Cobra 29 Problems

[Hilift]

I have a cobra 29 that I had a mosfet conversion done on about a year ago. I have the radio in my semi so it would have a good amount of vibration. It worked good until about a month ago when it would always be keyed. I did a little testing and the meter swings and the TX light comes on as soon as it is powered up. If I have a test radio on the bench next to it it will swing the meter and if I key the mic it modulates fine. But when I release the mic it stays keyed. However if I put something under the radio right under the meter to hold up that side of the radio and push down on the corner where the power cord comes in it flexes the board and it will work correctly.
I tried to see if I could see any bad solder joints or anything but I haven't gotten anywhere.

 

[midnight special]

try tightening the screws that hold the board to chassis... being loose sometimes makes a radio screwy,,,,

 

[Hilift]

Yes I did that. I also made sure my grounds where soldered good

 

[Handy Andy]

You may need to "shift" the radios' board too.

Check to make sure the case is "plumb" as in square. Just back off screws and re tighten them - do this also for the heat sink "squiggly" bracket that holds the Driver and Final - they too have riveted and soldered tabs - would be a good idea to "compare" the innards to the radio next to it.

The Keying event may be due to a loose mic socket or wire - so all those headers up by that Mic jack will need to be looked into. That can occur due to when the case was put together and a mic wire got pinched now time has passed and the mic wire's insulation has failed.

The "loom" or wire harness also has those TX and RX wires that go to the back of the board by the Final and Driver. Note the color of those wires, those may be your major culprit.



The main loom of wires are Zip tied to the side panel - an d most of the time they will outlast the life of the owner as far as needed maintenance - but as you can see those wires are pretty close to the speaker panel clamshell and even the bolts the hold the Front panel to side of chassis panels can get weak and "elongate" making the panel to case mess even more prone to failures from torque the case gets while you drive in bumpy roads and potholes - the sheer bouncing of something weighing over 2lbs - can and will - shake something loose sooner or later.

I know that you may also need to re-flow solder on parts that mount to the side panel - this includes that Audio Chip - not a problem at the moment but when you look into seeing if the board is "plumb" you may want to loosen the mounting screw the Audio chip tab uses holding it to the case, allow it to shift and then retighten making sure that the Mica insulator that keeps the tab from "shorting to case or chassis" is properly seated and the tab of the chip itself is centered.

 

[kaos513]

you may wanna get a microscope and check the board sometimes it can be a bad solder joint the naked eye cant see.

 

[Hilift]

Alright thanks for the help. I will take a look and see what I find.

 

[Hilift]

You may need to "shift" the radios' board too.

Check to make sure the case is "plumb" as in square. Just back off screws and re tighten them - do this also for the heat sink "squiggly" bracket that holds the Driver and Final - they too have riveted and soldered tabs - would be a good idea to "compare" the innards to the radio next to it.

The Keying event may be due to a loose mic socket or wire - so all those headers up by that Mic jack will need to be looked into. That can occur due to when the case was put together and a mic wire got pinched now time has passed and the mic wire's insulation has failed.

The "loom" or wire harness also has those TX and RX wires that go to the back of the board by the Final and Driver. Note the color of those wires, those may be your major culprit.

View attachment 28098

The main loom of wires are Zip tied to the side panel - an d most of the time they will outlast the life of the owner as far as needed maintenance - but as you can see those wires are pretty close to the speaker panel clamshell and even the bolts the hold the Front panel to side of chassis panels can get weak and "elongate" making the panel to case mess even more prone to failures from torque the case gets while you drive in bumpy roads and potholes - the sheer bouncing of something weighing over 2lbs - can and will - shake something loose sooner or later.

I know that you may also need to re-flow solder on parts that mount to the side panel - this includes that Audio Chip - not a problem at the moment but when you look into seeing if the board is "plumb" you may want to loosen the mounting screw the Audio chip tab uses holding it to the case, allow it to shift and then retighten making sure that the Mica insulator that keeps the tab from "shorting to case or chassis" is properly seated and the tab of the chip itself is centered.

Thanks for the suggestions. I looked at the radio before I left for work this morning. I cut the zip ties that hold the wire loom and checked for any that appeared to have been crimped. I also loosened the screws again and made sure the board was square in the case.

Kaos I dont have a decent glass to look at my board with so I haven't done that.

I did try pushing on the board to see if it would flex and go to normal operation but if I just push on it it doesn't. I have to torque the entire radio. Then the led goes back to green and the meter settles back and it works normally while I have the radio torqued.

If there are anymore suggestions that would be great and I will look at it closer this weekend or when I have some time.

