[Boatanchors] Heathkit SB-401 Alignment Problem - Fix

[Mark V Johnson]

I just spent a few weeks resurrecting a Heath SB-401 transmitter and thought
I’d share a fix for a problem I had when doing the alignment. 

 

The transmitter had all of the usual Heathkit problems that I resolved:
questionable solder joints, some sloppy work, replaced all of the
electrolytics, some bad resistors where there were questionable voltages,
etc. 

 

When I got to the alignment steps, I followed the manual and had good
progress until I tried to adjust the 10 meter, (28.0 mhz) driver grid and
driver plate coils. On all of the other bands I was able to successfully
peak the driver grid and driver plate coils for max output. The 28.0 mhz
driver grid coil would peak OK, but I could not get a peak from the driver
plate coil, L15. I could adjust it for max output but it would not peak. By
the way all of this was done as per the manual and with the final stage B+
removed at the feedthrough cap and the final stage screen voltages removed
by lifting R17 off of the screen grid wire that connects both 6146’s. . 

 

Very strange but as I was getting good final stage grid drive, I moved on. 

 

I then did the neutralization. Not able to get a nice null, or dip, in
output when adjusting the neutralization cap, C23. This is the small
variable cap in the final cage. The best dip, or null, occurred when the cap
was fully meshed. On spec I bypassed cap C24 which is in series with the
neutralization cap C23; cap’s in series divide and apparently I had
insufficient capacity as the variable cap was at full mesh and that was not
enough to neutralize the circuit. By doing so I was  then was able to get a
very nice dip when doing the neutralization. I replaced C24; a 7.5pf cap,
with a new 6.0 pf cap I had on hand and was able to get a nice pronounced
dip when adjusting the neutralization and C23 was at about ¾’s mesh. 

 

I then went back and adjusted the troublesome 28.0 mhz driver plate coil and
low and behold, it peaked up nicely. In addition I was now seeing maximum RF
output with the plate current at max dip.

 

Problem solved.

 

Mark V Johnson

VE3LU