A really great radio that needed a little love. I received this radio from a local Ham that said it had low output and just needed a good look-over.
Running a function test, I noted low output ( about 30 watts) but the receiver seemed to work okay.
I opened the radio for further inspection and found out someone had been here before. Plus, it needed a lot of love!
Someone had replaced the high voltage supply caps. Not only physically the wrong size but also the wrong voltage.
Also you can see the bleeder resistors have been hot. Checking them with an ohm meter, they were in fact, open. Time for them to go!
Old caps out. Hmmmm, foam and hot glue. What could possibly go wrong here.
New caps installed with the correct voltage rating and, that fit the mounting clamps also!
Opening the high voltage cage, found some terminally ill caps. Time for them to go too.
New bypass and plate caps installed. Lubed the fan and tightened RF connector. While looking at the RF board, I noticed some discoloration of the cathode resistors. I measured them and they showed roughly 8 ohms. Time for them to go too. New cathode resistors and a good cleaning of the high voltage cage.
Also found the ladder chain sprockets were cracked and loose on the shafts. Add those to the list.
I found new 3D printed sprockets at NR6C.com. Pretty nice sprockets with brass inserts. That should do!
Time to remove the RF board. In order to remove the sprockets from the air variables, the center one would need to be removed from the board. Get the iron hot!
While I had the RF board out, I thoroughly checked the solder connections on the bottom of the board, replace the two cracked nylon shaft couplers with new brass couplers from k4eea.com. These worked really good. I also cleaned the band switch.
Moving over to the other side of the radio, it was time to look at the screen supply caps and replace the four 1n60 diodes in the balanced modulator with 1N6263 Schottky diodes with a lower noise figure with an improvement in linearity.
With the IF board out and new parts installed, I checked all of the connector solder connectors and touched a few suspect one up with the iron before reinstalling the board.
IF board back in, new supply filters, ladder chain sprockets, brass shaft couplers, high voltage cage updated. Time for alignment.
Connected to the PLL board, time to adjust the Band Pass filter.
A view of the response from one of the filter stages after adjustment. Thank you to John, WA6IKO for helping me to understand my service monitor and bring my attention to the “offset” function using the spectrum analyzer and tracking generator. It make this routine really easy!
I used two tones from my Service monitor (700 Hz and 1900 Hz). I fed 7 mV to the Mic input jack. I first loaded the radio at 100 watts on 14.175 MHz. Then placed the radio in USB and set Mic gain for 50 watts. This allowed time to make VBT oscillator trimmer cap adjustments without taxing the Final tubes. I placed a 40 dB tap in the line from the SO-239 to the Dummy load then used the 40 dB tap to drive my O-scope. I then adjusted the trimmer cap for a zero-point crossing.
With a realignment and the radio back together, it was time from some on-air testing.
A special thanks to the guys over at the Kenwood-Hybrid groups.io page. Especially, Terry Wagoner, K9TW, for answering all of my dumb questions. He is wealth of knowledge on these old Hybrids!