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Low channel mod for UPD-2816
- Thread starter sus
- Start date
Hey there Sus, I don't know if this helps or not, but I've had this link tagged in my favorites for a while now.......Scroll down to your chip. It does have some data on your chip, but I believe it's through crystal conversions......It's a good reference for all kinds of PLL chips.......73's 
http://www.cbsite.nl/modif/onderdelen/pll.html
http://www.cbsite.nl/modif/onderdelen/pll.html
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.
73
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.
73
I know that it's possible I've seen it before it only produced loweres whish I'd copied it when I had thr radio here.
Did you see the radio actually produce this result?
Or just see it on the internet somewhere?
Inquiring minds want to know.
73
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.
Sounds pretty cool. Are you going to post the procedure here? I would love to try it out.
Went and looked at the FB group, only see for sale ads.......
He has been banned and his posts removed for Spam.
73
Jeff
He has been banned and his posts removed for Spam.
73
Jeff
Last edited:
It's a spammer begging for subscribers.
@Giffin put schematic with description here or don't bother us.
@Giffin put schematic with description here or don't bother us.
I wonder if this type of mod can also get to the "A" channels with out destroying the 455khz or drifting all over the place??
I hope the OP posts how you do the mod. I fear, without trying it, that this mod is roughly like trying to pick up a turd by the clean end.
I've got a fresh used 29 just waiting for some mods.
I hope the OP posts how you do the mod. I fear, without trying it, that this mod is roughly like trying to pick up a turd by the clean end.
I've got a fresh used 29 just waiting for some mods.
Many decades ago, when a CB needed two crystals for each channel the way to make a CB radio "stealthy" was to swap the receiver and transmit crystal. Only works on the radios that need two crystals for each channel you have. Only worked right in radios with a 455 kHz IF frequency in the receiver. This had the effect of moving your transmit frequency down 45 and a half channels. The receiver would realign okay to hear down there. Nobody listened below channel 1 in the sixties, so it was a private channel.
This trick is similar, and involves inverting the Transmit/Receive input to the PLL chip. The 2816 PLL jumps up in frequency when you transmit, by a factor of 455 kHz. Reversing this input to the PLL has the same effect as swapping the crystals in your Johnson Messenger I radio. And for pretty much the same reason.
Because you jumped 45 and a half channels, the 10.24 crystal gets tweaked to drop the radio 5 kHz when using the modified lower channels. Or you could just use the "zeros" and leave the crystal alone.
73
This trick is similar, and involves inverting the Transmit/Receive input to the PLL chip. The 2816 PLL jumps up in frequency when you transmit, by a factor of 455 kHz. Reversing this input to the PLL has the same effect as swapping the crystals in your Johnson Messenger I radio. And for pretty much the same reason.
Because you jumped 45 and a half channels, the 10.24 crystal gets tweaked to drop the radio 5 kHz when using the modified lower channels. Or you could just use the "zeros" and leave the crystal alone.
73
Thanks for the info Technicolo. I checked out the site and I think I'm going to get one to try out.