Dentron

• MLA-2500
  • The Problem: Makes full power on all bands but on 160 meters, the power falls off greatly in just a few seconds. 160 meters is the ONLY band where the problem shows up.
  • The Solution: Found that the bypass capacitor, C5 (500pF 5KV ceramic disc), for the end of the plate choke was bad. You could not detect any significant heat rise using a digital thermometer but the capacitor was definitely bad. Replacing it fixed this problem. Suggest two or more caps in parallel as a replacement to handle the RF current.
  • More: In the process of chasing this problem, several cold solder joints were found in the plate tank circuit where larger wires were soldered to lugs. Re-soldering these connections with a BIG iron improved power output.
  • The Problem: Low power output and high input SWR.
    • The Solution: The RF choke, RFC4, the cathode choke which is located on the ALC board, was bad (cooked and shorted). In this condition, input power was being shunted to ground by the .01uF bypass capacitor C24. Replaced the RF Choke and the problem was fixed.

Drake

• L4B
  • The Problem: The L4B made full power output on all bands but on 80 meters the power output would fall off within a few seconds. Re-touching the loading capacitor would bring the amp back into operating range but then if the amp was allowed to cool a few seconds, the amp wouldn't be matched properly and output would be down again.
    • The solution: The end of the plate tank coil, L7, which ties to the loading capacitors, C48, C49, C52 and C53, had a bad connection. This connection ran significantly warmer than the surroundings and could easily be seen with an IR thermometer but could also be detected by feel after the HV was off and discharged. This connection had to be completely re-made. Using brass nuts and a solder lug on the screw that connected to the capacitor improved the connection to the point it never got warm again and the problem was solved.