{"id":3381,"date":"2011-08-20T08:06:53","date_gmt":"2011-08-20T12:06:53","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=3928"},"modified":"2023-04-30T11:07:17","modified_gmt":"2023-04-30T15:07:17","slug":"the-harris-sx-5","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2011\/08\/the-harris-sx-5\/","title":{"rendered":"The Harris SX 5"},"content":{"rendered":"\n

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<\/a> in Pittsfield, MA. \u00a0 It has a few issues of late.<\/p>\n\n\n\n

\"Harris<\/a>
Harris SX 5 medium frequency AM transmitter<\/figcaption><\/figure>\n\n\n\n

The first of which is the unbalanced or out-of-ratio condition of the PA current and voltage. \u00a0When changing power levels, the PA current, and voltage are supposed to track together. \u00a0When they do not, it is an almost sure sign that one or several of the MOSFETS in the PA are shorted. \u00a0Shorted IRF-350 MOSFETS are indicated by blown fuses on the PA boards and should be replaced in pairs. \u00a0The reason for the damaged devices also needs to be investigated. \u00a0It is entirely possible that the site receives a lot of lightning, which can cause this damage. \u00a0It could also be heat related, as the site is not currently air-conditioned. \u00a0The other possibility is under drive conditions.<\/p>\n\n\n\n

The MOSFETS turn on and off at a rate of 1\/(carrier frequency (hertz)) times per second. \u00a0If they are under-driven, they will go full-on and short-circuit. \u00a0The 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. \u00a0Underdrive indicates an issue with the oscillator, which has its own set of peculiar failure modes.<\/p>\n\n\n\n

Since this is an older unit, all of the large electrolytic capacitors are also suspect and need to be replaced. \u00a0There are three power supply capacitors at the bottom of the transmitter, two 76,000 \u03bcF\u00a040 VDC for low voltage and one 7500 \u03bcF, 350 VDC for high voltage. \u00a0The modulator section also has six 5100 \u03bcF\u00a0350 VDC capacitors, collectively known as “dynamite sticks” due to their explosive potential if installed incorrectly.<\/p>\n\n\n\n

\"Harris<\/a>
Harris SX 5 modulator section<\/figcaption><\/figure>\n\n\n\n

While replacing the dynamite sticks, I noticed this PDM<\/a> 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.<\/p>\n\n\n\n

\"Harris<\/a>
Harris SX 5 transmitter damaged PDM pull-up board<\/figcaption><\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

\"Harris<\/a>
Harris SX5 PDM pull up board temporarily repaired with wire jumper<\/figcaption><\/figure>\n\n\n\n

The worst, and I mean worst possible situation with these transmitters is some type of control malfunction. \u00a0The control boards and oscillator are in that large vertical pull-out drawer. \u00a0God protect and preserve the digital control and S and M boards, as they are a major headache to troubleshoot. \u00a0They have 7300 TLL (5-volt logic) that controls all functions and only a little problem will cause the entire transmitter to shut down.<\/p>\n\n\n\n

Other SX series transmitter tips can be found here<\/a>.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Update: <\/strong>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.<\/p>\n\n\n\n

\"Small<\/a>
Small grey rat snake, electrocuted by Harris SX 5 transmitter<\/figcaption><\/figure>\n\n\n\n

Could have been chasing diner.  Overall, the site needs help.  The air conditioner is coming next week.<\/p>\n\n\n\n

\"Harris<\/a>
Harris SX 5 transmitter fully operational<\/figcaption><\/figure>\n\n\n\n

After repairs, the transmitter is back at full power and modulating +125% again.<\/p>\n","protected":false},"excerpt":{"rendered":"

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. \u00a0 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. … Continue reading The Harris SX 5<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[32],"class_list":["post-3381","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-am-transmitters"],"_links":{"self":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/3381","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/comments?post=3381"}],"version-history":[{"count":20,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/3381\/revisions"}],"predecessor-version":[{"id":11761,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/3381\/revisions\/11761"}],"wp:attachment":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media?parent=3381"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/categories?post=3381"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/tags?post=3381"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}