Click the pictures to enlarge.
Cushcraft R8 vertical at OH8SE 
The problem was in the broken upper Al-bar 
Detailed view of the broken bar. It looks like a long-term metal fatigue. 
New 2 mm thick Al-bar under construction 
New Al-bar ready and installed 
Internal check was not easy because of the corroded enclosure screws 
I used thin Dremel grinding wheel to cut the screws 
Removed screws 
Finally the box is open. See the brass inserts with screw remains. 
To remove the brass inserts deepen the plastic holes from the bottom side until a 6 mm drill is contacting the inserts. 
After that slam the inserts out with a suitable punch 
Removed brass inserts 
Inserts removed 
Internal check. All the cores seems to be two stacked FT-240-61 toroidal cores. The lower one is a choke balun with 9 turns of PTFE coax. The upper one is an 8+11 turns unun auto transformer, primary 8 turns, secondary total 19 turns. Impedance ratio abt 1:5.6 => load impedance abt 280 ohms. I measured the box RF behavior by connecting a 280 ohm resistor from the upper Al-bar to counterpoise screw on the bottom side. The SWR over the antenna band range looked acceptable.This confirmed that the matching box is intact. 
Reverse-engineered Cushcraft MN8 box schematics
Top cover reinstalled with quite thick sheet metal screws 
Bottom view with the counterpoise screw/nut on the left Update 2022-05-17 OH7SV Back to OH7SV projects |