Cobra 29 off Freq.

[longhaireddwb]

I have a Cobra 29 Philly model and on channels 1-7 it is fine but when on channels 8-40 its off freq by a lot! Like channel 19 is 26.9378 and channel 31 is 26.9275 Very odd!!!
I'm guessing this is an issue with the PLL chip. Can someone else give me an opinion if I'm going in the right direction with this? The chip is a D2816C and I don't have any of those. I have 6-8 PLL chips from other radios but don't know if they would work in its place. To start with the D2816C is a bit wider.

Please chime in with some opinions. Thanks.

Jerry

 

[Robb]

Check the pll voltage, ref osc xtal freq, and the ch switch itself.
I'd start there.

 

[longhaireddwb]

Check the pll voltage, ref osc xtal freq, and the ch switch itself.
I'd start there.

REF osc xtal (if this is X1) then it is at 10.240.
I've rotated the hell out of the switch and that didn't change anything.
Not sure how to test the switch.
As far as the IC1 chip (D2816C) voltages according to the book are off in a few places.
pin 4 should be 4.9 but seeing 0.5
5 4.9 0.5
10 1.9 1.4
17 1.8 2.1
18 1.8 2.1
19 2.6 1.7

These reading are in TX mode.

 

[longhaireddwb]

REF osc xtal (if this is X1) then it is at 10.240.
I've rotated the hell out of the switch and that didn't change anything.
Not sure how to test the switch.
As far as the IC1 chip (D2816C) voltages according to the book are off in a few places.
pin 4 should be 4.9 but seeing 0.5
5 4.9 0.5
10 1.9 1.4
17 1.8 2.1
18 1.8 2.1
19 2.6 1.7

These reading are in TX mode.

OK. Its not ic1, ic2, or ic3. I swapped parts from one radio to another and visa versa and the working one still works and the one off freq is still off freq.

Looking at the schematics it looked to me that ic2 is where pins 17, 18 and 19 get there voltages.

This all started when i installed a rfx75. So maybe the rfx75 is stealing voltage from somewhere and causing the issue.
Ill keep the world informed. But...

How can i test the channel selector?

 

[Robb]

Not sure how to test the switch.

Wiggle it when TXing and see if the freq changes - for one

 

[longhaireddwb]

Wiggle it when TXing and see if the freq changes - for one

well, i thought of that. i was hoping you had another way to do it, .
I did find out what it was, "RFI". All I had to do was move the wires/mini coax's and everything was good. After all of that it was the simple thing! Go figure. Live and learn, Check the little things first!

Thanks for the input Rob. Nice to get some help.
NOW, GET THIS,
After all of this I can now take the RFX75 out of the radio and put it back to normal. I was doing this as a test for a friend. He wanted to know if you could get high power out of a CB and was wondering about these RFX things. Now that he knows its possible (and has one) he can go about installing it into another radio. Not my old cobra. I know, This was a big waste of time but he paid for the parts and the BEER! I told home in the beginning I wasn't going to install it and leave it as that's just not cool if you know what I mean. He can go home and try and play all he want too now he knows it can be done by a non-professional! I'm not having anything to do with it myself. Not my cup of tea.

Should take me another hour and two more beers and all BETTER be good to go and back to normal .
Thanks again Rob for the help.