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Cobra 2000gtl Problem

J

johndell

Member
Jan 26, 2008
14
0
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I have a 2000 gtl that was hacked up and did not transmit. I have put everything back to normal (unlocked clarifier, extra channels, volted final). Still does not transmit, not clarifier and the channel selector does not work. If I lift the anode end of D31 I can transmit (TX and RX are both good). I can change frequency's if I adjust L19, but no voltage change at TP9. The clarifier still does not work. PLL Pin 6 shows .072v. Pin 1 shows 6.72V. Voltage changes on the PLL programming pins if you rotate the channel selector. I have 8 volts at R95, 8 volts at Pin 9 PLL, but .4 at R63 (The schematic shows 8 Volts at R63). MB 3756 voltages all check out.

Basically when D31 is lifted and I supply 8 volts to lifted anode of D31 the radio will TX and RX great, but no channel selector or clarifier.

Stumped.

Thanks for any ideas.
 

Pin 6 is the 'lock' pin.
If the pll is unlocked that pin will go lo.
It is part of the 'tx inhibit' function and there are other conditions that will cause that pin to go lo.
If you disconnect the cathode of d31 from the circuit then the anode should still be high.
If it is not high (8 volts) then there could be a problem with the rest of the 'tx inhibit circuit.'
The following step is only valid if the anode of d31 is 8 volts when the cathode is removed from pin 6.
I would put things back to norm with d31 and put the voltmeter on pin six and the slowly turn L19 and watch the voltmeter to see if pin 6 goes high at any point while turning the core of L19.
 
When the cathode is removed from pin 6 the anode side is still high, 8 volts. The voltage does not change on L19. When the cathode is connected the anode side shows .72 volts, .18 volts on cathode side.

PLL voltages in RX with channel selector set to 19 AM:
1: 6.76 2: 3.4 3: 0.03 4: 8.1 5: 8.1 6: .17 7: 4.43 8: 4.24 9: 8.14
10: 0.00 11:8.1 12: .01 13: .01 14: 8.08 15: .01 16: 8.08 17: 3.76 18: 0

VCO same conditions as above:

1: 7.76 2,3,4 Grnd 5: 6.76 6: 0 7: Grnd 8: 0 9: 8.14

Voltage Regulator:

1: 8.14
2: 13.36
3: 8.15
4: grnd
5: 13.36
6: 8.16
7: 0
8: .46 RX

I have 10.24 mhz at Pin 8 PLL and freq at TP1 is in range.

Meter lights dim on transmit

Thanks.
 
Whenever I had problems with the PLL/VCO, it always came down to three different causes:

1) Poor solder joints on the PLL from previous hacks (that includes the surrounding traces)
2) Wasted/damaged PLL from hacks
3) Bad electrolytic or tantalum cap (shorted) in these circuits (just old electronics)

In most cases, the symptom was L19 coil was stuck on just one output voltage and could not be adjusted.
 
Last edited:
johndell said:
When the cathode is removed from pin 6 the anode side is still high, 8 volts. The voltage does not change on L19. When the cathode is connected the anode side shows .72 volts, .18 volts on cathode side.

PLL voltages in RX with channel selector set to 19 AM:
1: 6.76 2: 3.4 3: 0.03 4: 8.1 5: 8.1 6: .17 7: 4.43 8: 4.24 9: 8.14
10: 0.00 11:8.1 12: .01 13: .01 14: 8.08 15: .01 16: 8.08 17: 3.76 18: 0

VCO same conditions as above:

1: 7.76 2,3,4 Grnd 5: 6.76 6: 0 7: Grnd 8: 0 9: 8.14

Voltage Regulator:

1: 8.14
2: 13.36
3: 8.15
4: grnd
5: 13.36
6: 8.16
7: 0
8: .46 RX

I have 10.24 mhz at Pin 8 PLL and freq at TP1 is in range.

Meter lights dim on transmit

Thanks.
Click to expand...

VCO same conditions as above:

1: 7.76 2,3,4 Grnd 5: 6.76 6: 0 7: Grnd 8: 0 9: 8.14

On the 'vco' ic pin 8 should be 8 volts.
I think it might be a typo but please check before we proceed.

I am puzzled that you did not indicate some of the 'tx' mode voltage readings.
From looking at the other voltage readings you have indicated I got a bad feeling the pll is toast.
There are two pins on the pll where the voltage you have indicated is in complete disagreement with my schematic.
But an excellent reason to put in the MB8719 chip
 
It was a typo. Pin 8 on the VCO is 8.13 RX Channel 19 AM

I only had time to post the RX as I had to leave for work:

TX Channel 19 AM (Cathode of D31 lifted - only way to TX)

PLL:
1: 6.76
2: 3.4
3: 0.03
4: 8.10
5: 8.10
6: .06
7: 4.43
8: 4.27
9: 8.17
10: 0
17: 3.78
18: 0

VCO

1: 7.77
2,3,4: Grnd
5: 6.76
6: 0
7: Grnd
8: 8.14

Voltage Regulator:

1: 8.17
2: 12.39
3: 8.15
4: Grnd
5: .05
6: .18
7: 0
8: 8.23

This is the second PLL. The first one produced the same results. I thought it was bad so I put in a new one. Also the second VCO.

