[Heathkit] SB-104A rx noise problem

[Jeramy Thibodeaux]

Sandy and Ulrich,

First you can find a great translator to help with the german to english 
here.  http://translate.google.com/

I have known about Kee's site for years.  Ulrich just provided another great 
source concerning this radio.  Thank you Ulrich for your hard work and 
sharing.

73,
Jeramy
KG4AZT

----- Original Message ----- 
From: "Ulrich Graf" <dk4sx.ug at googlemail.com>
To: <heathkit at mailman.qth.net>
Sent: Wednesday, January 08, 2014 12:46 PM
Subject: [Heathkit] SB-104A rx noise problem


> 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
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