Cobra 139 XLR resistor between B&E

[Low_Boy]

Why in the world would there ever be a resistor between the base and emitter on a 1307 final on a cobra 139 XLR?

 

[nomadradio]

To stabilize the bias circuit, pretty sure. Cobra published an update. Said to add that resistor and some other stuff to any radio getting a blown final replaced.

73

 

[Dmans]

These 2 Service Bulletins.

73
David

 

[Low_Boy]

Thank You.

 

[Handy Andy]

I do 180 ohm but - eh?

 

[Low_Boy]

Andy, possibly you did this one. I think that is what was in here. I will have to take another look.

 

[nomadradio]

The other thing it does is to make the setting of the bias trimpots less "touchy" to small movements. Typically the correct trimpot setting before the update is nearly all the way to one end. This puts all the stress onto a tiny portion of the trimpot's resistance element. Tends to wear out the trimmer pot. Any time this setting seems way too sensitive, it could be time for a new trimpot. We see a lot of them that need to be replaced.

Some things just don't last more than 40 years.

73

 

[Low_Boy]

Thanks Nomad. Yes with it in the bias set nicely. I noticed another problem one of the voltage regulators must be moved to the back of the radio. TR36 because it really heats up the resistor behind it.

 

[Dmans]

This service bulletin for that problem.

73
David

 

[Low_Boy]

This service bulletin for that problem.

73
David

I bet you have worked on one or two of those before. First time I was ever in one and it reminded me of the washington.