I have a Scope with a couple of 10x probes. I'll slap one on the counter and see if it makes a difference.
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Cobra 142GTL No Transmit or Receive
- Thread starter CSA1863
- Start date
Well I took a 10 M ohm probe from my Tektronix scope and put it on my counter. When probing TP1 I get no signal.
I decided to play with some resistors inline on the counter probe input and see if I could find a value that would reduce the sensitivity. Anything above 1.5 k ohm I got no signal. Between 300 ohm and 1.2 k ohm the frequency would fluctuate and never lock. At 270 ohm I was locked on the 70 Mhz signal.
So what to do from here?
Should I have continuity between pin 1 of the VCO/Mixer UHIC007 (IC1) to the secondary of L14 which has continuity with TP1?
Also here are the Voltages on the UHIC007
pin1 7.03 Should be 7.87
pin2,3,4 & 7 to ground
pin5 3.05 should be 3.40
pin6 0 should be 0
pin8 8.29 should be 8.17
pin9 7.5 should be 7.5
pin10 4.11 should be 4.03
I decided to play with some resistors inline on the counter probe input and see if I could find a value that would reduce the sensitivity. Anything above 1.5 k ohm I got no signal. Between 300 ohm and 1.2 k ohm the frequency would fluctuate and never lock. At 270 ohm I was locked on the 70 Mhz signal.
So what to do from here?
Should I have continuity between pin 1 of the VCO/Mixer UHIC007 (IC1) to the secondary of L14 which has continuity with TP1?
Also here are the Voltages on the UHIC007
pin1 7.03 Should be 7.87
pin2,3,4 & 7 to ground
pin5 3.05 should be 3.40
pin6 0 should be 0
pin8 8.29 should be 8.17
pin9 7.5 should be 7.5
pin10 4.11 should be 4.03
If that's the deal, the receiver sensitivity will be very poor and the slug in L14 will exhibit peak receiver sensitivity with the slug dead-even with the rim of the hole in the top of the can.
73
I am attempting to remove the mods on this radio before trying to do anymore troubleshooting on the 40 Mhz signal I am getting at TP1.
Can anyone tell me where the clarifier wires originally connected to the board?
Currently I have an orange (maybe red) wire running from the left side of the Voice Lock pot to the + side of C 66. An orange and white wire running from the right side of the Voice Lock pot to ground and an orange and white wire running from the center on the Voice Lock pot to a trace that connects to r187.
I know I will need to put R187 back, D36 back and remove the choke from the varactor Diode D37 and reconnect the cathode side to fully undo the clarifier mod once the wires from the Voice Lock pot are restored to their original place.
The channel mod has been removed and the 11.1125 Mhz crystal soldered back to the board.
Thanks!
Can anyone tell me where the clarifier wires originally connected to the board?
Currently I have an orange (maybe red) wire running from the left side of the Voice Lock pot to the + side of C 66. An orange and white wire running from the right side of the Voice Lock pot to ground and an orange and white wire running from the center on the Voice Lock pot to a trace that connects to r187.
I know I will need to put R187 back, D36 back and remove the choke from the varactor Diode D37 and reconnect the cathode side to fully undo the clarifier mod once the wires from the Voice Lock pot are restored to their original place.
The channel mod has been removed and the 11.1125 Mhz crystal soldered back to the board.
Thanks!
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I figured it out. Just had a hard time finding R415 & R417 since they were hidden on the board with the NB/ANL, CB/PA & MOD - SWR Switches.
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What has me curious is..... how can the PLL be "in lock" if the VCO is running at 70 MHz??? My brain pulls me back to that!
When you aligned the 11.125MHz oscillator for AM, did you use 11.125MHz? If so, that's not right. On AM, you need to use 11.111666.
Here is what I think is happening. If you set the oscillator to 11.125, that gets tripled to 33.375, add that to the loop mix of 1.87MHz and you have a VCO of 35.245. The second harmonic of that is 70.49MHz.
I suggest going back and making sure the AM oscillator is set to 11.111666MHz and see what happens because if your FC is picking up the second harmonic, it means the oscillator is at 11.12417MHz.
After that, we need to figure out why you have so much harmonic content there.
Here is what I think is happening. If you set the oscillator to 11.125, that gets tripled to 33.375, add that to the loop mix of 1.87MHz and you have a VCO of 35.245. The second harmonic of that is 70.49MHz.
I suggest going back and making sure the AM oscillator is set to 11.111666MHz and see what happens because if your FC is picking up the second harmonic, it means the oscillator is at 11.12417MHz.
After that, we need to figure out why you have so much harmonic content there.
