On Sun, 8 Feb 2009 17:36:01 -0500, Gary Poland wrote: >If I remember correctly the explaination I got from Drake for lowering the >power to 120 watts was for PA stability on 40 meters, keeping the predriver >below 1/2 turn.
I ran into the 40 meter instability problem on two rigs in the 3000 serial number range, my own TR7 with 2sc2879 replacement finals and one that I picked up recently for resale with a completely stock PA. Both had the version 2 predriver. I found that the situation could be improved somewhat by increasing the low frequency negative feedback at Q2201, by reducing the value of R2203. It's possible that doing the same thing to Q2202/R2212 might help as well. Even with this mod, the second TR7 required a delicate balance between ALC setting and predriver gain setting to avoid 40 Meter oscillation with specified power out. I think I ended up with the predriver gain set just below maximum by a little ways. There appears to be a broad but definite gain peak in the 9.3 to 9.7 MHz area, and I believe this is responsible for the instability problem. I don't know what the root cause is at this point, and it does not seem to affect all units. 73 -Jim -- Ham Radio NU0C TR7/RV7/R7A/L7, TR6/RV6, T4XC/R4C/L4B, NCL2000, SB104A, R390A, GT550A/RV550A, HyGain 3750, IBM PS/2 - all vintage, all the time! HyGain 3750 User's Group - http://groups.yahoo.com/group/HyGain_3750/ http://radiojim.exofire.net http://incolor.inetnebr.com/jshorney http://www.nebraskaghosts.org _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
