SB-400 Grid Drive Problem- FIXED!

[Diane Swynar]

Hi Mark...

Some VERY noteworthy achievements in your e-mail---certainly worth
retaining here for furture reference!

Thanks for sharing it with us...I appreciate it.

~73~ Eddy VE3CUI - VE3XZ



> 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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