SB-401 Excess Grid Drive

[W6CUX]

The balance modulator is probably out of null either because of faulty 
adjustment or bad parts. Most likely the 200 ohm balance pot. This part 
is infamous for opening up.

The fix is to replace the 200 pot with a .25 x .25 multiturn trim pot 
mounted on the top side of the board. These parts come in serveral 
styles; the one you want has the three leads in line out the bottom and 
the trim screw straight out the top.

While you are there you should replace the 220 end resistors and the 
diodes with 1N5711 Schottkey diodes.

These parts are all available from Mouser;
 the pots at http://www.mouser.com/catalog/625/530.pdf 
the diodes at http://www.mouser.com/catalog/625/352.pdf

73, Stu W6CUX


Allen Jones wrote:

>Last evening I started going through a very nice
>SB-401 I bought at Dayton a few years ago.  It is
>nearly spotless and shows few signs of use.  It has
>the crystal pack installed.
>
>I started by tuning into a 50 Ohm load on 80M and
>everything seemed to be fine with 100+ watts out and a
>decent note in the monitor receiver.  After a while I
>noticed the grid meter go full scale in either keydown
>or tune.  Reducing the level control had no effect. 
>The only way to reduce it was to detune the drive
>control.  40 and 20M also acted the same way.  I
>didn't try 15 or 10M.  I powered down for a while and
>tried again. Things were normal for a few minutes but
>the problem eventually started again.  Power out stays
>in the 100W region.
>
>I tried a different 6CL6 driver, cleaned the
>bandswitch contacts and the level control with no
>change.  I also checked to make sure all the hardware
>used to mount the pc boards was tight.
>
>Any ideas?
>
>Allen, W9DZ
>
>  
>

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.asp?list=HEATH
To post - HEATH at LISTSERV.TEMPE.GOV
Archives - http://listserv.tempe.gov/archives/HEATH.html