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

[Rick Poole]

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