Cobra 25 ltd classic noise blanker problem.

[speedracer6g]

I am working on a cobra 25 ltd classic radio works great. I hooked this up in a vehicle and the noise blanker does not work. I checked the voltages on tr20, tr21, tr22, and tr23 and they are all spot on. I adjusted L19 and still no change. Anyone have any ideas?

thanks,

MIKE

[Rick]

Try a second radio before you go looking at the NB circuit, as maybe the vehicle or wiring causing the fault.

[speedracer6g]

this is actually on the bench with a sevice monitor. the noise blanker makes no difference if it is on or off. I installed it in my service van and the same thing. I usually pick up some noise with the one that is normally in there. It is actually the same model that i have in my van.

mike

[Rick]

If you have a scope, check to see what is happing at the NB pulse amp and the NB shunt switch transistors, and check the coupling caps in and out of the NB circuit.

[speedracer6g]

I replaced the one coupling cap i cant remember witch one it was. I know it was a 47pf cap. I do have a scope. wich testpoints do i need to check and what should i be seeing.

thanks,

MIKE

[Rick]

An easy to check the NB is to use an hair dryer or electric drill, close the antenna socket and try tuning L10 for minimum noise, if you put your scope on the base of TR22 you should see the NB pulse, if not check the diode D3 it may test alright but may fall, also check C138 and C139.
Most of the problems with NB are cold solder joints.

[speedracer6g]

got it going rick thanks for the help. bad diode.

thanks,

MIKE