Cobra 29 No Keyup

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1096
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Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 4:09

Radio Make and Model: Cobra 29 LTD
PCB Number:
PLL Number:
Test gear available: Bird 43p, Voltmeter, RF Generator, and misc other things....
Symptoms: No Keyup.... Meter light blew and then I lost keyup

What is working: Final, Driver, Predriver are all good and working.... TR16 and IC3 checked out ok also.... TX/RX light changes on keyup.... Loud audio through PA and talkback ( radio has factory talkback )

What has been tried: Checked voltages at TR16 and IC3, Final, Driver and Predriver.... R88 only has 0.41 volts and adjusting L19 does not change the voltage.... Should have about 3.2 volts at R88 on channel 40....

Did it suddenly happen ?: Yes

Anything else that may be of help?:

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Re: Cobra 29 No Keyup

Post by The Defpom » Thursday 19th Mar 2009, 7:20

Check the voltage between pin 11 and pin 21 of the PLL, to make sure that it has a supply, it sounds like a DC supply issue at this point.

I would also check R124 for 5V from the main 5V supply.

It interests me that the meter bulb blew at the same time, I suggest that you check your DC power supply for a fault as well, to ensure it is not putting more than 13.8V out.

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 9:33

There is no voltage on any of the pins except for 2 on the PLL, the voltage is only 0.40 volts on those 2 pins.... Keyed or unkeyed.... I replaced the 10.240 mhz crystal and still nothing.... R88 only has 0.41 volts with no change when adjusting L19.... I'll check R124 and see what I get.... I think something may have gotten into the case and caused it to short

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 10:02

R124 has about 1.5 volts on one side and about 0.42 volts on the other.... D8 is good but i have yet to check the diode in back near the DC input.... Appears to be good visually but looks are deceiving....

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Re: Cobra 29 No Keyup

Post by The Defpom » Thursday 19th Mar 2009, 10:11

Check the voltage at R70, it should be at least 8V, if it is not then there is a problem with the main voltage regulator circuit, D22 TR23.

If the voltage is OK then it could be the 5V regulator circuit, Z10.

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 11:25

Let me try those and see what I come up with.... Thanks for the help, I think I'm finally getting somewhere....

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 12:38

D22 tests ok under a diode test but the diode it self seems to have what appears to be ink all over the inside.... I think it is blown.... R70 checks out ok.... TR23 checks out ok.... Where is the 5v regulator????

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 14:37

Another new and interesting twist, is that the channel 9/norm switch goes to channel 48 instead of channel 9.... PLL is probably dead.... No voltage at PLL....

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Re: Cobra 29 No Keyup

Post by The Defpom » Thursday 19th Mar 2009, 14:42

Go back and read my last post, I asked for voltages at some points, this is to determine where the fault may be.

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 14:43

Those checked out fine at Tr23 and D22.... Can't seem to find Z10

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Re: Cobra 29 No Keyup

Post by 1096 » Thursday 19th Mar 2009, 14:45

R124 had 0.42 on the leg closest to the PLL and @1.5 on the side furthest from the PLL... R70 had 8 volts....

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Re: Cobra 29 No Keyup

Post by The Defpom » Thursday 19th Mar 2009, 15:09

Z10 on the diagram I was using may be called D19 in your radio, it is a 5V Zener diode, you should be seeing 5V across it, if not then check R98 (56 Ohm), as that feeds the 8V to the Zener, so R98 should have 8V on one side and 5V on the other, if the 8V is present but the 5V is not, then it could be a fault with any of the following parts: R98, D19, C112, C113.

You could try lifting R124 to see if that returns 5V to D19, if it does then it could be dropping from excess current draw from a problem in the circuit that the 5V Zener supplies.

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Re: Cobra 29 No Keyup

Post by 1096 » Friday 20th Mar 2009, 1:26

I did lift R124 after I tested it and nothing hapened so we can rule that one out but I'm going to take a look at those others and see what I get....

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Re: Cobra 29 No Keyup

Post by 1096 » Friday 20th Mar 2009, 3:36

D19 has 1.5 volts and R98 has 8 volts on one side and 1.5 on the side that connects to the zener.... Replaced C113 and nothing.... Removed and checked R98 and it is in spec.... R124 is in spec also....
C112 has 0.42 volts at it but I replaced it and it still gives nothing.... When I check R124 and R98 they both show the correct value of the resistor but yet still nothing.... I swapped D19 out of a good working into this one and it didn't change.... Still have only 1.5 volts.... R98 checked in at 55.2 ohms and R124 checked in at 9.9 ohms....

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Re: Cobra 29 No Keyup

Post by The Defpom » Friday 20th Mar 2009, 6:57

Replace R98 anyway, as the current flow through it could be causing it to change value, I have seen that happen myself before.

If you do not get 5V at D19 with R124 lifted then the fault can only be R98 or D19.

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Re: Cobra 29 No Keyup

Post by 1096 » Friday 20th Mar 2009, 7:26

It has to be one of those.... I'll replace R98 and see what I get.... I replaced D19 but nothing....

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Re: Cobra 29 No Keyup

Post by 1096 » Friday 20th Mar 2009, 9:29

Replaced all of them and replaced D14 and D15 also and still only have 1.5 volts.... IC3 has 1.5 volts on pin 7 in TX mode which it is supposed to have 2 volts in TX....

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Re: Cobra 29 No Keyup

Post by 1096 » Friday 20th Mar 2009, 10:11

R106, R93, and R110 have no voltage at them and I couldn't find voltages for those points.... Your schematic must different from mine.... My channel display will also read channel 48 on channel 40 when I hit the channel 9/norm switch.... I think ultimately I may have a dead PLL.... No voltage whatsoever on the PLL....

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Re: Cobra 29 No Keyup

Post by RFEverywhere » Wednesday 4th Jun 2014, 10:51

Hi

Did you ever figure out what the problem was?

I have 2 Cobra 29's, one LTD the other LX. They were bought brand new several months ago. I have the exact same issue with them. On the LX I get the correct frequency at TR16 but non of the voltages past IC3 check out.

I have posted on a different thread also but am not sure if anyone even comes on these forums anymore or not.

I have gone bald trying to figure it out as if I did not have enough falling out to begin with. I have been going at it for a little over a month now with no luck. I bought the LTD to check all the resistor junctions for the proper voltage so I could fix the LX but the LTD ended up dying the first night I took it out of the box. I think I shorted something out by accident when my hand slipped as I was taking a measurement.

I have literally all the tools needed:
Frequency counter
Scope
DMM
LC meter
Power/SWR meters
Soldering irons
Desoldering pumps
Dummy loads
Frequency generator (audio)
RF generator
Handheld receiver
Antenna tuners
Antenna analyzers
Dipoles
Verticals
You name it, I have it..... Not to mention IF cans, transistors, resistors, pots, caps, diodes, everything so there should be no reason why I should not be able to repair any radio. I guess THE MAIN thing I am missing is detailed knowledge of how the system works. I do have a good general knowledge of it but obviously no enough to be able to troubleshoot my way out of this situation. I have gone through the voltage charts and the schematics on a daily basis and still nothing.

I have done several mods and things so I am capable but I really need some help PLEASE.......

Any and all advices is welcomed.

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