22r.) SSB reception for Kw.E.a

Optimizing the receiver for SSB.

The first thing to do when you have unscrewed the front and have done some possible repair*), is to rewire the RF gain (Störhöhe) such that it operates independent for CW/SSB reception. This is more important if you don't improve it for SSB in other way as mentioned below. The reason is that since the BFO injection is so weak you must decrease the RF gain to avoid overloading the detector process, and you must increase the audio gain by the same amount. It is not possible without this rewiring.

The original BFO circuit

AM/CW/SSB detector is the Audion-type.
The circuit diagrams shows that only the BFO unit is modified for SSB, but only a small circuit is added, and it is easily converted back to the original state. It is however, an advantage to decrease the IF level to the detector in CW/SSB mode further with an additional capacitor 330pF across '348' 300pF capacitor. The BFO filament RF choke was removed to provide some space for the BFO amplifier. The BFO amplifier is connected ahead of the screened cable and the 10pF capacitor ("222") and it is loaded with the resistor over the tuned circuit.

BFO circuit
It is quite complex and not really easy to read the circuit diagram. Different version of circuits shows different details,
so some details may be inaccurate, but it is difficult to say without closer inspection into the receiver. Note the function with selectivity switch pos."171" in position - will switch the BFO on - when the strap is connected for pot.150-151, instead of the normal way - pot.149-151 when it switched by the normal Tn/Tg switch pos."148".

AGC/MGC switch modification for SSB operation.
Originally, the RF gain is active only with AGC on, and it is not active without AGC, nor is the audio gain control, but another pot.meter on same axis as the audio gain operates as RF gain, although it is marked "Lautstärke". This is rather impractical for SSB, where audio gain is set to a certain level, and RF gain is used if the signal is too strong for the AGC to operate properly. For "CW" the audio gain is reduced by another switch.

See also SSB conversion notes on page f12

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Last update: 2005.01.22