Cobra 148 GTL Receives no Transmit. All checks Good?

[Low_Boy]

I am sort of running around in Circles. Driver, Final, Predriver all check good. All voltages at the PLL are good and 10.240 on pin 8. VCO block all voltages are good. Mixer all voltages are good. But my highest Freq. is I believe 34.XXX MHz? I do not understand that one. Transmit Inhibitor. switch is good. I see no Freq. at the final, driver or pre driver but all voltages are correct there. also the voltage regulator has all correct voltages and the AM regulator also. I checked a few Caps and what I checked were good. I will change them eventually once I find the problem. No one has ever been in this radio before. and the PA works and I can hear a change in tone on a test radio sitting next to the 148. What am I missing?
Edit. Forgot to mention the RX/TX light does turn from red to green.

 

[Tokin]

This may sound dumb but i have to ask, how many mics have you tried?

 

[Low_Boy]

2 and it does change the RX light to red when keyed.

 

[kaos513]

Audio chip maybe?

 

[nomadradio]

Time to start sniffing. If there isn't even enough 27 MHz energy coming out of the transmit-mixer chip to hear in another radio with the tip of a coax jumper's far end waved near the chip, time to see what is or isn't feeding into it. Either the carrier isn't getting there, or the PLL output isn't.

And if you CAN hear a transmit signal this way, that probably pins down the failure between the output of that chip and the antenna socket. L46, L47 and L48 are vulnerable to moisture or high humidity, causing the internal capacitor to croak. If any of those appears to show a peak with the end of the slug dead-even with the rim of the hole, suspect this.

The mixer chip is my favorite spot to settle the question of whether I should be looking upstream from it or downstream for the problem.

73

 

[Low_Boy]

I sort of figured it will put out power when it finally gets a signal. The mentioned coils where adjusted while keying the mic and no change. I will do some sniffing and post my findings. Thanks Nomad

 

[Low_Boy]

I retested this several ways. There may have been a misunderstanding. There is a signal coming in to the test radio when the 148 is keyed. I can get the sound to change by adjusting L46 L47 and L48. I can get the signal to decrease.

Next I took the end of the jumper from the test radio and put it near the mixer and L46 L47 and L48. I did not notice any difference.

I then tried the same thing with a VOM probe no difference.
Would it be wise to remove L46, L47 and L48 and check the internal Cap Even though I can get the signal to decrease by turning the slug? Like you said they go bad and I have removed one from another radio where the Cap was blown.
Thanks

 

[nomadradio]

How about the physical position of that "peak" on those three slugs?

If any of them has the slug dead even with the rim of the hole, that points to a failed cap inside that can.

73

 

[Low_Boy]

No all slugs get the loudest tone on the receive radio with the slug screwed down in a bit. None at the top or sticking out.

 

[Low_Boy]

I am getting someplace. I had a few minutes and compared the mixer with one on a known working radio. The mixer is a SO42P. I noticed on the working radio pin 2 shows the freq. of the set channel . Example Ch. 19, 2718.50. the non working radio shows no freq. on pin 2. I have to do a bit more research. I need to look at another data sheet. The one I looked at showed pin 2 to be supply voltage. I do not think that is right.

 

[Dmans]

Looks like Pin 5 is your 8.0v supply.
Pins 1, 4, 6, 9 & 14 are grounded.
Pin 2 is "output" (27Mhz ?)frequency to L46.



73
David

 

[Low_Boy]

I did not think the Sam's would help in this case but it did. I am seeing nothing from pin 1 through 9. Figuring L47 is the problem if I am looking at it correctly. Tomorrow I will pull L47 and see if the cap is bad.
Thank you.

 

[NODENS]

Have you checked both source oscillators for the mixer? Its clear that you tested VCO seeing that is generating the frecuency needed. Did you checked the Carrier Oscillator (the second one and only used in TX when in AM mode) to see if the second necesary freq to be mixed with VCO one, is being generated? You need both frecuencies or you will have the issue that you are describing.



Greetings

 

[Low_Boy]

Have you checked both source oscillators for the mixer? Its clear that you tested VCO seeing that is generating the frecuency needed. Did you checked the Carrier Oscillator (the second one and only used in TX in AM mode) to see if the second necesary freq to be mixed with VCO one, is being generated? You need both frecuencies or you will have the issue that you are describing.



Greetings

I will have to go back and check my schematic and my work. I am not near the radio at the moment. But I am thinking that is something I may have missed. I will keep posting until it is working. Thank You.​

 

[Low_Boy]

The only two oscillators I see are TR32 and TR9. Both check good voltages and tested out of the radio. If pin 5 of the mixer is supply voltage I am only getting .5v on test radio and non working. It seems like the only thing i am missing on the non working radio is the pin 2 output. Everything else seems to check the same as the working radio. So l tried a different mixer. Unless l got another bad mixer. All is the same. I can not figure out what is being overlooked.