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I have 11.111224 MHz at the base of TR24 (2SC1675) Oscillator. Not sure how to adjust this or if it's close enough. I'm using the Stalker IX/XV Service manual as I don't have the Service Manual for the 142 (Same board) and it doesn't show any thing on aligning the 11.1125 MHz Oscillator.
If you can point me to which pot to adjust I'll see if I can get it set to 11.111666 MHz
Thanks for the help!
If you can point me to which pot to adjust I'll see if I can get it set to 11.111666 MHz
Thanks for the help!
Also since removing all the mods I am reading 69.528917 MHz on Channel 1 and 70.408966 MHz on Channel 40 at TP1. Reading 10.238 MHz on Pin 8 of he MB8719
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When I get home, Ill upload the service manual for the uniden washington. Should be the same. I get home in about 2 hours.
Attached is the service manual for the President Washington. In this service manual, you will not find 11MHz shown because it is measured at the VCO (11MHz*3+loop freq).
Being you are seeing the second harmonic of the VCO output for some reason, you can either watch for the value that is double what is shown, or you can try to sort out why it is showing the second harmonic before doing the alignment. I would suggest the latter. In any case, the AM adjustment you are looking for is L20 and you want the VCO to output 35.205 on CH40 (or 70.41 in your case with the second harmonic).
This harmonic thing really should be sorted out first. Even harmonics come from asymmetric distortion, i.e. something clipping one side of the waveform. Being the UHIC-007 VCO chip is open collector and depends on external voltage through L14, I would expect that very result from a dislocated capacitor inside L14. Without that capacitor, it is not a tank circuit, and you will definitely have asymmetry in the waveform which would explain the harmonic thing.
The solution to this is to place a capacitor across that side of the transformer (adding it to the foil side). The question is, what size capacitor. I cannot answer that without tearing one apart. I would be willing to try, but you'll have to give me some time on that.
Being you are seeing the second harmonic of the VCO output for some reason, you can either watch for the value that is double what is shown, or you can try to sort out why it is showing the second harmonic before doing the alignment. I would suggest the latter. In any case, the AM adjustment you are looking for is L20 and you want the VCO to output 35.205 on CH40 (or 70.41 in your case with the second harmonic).
This harmonic thing really should be sorted out first. Even harmonics come from asymmetric distortion, i.e. something clipping one side of the waveform. Being the UHIC-007 VCO chip is open collector and depends on external voltage through L14, I would expect that very result from a dislocated capacitor inside L14. Without that capacitor, it is not a tank circuit, and you will definitely have asymmetry in the waveform which would explain the harmonic thing.
The solution to this is to place a capacitor across that side of the transformer (adding it to the foil side). The question is, what size capacitor. I cannot answer that without tearing one apart. I would be willing to try, but you'll have to give me some time on that.
And the magic number is 35pF. Place it across coil where the red circles are.
And if I am right about this, there is probably a couple pF of residual capacitance from the leads to the bad internal cap, so 33pF would be ideal. And I have those, so if you don't want to pay mouser shipping, I can toss one in the mail for you to try,
The cool part is if I am wrong, its really easy to undo lol
And if I am right about this, there is probably a couple pF of residual capacitance from the leads to the bad internal cap, so 33pF would be ideal. And I have those, so if you don't want to pay mouser shipping, I can toss one in the mail for you to try,
The cool part is if I am wrong, its really easy to undo lol
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I'm pretty sure I have some 33 pf disk capacitors. I will give that a try this evening when I can get to the shop and let you know how it goes.
Also I will post a picture of what I'm seeing on the Scope off TP1. Looks like it is showing both the 70 MHz and 35 MHz frequencies.
Also I will post a picture of what I'm seeing on the Scope off TP1. Looks like it is showing both the 70 MHz and 35 MHz frequencies.
Ok the 33 pf cap got me to 35.20#### the Hz are drifting around from 35.202 - 35.204. I can push the UHIC007 IC toward L13 & L14 and the drifting calms down to 35.20447#. Any ideas? I re-flowed the solder on the UHIC007
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Try to re-peak L14. Im guessing it might be a little off and not giving enough signal to the mixer.
My initial assumption about the cap being bad could be only half the story too. If it partially corroded from water, it may have some pF left in it. When I took mine apart, I was surprised to see the wires on that cap were 4x thicker than the coil windings. Maybe that metallization was partially ate and 33pF isn't perfect. The good news is that we are onto the problem.
My initial assumption about the cap being bad could be only half the story too. If it partially corroded from water, it may have some pF left in it. When I took mine apart, I was surprised to see the wires on that cap were 4x thicker than the coil windings. Maybe that metallization was partially ate and 33pF isn't perfect. The good news is that we are onto the problem.