Uniden Madison MB8719 All Channels 10 khz down

[Gman]

I always get the really weird ones. Bought a basket case Uniden Madison off ebay. Mods everywhere. Undid all I could figure out. Was missing the 11.3258 crystal so I added that. All LEDs burnt out (AM, USB, LSB, CH9). Channel LED missing. Mic and RF gain pots missing. Speaker and PA jacks missing. Shall I go on? Well I got almost everything added back and running. But get this: all channels are 10 khz down. Transmit and receive. N codes into MB8719 correct and verified, I even swapped out the MB8719 IC too. 10.24 mhz and 11.3258 mhz crystals good. Underneath the MB8719 looks clean, no mods or bad traces. Am I missing a switching diode somewhere? Wrong varactor diode tied to the 11.3258 xtal? Any ideas anyone? Thanks.

 

[nomadradio]

I have done this to Galaxy radios by being sloppy. Watching the right-hand digits on the frequency counter. Failing to pay attention to the channel you're on can put you one whole channel away, but dead on center of THAT channel, 10 kHz too low.

The three clarifier-center trimmer coils have a wide range, but I'm not sure if it's a whole channel on that model. The inductance of those coils is highest with the slug dead even with the rim of the opening. This will show the lowest frequency it can reach. Can't remember the callout number, but see if turning the slug of the AM mode coil clockwise and won't take you back up one channel. If the clarifier range was stretched, you might need to position the clarifier knob at 3 o'clock to achieve channel center.

Sure what it sounds like.

73

 

[Gman]

Nomad, thank you. Been awhile since we talked. I will relook the tuning but almost sure its not this. The radio was heavily butchered. There was even an old DX FM module in there. I recall having another base doing this 10 khz off thing, and the cause was a switching diode or varactor pulled. Reinstalling it corrected the problem. But so far, cant find any missing diodes. Maybe an incorrect varactor is in there.

 

[SuperLid]

Nomad, thank you. Been awhile since we talked. I will relook the tuning but almost sure its not this. The radio was heavily butchered. There was even an old DX FM module in there. I recall having another base doing this 10 khz off thing, and the cause was a switching diode or varactor pulled. Reinstalling it corrected the problem. But so far, cant find any missing diodes. Maybe an incorrect varactor is in there.

Try removing 8 volts (or whatever someone tied to it) from the clarifier circuit. See if the freq shifts. That should tell you if you are on track with a varactor. If it is a switching diode, it will most likely be in the PLL programming. But, you said you have already been through that.

 

[Gman]

Thank you. Will try all and report back.

 

[Gman]

OK, removed 8v source from clarifier. No change. But now, I'm thinking, Nomad cant be off. Well, it does seem to be tuning. But I think I found the problem. I don't have another Madison to pull parts from, but I do have 148 cobras for parts. And I keep reading Madison and 148's are identical boards. I've been using the 11.325 mhz crystal from the 148, and Nomad's insight, I retuned with the clarifier at 3 o'clock. Still bad. But, I then tried retuning with the clarifier at 9 o'clock, I was able to correct the 10 khz off problem, but I did have to remove the 10pf padding cap across the trim cap. And now the trimmer cap for the 11.325 mhz crystal is at the end, and so is the tripler coil. Hmmm.... double checking, Madison uses 11.3258 mhz crystal, 148 uses 11.325 mhz crystal. Short 800 hz ???

 

[SuperLid]

OK, removed 8v source from clarifier. No change. But now, I'm thinking, Nomad cant be off. Well, it does seem to be tuning. But I think I found the problem. I don't have another Madison to pull parts from, but I do have 148 cobras for parts. And I keep reading Madison and 148's are identical boards. I've been using the 11.325 mhz crystal from the 148, and Nomad's insight, I retuned with the clarifier at 3 o'clock. Still bad. But, I then tried retuning with the clarifier at 9 o'clock, I was able to correct the 10 khz off problem, but I did have to remove the 10pf padding cap across the trim cap. And now the trimmer cap for the 11.325 mhz crystal is at the end, and so is the tripler coil. Hmmm.... double checking, Madison uses 11.3258 mhz crystal, 148 uses 11.325 mhz crystal. Short 800 hz ???

They are NOT identical. Close, but not identical. The Uniden has the 11.3258 HC/49 crystal. The Cobra's may have the 11.1125 crystal, or a larger 11.325.

 

[Gman]

And there it is. Thank you very much SuperLid.

 

[brandon7861]

I don't think the difference in those crystal values can cause a 10kHz difference. Triple 11.325 you get 33.975, triple 11.3258 you get 33.9774. Those are not 10kHz apart, they are 2.4kHz apart.

As for the 11.325 in place of the 11.3258, in case you don't have a 11.3258 crystal, there are a few ways to move a crystal's pullability one way or another. IIRC, that radio is using one method that is working against you. Compared to a crystal with a series capacitor only, adding an inductor in series increases pullability, but mainly in the negative direction (it lets you get lower frequencies for the same range of capacitance). Adding nearly identical crystals in parallel will increase pullability in the positive direction (getting to higher frequencies).

Oscillators can become unstable with too low of capacitance. That padding cap helped stabilize the oscillator and make the variable cap adjustment not so touchy. Depending on the pF at the minimum setting on that variable, it could be on the unstable side. The lower that capacitance (the higher the reactance in series with the crystal), the more it can be influenced by parasitic effects. In other words, putting your finger near it may change the frequency.

I wonder what results you might get if you short across the inductor and use the trim cap alone (which will hopefully not be set at its minimum any more). Maybe you can get that extra 800Hz without removing the padding cap.

 

[Gman]

I ordered an 11.3258 mhz crystal. In the mean time I will try shorting the coil and see if it raises the frequency at all. Thank you for the info.

 

[SuperLid]

I ordered an 11.3258 mhz crystal. In the mean time I will try shorting the coil and see if it raises the frequency at all. Thank you for the info.

Where did you get the rock from?

 

[Gman]

Ebay.

 

[SuperLid]

Ebay....cool

 

[Gman]

Going thru my notes, i found an old trick my elmer showed me. And it works. I paralled two 11.325 mhz crystals, and the darn thing aligned right up as if i had just the 11.3258 crystal in there. Until the correct crystal comes in, i can sleep at night knowing the clarifier is a 1 pm and not 9 pm.

 

[nomadradio]

I paralled two 11.325 mhz crystals,

Now there's a trick I never tried.

Cool!

73