Cobra 139XLR clarifier lock....not quite on LSB

[Lkaskel]

Hi Everyone,

I used the directions on here to lock the clarifier on a 139XLR.
1) Remove D30
2) Remove R117,
3) Solder the PURPLE wire to an 8 VDC source.
5) Solder the BROWN wire to GROUND.

The purple wire is on a constant 7.82vdc point. AM and USB align perfectly with the clarifier in 12 o'clock. LSB will only get to 27.183200 and drops at that peak. If I turn the clarifier counter clockwise to about 11:30 it is at 27.184.

Any suggestions?

Thanks as always!

 

[sp5it]

Retune LSB can(s)

 

[Lkaskel]

Retune LSB can(s)

Ummmm. I did that. C5 will not get me to 27.184...

 

[999]

My guess would be that the capacitor in the can has gone south and you need a new can for LSB.

 

[Lkaskel]

This is a Uniden 858 board. No LSB specific can. Only 2 varicaps for LSB.

 

[tramd]

Sounds like the LSB crystal is out of tolerance and the trimmer can't bring it onto the correct frequency. I've seen that quite a few times in those 858 PLL radios. They are 45 or 46 years old now.

 

[DoctorAudio]

I believe the LSB/USB/AM crystals are in sockets. Swap X3 and X4 crystal (LSB and USB) and see what happens. If the LSB crystal is bad/off frequency the off center frequency problem will switch to USB mode. If the off center condition stays on the LSB side, the crystal is fine and there is something wrong in the LSB amp/oscillator circuit. (TR38/D43)

 

[Lkaskel]

I believe the LSB/USB/AM crystals are in sockets. Swap X3 and X4 crystal (LSB and USB) and see what happens. If the LSB crystal is bad/off frequency the off center frequency problem will switch to USB mode. If the off center condition stays on the LSB side, the crystal is fine and there is something wrong in the LSB amp/oscillator circuit. (TR38)

Here is the thing Doc. It will align fine when the clarifier is factory. When I lock it with the above method AM and USB align fine. LSB will only get to within 800Hz.

 

[DoctorAudio]

I wonder if your 8 volt source is sagging just a hair (.01 or .02 volts) only on the LSB setting.

 

[Lkaskel]

I wonder if your 8 volt source is sagging just a hair (.01 or .02 volts) only on the LSB setting.

Interesting thought Doc. I'll need to check that!!!

 

[DoctorAudio]

If the 8 volt source is zener regulated with a 1 watt zener and a series current limiting resistor (real common in CB's) they can start sagging when they get close to the limit. When you switch from AM/USB/LSB, there is a varying load on the 5 and 8 volt sources as the different circuits are switched in and out. The 8 volt source you chose might be sagging just a hair which in normal operation may not be a problem for the normal circuits it's feeding but for a sensitive clarifier circuit, a .01V drop could cause a problem. It's worth a check.

 

[DoctorAudio]

Which constant source are you using, D25?

 

[Lkaskel]

Which constant source are you using, D25?

That's correct. The 7.82 vdc source. The voltage is at 7.923 and only drops .002 on xmit. I did rasise the voltage on the PS from 13.84 to 14.01 and it has no difference. Man this is strange. Also, the frequency as measured at TP8 does not change from rcv to xmt. It is stil labout 500Hz low on LSB unless I move the clarifier. Man this is one has me puzzled......

 

[DoctorAudio]

It is a puzzling one...
You could blank out the word "Cobra" on the front and change it to "Galaxy", at least then it would have an excuse to having a strange frequency problem that's hard to find...

 

[brandon7861]

Do you have a block diagram?
Edit: nm, dont need it, sorry