SB-400 Grid Drive Problem- FIXED!

[Mark Johnson/Ontario/IBM]

Heath team;
Thought I'd share my fix for the SB-400 grid drive problem I've been
struggling with. This bug has been with me for about 2 years and I've taken
shots at it off and on over that time with minimal success.
The problem: As the unit heats up, the grid drive on 40 meters drops off to
the point where you have to crank the CW level control to full up. After
much scoping I found the drive drops on the other bands as well, but its
less significant. Also, as I do mostly 40 CW in the 1st 50kc's of the band,
I also noticed it was worst in that area. Going up band on 40, the grid
drive recovered but was still too low.
The problem turned out to be resistor R71. A 68K 1/2 watt resistor that
feeds B+ to part of the rig. On the SB-401, Heath used a 1 watt resistor as
this little baby gets hot! Once I replaced the 1/2 watt(R71) with a 1 watt,
(new resistor) problem solved!! For reference, the resistor in question is
R402 on the SB-401.

Some things I learned during this journey:
There are differences between the SB-400 and SB-401 beyond the LMO
switching changes.
When adjusting the 8.6 mhz trap, use the procedure from the SB-401 manual.
Its easier and just as effective.
I found myself fighting an intermittent problem with total power loss along
the way that turned out to be a bad solder joint on the bandpass board. The
lesson there is the usual....Always check those soldered connections.
Not surprising, tubes get weak after 35  years. Don't do a "one at a time
replace", if the rig is a keeper, replace them all at once. Less hastle and
less fighting of multiple problems.
When adjusting the grid and plate drivers for 10m, I used the 29.0 mhz
band, and did adjustments there. This way I get 80 watts output on the band
edges and around 100 watts on in the centre! I did not have to do this on
my SB-401. It adjusted to spec as per the manual.
Use this reflector for advice and council. I got to this problem indirectly
through a tip I got from Stu, W6CUX. Stu's tip had to do with the carrier
unbalance resistor (R39 SB-400, R401 SB-401). I checked this out and that
took me right to the "Hot" resistor in the same circuit, R71.  My thanks to
Stu for his advice and for taking the time to reply.

Many thanks! Feedback and further suggestions are most welcome!


Mark V. Johnson
VE3DJU/VE3DDI

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