• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Low channel mod for UPD-2816


Jul 20, 2005

I was wondering if some body could post the mod for low
channels in a PC-76XL without xtals just by working the

Thank You!


The only "non-crystal" mod I've ever heard of won't move you down 45 channels, it moves you down 45-and-a-half channels. This trick involves taking the transmit/receive pin (#9) on the 2816 loose, and "inverting" the signal that feeds into it. Takes a DPDT switch, a NPN transistor like a PN2222 and three resistors.

What this does is to change the PLL chip to "receive" frequency when you key the mike and to "transmit" frequency while receiving. So they say. Kinda like the old days, when a CB had two crystals for each channel, one for transmit and one for the receiver, each for one channel. Some of those radios, like the Johnson Messenger would go below channel 1 by putting the transmit crystal in the receiver socket, and the receiver crystal in the transmit socket. Seems to me the KKK made this trick famous, to keep their communications "private".

Never have tried the "invert pin 9" trick. The additional stuff you have to do to shift the radio one half of a channel, and get it on frequency looks like a pain. By the time you do all that, the 'crystal' kit looks attractive in comparison.

The reason you can't just use switches on the input pins to the chip is that the 2816 has in internal "ROM", or Read-Only Memory. The actual binary code that sets the channel frequency is INSIDE that part of the chip where you can't change it. All the switch wires do is to select which "legal" binary code gets fed into the actual PLL circuit. Changing the wires from the channel switch only gets you a different legal channel.

The "crystal" kit used for some SSB radios won't cut the mustard with this radio. A kit for the 2816 must contain the whole oscillator circuit, along with the crystal, one for each added band. The output from a "A", "A+", or "N" kit overrides the mixing signal that normally sets the PLL to a legal-only channel frequency. A crystal alone can't do this.

This chip was meant to be 'channel proof', but turned out only to be 'channel-resistant'. Newer PLL chip types are even tougher to convert. Cheaper AM-only CB radios truly are "channel-proof", unless you spend twice the radio's sale price to convert it. By the time you spend that much, you can afford a radio that comes with more than three sets of 40 channels. Mods that cost twice what the radio did just aren't very popular.

My Cobra 1000 has kinda that mod. Where a 10.695 oscillator replaced the 10.24 on TX, a "slideable" 5mhz oscillator was used in place of the 15.36. It works great but the drawback is the channels don't line up with the channel selector but a freq counter can be added like one of those 11 dollar jobs from eBay. Another plus is the A channels are available on the "slide". I get 26.595 to 27.515. It's a trip to see a Cobra 1000 on 27.095. I'm not sure but I think the lescom mod don't hit A channels.

Help Users
  • No one is chatting at the moment.