Cobra 139XLR USB will not align...

[Lkaskel]

Hi Everyone,
I know that I have a few posts on here about various radios. One of them has parts ordered and one of them I am in the step by step process with it. Today I have a Cobra 139XLR that had some PLL mods (extra channels) as well as a strange clarifier mod. I removed the channel and clarifier mod and did the following clarifier mod https://grumpy.proboards.com/thread/7846/unlock-clarifier-uniden-upd858-chip. The radio will align perfectly on AM and LSB but USB will only get to 38.9877XXMHz. So, it only gets to about 300Hz when adjusting CT4. The correct cyrstals are in it and the radio has been re-capped. Any thoughts as to why USB would do this?

Thanks!

 

[doffo]

The clarifier mod I did on mine seems different that the one on the link you provided... Let me crack open my President Washington and take some pictures. I swore there was a wire you had to desolder and solder it to a ground. One moment...

 

[doffo]

Sorry for the delay. I got sidetracked.

So as you already did, I just desoldered, lifted one side of each. For both R119 and D30



I believe where I pointed at for the purple wire, I desoldered it and grounded it out, but follow the wire from your clarifier knob that when its fully turned down, its that wire. The middle wire is left alone on that knob, and then your right wire off the knob is where you solder to an 8v source. But just to be sure, look to see what wire is in that are where I have the arrow to. If its the brown wire, then that one needs to be grounded. You will know if its wired backwards is if you go fully clockwise, it slides lower vs higher. Just reverse the two and you will be good.

Didn't have issues after the fact. Maybe the mod you did is doing the same thing, but what seemed is when it had you jump that one part with a resistor, and that was it. But hope you get it sorted.

 

[Lkaskel]

There are still issues here but here is what I did. I restored the clarifier back to stock. I have a second radio so I comapred it to that radio AND the schematic to be sure that the resistors, diodes and the clarifier wires are correct. VR9 now works to adjust the transmit frequency. I also discovered that the crystal for USB is bad so I am forgetting about USB for now. In fact, the owner may be fine with that. I completed the PLL/Osc alignment again. Am and LSB will align fine. The challenge is when I adjust VR9 for the correct TX frequency on LSB (per the alignment instructions) AM TX is 3KHz high. On CH19 AM will TX at 27.188XXXMHz. LSB will be dead on. VR9 is adjusted almost in the middle of it so it seems like it should be right.

What would cause AM to be 3KHz high on TX? I did check the voltages on TR39 and TR34 as well as the voltage on each stage when you select AM/USB/LSB and they are all good.

Thanks for any input!!

 

[tramd]

AM transmit frequency is adjusted in AM mode. Try using this. The SAMS book states to connect a frequency counter to the antenna jack with radio on channel 1 AM transmit and adjust VR9 to 26.965 MHz. Makes sense right? If I remember correctly, the AM crystal only sets the AM receive. The USB crystal sets the USB and AM transmit, but AM will be slightly off-frequency. That is why VR9 is there to get AM transmit on frequency. When clarifier mods are done, VR9 is disabled so the AM transmit is only being controlled by the USB crystal and will not be on frequency when the clarifier is centered like it is for USB and LSB. Then usually the clarifier needs to be at the 12:30 position or a little more to get AM transmit on frequency.

 

[Lkaskel]

AM transmit frequency is adjusted in AM mode. Try using this. The SAMS book states to connect a frequency counter to the antenna jack with radio on channel 1 AM transmit and adjust VR9 to 26.965 MHz. Makes sense right? If I remember correctly, the AM crystal only sets the AM receive. The USB crystal sets the USB and AM transmit, but AM will be slightly off-frequency. That is why VR9 is there to get AM transmit on frequency. When clarifier mods are done, VR9 is disabled so the AM transmit is only being controlled by the USB crystal and will not be on frequency when the clarifier is centered like it is for USB and LSB. Then usually the clarifier needs to be at the 12:30 position or a little more to get AM transmit on frequency.

Very interesting!!!! So it is possible that because the USB crystal is bad that is causing the AM TX issues? I will swap in a good USB crystal and try that. Actually, based on some memories of alignment proceedures referencing USB and AM together that makes sense. Not that I understand how the radio is using the USB crystal for AM TX but I trust the logic!!!

Thanks!!

 

[Lkaskel]

Well, it's fixed..kind of Tramd was correct that the USB crystal was the issue. I was able to use a known good crystal and the radio aligned perfectly. Now, for the final repair, I need a 11.2858 crystal as the good one went back in my radio.

Thanks so much Tramd!!!

 

[brandon7861]

Well, it's fixed..kind of Tramd was correct that the USB crystal was the issue. I was able to use a known good crystal and the radio aligned perfectly. Now, for the final repair, I need a 11.2858 crystal as the good one went back in my radio.

Thanks so much Tramd!!!

Kens lists that crystal

 

[DTB Radio]

You can also use some capacitor trickery to nudge the frequency up or down. To nudge up, cut a trace to the crystal and insert a cap. To nudge down, place additional capacitance across the existing tuning cap (this will reduce the affect the tuning cap has, so some experimentation is in order). use rf-stable caps for best frequency stability.