Don't overlook the PS7 as part of the feedback loop. There is a factory mod to put a .01uf (I think) cap across the 12 volt aux port. Even without any aux devices on my PS7 at the time, mine would go into oscillation on 40 meters and generally trip the PS7 off. Not exactly the same symptom, but RF feedback can come from some strange paths.
73, Ron WD8SBB --- On Tue, 12/28/10, Jim Shorney <[email protected]> wrote: > From: Jim Shorney <[email protected]> > Subject: Re: [Drakelist] MN2700 Tuner > To: "[email protected]" <[email protected]> > Date: Tuesday, December 28, 2010, 5:17 PM > 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 > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
