On Tue, 28 Dec 2010 12:13:39 -0500, Michael & Sue Trussell wrote: >The TR7 appears to be oscillating on 40 meters only when used with an external >antenna. This does happen when I use a dummy load.
<snip> > Not having a lot of experience with the internal working of this rig I wanted > ask to see if anybody else has had this experience, and what action was taken > to remedy this condition? Hi Michael, As we already discussed, there are two "fixes" for this problem. One is to decrease the maximum power level available from the radio by adjusting down the ALC control in the bottom of the radio. This is a pot that should be accessible through a hole in the bottom panel near the front right side. Adjustment is described in the service manual. Adjustment of the predriver gain pot may also be necessary for proper output on 10 Meters afterwards. A perhaps better fix is described in my note here: http://zerobeat.net/drakelist/smf/index.php/topic,224.msg440.html#msg440 This involves adding a resistor in parallel with a resistor on the Version 2 predriver board, to increase the negative feedback at lower frequencies. On two units I have had on my bench with this problem, this solved the 40 Meter oscillation problem while still allowing the maximum output power level to be set to spec. Once the resistor is added, the ALC and predriver gain adjustments will need to be performed as above. Note that this mod applies only to the Version 2 predriver board; I have not had a sample with the Version 1 board to play with yet. If you don't have a service manual, usable PDF copies can be found on the internet for free download, and high quality copies can be purchased from internet vendors. 73 -Jim -- Ham Radio NU0C Lincoln, Nebraska, U.S.S.A. TR7/RV7/R7A/L7, TR6/RV6, T4XC/R4C/L4B, NCL2000, SB104A, R390A, GT550A/RV550A, HyGain 3750, IBM PS/2 - all vintage, all the time! "Give a man a URL, and he will learn for an hour; teach him to Google, and he will learn for a lifetime." 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 [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
