Cobra Mods


This page describes modifications for the Cobra 148 GTL (PC-412)


The PCB in this radio is almost identical to the Grant (PC-409) and the Madison (PC-411) and it can be used for comparisons


This set is much the same as the Grant/Madison/Washington, as it uses the MB8719 PLL so adding another 40 channels is easy !


Frequency conversion by reprogramming the PLL pins


Pull pin 10 of the PLL low to go up 64 channels and start another band.

All you have to do is use one of the switches (the tone switch is best) and re-wire it so that it controls pin 10 of the PLL, one side of the tone switch currently goes to earth (should be the common pin), the other side goes to C77 which is shunted across the audio path to cut off the high frequencies to reduce the tone, all you need to do is cut the wire that goes to C77 and run from the switch to pin 10 instead, when the switch is in the low tone position it will connect pin 10 with earth, which will make the radio go up be 640kHz (64 channels).

Re tune if needed


Frequency conversions by crystal changes


This mod came from a forum I am a moderator on:

(You need to change the crystal at location X3)


Quote
" 11.6908MHz gives 28.060MHz to 28.500Mhz (Ham 10 Meter) operation in Cobra/Uniden CB's that originally had 11.3258MHz in them "
You really just need 28.300 to 28.500 for voice, the lower part of the band is CW.

You can use a 11.4758MHz to get the high band, or a 11.1758MHz to get the low band instead if that is what you need.

You can get new crystals from Kens Electronics:

http://www.kenselectronics.com/lists/cbxtal.htm


Modulation Boosts



1. DONT Remove TR24!

There are two methods to improve the AMC, you can change R130 (a 1.5KOhm for a 500 Ohm, it may be in a diode location close to TR26)

Or you can add a 1K resistor in series on the the middle leg of TR24, to do this just cut the track for the leg and put the resistor across the cut.
2. Change R126 (10k) to a 4.7K
3. Change R124 (10k) to a 4.7K
4. Re-tune L37 and L38 for maximum AM forward power.


Unlocking the clarifier


This mod came from a forum I am a moderator on:


This is what I have:
1 Cut R44
2 Cut D52
3 Solder a jumper wire across R 174 (or remove it and put a jumper in its place)
4 Remove red wire that comes from the Fine tune pot from the board and hook it to the + side of C84
5 Change R175 to a 1KOhm

Jeff


Dual Clarifier Mod


This is the dual clarifier mod, I do not know who originaly wrote this, as it was posted on my forum.


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