160m Mods

160m Mods. The 160m mods involve working on: 1. the input PI section, 2. the PA output PI section 3. changing one of the bandswitch stopps.

It's a tight fit inside the input circuit compartment. 160m variable coil is bottom left. The other 160m coil and SM caps are tucked inside the aluminum box.

NB. It appears there are two different 30S1 input circuits depending on the age of the amp. The 160m mod is practical only for input circuits using simple wafers with contacts on one side only. If there is a grounding ring in addition to a selector ring then things get rather complicated. The mod I am doing does not deal with the double sided grounding wafers for obvious complex fabrication reasons.

Also it is my intention to upgrade to 1 KV SM if I can find any (I am using 500V). Collins uses 1KV disk ceramic. I tested 500V SMs i am using and they showed no leakage up to 1.5KV, the max range on the 576 Tek curve tracer, so they maybe ok under 100W. I noticed Alpha 77Sx uses compression micas, which have higher Q than ceramics.

Schematic showing 160m additions to the PA

160m addition to the input-Pi circuit

Accessing the rear PA band wafer for removal

• Switch-Wafer modifications
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160m new, added input-switch wafer

160m switch wafer with arrow

160m new, added vacuum relay

input-switch-wafer modifications for 160m

input-switch-wafer mods 2nd view, for 160m

VNA showing S11 for 160m input-circuit