Cobra 2000gtl Problem

[RADIOOMAN]

On my schematic I have different voltages indicated for pins 2,3,4,5 on the pll chip.
Maybe we should assume that these are not proper on this radio because the pll is unlocked.
We need to confirm/verify what is at tp10.
This is the loop back freq/signal and if it is not correct the pll will be unlocked.

On my schematic I have :
schematic .............. yours
pin 2 = 4.28 .............. pin 2 = 3.4
pin 3 = 5.15 .............. pin 3 = 0.03
pin 4 = 3.44 .............. pin 4 = 8.1
pin 5 = 3.44 .............. pin 5 = 8.1

Maybe you should scrutinize c81 and c82

 

[johndell]

C81 ok, pulled and tested. C82 is missing. The SAMS shows C82 as a .047 50V, but the schematic does not show it. It also does not show on the pictures of the part locations. Not sure.

I tried my hand at the scope on TP10. I set the probe to X10 - 50V/Div, 1uS/DIV - it showed the vpp jumping around between 1.x volts and 3.8 volts. The scope showed an almost completely flat line, just a little bit of movement. It is highly possible I did something wrong.

Thanks

 

[RADIOOMAN]

C80 & C81 (my bad)
2.2uf.... in series with each other..... they could be tantalums
signal at TP10 should be 0.34 vpp.
You need ~0.1 vpp sensitivity setting on the scope.
What model scope?
Do you have a frequency counter?

 

[johndell]

Pulled C80 and checked ok. The scope is a DSO Nano V2. I do have a frequency counter.

 

[Shockwave]

Always suspect them tantilum caps. They just love to short out and drag the DC line down to zero.

 

[RADIOOMAN]

From what I have learned about that scope on the internet I don't think it will be suitable for this application.
The information about that scope was kind of unclear but what the conclusion I came to is that it will allow you to look at signals below 1mcps, after that it is pretty much useless.
And the signal at TP10 should be 33 mcps.

If you frequency counter is sensitive enough and you have a x10 probe for it you can try to get a reading from TP10.
If it displays 33.xx mcps then we could probably assume that is not the problem.
If you don't have a x10 probe for the freq' counter then you could try with a x1 probe and if you get a 33.xx mcps reading then we can probably move on with the trouble shooting process and move to another part of the circuit.

 

[RADIOOMAN]

Always suspect them tantilum caps. They just love to short out and drag the DC line down to zero.

Absolutely agree.
Unfornately that failure mode is not supported by the voltage readings in this case.
But that still does not mean they are non-defective.
But good information for an aspiring technician to remember.

 

[Robb]

Worked on a President Madison (858/PLL) just last week. The final and driver would not hold bias. C17/tantalum used for biasing checked out OK off the board. Tossed in a new 2.2uf/50v electrolytic cap and it just biased fine after that. Those tantalum caps are tricky . . .

JohnD:

You said that you pulled a board that had crystals out of the radio used for extra freq's.
Did you check to see if the the holes this board was hooked to had the right crystal value in it? Did you try a different crystal in that spot to see if the other crystal had failed?

You said that you also removed the unlocked clarifier mod.
Did you restore that cut/removed diode - or see if the diode that was cut/removed for that mod was replaced?

I look at the PLL circuit as a bicycle chain; one broken link and the whole thing stops

 

[RADIOOMAN]

Worked on a President Madison (858/PLL) just last week. The final and driver would not hold bias. C17/tantalum used for biasing checked out OK off the board. Tossed in a 2.2uf/50v and it biased fine after that. Those tantalum caps are tricky . . .

Curious about what method/equipment you use to trouble shoot capacitors problems?

 

[Robb]

Curious about what method/equipment you use to trouble shoot capacitors problems?

I don't have an ESR tester - if that is what you mean.
I do use a DVM that will test cap function with values from 200pf to 200uf.

 

[RADIOOMAN]

I don't have an ESR tester - if that is what you mean.
I do use a DVM that will test cap function with values from 200pf to 200uf.

Well I was just curious.
Thought maybe you had a method I was unawares of.
Had you told me that you had a ESR meter I would have asked you what brand/model and what you thought of it and its effectiveness.

 

[johndell]

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

 

[RADIOOMAN]

I don't have an ESR tester - if that is what you mean.
I do use a DVM that will test cap function with values from 200pf to 200uf.

I was just curious.
Thought maybe you had a method that I was unawares of that you didn't mind to share.

 

[johndell]

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

 

[Robb]

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

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. They have very fine retractable tips and will keep you from shorting against something; alligator clips are just too big and can/will cause a short. Use your probe for testing the 10.240mhz refernce oscillator crystal from the PLL; but use the clips to hook up to your TP and board ground while keeping 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. Otherwise, it can also drag down the freq if it isn't there.



RADIOMAN said:

I was just curious.
Thought maybe you had a method that I was unawares of that you didn't mind to share.

Actually I do. If the cap is old in a circuit that is having a problem; I remove, test, and just replace it. Don't work on any new stuff; so the cap is always my first best bet. 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 . . .