Cobra 2000gtl ssb problem.

[King Mudduck]

My 2000 swings to around 20 watts on usb but only to 7 watts on lsb. I dont understand why its so weak on lsb, i dont expect a power house but looks to me like power should be the same on usb and lsb. I cant find any service information on the net for this radio which really surprises me since these radios are so popular so i dont know where to start looking. My guess is that its a little out of tune. Can anyone point me in the right direction?

 

[Moleculo]

It just needs an alignment. If you have the equipment to do it yourself, just look for the procedures for the 148gtl. It's the same radio.

 

[King Mudduck]

Update.....I know i should not be messing with this but i found imporvment by adjusting L-31. Now there is no defference between uper side band or lower side band.

 

[Moleculo]

Do you have the ability to do a full PLL alignment?

 

[King Mudduck]

Well, 148 gtl you say.......

 

[bigred222]

that was one of the weirdest problems i had ever heard. Sounds like you got it all fixed up though.
73's enjoy that skip tonight while its around i know i hear everyone on the east all the way out to the "smokey hills" bombing into my recieve

 

[Moleculo]

It's not that wierd of a problem. The ssb offsets or mixing frequencies get out of whack when someone with a screwdriver doesn't know what they're turning, and then you get a problem like this.

Now you might have full power, but is it on frequency? You really shouldn't be playing with this if you don't have the gear to do the full alignment.

 

[AudioShockwav]

L-31 is in adjusted for 7.8Mhz at test point TP 14 in AM mode, Ch 19
L-30 Is adjusted for 7.7985Mhz at test point TP 14 in LSB mode, Ch 19
CT 2 is adjusted for 7.8015 Mhz at test point TP 14 in USB mode, Ch 19
Mole is 100% correct, you need more than a watt meter to do the work.

Have a look here:

(Covers 148GTL, 148GTL ST, 148 NW ST) (dated 1997 1998)



http://cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf

Thanks to Bennie of CBTricks.com for the info.

73
Jeff

 

[loosecannon]

dont forget that L31 should be adjusted in TX mode!

also, am i mistaken?
i thought that L30 and CT2 were adjusted with the freq. counter at TP3.

i do remember having trouble sometimes getting a reading at TP3, so maybe i should try TP14?

hmmm...

thirded; the IF cans are not to be messed with unless you have an accurate freq. counter. (the kind with a BNC on the front)

later,
LC

 

[ElectrikTec]

If the problem is on both USB and LSB here is what I would suspect. It could be the carrier OSC. out of tune as mentioned in a previous post by Molecule, Or there could be another problem. On AM the Final and Driver are "turned down" to about 6 volts or so (depending on what the carrier is set at) And on SSB there is supposed to be 13 volts on both the final and driver, If there isn't 13 volts on SSB, then that is where your problem is. I do not remember what transistor number it is, but there is a 2sc945 that will go bad near the current regulater <-(usually a 2sc1419). Hope this Helps

 

[Moleculo]

the IF cans are not to be messed with unless you have an accurate freq. counter. (the kind with a BNC on the front)

Yeah, you need a freq counter with sensitivity of about 45mV or so (going off the top of my head here), and of course the proper test leads.

The last time I tried to do a PLL alignment for a 148 using that info on CBTricks, it didn't work out for me. Maybe I just missed something.

Here's the complete PLL sequence I use now, which has worked flawlessly:


Input of Freq. Counter To TP-13 (IC-1 Pin 8)
CH-19 - Check for 10.240Mhz.

Input of Scope To TP-10
CH-19 AM - Adjust L-21 for Maximum RF (3-4vPP Typical)

Input of DC Meter To TP-9
CH-40 AM
Adjust L-19 for 3.2 Volts DC - 2.2 Volts DC on CH-1

Input of Scope To TP-1
CH-19 AM - Adjust L-20 for Max RF (190 MvPP Typical)

Input of Frequency Counter To TP-1
CH-1 AM - Adjust L-23 for 34.7650 Mhz. +/- 20 Hz.
CH-1 USB - Adjust L-59 for 34.7665 Mhz. +/- 20 Hz.
CH-1 LSB - Adjust L-22 for 34.7635 Mhz. +/- 20 Hz.

Input of Frequency Counter To TP-10
CH-1 AM - Check for .790Mhz.

Input of Frequency Counter To TP-3
CH-19 USB - Adjust CT-2 for 7.8015 Mhz. +5 Hz. -0 Hz.
CH-19 LSB - Adjust L-30 for 7.7985 Mhz. +0 Hz. -5 Hz.
CH-19 AM XMT - Disconnect TP-7, TP-8 and transmit, Adjust L-31 for 7.8000 Mhz. +/- 5 Hz.

Input of Frequency Counter To TP-15 (FET-1 Gate 1)
CH-19 AM - Check for 7.3450Mhz.
Input of Frequency Counter To Antenna Input
CH-1 AM - Adjust VR-5 for 26.965Mhz.