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Cobra 139xlr help!!!!!
- Thread starter 989
- Start date
check TR36, a 2SC1419 transistor. physically it looks like the driver or final, meaning it has a metal tab on it with a small bolt through it and will be mounted to a part of the metal chassis.
this particular transistor is mounted on that little metal wall that goes around the PLL section (the UPD858 chip) look on the rear right corner of this metal wall with the radio facing you.
this transistor is known for overheating and causing drift on SSB because its located so close to the IF crystals.
secret CB used to sell a kit to move the transistor to a better location, and they did a write up on it in Secret CB vol.2. here is a link to that write up:
http://www.cbtricks.com/pub/secret_cb/vol_02/graphics/secret_cb_vol_02_pg45_50.pdf
you can use this write up to move your transistor after you replace it.
there is one thing though. the 2SC1419 is known to be underrated for its purposes inside these radios and they should all be replaced with stronger parts.
there are four of them in a 139XLR. one on the power supply board, one mounted to a heatsink on the rear of the chassis, one on the same wall as the driver and final, and the one we are talking about, TR36, which is near the PLL.
they should all be replaced with 2N6487 transistors, NTE152 transistors, or NTE291 transistors.
all of these will work fine, and it doesnt have to be the same part in each location. they are pretty easy to find also.
if replacing TR36 doesnt fix it, check TR37 and R153.
good luck,
LC
this particular transistor is mounted on that little metal wall that goes around the PLL section (the UPD858 chip) look on the rear right corner of this metal wall with the radio facing you.
this transistor is known for overheating and causing drift on SSB because its located so close to the IF crystals.
secret CB used to sell a kit to move the transistor to a better location, and they did a write up on it in Secret CB vol.2. here is a link to that write up:
http://www.cbtricks.com/pub/secret_cb/vol_02/graphics/secret_cb_vol_02_pg45_50.pdf
you can use this write up to move your transistor after you replace it.
there is one thing though. the 2SC1419 is known to be underrated for its purposes inside these radios and they should all be replaced with stronger parts.
there are four of them in a 139XLR. one on the power supply board, one mounted to a heatsink on the rear of the chassis, one on the same wall as the driver and final, and the one we are talking about, TR36, which is near the PLL.
they should all be replaced with 2N6487 transistors, NTE152 transistors, or NTE291 transistors.
all of these will work fine, and it doesnt have to be the same part in each location. they are pretty easy to find also.
if replacing TR36 doesnt fix it, check TR37 and R153.
good luck,
LC
you got it.
be sure to move TR36 to that heatsink on the back wall, as it does cause SSB drift when its mounted in the stock location.
i like to solder R153 (i replace it with a 5 watt resistor) right to the leg of the new transistor so that it isnt causing the drift when it heats up.
LC
be sure to move TR36 to that heatsink on the back wall, as it does cause SSB drift when its mounted in the stock location.
i like to solder R153 (i replace it with a 5 watt resistor) right to the leg of the new transistor so that it isnt causing the drift when it heats up.
LC
I've had to do 3 radios this week where TR-36 was mounted next to the crystal, and had to move it, along with R-153. Here's a Cobra service bulletin on that very same problem.
http://www.cbtricks.com/radios/cobra/139xlr/graphics/cobra_139xlr_tb1218.pdf
Also, you can use 2SC1061 to replace the 2SC1419's too.
LC's got you in the right direction. Good luck!
~Cheers~
http://www.cbtricks.com/radios/cobra/139xlr/graphics/cobra_139xlr_tb1218.pdf
Also, you can use 2SC1061 to replace the 2SC1419's too.
LC's got you in the right direction. Good luck!
~Cheers~
found the post finally that suggested the 2n6487 but when you start trying to cross it it doesn't come back to any of the other mentioned transistors so my question is was this an error on the part number Loosecannon?
no, 2N6487 was the part i was using back then.
This is a pretty general application for this type of transistor so there are many that could work.
I was steered toward that part by another tech here on the forum back then, but i have since moved over to using TIP regulators pretty much exclusively.
remember that it's not the transistors current rating that determines its running temperature. Its the current it has to pass and the heatsink it's attached to that determine temp.
any of the transistors listed in this thread will work just fine.
LC
This is a pretty general application for this type of transistor so there are many that could work.
I was steered toward that part by another tech here on the forum back then, but i have since moved over to using TIP regulators pretty much exclusively.
remember that it's not the transistors current rating that determines its running temperature. Its the current it has to pass and the heatsink it's attached to that determine temp.
any of the transistors listed in this thread will work just fine.
LC
Reason I asked it I did a full recap of a Robyn sb520d worked okish before also changed all the c1419s to that part and now have no am modulation I read that tr26 I think it is can cause this just wanted to confirm this part isn't the issue.
yeah TR26 is the AM regulator, so it could cause a problem, but the part selection isn't it i don't think.
i know how it is when you get to restoring a radio and don't check it until you've replaced a bunch of parts and now aren't sure which part might have caused the problem.
what are the voltages you are reading on TR26 and what is your deadkey set?
LC
i know how it is when you get to restoring a radio and don't check it until you've replaced a bunch of parts and now aren't sure which part might have caused the problem.
what are the voltages you are reading on TR26 and what is your deadkey set?
LC
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