At 09:42 AM 06/10/07, you wrote: >Mark, > >Try replacing R5, the 1000 ohm voltage adjustment pot. This is a cheap >composition-element pot that often gets noisy after the wiper is in the same >position for years, in a hot environment. If a poor contact between the >wiper and the substrate occurs, the regulator chip thinks the output voltage >has dropped and it will cause a spike. > >You can verify whether the pot is bad by first noting its current position, >then sweep it back and forth several times to "polish" the wiper, and leave >it at a slightly different position than before. If the output voltage is >now stable (it will be higher or lower than before, of course), you've found >the problem. You might think about remotely mounting a high-quality >wirewound pot on the front panel, where it's convenient to adjust and away >from the hot environment. > >If the pot doesn't solve the problem, call Astron at 949-458-7277 and ask >for Fred in Tech Support. > >73, Eric Lemmon WB6FLY
If you remote-mount the pot, do NOT use 6 inch long wires. I had to look at a modified power supply that was acting up at a repeater site and the person had left the wires long enough to provide a service loop. Unfortunately the service loop was resonant at the UHF transmit frequency. I replaced the three wires with a piece of three-conductor-plus-braided-shield cable (if you haven't looked at the Astron regulator schematic all three terminals on the voltage adjust pot are hot above ground). I made sure to ground the braided shield at the PCB end and at the pot end and the problem went away. Mike WA6ILQ
