really get it going when you put your non metallic screwdriver on it to adjust, and also doing a voltage check on the Transistors, the buzzing is really loud on TR8,TR9,TR10. This is all the things I have encounters, something different and odd compared to other 29's. If you have any other questions, I will check back every half hour to see what's up.

 

[Mark A Bradley]

OK,I replaced the main Brain,, Now, the D2816C the PLL,, now I have a 30.000 +- to 32.000 +-,OK, it is not going all over ,it is staying steady between those frequency's. Now I still cannot adjust the frequency, turn the pots, and still no movement. This is almost like the last disaster I worked on. I will post back when I make a discovery. It is 5.36 Central time, hope I can post before to long, or shall I say, I hope I fix it before to long.

 

[Mark A Bradley]

Where

Where do you connect freq meter?

Welcome aboard, I am still at it, making some progress, you can read my new post. Yesterday I install 2 mixers and 2 crystals,TA7310PA, no difference, and the Crystals, no difference. Today I tried a PLL, make a bit of a difference, just go down and the post will explain.Any ideas at this timeSP5IT. I am going to start checking voltages again at the transistors, and poke aroundatthe solder joints now.
do you connect freq meter?

 

[Mark A Bradley]

Quicky, both mixers and the PLL have the same readings as the frequency counter

 

[Mark A Bradley]

UPDATE !!!!!!! For some reason, I walked away, came back and now the radio is working????? I am wondering what happened, I have my L19 and L23 adjustment, and I have my shift also. Now I am going to align receive, I have transmit, but I will go over it,I did make a quick tune, but it needs more. Updatenin a few.

 

[Mark A Bradley]

UPDATE !!!!!!! For some reason, I walked away, came back and now the radio is working????? I am wondering what happened, I have my L19 and L23 adjustment, and I have my shift also. Now I am going to align receive, I have transmit, but I will go over it,I did make a quick tune, but it needs more. Updatenin a few.

OK, now I can see that L22 and L23 are not tuning at all, time for the schematics

 

[freecell]

Service Manual For 29LTD:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm.pdf

the problem appears to be that the tx and rcve mixers are operating intermittently. that would indicate a circuit fault in the area of the (vco) crystal oscillator and the mixer inputs, causing the pll to fall into and out of lock. i've seen this before and it's usually due to a cold, dry or fractured solder connection which, if this is the case, will cause intermittent operation when the board is lightly stressed (top or bottom) in the area of the 10.240 crystal. see if any of that fits your sense experience, since i have no idea what you have in the way of test equipment. stop turning stuff.

 

[Mark A Bradley]

Service Manual For 29LTD:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd_sm.pdf

the problem appears to be that the tx and rcve mixers are operating intermittently. that would indicate a circuit fault in the area of the (vco) crystal oscillator and the mixer inputs, causing the pll to fall into and out of lock. i've seen this before and it's usually due to a cold, dry or fractured solder connection which, if this is the case, will cause intermittent operation when the board is lightly stressed (top or bottom) in the area of the 10.240 crystal. see if any of that fits your sense experience, since i have no idea what you have in the way of test equipment. stop turning stuff.

I have everything you need, Oscilloscope,100MHz, Wave Generator, 3 frequency counters, Watt SWR Mod meter, I have enough equipment to do what we need to do. I am a 1 year learner, lots of reading, hands on, learning on my own. I am a little behind on Injecting, but I am in the middle of learning, reading first, then try hands on. All I have to say about the radio, it started again, I think it was a fluke that the frequency went back to normal, but not for about 2 minutes, enough time for me to post it was working, then it started again.
I have have noise and white sound, but there is the buzzing with the cans and the transistors that I don't understand. And I am not playing with the cans did a quick receive align to see if I could get noise, and boy do I have noise, I did put in a 3 leg 10.695MHz filter, slash Crystal in a,so, no difference, and no difference with the 10.240 MHz crystal,,all I posted is where I am at now. Thanks for the return message.

 

[nomadradio]

One recurring headache with this kind of radio occurs with a particular slug-tuned RF transformer in a 10mm 'can'. Schematic callout is L22. Has a small ferrite tuning slug.

A 'scope on pin 22 of the PLL chip will show the input signal to the PLL chip's divider. A volt or two peak-to-peak more or less. The slug in L22 gets adjusted for max amplitude at pin 22. Other methods that connect directly to L22 can disrupt the adjustment when you unhook a probe from it. Doing it this way isolates the 'scope probe from the tuned circuit being peaked.

Might not be the root of your mystery fault. But if you see the amplitude at pin 22 increase in a significant way after peaking L22, this could have been the problem.

73

 

[Handy Andy]

R85 is where you "tap" to adjust L22 to peak.

​

Understand too, that not all the latest radios from Cobra have used the TA7310 - they're using YOUDA knockoffs which is why there is a loop wire versus a coil - they are narrowing the resonance region of the loop between L22 and the Varactor - hence the inductive effects of that jumper wire which is a specific-length and AWG of stranded gauge of wire.

You may also have "wax build up" so bad, that you get intermittent problems with contacts - even the Resistor leads themselves.

I've have fixed tune ups cause by BROKEN Resistors -that someone tried to unbury to get a good signal off of only to crack the resistors is TWO trying to obtain it. They pulled the resistor apart using the test leads to try and get something off of the Test point bare lead it's for.

 

[Handy Andy]

Per your 29 board...

​

For the "Varactor" you'll need to flip the board upside down and look for two SMD parts soldered to the VARACTOR symbols silkscreened on the top (component) side of the board ...

Make sure you don't have "Wax Creep" where it
can affect the ability of these components to
stay connected to the foil traces.​

 

[baconater4749]

Per your 29 board...

View attachment 46271​

For the "Varactor" you'll need to flip the board upside down and look for two SMD parts soldered to the VARACTOR symbols silkscreened on the top (component) side of the board ...Hey, how you doing, I may have a few SMD Varactors, rhese were Givin to me on a Transistor Radio Recap, a couple on Caps. Have to tell you, replacing the Caps made a big difference on a little Transistor Radio. I remember I had one when I was small, I put the radio in the water of my swimming pool, put my ear in the water, and wanted to see if I could hear it playing, well, do you think I heard any music from the Transistor Radio under water, FYI, I still think it was water proof, LMFAO. Boy, did I get the belt on my rear end, Father was a bit angry. OK, I am going to look for the Varactors, this may have something to do with the tuning problem, but no conclusions yet. OK, update in a short, only 7:40PM now, Central time,, Hoosier Time.. Not only corn in Indiana, lots of Marijuana though. Enough of that, but it is easy to grow in your back yard, in between the Tomatoes. This is so the Helicopter that buzzes the houses in early fall don't see it. If it gets really tall, you need a netting over it, military design. Going to search for the Varactors, and go from there, going to check cold solder joints, check my Crystal solder and the 10.695 filter also. Going to go.... Thanks for the info, the Varactors spot is new to me, making a note in my Journal, about the Varactors position on a 29. Out of here, dinner is also ready. Later

View attachment 46270
Make sure you don't have "Wax Creep" where it
can affect the ability of these components to
stay connected to the foil traces.​

by

 

[Handy Andy]

- I saw what you did!