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Cobra 142GTL No Transmit or Receive

Alright! Defiantly getting somewhere now. I have 34.985000 on Channel 19 per the alignment directions with no drifting. From CH 1 to CH 40 are all within 10 Hz of where they should be.

20250613_1.jpg

I am having an issue with aligning CH 19 TX LSB to 34.982500 Mhz I have VR3 all the way to one side and it is reading 34.982065. I assume I need to change the resistance value of VR3?
 
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I will put a little gray matter towards it in just a bit. I am in the middle of cleaning the lash adjusters in my explorer and I want to get one side buttoned up before It gets dark out. Just came in for a quick bite to eat as my love cooked me a delicious grilled chicken sandwich. I'll be back in a little bit.
 
Could you get a voltage on all three legs of VR3 in LSB TX? Don't bother writing down which leg is which voltage, just get me the three voltages. It will be obvious which is which unless something is wrong.

When you do this, you will be transmitting, so I hope that you either have the radio on a dummy load or have the final jumpers unhooked. We don't want to cook finals sitting on the key trying to sort this out.
 
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CSA1863 said:
Voltage on VR3 5.35 | 1.29 | 6.54

Actually got the counter cheap at an estate sale. No deep pockets here LOL
Click to expand...
Lucky, scored a heck of a deal there!!!

Those voltages look like they are in the expected range for a good VR, so I don't think VR3 is bad.

Forgive me for asking, but I have to cross off the the usual causes before continuing. Was the clarifier centered when you did the alignment?

And is there any clarifier mods done to the radio? I need to know if we are working with the schematic circuit or someone else's circuit.
 
If VR3 has no effect, check the diode D36 between VR3's wiper and the varactor circuit. Makes it sound as if a previous mod to "lock" transmit and receive frequencies together was done. That would require disabling VR3.

If you want the radio's transmit and receive frequencies to always be different from each other, restoring VR3 and disabling the clarifier on transmit is the way to go.

73
 
nomadradio said:
If VR3 has no effect, check the diode D36 between VR3's wiper and the varactor circuit. Makes it sound as if a previous mod to "lock" transmit and receive frequencies together was done. That would require disabling VR3.

If you want the radio's transmit and receive frequencies to always be different from each other, restoring VR3 and disabling the clarifier on transmit is the way to go.

73
Click to expand...
I made the assumption that VR3 had an effect due to the fact he ended up on the high voltage end of the pot. But yea, I agree with you here. My next question was going to be the voltage at the varactor in RX and TX which would tell us about both diodes.
 
VR3 is powered by the transmit-only 8 Volt supply. Unless someone unhooks it. More-typical way is to lift one end of D36. VR3 takes the place of the front-panel clarifier while transmitting in the factory-stock setup of the radio.

73
 
Yes the clarifier was centered when I took the voltage readings. It was modded but I removed it and put everything back to stock the best I could tell from the schematic. I put R187 back , D36 was put back, I pulled the choke off the Varactor D37 and soldered the cathode end back to the board and put the wires from the clarifier back to R415 and R417 on the NB/ANL, CB/PA SWR/MOD switch board.

I will check the voltages at D37 tomorrow.

Thanks!
 
Since VR3 is good (based on the voltages you measured), and now knowing you put D36 back, I believe it is safe to say that something is still amiss in the the clarifier circuit leading to CT3 being at such an extreme during the RX alignment that the varactor cannot compensate via VR3 in TX.

The voltages will tell the story. In the meantime, I will look at the schematic and try to calculate what voltage should be at the varactor in RX with the clarifier centered. If the two numbers don't jive, we will have narrowed it down.
 
Should be about 3.26v at the cathode of D35/D37 in RX with the clarifier centered, assuming 8v on the RX rail.

And yea, I cheated, the diode complicated the math and I'm a few drinks in. And I forgot to label R3, that's R415
Screenshot from 2025-06-14 01-17-08.png
 
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Voltages for the diodes with clarifier centered
D35 Anode: 8.28 Cathode: 7.37
D36 Anode: 0 Cathode: 7.37
D37 Anode: 0 Cathode: 7.37

Obviously something isn't right. Verified I put a 1n4148 diode back at D36 and a 22 Ohm resistor back at R187. Is it possible I reversed the wires coming off the clarifier pot when I put them back?

20250614_130542.jpg
20250614_130603.jpg
 
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After you swap R187 for the right value, start back at the top of the alignment procedure as the RX frequencies will change. It is supposed to be 22k, not 22,

Edited to combine both posts into one.
 

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