 

[Handy Andy]

Then you have a lifted trace of copper foil - more than likely near an edge of the board.

I would start by using a marker or simple stylus "pen" they used to offer for Phones and Tablets using it's tip as a press probe and press down on the foil side - starting around those headers or wiring that goes into the board from the Mic socket.

IF you can could you post a pic of the foil side, try for good steady shot for resolution and post it here - at least me and some others can go over it to possibly shorten the length of time you gotta research this...

For you to "torque" the board tells me there are areas on the foil side that separated when the board is left to "spring back" into original shape before it gets mounted - then mounting places the torque and we have to locate the areas with that effect the most pronounced and usually that is in areas around the perimeter of the board near the edges.

This isn't an easy task and we cannot guarantee any results due to the physical nature of Copper foil and it's ability to tear - shred itself off of soldered components.

Since RX and TX - the RX "forces off" TX, else if left alone and pin 4 is left to float, TX comes on due to a resistor that "shunts" power off the line that tells the TR21 to turn on, that normally does not have power thru it when PTT is shorting Pin 4 of Mic socket to Pin 1 (Ground) as in the Mic PTT switch opens Pin 4 kills speaker but grounds Pin 3 which turns off (starves) a transistor and routes power from RX sections thru diodes steering it into ground - but allows the TX transistor (TR21) to receive power normally dropped thru a resistor that grounds thru Pin 4 and it turns on the TX side.

 

[Hilift]

Then you have a lifted trace of copper foil - more than likely near an edge of the board.

I would start by using a marker or simple stylus "pen" they used to offer for Phones and Tablets using it's tip as a press probe and press down on the foil side - starting around those headers or wiring that goes into the board from the Mic socket.

IF you can could you post a pic of the foil side, try for good steady shot for resolution and post it here - at least me and some others can go over it to possibly shorten the length of time you gotta research this...

For you to "torque" the board tells me there are areas on the foil side that separated when the board is left to "spring back" into original shape before it gets mounted - then mounting places the torque and we have to locate the areas with that effect the most pronounced and usually that is in areas around the perimeter of the board near the edges.

This isn't an easy task and we cannot guarantee any results due to the physical nature of Copper foil and it's ability to tear - shred itself off of soldered components.

Since RX and TX - the RX "forces off" TX, else if left alone and pin 4 is left to float, TX comes on due to a resistor that "shunts" power off the line that tells the TR21 to turn on, that normally does not have power thru it when PTT is shorting Pin 4 of Mic socket to Pin 1 (Ground) as in the Mic PTT switch opens Pin 4 kills speaker but grounds Pin 3 which turns off (starves) a transistor and routes power from RX sections thru diodes steering it into ground - but allows the TX transistor (TR21) to receive power normally dropped thru a resistor that grounds thru Pin 4 and it turns on the TX side.

Here you go. I hope that helps

 

[sonoma]

I do not know if it is me or not but blowing the pic up I see a lot of solder joints I would be resoldering since some of the legs do not look like they are soldered good . a bunch of them look very thin on solder with what looks like very little solder on them. some look like legs that should not be soldered have solder on them also.

 

[Crossbow]

I do not know if it is me or not but blowing the pic up I see a lot of solder joints I would be resoldering since some of the legs do not look like they are soldered good . a bunch of them look very thin on solder with what looks like very little solder on them. some look like legs that should not be soldered have solder on them also.

You got that right!

 

[Hilift]

So a friend of mine came over and we were working on it. It seems like it is in the face. We took the face off and you can leave the radio lay on the bench and just torque the face and it reacts the same. He goes by 6.0 litre on here and will maybe say something I missed.

 

[Hilift]

So we seemed to fix it. The screw you can see in the picture was backed out and against the wire the the screwdriver is pointing to. So we straightened that pot out "delta tune" and could duplicate it by using something to ground that leg of the pot.

So I thought you could see the screw but it is just going into the face of the radio

 

[Handy Andy]

Glad to see you have it fixed, but you may have more problems down the road - might want to take a moment and double check some soldered areas.

I found some "pucker" areas of solder that indicates the board shifted either in the machine or the board got pulled when parts still had molten solder that had not cooled and solidified. - this happens on Tank Cans and larger heavier coil and discrete parts. Nothing new, but should have been caught at the factory before you had to Bend Fold Spindle And Mutilate the radio to get it to work again.


Not a flame to anyone, just these things are delicate and the ones that soldered in the Varactors should have caught the error and simply reflowed - as said earlier in the post, you may have similar problems again with this one later...

 

[Hilift]

So would you suggest re-soldering them?