I'm in need of a schematic for a Cobra 19+ with a C5121 PLL

Need more info, Pic of the board?

Some used a uPD too - very early PLUS (Note CAPITALS) units used a microcontroller (back then about like the 6502) to program pins for the PLL and handle Display functions.

You'd have 2 chips - else 7132 might be more close...

either way - we're guessing...

5121 is CMOS chip, be careful, it is very easy to kill by static or not grounded soldering iron.
See schematic attached. Different radios, but c5121 application visible.
Mike

I’d look at removing all the corrosive glue used on those sets. Both versions of the 19+ radios are notorious for the damage caused by it, especially in the PLL area. The crystal is usually glued to the top of the PLL chip as well as glue spread all over the print side under the chip. Another common problem involves the small circuit board attached to the two switches on the front. Pressure applied to those switches over time causes the solder connections to break loose away from the main board. If you decide they’re beyond reasonable repair, among other parts, save the LED display. The LED is compatible with common anode 16 pin displays found in the Cobra 29 and others.

Well, if you get a display - like the channel and the buttons work - like up and down - then the 10.240 is working. It's needs that to even "Calculate" - that's it's - "clock"

I'd look further into the "Exciter" portion of the radio - that is in the tin box on board.

It also pays to unsolder any of those tin foil shield plates underneath too. You may need to look under the foil to gain access to the PCB board under it and check for cold solder joints and any debris that may have sandwiched itself there. The "Exciter" I'm talking about, is that radios VCO section.

You may get it working again if you reflow solder. If you have the board I think you do, then you've got a side edge card riser for the channel display meter and up/down buttons - if those crack open or the radios been dropped - these break and if that occurred, you're gonna be hard pressed to try and reflow solder on torn-off foil pads.

If the channel display works, you may be in a better position than you think. It just may be from age and a fresh Electrolytic cap or two, along with a tweak of the exciter, just to get it working again...

Look over the attachments - see if it's the radio you are working on...

Note the bottom - Foilside - the edge connector pads can be seen in the photo just to the front panel side of that insulator.

When you say you "check" - how or what are you using to do this? There's not a lot of places labeled TEST POINT on that board...

Did you see this?

See Below...


The Xtal itself may work - but remember the Exciter is also trying to do something too. It's attempting to "ring" and start the loop feedback process.

To start, look at the Blue Box section, the Xtal and there are two small caps, one is a trimmer cap to the right of the Xtal on the board (below green braid) and to the left of the Xtal is a square cap - these "trim" that crystal to a frequency close to 10.240 - but not exact - the CX trims to further fine tune it.

The Yellow Box shows two TANTALUM caps the PLL uses in a loop to two speeds - one high, the other low, they clock thru the PLL and as they "sync" together - this signal is a lot like a frequency. The PLL then determines this exciter and the divider inside itself if the channels are able to be tuned to - it uses the Exciter as the other half to check for "sync" and sends a error correction signal as part of this "Check gating for sync" process.

This is what made this radio unique, most of the other 5121 or 7132's used single cap designs for signal - ok I guess but was not the best smooth waveform the radio can use for IF processing - which the line sampled from that output of the Exciter - so if it was noisy - so went the IF...

Cobra even had a TSB for this...


Now, how do I know about that clock - is because if it doesn't lock in or find any frequency on any pin, it'll just keep the display blank - it's for the reason of servicing - to show something has failed internally.

Did you check the Exciter coil - any "tampering" of the wax?

This thing is old, but there are spots on this board, that use older electrolytic caps that can dry out. That also can affect the signal you're trying to find. IF the coil is ok, but the parts have drifted off. That wax section will need some of the work you may have to "replace parts" done to it. That means removing as much wax physically before you start to reheat and remove/unsolder parts. The wax needs to be warmed, slightly, and removed so you can get at the parts, just don't apply too much heat at first so you can soften it then remove as much as you can in one piece.



So look into what I've listed above in a previous post - clean it up and reheat soldering joints in and around the VCO/Exciter section to see if you can get it to start oscillating (Ring). The Xtal itself may just need to be cleaned up or get the "glue" bonding it to the board/chip removed - so it can send more signal you can detect.

Something tells me that the Xtal may not be the problem but the caps used to "trim" and the dirt that the thing can collect around the glue can start siphoning off signal. See the schematic reference graphic above on the right.

Considering the size of these things, they can be put just about anywhere in a small car onto a Chrysler Mini-van and were used like CB's onto even French Fry and Cup Holders.


So some can be abused and may not even work - it's the law of diminishing returns - you can only go so far before you have to give up on the cost or the effort or both.

Right now, I'm not in the market - so I'm the wrong guy to ask - all I got is an older 60MHz scope - bought used.

It works, but you do mot need "fancy equipment" - don't succumb to that pressure. This is CB, it's AM and about as simple as it gets.

You just need to remember a few tricks to help you...

Remember where you started, because at Square one, is where you and it, got it all together and started...

Work in small increments. Rome was not built in a day. So don't rush - you can make some serious mistakes.

Log your work. Document! I mean; take notes or run a camera to show you in action on that radio - so you can play it back and find where you need to go back to and do over if needed.

So it is not a lost case, just make sure you have the right microphone for it.

These things used a 5Pin DIN Radio Shack style mic plug and jack for this radio - but it used power from the radio itself to power the Electret condenser element in the handset.- so Grey Key mike for the radio and only this kind of radio. It's how this radio works.

I gotta' do some stuff for the wife, but will continue shortly...

Correct...

Longer version...
That is how these things work. The PLL will send a signal to a pin, raising or lowering a voltage.

The voltage goes to a Varactor - a type of diode that has a special doping...It is designed to work like a diode but it's got a really thick substrate - read wafer, read chip, read doping material and SEE SANDWICH...

You can read more about this device at this link

Only this one, when you put it in a circuit the right way, changes capacitance based upon a voltage.

So L6, That Diode - and the voltage from the PLL - along with a 10.240 Xtal - is really all that you need to focus on.

The PLL GENERATES a signal for the EXCITER using the 10.240MHz signal as a REFERENCE.

• The Radio USES The EXCITER to power one part of the Receivers three stages to receive AM signals
• The Radio USES the 10.240MHz Xtal to Power another part of the Receiver's three stages to receive AM Signals.
• The Radio USES BOTH EXCITER and 10.240MHz Xtal IN a method of SUBTRACTION to generate a 3rd signal used in the 3rd stage to Receive AM Signals.
• The Radio USES BOTH SIGNALS to Power TX - COMBINED together to make 27MHz.
• The Radio USES a process know as Heterodyne to obtain signals for further processing in High-Gain amplifier stages.
• Heterodyne can be used to COMBINE as well as SUBTRACT two frequencies to generate another.

You can read more about this process at this link

Once you understand the process of MIXING and deriving two other signals - One ABOVE the two frequencies (Summed together) and another BELOW the two frequencies (Subtracted from each other = Difference) you can then apply this to your situation of arriving to the correct frequency and letting the PLL do the correction work for you.

The Exciter - that L6 coil - is a free running oscillator that uses that tiny voltage change from the PLL to adjust its' free-running frequency - it's checked by the PLL using a "gating process".

• The PLL "Checks itself" using a ROM program to set a counter and the frequency it looks at from the Exciter then is used as the actual "count" as a process of dividing down and seeing the that result is good - hence the Gate - Open or Closed - and corrects accordingly depending on the state of two gates it checks this result with. An Error signal is sent to the exciter as a correction - a looping feedback process.

Will add more as time permits - you have a lot to read here - just enjoy the post as fun for a change...