VR-11 ??

[1iwilly]

hello to all i have a Cobra 2k that vr 11 ssb power pot does nothing when adjusting took it out and checked with the meter it's working as it should
the second issue let's say on am the dead key is 4 watts apply modulation goes to 5 1/2 watts then out of the blue 6 1/2 check a few times to verify it
it does it if powered on by internal power supply or external power supply

i notice that c132 on my board is a 2.2uf electrolytic cap and not a .0047 disc cap
also, R161 has 6 volts all the time where coming from the middle leg of tr34 without xmt

 

[Dr. Frankenstein]

You say C132 is wrong, it may be causing the VR11 problem, either way, you will have to put the 0.0047 back in place of the 2.2uf. The .0047 would have a marking of 472 on it.

VR11 is the ALC adjustment and is meant to stop over-modulation on SSB, and does not affect AM mode that is controlled by the AMC (VR12). These radios can be made to over-modulate very easily, and they don't sound good.

Turning VR11 counterclockwise lowers your modulation level and power level so that you don't overmodulate. It is better seen on a scope, that is if the audio limiters are in place.

I usually set VR11 to around 10:00 and run the Mic Gain at around 2:00 on the front of the radio for the best scope pattern.

When set up right while on SSB you can see the watt meter kind of hold back after your first few works. Kind of like lowing to its best RF point showing that the ALC is working. Sure you can use the Mic Gain for this, but if set up right the Mic Gain can be cranked to full Clockwise and it still will not over-modulate due to the ALC holding the level down.

Getting to the 6 volts at the collector of TR34. Try putting the channel selector in between channels to simulate an "Out-of-Lock" and see if the 6 volts goes away. If it does the 6 volts is coming from pin-1 of IC4 by way of TR25. and on the other side of R161.

Good luck...........

 

[1iwilly]

You say C132 is wrong, it may be causing the VR11 problem, either way, you will have to put the 0.0047 back in place of the 2.2uf. The .0047 would have a marking of 472 on it.

VR11 is the ALC adjustment and is meant to stop over-modulation on SSB, and does not affect AM mode that is controlled by the AMC (VR12). These radios can be made to over-modulate very easily, and they don't sound good.

Turning VR11 counterclockwise lowers your modulation level and power level so that you don't overmodulate. It is better seen on a scope, that is if the audio limiters are in place.

I usually set VR11 to around 10:00 and run the Mic Gain at around 2:00 on the front of the radio for the best scope pattern.

When set up right while on SSB you can see the watt meter kind of hold back after your first few works. Kind of like lowing to its best RF point showing that the ALC is working. Sure you can use the Mic Gain for this, but if set up right the Mic Gain can be cranked to full Clockwise and it still will not over-modulate due to the ALC holding the level down.

Getting to the 6 volts at the collector of TR34. Try putting the channel selector in between channels to simulate an "Out-of-Lock" and see if the 6 volts goes away. If it does the 6 volts is coming from pin-1 of IC4 by way of TR25. and on the other side of R161.

Good luck...........

thanks will look at it when the temp goes down a bit i replace tr34 cause the middle leg had been cut then solder back but no change

 

[nomadradio]

Ahh, make sure TR24 hasn't been plucked. This is the attenuator transistor that reduces mike gain when driven by either AM or SSB limiter circuits.

I run into non-functioning sideband ALC from time to time, and learned to follow the limiter function stage by stage with a 'scope starting at the collector of TR34, then to the collector of TR25. Won't see much more than 6/10 of a Volt there, but if it's missing, no sideband limiter.

73

 

[1iwilly]

Ahh, make sure TR24 hasn't been plucked. This is the attenuator transistor that reduces mike gain when driven by either AM or SSB limiter circuits.

I run into non-functioning sideband ALC from time to time, and learned to follow the limiter function stage by stage with a 'scope starting at the collector of TR34, then to the collector of TR25. Won't see much more than 6/10 of a Volt there, but if it's missing, no sideband limiter.

73

thanks will check it out

 

[1iwilly]

well, she is all fixed turns out tr-24 was put in backward. i mean there is a image on the board that shows the orientation of it.

 

[TM86]

well, she is all fixed turns out tr-24 was put in backward. i mean there is a image on the board that shows the orientation of it.

Kinda the opposite of the problem I had with another radio where the image on the board was backwards, and someone put the transistor in following the image.

But the main thing is, it works again!

 

[nomadradio]

it works again!

Rock on! That's what matters.

73