The Harris SX 5

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.

Harris SX 5 medium frequency AM transmitter
Harris SX 5 medium frequency AM transmitter

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.

Harris SX 5 modulator section
Harris SX 5 modulator section

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.

Harris SX 5 transmitter damaged PDM pull up board
Harris SX 5 transmitter damaged PDM pull-up board

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.

Harris SX5 PDM pull up board temporarily repaired with wire jumper
Harris SX5 PDM pull up board temporarily repaired with wire jumper

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.

Small grey rat snake, electrocuted by Harris SX 5 transmitter
Small grey rat snake, electrocuted by Harris SX 5 transmitter

Could have been chasing diner.  Overall, the site needs help.  The air conditioner is coming next week.

Harris SX 5 transmitter fully operational
Harris SX 5 transmitter fully operational

After repairs, the transmitter is back at full power and modulating +125% again.

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6 thoughts on “The Harris SX 5”

  1. Thank you for the article – I have a Harris SX 5 and some other older equipment that was in an older building I purchased that served as a radio station for over 50 years – had no clue what it was until reading your article. Is there much of a market for this equipment?

  2. These transmitters are notoriously difficult to re-tune, so it may be worth something to somebody who is on the same frequency, otherwise it is likely more trouble than it is worth

  3. While if it’s not running, yes, it can be more trouble than its’ worth, especially if you don’t have a radio station!

    However, about 90% of this thing is now manufactured from “unobtanium”. Very few of it’s ‘custom’ internals are available anywhere (Harris dumped support in the late 90’s), and existing parts stocks were gone shortly afterwards.

    For the few companies that rebuild and repair them, and the several hundred owners that still run them commercially, you have a box of gold.

    There are many still in Backup service, and even a few in “Main and Only” service, playing pots and a making money every day.

    From personal experience, a Rebuilder (Michael Patton And Associtates) got outs running again (and we had evil, low-level-logic troubles) for note TENTH the price of a new, similarly size rig.

    For a station that could never hope to pay off a new 5, 2.5, or 1 kW transmitter, the insides of that thing are worth far more than the scrap value.

  4. FYI….if you know of anyone in need of parts from an SX-5 we pulled ours off air…replacing with a new Nautel transmitter. We’ve got plenty of parts for sale…many nearly new. Contact me via email if you have a need…

  5. Wanted to add that if you own one of these, and need to keep it making money, you MUST invest in the new display offered by Michael Patton Associates.
    I put one in a VERY sick transmitter today, and between a few calls to Mike and this amazing new display, this rig will live again.

    The display is AMAZING. Forget for a moment, the need for the laminated “normal readings” card, doing Binary Math to determine your actual overload, and a face full of half-digits and missing segments.

    The screen they picked for this creation is SO good, you can be sitting on the floor adjusting the AC controller, and STILL READ the display!
    Install was super easy, but just throw out that paper ribbon cable between the display and keyboard.Any decent ribbon, or even individual wires, is less painfull than re-using the original SX part.

    This thing is just brilliant, and while the SX’s had extensive metering (to justify the microprocessor nonsense, by now, those TI displays are done.Even if you HAVE a full original display, is it going to read you the measurement in plain english?!? And tell you what they should be?

    Sure, $800 is a little sticker-shock-y if it’s “just” your aux, but so many SX’s are still Mains today…and really, how much time do you want to spend on your Aux? This ingenious module is a MUST HAVE, and I mean a REALLY Must Have if you EVER need to push an SX series into ANY kind of service.

    It is 1000x better than even a PERFECT original display.

    Full disclaimer: No interest other than I’m a happy customer, and am most pleased to get the SX’s guts off the floor and back in the cabinet.And at least have a shot at a few grand in repairs, as opposed to no aux, or even a Part 15 aux, for a station that’d take decades to pay off a brand new 5kW anything, even a space heater.

    This display / controller takes the u out of the SX.

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