RCI 2970N2 " no bias present" issue

[Danzik]

Hi, I am kinda just getting started working on radios of this sort anyways and I have a RCI 2970N2 board model EPT695015BR the radio has hardly any output wattage at all but I can hear it on another radio close by, the things I notice that might be a hint to what's probably wrong with it is that after pulling the mirror plate and checking all 3 prongs there is no bias showing at all so I cannot adjust them to 50ma. Anyone know what components I might need to be checking here ? I do have and can adjust the 3 bias pots for 4 volts on the resistors next to the finals and have done this to test the radio but no bias reading off the prongs. I do also have 6 vdc on the center pin of the driver and both final transistors. This radio is kinda a second hand basket case but I figure it is a good one to learn on ! It also has a top gun MD-1 installed in it which looks like it was installed correctly but have thought about removing it just to make sure it is not faulty, any thoughts or suggestions would be greatly appreciated, thank you

 

[sp5it]

Use RF probe to find where power disappear.
Check bias diodes, and driver/finals.
Mike

 

[Danzik]

Use RF probe to find where power disappear.
Check bias diodes, and driver/finals.
Mike

I have one I think a old dog ! But am going to have to learn more about how to use it and where to start and end to look for the rf power. I have removed the driver, finals and modulator/regulator and they checked out good along with the diodes right next to the final transistors and they were good

 

[Danzik]

I now have at least some power out as I found a grounding tab shorting out C221 but it still isn't putting out what it should and still not getting a ma reading off the mirror prongs

 

[nomadradio]

I think that one is new enough to have MOSFET final transistors.

Or maybe it has the pre-2007 bipolar RF transistors? Has a big bearing on what to check next.

Any chance the two finals have been replaced at some time in the past? When they fail, parts in the finals' input circuits can be damaged. That can prevent seeing proper bias current.

73

 

[Danzik]

I think that one is new enough to have MOSFET final transistors.

Or maybe it has the pre-2007 bipolar RF transistors? Has a big bearing on what to check next.

Any chance the two finals have been replaced at some time in the past? When they fail, parts in the finals' input circuits can be damaged. That can prevent seeing proper bias current.

73

Yeah, they probably had been replaced or at least messed with. They are irf520. I did end up getting the radio working last night! Which is great the main problem was that grounding tab grounding out that disc capacitor as they had that grounding tab on the component side of the board and it was supposed to be on the bottom, the other problem I had I must have accidentally broke off the center pin on the modulator/ regulator transistor while I was soldering them back down to the board after testing it, I couldn't see it because the top gun md-1 wire was in the way, anyways it is now putting out great power however still no bias reading on those prongs and I have to repair or replace the CPU as the display is messed up other than that it's all good !