Channel Board For Cobra 146GTL & Uniden PC-122XL

The Cobra 146GTL and Uniden PC- 122XL are low-cost AM/S SB radios using a gPD2824 PLL chip. The Uniden PC-122XL is the same radio as the PC-122. They changed to the XL series when they went to the Cobra look front panels. Uniden also made the PRO-810e, a base radio with the same board as the 122. When this chassis initially came out, there was no known way to add extras.

This was viewed as just another challenge for the CB hackers, and after a short while, there were two methods of expanding coverage.

One uses two crystals that are switched into the more triple coil, and this yields 40 extras above and below the center off position at a regular 40, but it created a problem on the sideband. This scheme rendered the clarifier inoperative in the expanded positions.

A wire from the clarifier control to the expander kit was added to get some clarifier range, but it was limited, and all three modes had three different center slots. These kits also work on the Cobra 29-GTL and Uniden PC-76. These radios, being AM-only, work fine, and these kits are the preferred method of expansion.

The other kit doesn’t use crystals. It uses two ICs that generate a signal for the triple coil in the PLL circuit. The clarifier works the same in all positions of the expansion switch.

This makes for ease of operation. The only drawback is the total channel coverage is reduced. It doesn’t yield any lower frequencies, but it does make the radio cover up to 28.045 Mhz.

Warning: The modification of the radio in the U.S. voids the FCC-type acceptance and makes the equipment illegal! The installation of the circuit board is easy. The leads should be as short as possible. Locate the PC board as close to the 2824 PLL chip as possible. Either an SPDT center of a toggle switch or a three-position rotary switch can be used.

If you buy a kit, it comes with the toggle switch mounted to the PC board. It can be removed or used to secure the board and switch in one step. If you choose to build the PC board, you can use a multipurpose PC board such as Radio Shack #276-150 or #276-159. Position all the components as close together as possible.

Hand-wire all connections using short wires. Mount the board securely, and be sure that nothing metallic comes in contact with it, including the speaker on the bottom cover.

Once the PC board is installed and wired to the switch, four short wires need to be connected to the PLL circuit. First, run a wire from the connection with the ground symbol on the expansion PC board to the can of L15. Next, connect the lifted side of C67 to the (OUT) connection of the expansion board.

Connect IC2 pin 10 to the expansion board connection marked (IN). Then, connect IC2 pin 11 to the expansion board connection marked (+5 VOLTS).

To ensure complete VCO coverage, solder a 27pf capacitor across C72 on the backside of the radio circuit board. Make sure to lay it down flat and keep the leads short. Then double-check your work, looking for solder bridges and correct connections.

Next is the alignment procedure. Connect a wattmeter, frequency counter, and dummy load to the radio.

1. Put the radio in the AM mode on channel 1 with the expansion switch in the normal position and transmit. You should read 26.965 on the frequency counter. If the radio doesn’t transmit, there may be a wiring mistake somewhere or the VCO may need adjustment. Adding the capacitor across C72 does change the adjustment slightly, but not usually enough to lose channel 1. If everything looks OK then melt the wax in L38 and adjust it slowly in either direction no more than 3/4 of a turn. You should now transmit on channel 1.
2. Put the expansion switch in one of the upper channel selections and adjust L13 until you read either 27.285 or 27.605 on the frequency counter.
3. Put the expansion switch in the opposite expanded position, and you should read the other frequency listed above. You should read 27.285 in one position and 27.605 in the other position. If not, tweak L13 a little until you do.
4. Check to see that you still read 26.965 in the normal position.
5. Switch to the position that yields 27.605 and switch the channel selector to channel 40. The counter should read 28.045 if it doesn’t, melt the wax in L38 and slowly adjust it until you read the 28.045 on your counter.
6. Go back to the normal channel I and ensure that you read 26.965 on the counter. If everything goes well, you have a radio that goes up to 28.045. Of course, we know you will only receive these frequencies and never transmit above 27.405. Remember, 28.000 is the beginning of 10 Meters.

I’m told for the Ham operators out there that moving the wire on the expander from pin 8 to pin 9 on the 7493 IC and adjusting L13 and L38 will give you coverage in the novice portion. This article was supplied for information purposes, and I hope you enjoyed it.

Even if you don’t tinker with radio equipment, it’s always good to be aware of what’s available. Have fun!