I give you joy, the unmitigated joy, and sheer pleasure of the Harris SX 5 AM transmitter. This particular unit dates from 1984 and is installed at WUPE in Pittsfield, MA. It has a few issues of late.
The first of which is the unbalanced or out-of-ratio condition of the PA current and voltage. When changing power levels, the PA current, and voltage are supposed to track together. When they do not, it is an almost sure sign that one or several of the MOSFETS in the PA are shorted. Shorted IRF-350 MOSFETS are indicated by blown fuses on the PA boards and should be replaced in pairs. The reason for the damaged devices also needs to be investigated. It is entirely possible that the site receives a lot of lightning, which can cause this damage. It could also be heat related, as the site is not currently air-conditioned. The other possibility is under drive conditions.
The MOSFETS turn on and off at a rate of 1/(carrier frequency (hertz)) times per second. If they are under-driven, they will go full-on and short-circuit. The minimum drive is 27.5 volts peak to peak, anything less than that is marginal and can lead to the destruction of the PA devices. Underdrive indicates an issue with the oscillator, which has its own set of peculiar failure modes.
Since this is an older unit, all of the large electrolytic capacitors are also suspect and need to be replaced. There are three power supply capacitors at the bottom of the transmitter, two 76,000 μF 40 VDC for low voltage and one 7500 μF, 350 VDC for high voltage. The modulator section also has six 5100 μF 350 VDC capacitors, collectively known as “dynamite sticks” due to their explosive potential if installed incorrectly.
While replacing the dynamite sticks, I noticed this PDM pickup board has a whole burned through it. This is a part of the modulator section and if it burned completely open, would likely cause all sorts of problems with this transmitter, likely spurs all around the dial, distorted modulation or perhaps overload and fail altogether.
I managed to fix it with a jumper between what is left of the circuit board trace and the capacitor mounting bracket. I soldered the jumper to the board face and soldered the wire lug. After scraping all the oxidized metal off of the capacitor mounting bracket, I attached with a screw. The board itself needs to be replaced, if it is still available from Harris, which it may not be as support for this transmitter was dropped in 2008.
The worst, and I mean worst possible situation with these transmitters is some type of control malfunction. The control boards and oscillator are in that large vertical pull-out drawer. God protect and preserve the digital control and S and M boards, as they are a major headache to troubleshoot. They have 7300 TLL (5-volt logic) that controls all functions and only a little problem will cause the entire transmitter to shut down.
Other SX series transmitter tips can be found here.
I didn’t get to replacing the blown devices because of a looming electrical storm, which precludes working inside of transmitters. I’ll get back there next week and finish the job.
Update: I finished the repair job today 8/24. There were 16 blown MOSFETS on the PA boards. I checked the drive levels on the input side of the RF torrid load resistors and it is with normal range. I also found this snake in the bottom of the transmitter across the HV shorting bar.
Could have been chasing diner. Overall, the site needs help. The air conditioner is coming next week.
After repairs, the transmitter is back at full power and modulating +125% again.