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 stops.

It's a tight fit inside the input circuit compartment. Facing the front, the 160m variable coil is mounted on the left side next to the 80m coil. The fixed coil and SM caps are tucked inside the semi closed 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.

Collins uses 1KV disk ceramic so ideally the 160m capacitors should be 1 KV SM but space is tight. I tested the 500V SMs i am using and they showed no leakage up to 1.5KV, the max range on a 576 Tek curve tracer, so they should be ok up to 100W. I noticed Alpha 77Sx uses compression micas, which have higher Q than ceramics.

Fig.1 Schematic showing 160m additions to the PA

Fig.2 160m addition to the input-Pi circuit

Fig.3 Clearing a path to the rear PA band wafer for removal

• PA-Wafer. Adding 80m and 160m tabs
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Fig.4 160m added ordinary wafer for 160m PA vacuum relay switching

Fig.5 160m switch wafer with stop drilled out

Fig.6 160m new, added vacuum relay

Fig.7 input-switch-wafer modifications for 160m

Fig.8 input-switch-wafer mods 2nd view, for 160m

Fig.9 VNA showing S11 for 160m input-circuit Zload=50 Ohms