Cobra 142GTL No Transmit or Receive

[CSA1863]

I have a 142GTL with no transmit or receive. Radio powers up. Can key the mic and the transmit light come on but no rf out. I have checked the voltage on pin 6 of the PLL and have 8.25v. Pin 8 is reading 10.238 MHz. There is rf voltage at TP1. There is 4.8v at TP9.

The problem seems to be in the VCO as the frequency at TP1 is 70.485 on Channel 40 where it should be 35.205.

Voltages on the VCO/Mixer IC UHIC-007 look right.

Radio has what looks to be a expo 100 channel mod and the clarifier mod.

Electrolytic capacitors have been replaced.

Any help would be greatly appreciated

Thanks!

 

[nomadradio]

Hmmm. The light turns red on the front panel when you key the mike. This eliminates the PLL as the root of the trouble. If it falls out of lock it will inhibit the LED from turning red.

Does lead to the suspicion that there is more than "just one" thing gone wrong. The one thing the receiver and transmitter share is the PLL. If it falls out of lock it disables receiver audio and the transmit keying circuit both. A radio that won't talk or hear points in that direction. At first, anyway.

Start with the "Manual Analog Digital Signal Injector". Your finger laying alongside the shaft of a small screwdriver. Touch the tip to each of the two "hot" foil pads under the crystal filter. You should hear RF noises in the speaker. One of the two pads will be louder than the other, but you should hear noise from each.

The S-meter is useful to see if the receiver problem is in the radio's RF or IF sections. If the meter kicks around and you hear nothing, that points to an audio-only problem. It's common for the needle's pivot bearing to stiffen with age. If the, meter's pointer is seized, that's one less tool you have to work with.

If you have another CB on hand, hook it up on the same channel with a coax jumper connected. Thread the plug's shell on the far end of the jumper back over the plug body. This exposes the center pin. Makes it a half-inch long "RF sniffing" antenna. Hold it close to the radio's transmit mixer chip, marked "S042" while you key the mike. You should at least hear a weak carrier if the radio is producing a transmit signal at all. If not, the trouble is upstream from that chip.

Never underestimate the odds of a solder blob bridging across adjacent foil pads on the solder side of the PC board, Changing all the electrolytics creates this risk. A bright light and a magnifier are a big help finding one of those.

73

 

[CSA1863]

Thanks for the reply.

I kind of assumed it was the in the PLL circuit since there was no transmit or receive and wasn't getting the right frequency at TP1. I will try your suggestions and post what I find.

The radio had the same issue prior to replacing the Caps. Also when replacing the capacitors I used a magnifying lamp and verified there was no continuity between the solder points. I feel fairly confident there are no solder bridges but I will go back and look.

 

[CSA1863]

So I checked checked for any solder bridges and didn't find any.

I took a small screwdriver and heard a "popping" sound when initially touching the "hot" foil pads under the crystal filter. If I kept the tip of the screwdriver on the pad the "popping" stopped. No movement on the s/rf meter.

I did get a carrier on a working radio when probing the SO42 IC.

I guess this eliminates the PLL and you are probably dead on by saying there may be more than one thing wrong...

Any more tips on locating/isolating the issue(s)?

Thanks!

 

[brandon7861]

I'll take a guess, but that's all it is.

We know the PLL is locked, we know the VCO is creating RF, and we know that the loop mixer is doing something since it is locked. Your frequency is almost exactly twice what it should be, almost. Almost rules out any divider error and I highly doubt the VCO has enough tuning range to make it to 70MHz normally.

That leads me to believe the VCO tuning coil is either bad or has a bad solder joint leaving the VCO to operate on the self-resonance of the tank capacitors alone and the mixer is producing something the PD can lock with - maybe the 70MHz VCO - 2x the tripler, or something like it.

Again, I am just guessing.

edit: Having a carrier on the TX mixer shoots my theory down, but I don't see how you can even have that with a 70MHz VCO.

 

[nomadradio]

almost exactly twice what it should be

If the counter has a way to select the input sensitivity,try turning the level down. You may find that the counter's input circuit is doubling the PLL frequency.

There are more ways to get a wrong reading from a counter than there are to get the right one. I think the PLL is on frequency, but the counter is displaying the second harmonic.

73

 

[CSA1863]

Will check the VCO tuning coil (L13) for bad solder joints when I get in tomorrow evening.

