Uniden Grant Help

[Yogi]

Ok guys I picked this mobile up at a flea market, It has the PC409 board with the MB8719 chip. it has 3 SPST switches mounted in the case, and the clairifier has been opened. here's the problem Switch one connects pin 11-12, switch 2 connects pin 10 to board ground, switch 3 connects pin 16 to ground. switch one gives me low channels starting at 26.815, and according to the cb tune up master edition volume2 pg 24 is correct but switch 2 does absolutly nothing. switch 3 is not shown but on certain channels gives me a 10kc jump. Me being a sideband user would really like to have the upper freqs everything is wired correctly, i checked it twice. all othe channels work and it gets great reports. any ideas on why it's not getting the upper frequecies. Thanks for any input.

 

[ExitThirteen]

Interesting that switch 2 (pin 10 to ground) doesn't do anything.... by itself, it's suppose to give you 27.605 (ch 1) to 28.045 (ch 40). If you use switches 1 & 2 together, that should give you 27.455 (ch 15) to 27.595 (ch 27). Sounds like a wire on switch 2 is messed up, or the switch itself could possibly be bad. Let us know how you get on.


BTW, the 3rd switch does give you "some" of the "alpha" or "RC" channels.

(edit) There is a possibility that the VCO will need to be adjusted slightly to lock in the upper channels. Look for L19 on the board, it will be just left of a crystal "pack" and directly behind the PLL chip. Might have to tune it a touch to get the uppers.


~Cheers~

 

[Yogi]

Ok, I rewired and ohmed out all the switches, changed the old wires with new wires (some cat5 cable i had extra). tried adjusting L19 but nothing.
It's like the switch does nothing at all, no frequency change at all. It remains on the same frequency with the switch on or off. Did they use different kinds (from different companies?) of MB8917 chips in these PC409 boards?

 

[ExitThirteen]

Really strange. There is a *slight* chance they used an MB8719 from a different company, but on the early Uniden Grants, I wouldn't think Uniden would have changed brands on the PLL chip. I wonder if there is a possibility that the loop mixing crystal was replaced with something different. Those Grants usually came with an 11.325 or 11.3258 mixing crystal. There is a possibility that got changed to an 11.1125 crystal. If that's the case, you won't get upper channels. To get uppers in this instance, you have to put the switch on pins 10 and 17, instead of pins 10 and ground. If it has the 11.325 or 11.3258 crystal, then it sounds like pin 10 isn't functioning as it should. Double check the crystal, and let us know how you get on.



~Cheers~

 

[Yogi]

Ok, I took a look and it has the 11.3258 mixing crystal so it sounds as if there is a problem with pin 10. So what would cause this problem? I'm not a tech but I do have a few bench tools I inherited, if they will be any help.

 

[Robb]

I would re-check all of the solder joints from the board to the switches just the same. These newer switches available are pure junk. Sometimes they work; sometimes they don't. Had this problem before. If the radio is working on frequency as it should and only one switch isn't working; then it must be the wiring or that switch.

When you eliminate all of the other possibilities - that is all that is left!

 

[Yogi]

Ok, I will give that a try, I happen to have a new one I piked up at the rat shack last week.

 

[ExitThirteen]

Yep, Robb is right - the switches these days can be junk. Here's something to try. Since pin 10 is isolated from ground on that particular board (because it has the 11.3258 loop mixer crystal) ground out pin 10 with a small dab of solder. Take the switch and wires out of line completely. If you get 27.605 to 28.045 with the dab of solder from pin 10 to ground, then you know it's the switch or wires. If you do not get these frequencies (even after adjusting L19, the VCO) then I'd suspect that pin 10 on your PLL chip is not functioning, and should be replaced. You can remove the dab of solder from pin 10 and ground when you are done.

Also, make sure that there's an MB8719 installed, and not an MB8734. Pin 10 on the MB8734 PLLs are not connected internally, and are not functional. Let us know how you get on.


~Cheers~

 

[Yogi]

Ok guys no joy replaced switch no change, wired pin 10 directly to board ground still no change. PLL is MB8719 (Japan model) I think before I go through all the hassle of finding, puchasing, and replacing the pll I will just find another mobile. Being I've been out of the loop so long any recomendations?

 

[iceman78]

Put it back to stock. Then if you want to add the channels you may do so of your own volition; not needing to worry about what someone previously did which may have been haphazard.

 

[Yogi]

I will take it back to stock, lucky enough it looks as if nothing has actually been clipped out as far as tune ups, the clarifier mod is ok to the point that transmit and recv is alligned although the clarifier is not centered, but a little research should bring forth results on how to do that. Thanks for all your input, and sharing of your knowledge. I'm going to start from scratch and see what happens.
I'll keep you posted.

 

[iceman78]

Good luck!

 

[Yogi]

ok I had time to sit down and work on this thing today, I took all mods out including clairifier mod, all the components was still there they just lifted one leg and soldered jumper on bottom of board. evrything lined back up and radio worked flawlessly. So now I'm going to start over on the mods. I don't really care about the lower frequencies. so should I start with the frequency mod first or opening the clairifier?

 

[Robb]

Definitely open the clarifier if nothing else - IMO . . .

 

[ExitThirteen]

I'd start with the clarifier first. Get that all done, and get it lined up in all modes. That way you can lock on someone without them having to re-tune back to you.

Oh, and make sure TR24 is still in place, the modulation limiter. The "Golden Screwdriver" people are notorious for removing that transistor.

Let us know how you get on.


~Cheers~