SB-104 Display problem: WWV and 20m

[JF]

Alexander N. Gerli, Sr. wrote:
> My SB-104A has a little problem: on the 20M and WWV bandswitch positions,
> the display reads, get this, "34" then the correct kHz; on WWV, "35" then
> the correct kHz. The rig's working fine otherwise.
> Of course, that "3" should be a "1" in each case. Anyone have an idea what
> would be making this happen? Only two bandswitch positions that do this, and
> the contacts have all been cleaned and the boards reseated, etc.
> 73,  Sandy AC1Y


Does it display a "2" correctly for the 1st digit on on the 10
and 15 meter bands?

Was it like this from when it was built, or did it develop this
problem later? If it always had it then it is most likely a
wiring error or solder bridge on the display board or from the
band switch (which controls the 1st digit, not the Frequency
Counter Board "A"). If it developed this problem later then it
can only be a bad display board driver IC, numerical readout,
a band switch problem, or wiring that got damaged.

Has any crud collected on or behind the display board, or on
the band switch? The display board ICs were socketed, so first
you could try switching the segment driver IC (IC-156) for the
1st digit with one of the others to see if that has any effect.
If not, try switching the numerical readout (V-153) for the 1st
2 digits with one of the others.

For 20M & WWV the band switch supplies a ground to connector
"C" on the display board (10 MHZ), which then goes to pin one
on IC-156: check to verify that ground is there when the band
switch is on those 2 positions. If a ground were there for all
band switch positions the display segments for #1 would always
be on, indicating either incorrect wiring to the band switch
or a faulty switch. From your description that does not seem
to be the case.

Simultaneously with the pin 1 ground on 20M & WWV, pin 4 of
that IC (10 MHZ BLANK) should have ground removed, as should
pin 7 (20 MHZ). Pins 2, 6 & 8 should have permanent grounds,
pin 5 has no connection. Pin 16 should have a constant +5VDC
(power), and pin 3 should be connected to +5VDC through a 10K
resistor (R-157). If there is no ground to pin 1 while on 20M
& WWV, then the connection to the band switch or the switch
itself is bad or mis-wired. If that is the case try connecting
a temporary ground to IC-156 pin 1 to see if the #1-segments
light on the 1st numerical display.

All the socketed ICs in the 104s tended to develop corrosion
(or intermittent connections if all construction solder flux
wasn't removed from INSIDE the sockets, where it creeps to)
on the pins which could recur even after cleaning them and
re-seating the ICs. The Display and "A" counter board IC
sockets (cheap, tinned, not gold plated) can be so prone to
this that the only remedies are to either replace them with
high quality sockets, or remove them and solder all the IC
pins directly to the PCB. However, that makes plug-and-try
troubleshooting more difficult.

Those numerical displays (neon powered by 180V, not LED)
are now hard to get, expensive if you can find them, and
usually not long-lived because the gas eventually leaks out
between the layers, so if you get to the point of replacing
them it would be better to get an entire replacement kit
which comes with a new board and LED readouts. Mike Bryce,
WB8VGE, SunLight Energy Systems, sold these kits a few
years ago, but I don't know if he still is. You could look
  at
         http://www.theheathkitshop.com/page29/page29.html

If you get one the readouts are very bright, so you might
consider using higher-value segment supply resistors.

73
JF  KE5ZB

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