Cobra 29LTD classic help no tx/rec

[dss56]

Hi all

I have a Cobra 29LTD and it has no transmit and receive.
There is great volume up and down
no receive or transmit

I checked the pll with a known good one and its ok.
I do have 10.240 at the vco.

I put a radio next to it and 29 will not transmit any signal to the other radio.
And other radio will not transmit a signal the the 29 no movement on radio.

Again the audio in radio is fine just dead signal and receive.

thanks any suggestions

 

[dss56]

I just tried a possible fix as Its in the vco circuit I'm sure.

I have an oscillator board and I injected a signal into C87 the leg towards the front of radio and I was able to get the radio back to transmit and receive.
So its in the oscillator and I just cant seem to pinpoint it. I replaced the electro caps in that area and checked a few ceramic caps nothing.

Any suggestions

thanks

 

[nomadradio]

So, which is it?

"Great volume up or down"?

Or "No receive"?

How do you get both great volume and no receive?

Or does this mean you have a noise level in the receiver, but it won't tune in the channel you select?

Do you have a meter to check the radio's VCO tuning voltage? If the DC voltage at test point 1 stays the same no matter where you set the channel knob, the PLL is not locked.

The symptoms sound like the PLL is out of lock. First place to check is pin 11 on the 2816 PLL chip. Should be 5 Volts DC. The zener diode D19 is probably shorted if you show about zero Volts there. If D19 is shorted, a new 5-Volt zener might bring it back to life.

This radio was in production for a long time, and not every version is the same on the inside. A whole list of diagrams for various 29LTD radios is here: http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/index.htm

My favorite is this one: http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29_ltd_classic_sch.pdf

And if your radio is newer, there will be differences.

But it really sounds as if the PLL is not locked onto the channel frequency. When that happens, the transmit side shuts down.

73

 

[dss56]

I have the version with no D19 and I do have 4.9v at pin 11.
As my second post says I was able to use an oscillator board and inject it to the leg or C87 and the radio works transmit and receive. If i remove the rf wire to C87 radio just has volume and no transmit or receive.

Seems I'm missing 4.9 v on pin 15 and 16 on the pll. I did check the 4 electro caps in that area but all were good.

 

[dss56]

In picture red arrow if i inject a expo 100 a kit there the radio will work I remove and nothing just volume with no receive or transmit.

 

[nomadradio]

Whups!

Cross-posted. Typed my reply to your first post while you were updating it.

C87 feeds 15.36 MHz from L22 into pin 4 of IC2. L22 has a 5.12 MHz square wave from pin 10 of the PLL feeding into it. L22 tunes the third harmonic of 5.12, feeds it into C87.

Lost track of how many times I have seen L22 go bad. There is a capacitor inside L22 that tunes it to 15.36 MHz. Seems to be the most-common fault when L22 goes bad. No way to take L22 apart to change the bad cap. One trick that sometimes works is to put a 40pf or 50 pf trimmer cap on the solder side of the pc board can take its place in the circuit. One side of L22 has two pins, the other side has three. Pretty sure you put the trimmer cap across the two outside pins on the 3-pin side.

Of course, now you have to peak the slug in L22 and the trimmer cap both for max 15.36 MHz output from L22 to make the PLL lock properly.

Easiest place to see this peak is on pin 22 of the 2816 PLL chip. Only way I know is to probe pin 22 with a 'scope. Don't have a procedure to do this without one.

73

 

[dss56]

I will give that a try and let you know about the trimmer cap. I guess I would never see the bad cap to check in L22.

thanks for the info. If it does not do the job Ill just leave the expo 100 A kit in radio.

 

[nomadradio]

And if you want the normal channels 1-40 back you can always substitute a 15.36 MHz crystal for one of the crystals in the Expo kit.

Barkett should sell the one used in some Galaxy radios.

73

 

[dss56]

I have all the xtals I need. I just tried the trimmer cap and I did get some transmission output I measured the 2 legged side and it was 3ohm the 3 legged side nothing so i tried a vari cap across the 2 legs you mentioned and nothing.
I tried a 10ohm res closest to the 3 ohm reading and that is what gave me about 4 watts out and all the channels. Only thing was the audio out was all static. So I think L22 is bad and I will just leave the 3 xtal expo board in the radio. It has 40 up 40 reg and 40 down when toggle is moved and just by pass L22.

Pictured is a Expo 100 A kit which I sell and make also other Expo kits.

I just gave it a try as something in the oscillator/vco area was bad.

I guess I could use 1 of these as a test for radios with bad vco and inject the signal into the radio and see if it works as I did here.

Thanks for some tips.

1 thing on the schematic is r92 I get 5 v on 1 side and nothing on the other side. I replaced and same with new resistor.

Well again i will jest keep the channel kit in radio as it will work since I dont have a L22 replacement.

 

[NZ8N]

View attachment 28101

Pictured is a Expo 100 A kit which I sell and make also other Expo kits.

Looks like the picture from your Dual S Kit or did you start making your A kits with the same board too? That is a lot cleaner looking kit than the copies of the original Expo kits.

 

[dss56]

I have both kits the 3 xtal or 2 xtal