Thanks.
 
TP10 has 8.12

PLL Pin 7: 3.78

TR20 E: 3.4 C: 8.12 B: 4.04

C87 3.16 and 8.13
L17 8.14 and 8.14
L16 8.12 and 8.13
C86 8.13 and 0
R101 8.14 and 8.14
C89 0 and 8.13

I have the SAMS and the 2000GTL schematic
 
johndell said:
I will try to get you a scope reading from TP10. I'm not good using the scope yet.
Click to expand...

Some advice so you don't get erroneous results; Always use a x10 scope probe in the x10 position.
x1 will load some circuits to much and can even cause some circuits to quit working properly.
Do you know how to adjust the compensation trimmer cap in the x10 probe?
If you don't adjust that your waveform might not look the way it should.
 
I have to admit I know next to nothing about the scope. I have every other piece of equipment. I was interested in learning and a friend of mine loaned me his DSO Nano. I played around with it and got it to read frequency's. I had basic electrical engineering in college so I can read schematics and I understand basic electronics and simple circuits. Been soldering and messing with electronics since the eighth grade. Life took me in another direction, but I still like working with it.

With that in mind, I would like to keep working at this radio. If you think it is above my head, let me know.

Thanks
 
I have no formal education in electronics at all, and I can fix radios. Might take me a bit longer to fix something; but I sure don't give up until I do fix it.

Don't give up; you are just getting warmed up!

I look for the most likely failures in radios and that is how I fix them. Working on a hacked-up radio really makes it far more difficult when a schematic is the best and only guide. Put it back to stock first; that should get you going. The most common failures I've found are in my first post.

You are working on a radio that has old caps in it and has probably been handled by Golden Screwdriver operators/modifiers; keeping that in mind is what I would do when working on that thing.

Keep us posted please.
 
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It sounds like you have an advantage over most people on the radio.
I think you can get thru this, maybe without a scope.
Had I known that a scope was not available I might have taken this in another direction; And going in another direction with the trouble shooting techniques is still and option.
The poster by the name of Robb had excellent advice but I think we have worked our way through his suggestions.
Information is key in the process.
I find myself wondering about the replacement parts. (pll & vco)
What I mean to say; Is it possible that they were defective or were they know to be good?
Did you remove these parts from another radio, or were they new and known to be good.
How are your soldering/desoldering skills.
Maybe you could kind of fill me in on some of the background information.
Another question I have is "Do you think it is appropiate to move our communications efforts to Pm?"
 
Robb said:
I have no formal education in electronics at all, and I can fix radios. Might take me a bit longer to fix something; but I sure don't give up until I do fix it.

Don't give up; you are just getting warmed up!

I look for the most likely failures in radios and that is how I fix them. Working on a hacked-up radio really makes it far more difficult when a schematic is the best and only guide. Put it back to stock first; that should get you going. The most common failures I've found are in my first post.

You are working on a radio that has old caps in it and has probably been handled by Golden Screwdriver operators/modifiers; keeping that in mind is what I would do when working on that thing.

Keep us posted please.
Click to expand...

I absolutely agree with this.
When you have a lot of experience you become familiar with typical failure modes and when you find yourself working on something that someone else has worked on before you, you are not familiar with this and makes it much more difficult to anticipate what the problem might be. ( when you work on something that has been worked on before by an inexperienced person you tend to find really odd and bizarre things that have been done to the radio )
Since these are outside of what you see for typical failure modes they usually involve much more time and effort to find what has been done to the radio.
Sometimes these things that have been done by an inexperience person make no sense at all.
For example: Most typical failure modes are component, solder connections, over-voltage and operator error.
Typical components to fail are electrolytic capacitors. (it used to depend on age )
Intermittant solder connections.
Lightning damage or mains overvoltage. (applicable mostly to mains powered equipment)
And the last most common failure is related to operator induced failures.
Reverse polarity from power supply.
Hooking up something they should not have hooked up, etc.
 
I have no problem with Pm. My only thing is others might benefit from our dialogue. The PLL was purchased "new" from Barkett Electronics. The VCO was from a working 148GTL. I will describe what I found when I opened the radio.

Volted final: Reversed

Opened clarifier: Reversed (looked like the standard hack)

Tone switch was removed and in its place was a 3 position rotary switch with three crystals installed on a small pc board. No marking on the pc board. one crystal was a 11.3258, one was 11.4758, and one was a 11.1758 (?). There are two small trimmer pots on the pc board. The X3 crystal was removed from the radio and a trace on pc board directly opposite (underneath) was cut. All was returned to normal.

Several traces were cut on the back of the slide switch board. All were returned to normal.

There was also an extra audio jack added in the rear. That was removed.

C146 and C147 were removed, and a 510 "poly" capacitor was installed on the solder side. Returned to normal.

TR24 was removed. Replaced

Several resistors were cut. They were all fixed.

D52 was removed. Replaced

I hope this offers some "clue". Thanks for all the help.
 

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