Hi Gary, The concern here is RF bypassing. Not to keep RF out of the device, but to keep RF from GETTING out of the device. Which means smaller value ceramic capacitors in addition to any electrolytics used. Diode junctions can produce wideband RF noise that could theoretically impact receiver performance. If you look at the manufacturer data sheets for the 3-terminal devices, they recommend low value bypass caps on both input and output, as physically close to the device as possible. I figger that on the device legs is pretty close... This is the logic behind bypassing the Zeners. There's a repro of the QST note that the OP referenced on the Drake Museum web site.
Otherwise, I saw an LM-317 based home built supply years ago that the builder had failed to include the manufacturer's recommended bypass caps in. It was very spikey, going from TX to RX on a 5 watt transciever caused the lamps in the radio to get real bright for a second. Installing one critical cap fixed that. Ever since then, I've been almost evangelical about following the data sheet recommendations for bypass capacitors. 73 -Jim On Sat, 24 Apr 2010 09:22:15 -0400, Garey Barrell wrote: >Ron & Jim - > >Maybe I'm missing something, but according to the documentation I have, >all of the regulators and Zeners seem to have input and output bypass >capacitors. > >Bypassing multiple pins of a 723 might be counterproductive, actually >degrading noise reduction on the output. There is fast, high gain >amplification going on in there! This is still one of the best >regulators around. > >Most caps seem to be larger than datasheet values, but based on other >Drake gear I assume they are tantalum and so work as well as lower value >caps at the higher frequencies. > >I don't have any numbers, but I suspect that a 1 uF tantalum is just as >good (low dissipation and inductance) as a multilayer ceramic. > >I haven't worked on a lot of TR-7s, usually find that reseating the >cards "fixes" them! :-) Perhaps early units didn't have these caps? >Doesn't seem likely. > >Certainly added caps wouldn't make things worse, with the possible >exception of the 723 "Christmas tree", but I question the value..... > >73, Garey - K4OAH >Glen Allen, VA > >Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs ><www.k4oah.com> > > >Jim Shorney wrote: >> I did do some of the Zener bypassing, but did not measure the results. It >> didn't make things any worse, I will say that, and I have pretty good MDS >> numbers on my 7. In addition, I also bypassed the 3-terminal regulator ICs to >> manufacturer's spec with some surface mount caps on the legs of the IC right >> next to the body. Again, no measurements, but it didn't hurt, and I've >> learned >> over the years that they tell you to put those caps there for some very good >> reasons. >> >> I did find it interesting that all of the TR-7s that I have worked on so far >> have had the IF gain control maxed out already anyway, in some cases >> apparently >> by the good folks at factory service. >> >> 73 >> >> -Jim >> >> On Fri, 23 Apr 2010 17:30:55 -0700 (PDT), Ron wrote: >> >> >>> Has anyone actually done and measured the results of bypassing the zener >>> diodes and regulator chips as discussed on QST Feb 1989? The author had >>> "anecdotal" data only. Was just wondering if there was any instrument >>> measured data floating around out there. Not really planning on doing >>> this, just more of a curiosity. >>> >>> 73, >>> Ron WD8SBB >>> >> > >_______________________________________________ >Drakelist mailing list >Drakelist@zerobeat.net >http://mailman.zerobeat.net/mailman/listinfo/drakelist -- 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 Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
