[AMRadio] DX-100 / Apache modifications on the Web

[Mahlon Haunschild]

All,

What with the recent discussion on the subject of Apache modifications
on the list, I thought I would share the links to the source material I
found on the Web for this subject when I embarked on re-working my
Apache last year.

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The W3SCC / W3YGC Apache audio circuit can be found at:

http://www.amfone.net/AMPX/71_2.gif

I applied this circuit to my Apache without change, with great success. 
The whole thing can be built on a strip of perfboard and wired into the
Apache in place of the existing audio circuit, making this mod easily
reversible.  Note that the heater pins of the 12BY7A are mis-labeled on
the diagram (no biggie).

Andy Bell, KC4FCU, wrote some comments in an old issue of the AM
Press/Exchange outlining some simple changes:

http://www.amfone.net/AMPX/114.html

The Self-Modulated Screen Grid by Bacon, WA3WDR, advocates moving the
Apache final amplifier screen supply point from the output of the mod
transformer to the HV-input-side.  This allows the screens to
self-modulate because of the impedance of the screen dropping resistor. 
This also gets approx. 20 mA DC out of the modulation transformer
secondary, which can only improve its linearity:

http://www.amfone.net/AMPX/98.htm

Factory mods/updates for the Apache and SB-10 SSB adapter (transcribed
from the original Heath notes by VE3FAB).  Note that the final
amplifiers in early Apaches weren't neutralized; unfortunately,
retrofitting this in the factory way requires a different mode switch
wafer:

http://www.mines.uidaho.edu/ftp/pub/Glowbugs/BAarchives/heath.apache.bulletin

DX-100 Modifications by Ed, AA8TV.  Useful material because the DX-100
modulator is similar in concept to the Apache's.  This write-up and the
following one got me to thinking of the remaining mods (few though they
may be) that I might perform on my Apache.  Of particular interest is
his negative feedback circuit:

http://www.amwindow.org/tech/htm/dx100/dx100.htm

WA1HLR's DX-100 Modifications (again, included for the same reason as
the preceding link):

http://www.amwindow.org/tech/htm/dx100.htm

DX-100 Screen Optimization by Dean McGorill (again, included here for
the sake of completeness):

http://www.amwindow.org/tech/htm/scrnmod.htm

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As I have previously written I can personally attest to the W3SCC /
W3YGC Apache audio circuit as well as replacing all selenium rectifiers
with silicon parts and doubling the value of all filter capacitors.  My
Apache ROCKS on AM now!  Because it's so good at the moment, I've been
reluctant to open it up again to apply a negative feedback circuit, but
two friends of mine have done this to their Apaches and have spoken
highly of it.

A few of other thoughts:  I use 6146Bs in my Apache without any
problems, but mine has the neutralization update.  Svetlana 6CA7/EL34s
work fine in lieu of the stock (unobtainable) Mullard tubes.  Do be a
pal and operate the transmitter and modulator within the manual's
specifications.

HTH.  regards,

Mahlon - K4OQ