They say the first thirty years are the hardest, perhaps it is true. This Harris MW1A transmitter turns 31 this year:
Harris MW1A AM transmitter, WINE, Brookfield, CT
It is on the air as the main transmitter for WINE-AM in Brookfield, CT. These are not necessarily bad transmitters, although they do seem to require regular infusions of MOSFETS to stay at full power. This is Harris’s first solid state AM transmitter design, based on the work of Himmler Swanson. This is not a PDM transmitter, rather, each module has RF MOSFETS and audio transistors. The output of all twelve modules are combined for a carrier output of 1,ooo watts with +125% modulation. Thus, I would call it low level AM modulation.
This is also the only transmitter that I know of where blown fuses can cause damage to the RF devices.
The RF output transistors and audio transistors are still available from Harris. Non-OEM parts for this include the 2N5038 for the RF MOSFETS and the MJ15011 for the audio transistor. Inside the front of the transmitter is a row if incandescent light bulbs that glow increasingly as the various MOSFETS go bad. At 1,000 watts carrier power, the ratio of PA volts to PA amps is 52.5/22.5 respectively. If that ratio is off by any measure, there is a problem.
The other thing with this transmitter is it is very sensitive to any kind of VSWR. Any change in the output impedance will quickly make itself apparent. My Harrisburg MW1A had two ATU settings, one for winter and one for summer. It was a slightly tall tower on 1230 KHz, thus any change in the ground system (e.g. snow cover) would upset the tower base impedance.
The other thing that goes bad is the large Rotron fan in the bottom of the cabinet. They go bad about every 10-15 years or so.
The owner has spent some money on this particular unit, rebuilding and replacing several modules with new MOSFETS etc. Will it last another thirty years? Depends on if the RF and audio devices remain in production.
I had a Harris MW-1 (not the “A” updated version) when I was chief at the now defunct WMMM 1260 AM in Westport. Even when taking the greatest of care in using my Sencore Cricket to match the transistors in the rebuilt modules I would always be plagued with one of the module indicators glowing a small amount. Other than that the transmitter served the station quite well. If I recall correctly, their box was modified by the late Art Silver of Harris to add an additional power position for 50 watts in addition to the 500 and 1000 watt levels.
I have to agree with this transmitter being a bit fussy with high VSWR. One Saturday I was paged as the transmitter was popping itself off the air at random (but retuning to the air a second or so after it went off). This was caused by a number of ty wraps holding the RF lines for the STL antennas failing, allowing the lines to sway out away from the tower in the breeze. The RCA BTA-1R backup was pressed into operation until Pat Allen’s tower crew arrived since I didn’t have a climbing belt nor was I insured for climbing.
As far as I know the MW-1 transmitter is still in service at WSHU-AM’s Norwalk transmitter site.
I did this modification found in Radio World to the WINE MW1A transmitter when I worked for Cumulus. Do not know if it helped or not. at that time I did replace many of the audio output transistors.
http://radioworld.com/article/add-a-relay-save-a-transistor/2980
@Ed, I think the RF MOSFETS go regardless of this modification. I was not aware of the audio transistor issue. As I remember, this transmitter needed to be extensively modified to pass the original NRSC-2 requirements in the early 90′s (including removing the “modulation enhancer”). I am sure there are many other mods floating around for this transmitter
@Paul: I surprised anybody would have still been using the ME-1 modulation enhancer on there. Back in the 80′s on a tour of several different sites the MW-1/1a’s I saw had them out of service. I actually had the one from WMMM’s MW-1 connected to the LPB TX 2-20 for the 9 watt nighttime power to help match the levels between it and the Harris. To this day I still have one sitting on the shelf, mostly for nostalgia.
I do volunteer work at a radio station that had a MW-1A for backup. It had been replaced by a Harris The One, but they wanted the MW-1 to be refurbished so I set out to do so. Using a B & K transistor tester, I matches the two RF’s and the two audio transistors. Many of the resistors were out of tolerance, and were replaced. Being a ham operator for many years, I found a source for the toroids used as the wires were very brittle and would break off. Matching the wire size and copying the turns proved to work out well. The PA tuning rig needed lubrication and worked smoothly after work. The filter caps were changed in the power supply. As there was a whole bag of transistors for me to work with, I did not have to buy any but I checked on Mouser’s website and 2N5038′s go for 500 @ $504 and the MJ15011 are 500@ $234 which will keep you in business for a long time. I am going by memory, but there is a diode, that is no longer available, but I was able to find a replacement. I have packed away all the info but can dig it out if people need info. Contact me at K1MJP@Hotmail.com.