[Heathkit] SB-104A rx noise problem

[Ulrich Graf]

Hello Sandy,

unfortunately the receiving section of the SB-104 is one of the worst of
all receivers I know (and I know very many!). I have restored and improved
an SB-104 and an SB-104A so far and was diving very deeply into schematic
and circuitry. The results you may see on my web site

www.dk4sx.darc.de

and there in the section "Restauration". Just click on the appropriate type
name. Text is in German but schematic details are informative.

With the SB-104A Heathkit tried to improve the very weak large signal
capability of the receiver, neglecting all the additional effects causing
heavy intermodulation aside of the mixers, such as too little current
through filter switching diodes and transverter output switching diodes,
noise blanker circuitry, and worst of all, the circuitry around the two
crystal filters. Especially the last amplifier stage in front of the
filters runs at very high gain with very little current. This stage causes
more intermod than the whole rx frontend! I remodeled the SB-104A with
moderate modifications to be able to work on the overloaded 40-m-band over
here without noticeable distortion while I built a new receiver frontend
for my SB-104 and redesigned the IF-board circuitry. You may see all the
mods on my web site. I gess the first if stage before the filters is
oscillating causing this strange effects you are able to notice during
listening periods.

73, Uli, DK4SX