DX-100B Problems - Final Report?

[Mike Hardie]

Preliminary tests indicate the DX-100B is working fine. The circuit isn't 
exactly original, more of that follows. Many thanks to the many list members 
who provided help, especially to Ken who sent a series of step by step 
emails that led to the "fix". The emails also reminded me that half of 
trouble shooting time should be spent thinking, and the other half doing.

Voltages were re-checked at all the tube pins and no large variations from 
an old reference list found. (I can't remember who gave me the list - it was 
some time ago, I can try to have it scanned if anyone wants a copy.) I didn't 
(And still don't) have any information on the amplitude of signals through 
the rig, poking around with a 'scope probe showed a signal was getting 
through, but whether or not it was the correct amplitude was unknown. The 
output from the buffer stage looked distorted on 160 and 80 meters, but the 
low output was occurring on all bands.

Initially attention was focused on the VFO stage. Taking the cover off the 
VFO wasn't as bad as anticipated, but a special "tool" had to be made for 
the screw that's not readily accessible due to the two large filter caps. 
(Screw closest to the chassis, non meter side.) A ¼" piece of steel rod 5" 
long was bent into sort of a "Z" shape. The top and bottom of the "Z" were 
¾" long and at 90° to the center 3 ½" part, then the ends were ground down 
to a screwdriver blade shape, one end in line with the center length and the 
other end at 90° to it. By reversing the tool back and forth the problem 
screw was removed. Also the shaft on the driver tuning control was removed 
by loosening at the insulated coupling and pulling it out. After removing 
the VFO cover no wiring errors were found but the soldering job was a little 
sub standard, so the VFO assembly was removed and "cleaned up". When 
re-installing the VFO I noticed that tightening the bandswitch nut pulled 
the top of the top of the VFO box down. So an extra washer was put as a shim 
between the bandswitch and the top of the chassis. Testing after re-assembly 
resulted in no difference in output.

Next the buffer stage. The two small slug tuned coils were suspect but 
unsoldering the connections between them and checking inductance indicated 
things were normal. It's very possible there was a cold solder joint that 
got fixed during the re-soldering of the connections but unfortunately no 
testing was done before further work.

A schematic for the Apache was looked at and it had a 1.1 mH choke between 
pin 8 of the 5763 driver tube and the resistor that leads to the power 
supply. (The resistor was a different value also but the Apache keying 
arrangement is completely different from the DX-100B.) My assumption was 
that with only a 27K resistor some pre-driver signal was being lost to the 
power supply. A choke was removed from a salvaged DX-100 final sub-chassis 
and installed between the 27K resistor and the terminal strip on the driver 
pot bracket. The rig was tried out and power output was 100 watts plus, but 
a new problem was present.

When rotating the amplifier tuning control the point of maximum output and 
the point of minimum plate current were about 10 dial divisions apart. Quite 
a while ago I'd read that this condition is indicative of a neutralization 
problem, and can be cured by soldering a length of wire to the unused tab on 
the air variable 75 mmF driver cap stator (C40) then leading it through the 
hole in the shield to alongside the 6146 tube. I used a 3 ½" piece of wire 
and after a couple of tries bending the wire closer to the 6146 the point of 
minimum plate current and maximum power output were made exactly the same. 
This also resulted in being able to get a little more power output because a 
little more drive could be used.

Things were looking good so the choke addition was made permanent. The bare 
wire from one end of the driver pot, the end of the 27K resistor at pin 8 of 
the 5763, and a green wire from the power supply were de-soldered, and the 
ternminal strip removed. Fortunately a 3 terminal strip with the end 
terminal having the mounting lug (i.e. end terminal grounded) was found in 
the junk box, and installed on the pot bracket where the two terminal strip 
had been removed. A piece of Teflon tubing was slid over the bare wire from 
the pot and this with one end of a .005 cap soldered onto the chassis end 
(grounded terminal) of the terminal strip. Another piece of Teflon tubing 
was slid over one of the choke wires and this soldered to pin 8 of the 5763 
socket. The other end of the choke was wrapped through the other end lug of 
the terminal strip, with the chassis upside down this was the uppermost lug. 
The free end of the .005 cap was also put through the top lug. One end of a 
27K resistor was put through the top lug and the lug soldered. The free end 
of the 27K resistor and the green wire from the power supply were put 
through the middle lug and soldered.

I hope the choke addition has been described accurately.  It worked on this 
rig but whether or not it will on others is not known.  If anyone can shed 
some light on where the problem really is or why the choke has eliminated it 
please post to the list.

A word of caution, when I got this rig it was in poor condition.  But maybe 
worst of all there had been a fire in the bias supply and the previous owner 
had jury rigged a repair that did not include terminal strips.  The bias 
supply consisted of a bunch of components soldered together sort of floating 
in mid air under the chassis suspended by the connecting wires.  That plus 
the old 2 wire plug could have resulted in a nasty surprise.  It definately 
pays to make repairs in a professional manner, and to check over new 
acquisitions carefully.

73, Mike VE7MMH

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