I very much need the Robyn SB-540D channel mod confirmed with details PLEASE...

[Retro CB Guy]

Okay folks, I just had the mod done for upper channels on my Robyn 540D but it didn't quite turn out right. I get the range of channels 41-85 but on the upper extra channels, they won't line up properly on frequency... He was able to get much more range on going left on side of the clarifer dropping down to for instance (channel 1 readout = 41.4167) and it's like that all through the channels !!!!!!!! So I need help still please ! I need someone who's experienced this first hand and was able to solve this issue.

So being that far off I can still use those new upper channels but it is annoying to not be able to tune in folks, and they will have to tune to me as I can NOT get any further on my clarifer.... Sending a few pics !

SOS - SOS - ----------------------------------------- SOS - SOS

Somebody is bound to have experienced and knows how to solve it 100% on this model radio so please reach out to me.

Retro on stand by anxiously waiting 73's to all ~

 

[Retro CB Guy]

This is the readout for channel 41...

 

[Retro CB Guy]

Here's the PLL info from Lou Franklin's "The PLL Data Book"
http://robco.ath.cx/cb/PLLpinout.htm#LC7120

Here's the uPD858 "truth chart", which is the same as the LC710, N codes are the same (91-135)... and in many rigs or similar design the 858 was used. Two things to keep in mind:
1. The PLL (and channel selector/selection) are BCD, not straight binary
2. IIRC, the LC7120 only has 6 programming pins:
http://robco.ath.cx/cb/upd858truthchart.htm

Here is where I documented the channel mod for the uPD861... a SIMILAR chip. This will likely be the most helpful for making your switches, etc., and has truth charts, wiring diagrams, etc.
http://robco.ath.cx/cb/upd861truthchart.htm

Please reply to my conversation I started with you bro...

Retro

 

[Retro CB Guy]

After the upper channel expansion, the sideband doesn't line up as regular CB channels and the closest the clarifer my tech could get is 1.7mhz away which makes it next to impossible to use unless someone knows I'm there enough to always tune to me...

I FIND IT HARD TO BELIEVE THAT NO ONE KNOWS HOW TO CORRECT THIS ISSUE AND HAS A PROVEN CONFIRMED METHOD !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Please help with details, but note that my upper channels are half channels to start with, as that is how it turned out for some reason...........

I would appriciate some help ASAP please good folks, JIM (Retro)

 

[BBB]

I FIND IT HARD TO BELIEVE THAT NO ONE KNOWS HOW TO CORRECT THIS ISSUE AND HAS A PROVEN CONFIRMED METHOD !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Sheesh... I need, I need, I need. No need to shout.

 

[Retro CB Guy]

Sheesh... I need, I need, I need. No need to shout.

Yeah I hear ya BBB. Hopefully sometime soon someone will have a proven method to get me going on this issue, which would be so very awesome considering I already paid my veteran tech to do the job and it's still not right. Very annoying atm for me but I am very optimistic about it and life in general.

73's, Retro

 

[ExitThirteen]

Looks like there's two methods, changing the programming, and there is a possible use of using the T/R pin. Guess I don't see what the fuss is about.

If I had the radio in front of me, I could figure it out. Good luck.

~Cheers~

 

[unit_399]

If I had the radio in front of me, I could figure it out. Good luck.
~Cheers~

That's because you really are a "veteran tech."

- 399

 

[Retro CB Guy]

Looks like there's two methods, changing the programming, and there is a possible use of using the T/R pin. Guess I don't see what the fuss is about.

If I had the radio in front of me, I could figure it out. Good luck.

~Cheers~

The upper channels are from 41-84 and they are all on half channel or in between channels bro. As in first one is 27.4200 and he managed to get the clarifier to slide down the most he could to the left being 27.4167 and it's that way all through the upper channels, which make it near impossible to really use them in a good way...

I just need help in getting more slide to be able to get to center slot and be on frequency or maybe another method completely. He used that stuff from CB Tricks secret books there to get this result and figured out how to get the clarifer to slide alot more but NOT enough !

Please help, Retro 73's ExitThirteen

 

[ExitThirteen]

Sounds like the mod in the CB Secrets book on CB Tricks involves the T/R pin on the PLL, pin 8 in this case. Depending on the state of this pin, it can either move 455khz UP or 455khz DOWN. Whether it moves up or down is determined by either grounding this pin, or putting the PLL control voltage to it (in this case, I believe it's +5V). Most likely to get the 40 1/2 channels that you're getting, pin 8 is likely being grounded.

I just looked and couldn't find anything on this model on CB Tricks, other than a "10 meter amateur" mod.

If you're getting 27.420 - 27.860, what's got to happen is you need to slide back 5khz + to get to 27.415 - 27.855. Your tech needs to remove D631, and either install a "super diode" in its place or install a 5.6 microhenry (uH) choke inline with the existing diode on the cathode (banded) side. That should get you enough slide to get into 27.415 - 27.855, provided the rest of the clarifier circuit is unlocked correctly. Give this a try.

Keep in mind that once this is done, you'll have a different "center slot" on the normal 40 channels VS. the upper 40 channels.


~Cheers~

 

[ButtFuzz]

Oh, the inductance of it all...

 

[Retro CB Guy]

There is way too little great CB techs left around from the golden age lol... But I believe I'm getting somewhere now, I sure hope. I am meeting up with my tech tommorow with that new info ExitThirteen and Buttfuzz you're a good ole fella appriciate the input. Together we can get errrrrr DONE lmao !
Yeah my tech was on this forum some time back too and a well respected fella ...

Retro hanging loose for a bit and hopeful 73's my brothers !!!!!!

 

[Retro CB Guy]

Here she is currently very purdy just waiting to be finished up with her correct frequency range !!!

 

[Retro CB Guy]

** UPDATE **

After comming back from my local techs today and sharing ExitThirteen's info, I am happy to report that after some trial and error also that he was able to get my upper frequencys ranging from channel 41 to 85 to be within the clarifer range !!! So broadbanded nearly all of that to about channel 75 or so... So needless to say, I'm very happy and extremely pleased with the results I'm now getting !!!!!!

And as a few notes from this experience, he added a choke, varactor to move the frequency down 5 khz on the upper channels...

I'm giving a BIG shout out to everyone who tried to assist me in this endeavor and special thanks to ExitThirteen brother but everyone who gave their support also thanks for your support and time and that's what it's about a good network of CB folks here and some quite experienced techs and I want to thank each and everyone of you now !!!

He's documented this too, so for the next folks it will make the process even somewhat easier. But these rare radios I love alot of them and they sure don't make them like that anymore.

Retro signing off for now 73's ***

 

[ExitThirteen]

Sweet! Good Stuff!

~Cheers~