[Heathkit] SB-104

[Donald Spoon]

Uli,

I am very interested in your findings, although I don't have much to offer.

My SB-104A has recently started muting intermittently on receive... i.e. 
something is triggering the T/R circuit other than VOX or PTT.  It is 
independent of the HI or LO power setting.  If I am in LO power when it 
happens, when I switch to HI the relay will click as if the PTT has been 
pushed.  This effect can last from seconds to hours here!  The first 
time I noticed  it this round was when I was switching between the 
"long" and "short" AVC setting.  It started Sunday and today I managed 
to keep the rig on for about 3 hours without it happening... I am hopeful.

I have spent hours with the covers off , wiggling boards and connections 
as well as taking voltage measurements.  The only thing that seems to 
help the problem, so-far, are meticulous cleaning of all the pins and 
receptacles on all the boards.   Along with the cleaning I have also 
started bending in the spring "clip" on all the card's female Molex 
connectors.

I mentioned "this round" because I had a similar experience a while 
back. It turned out to be a leaky 10uFd electrolytic (C1) on the line 
from the +RX Pin (Pin 7) on board "B" to pin 7 on board "C" (+RX) .  
Replacing this capacitor cured the problem then, but it isn't a "cure" 
now.  The previous time the voltages on pin 7 of board "B" appeared to 
be floating between the expected T and the R values.  Sometimes it would 
drop into the "T"range and trigger the switching to the Transmit mode.  
Worth a look for you...   My original capacitor had a 16Volt rating, 
which is a bit close to the working voltages, so I replaced it with a 35 
Volt unit.

Good Luck & Cheers,
-Don Spoon-
K0APK





On 3/21/2012 6:37 AM, Ulrich Graf wrote:
> Hello to the list,
>
> I'm in the process of restauring an SB-104. Receiver and transmitter are
> working very nicely already. There is only one effect remaining: When I
> release PTT receiver agc gets some kind of pulse that, with slow agc time
> constant, blocks the receiver for some seconds. This pulse is obviously
> excited by the receive/transmit relay (it occurs when I hear it click) but
> not caused via the rf path. I can shortcut the rf connections to the
> frontend or the if board and the effect remains. It also occurs when I
> switch the meter to RF or 13.8 V - S-meter doesn't move but receiver is
> nevertheless blocked. So it does not come via the AGC/Meter connection.
>
> Did anybody also find this effect or knows some kind of cure? Any idea very
> welcome.
>
> vy 73, Uli, DK4SX
>