Cobra 2000gtl Problem

[johndell]

They look like the ones I have. It came with small alligator clips, so I bought the other ones.

 

[RADIOOMAN]

Frequency counter is a Victor Frequency Counter VC3165. It shows 34.6 and it shows 29 for period. Probe is x1.

Q20 ( TP10 ) is where the tripled 11.325 mcps crystal is mixed with the 34.765 from the vco (ch. 1 am) and that is fed back to the pll for comparison.
There will be three signals present.
33.975mcps, 34.765 mcps, (ch. 1 am) and the difference between the two which is 0.790 mcps.
The 0.790 is sent back to pll chip for comparison.
This is difficult for some feq. counters to measure due to three signals being present.
It confuses the counter and you get a display the keeps changine.
I use a tuned frequency counter (texscan tfc450) and am able to tune it to the fequency I want to measure.
Would be nice to be able to view the waveform at this point with a scope.
Have you performed any of the adjustments associated with a 'pll alignment' in this radio?

 

[RADIOOMAN]

That counter should be just fine. Might want to get a pair of test leads like this for doing freq alignments:

Bought mine at Fry's Electronis for about $12.
Worth every penny - IMO. Use your probe for testing the 10.350 refernce oscillator from the PLL; but use the clips to keep your hands-free to adjust the coils. Also use a .047uf disc cap on the end of the "+" clip lead to attach to your freq test points for DC isolation.



RADIOMAN said:


Actually I do. If the cap is old in a circuit that is having a problem; I remove, test, and just replace it. If you have a better way, let me know. Those ESR testers are not too expensive and can save time. You still have to hunt down the cap traces to hook it to; just a step away from removing and replacing it at the same time. Still on the fence for buying an ESR unit for ~$125 that measures capacitance AND the ESR value . . .

I have been using a ESR meter for over 10 years and would not be without it.
Most of the time you don't have to remove the 'cap' from the circuit to test it.
I would also consider the time/money replacing parts that were not the source of the malfuncion. ( unless it was aged equipment and then I don't have a problem with replacing electrolytic capacitors due to the fact that 'they might be good now, but what about later' )
There are some schematics on the internet you could use if you were interested in making your own ESR meter.
If I had to buy another ESR meter today I think I would consider the 'PEAK' brand.

 

[johndell]

the only way for me to change frequency is to adjust L19. I tuned the radio to Channel 1 26.96550. I took a frequency reading at TP10 on AM, USB, and LSB. Checked the truth chart and I have those frequency's.

I have not made any PLL adjustments. I do have 10.240 MHz at TP13.

Adjusting L19 only changes the frequency, there is no voltage change at TP9.

Synthesizer Alignment Reading: L19 adjusted for Channel 19

TP10 34.09 MHz.

TP3 USB 7.8010 MHz
TP3 LSB 7.7980 MHz

TP14 (IC3 Pin 7) AM TX 7.8000 MHz

 

[johndell]

Sorry, I took the readings in the first paragraph at TP1, not TP10 (typo).