[AMRadio] SB-10 Modification (was DX-100B on Phone - bad audio distortion / oscillation)

Sat Dec 10 14:35:33 EST 2011

OK Rick,  I  have a TV-7 checker,   yours is likely OK.  Sometimes  people  run excessive  grid   drive.   Running 6ma. is what you  want to  get  for 2 6146 finals.  I can  peg the meter with the drive  cranked up
all the way.  Correct  all the mods.  Check connections-  yes there are  real bad builders.  [excessive  solder]   Check  resistors-  they  should be as marked-  most  are 10% tolerance.  If out  of  range, replace them.
no Radio Shack?  where do you  live?  I  have  2 in my  area- Erie PA.  is not a  big town.    Drive pot-  these  are  known to be a problem-  check it with an ohmmeter for  operation-  clean it  if necessary.  Boatanchors 
are  always fun-  just  be  patient-  you will soon have the bugs worked out.  Just look on the brite side-  its not a  rice box-  you   don't have to send it  to  the factory  for repair...73 Russ.

--- On Sat, 12/10/11, Rick Poole <wa1rkt at arrl.net> wrote:

> From: Rick Poole <wa1rkt at arrl.net>
> Subject: Re: [AMRadio] SB-10 Modification (was DX-100B on Phone - bad audio distortion / oscillation)
> To: amradio at mailman.qth.net
> Date: Saturday, December 10, 2011, 7:40 AM
> At 07:25 AM 12/10/2011, wb3fau at att.net
> wrote:
> 
>  >>>>>
> >very  interesting Rick.  Not  enuf 
> drive.  Clamper not  working  right.
> <<<<<
> 
> Good morning, Russ.
> 
> Since I started removing the SB-10 mod, and in particular
> after I 
> removed the long run of coax that goes from the driver
> output out the 
> back of the unit, and replaced the 1-inch length of bare
> wire to 
> reconnect the driver to the finals, I am now getting
> (barely) enough 
> grid drive, can prod it up to 4.5-5 ma.  And the
> clamping pot now 
> works.  So, making progress.  :-)
> 
> There are, of course, some undocumented mods to this, as
> with most 
> Heathkits it seems.  For example, there is a 27K
> resistor that goes 
> from the bias supply to pin 1 of one of the 5V4 (pin 1 is
> uncommitted 
> in that tube so the builder presumably just wanted to use
> it as a tie 
> point).  Then it goes, via shielded wire the shield of
> which is not 
> grounded or connected anywhere, halfway across the
> transmitter to 
> another 27K resistor that connects to two capacitors in
> parallel to 
> ground, then the input end of that RC combination goes to
> the far end 
> of the transmitter and connects... nowhere... it just
> connects to a 
> terminal on a terminal strip and that is all.  I have
> no idea what that is for.
> 
> I think the original builder of this thing must have owned
> a solder 
> factory.  Large... huge... blobs of solder
> everywhere.  I'm cleaning 
> them up in areas where I'm working and in areas where they
> look 
> particularly bad and for now am leaving the rest of them
> alone.
> 
>  >>>>>
> >  Are there  any  waxy coupling/ bypass
> capacitors  anywhere in 
> > these  circuits?
> <<<<<
> 
> Most of that stuff seems to have been replaced.
> 
>  >>>>>
> >Do  you have any spare  12BY7s  or 5763
> tubes?
> <<<<<
> 
> I have some 12BY7's, but no 5763's.  I suppose I
> should buy one or 
> two.  I do have a TV-7D/U tube tester (which I doubt
> has been 
> calibrated since the dawn of time) and the tubes all check
> OK in that.
> 
>  >>>>>
> >Less grid drive  on 10m  is common.
> >But  you should get plenty on 40m.
> <<<<<
> 
> Much of the SB-10 mod was in the bias circuit to the
> finals, to put 
> them into class ABx during SSB operation.  That mod
> wasn't done 
> exactly as specified in the Heathkit modification kit...
> for example, 
> there is an 8.2K resistor in series with the pot that
> adjusts the 
> bias, which isn't documented anywhere.  I'm pretty
> confident that 
> once I get that circuit back to original factory
> configuration, the 
> grid current issue will go away.  Assuming I do it
> right.  :-)
> 
> Big problem right now is getting parts.  I've reached
> the point where 
> I can't continue for a while, for lack of a 2.2K
> resistor.  Stop 
> work, order some parts, wait for them to come in, resume
> work, find 
> another part that needs replaced, stop work, order more
> parts, 
> lather, rinse, repeat.  You guys know the drill. 
> I try to figure out 
> everything I need but there is always that something I
> missed, which 
> leads to other "somethings" I missed.  I order
> resistor and capacitor 
> assortments but of course I use up a certain value on other
> projects 
> and when I need one it's not there.  I have ordered an
> assortment of 
> 1/2W, 1W and 2W resistors from JPM Supply; they should be
> here 
> sometime next week and then I'll be able to continue. 
> I could build 
> a 2.2K resistor out of a bunch of others in parallel but I
> don't want 
> to do that if I can avoid it.  And I can't just go
> down to the local 
> Radio Shaft anymore... :-(
> 
> Rick WA1RKT
> 
> 
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