Will I need to remove the VCO tuning coil to test/check or can this be done with it in the circuit?

Thanks!

 

[CSA1863]

I will check and see if the input sensitivity can be turned down on the counter I am using. I have 2 other counters that I can check also...

 

[brandon7861]

Definitely investigate nomads suggestion before attacking that coil. If you really do have a carrier at the TX mixer, he is almost certainly right. Come to think of it, my counter has done that before.

Removing that coil is a last resort as they are easily damaged during removal. Odds of it being bad are pretty low in my opinion, I'm just brainstorming possible explanations. Counter sensitivity didn't even cross my mind.

 

[CSA1863]

Well I couldn't find a way on either of my 2 working counters to reduce the sensitivity. Could there be something in the radio creating the second harmonic frequency or do you think my counters are just to sensitive?

Also L13 (VCO coil) apparently has never been "turned" as it still has wax in it. Did verify it had good solder joints.

I checked the CB/PA switch just to make sure it wasn't stuck in the PA mode. Switch works fine but I have no audio when keying the mic in PA. Checked the voltages on the TA7222P chip and they look to be in range.

I do have an audio "hiss" in the receive mode that increases when I turn up the volume.

I also removed the S/RF meter and the needle was definitely seized up. I was able to free the needle up and get the meter working. I probed the "hot" foil pads under the crystal filter again and the needle jumps up maybe one s unit.

 

[loosecannon]

the service manual on cbtricks.org for the realistic TRC-450 should have an IC voltage chart in it, where you can see the correct voltages of each pin on the MB8719 and UHIC-007 vco chip.

measure the voltages on both IC chips and let us know what you find.

pin 10 on the PLL might be different as there were a couple of models where they used the 11.3258 crystal in a single conversion chassis, but i would imagine pins 1-9 would be the same.
LC

 

[nomadradio]

If the transmit light changes to red when you key the mike, there is no reason to be mucking around in the PLL section. If it were not locked onto the channel, that light would go dark when you key the mike. The PLL is working odds are.

Reading between the lines, I'm guessing this is a radio that broke and got "looked at" by someone who failed to get it fixed. Too often the guy who couldn't fix it also creates some new fault while poking around. But since the first fault kept it from working, you can't tell it now has two real faults, not just the original.

The next guy who tries and fails to get it revived probably adds another boo-boo to the radio.

A radio that has a trustworthy history, like: "Worked every day for years, then the lights blinked on and off during a storm. Hasn't worked since".

That kind of first-hand picture would suggest just one thing wrong, maybe.

Unless you have that kind first-hand history of the radio, never assume there is only one fault.

73

 

[CSA1863]

When I get some time tomorrow I will take an RF probe and see if I have anything on the pre drive TR38 (2SC1973) if not I'll check voltages and if they are off test the TR. If everything looks good there I will move on to the TX drive TR39 (2SC1306) and then the TX final (2SC1969) If I should be doing this different let me know.

Back to the counters reading the second harmonic. I pulled the cover on a President Grant (d858 PLL) and it is reading 34.98550 on CH19 at TP8. I'm just trying to understand why the counters read fine on the Grant but are reading the second harmonic on the 142. Is it truly a counter issue or is there something up with the 142 causing the second harmonic

 

[nomadradio]

Most frequency counters use diodes in the input circuit to limit the input level. Driving those diodes hard enough will distort the RF waveform enough to produce 2nd-harmonic energy at twice the input frequency. The run-of-the-mill bench counters would frequently have a high/low or high/med/low sensitivity selector. Fancy ones had an attenuator knob on a pot.

Pretty sure that's what you're seeing is a second harmonic resulting from excess input drive to the counter. We got in the habit of using a times-ten (X10) 'scope probe for frequency counters. Attenuates the signal by a factor of ten. This also has the advantage of reducing the circuit load to ground on the probe's tip. A direct probe places the entire center-to-shield capacitance of the probe's cable in parallel with the circuit you're measuring. This can disrupt a circuit's behavior, placing the equivalent of a 100 or 200 pf capacitor into a sensitive RF circuit like a crystal oscillator when the probe tip makes contact.

73

 

[brandon7861]

Having enough 70MHz come out of the low impedance side of a tuned transformer for a counter to lock on to makes me wonder if L4's internal cap didn't give out. Sounds like it is acting as a broadband transformer more than a tuned circuit.