Transmitter trips main breaker

Received a call last night, after a particularly bad thunderstorm, that WGHQ in Kingston, NY was off the air.  Earlier in the day, the transmitter had tripped the main breaker after a thunderstorm.  I arrived at the transmitter site and found the breaker tripped again.  Once the breaker was reset, the transmitter came back on and ran without any overload indications.  The transmitter is a 10-year-old Nautel ND-5.

WGHQ Nautel ND-5 transmitter
WGHQ Nautel ND-5 transmitter

I was thinking of breaker fatigue as the breaker is the original 1960 breaker installed when the building was built.  I reset the breaker and turned the power output down to 3 KW, thinking the reduced load might not trip the breaker until we could get a replacement.  The transmitter was on the air running as I was about to lock up and go home when I heard, but more felt through the floor, a THUMP! There I stood and watched the transmitter go dark.

At least it happened when I was there looking at it.  Because of the lightning, I was thinking of something in the output network.  I reset the breaker and once again, no faults, and the transmitter came back on.  Strange.  Obviously some sort of power supply issue.  Here are the clues:

  1. The B- voltage was right where it should be at 72 volts.
  2. All other readings, reflected power, forward power, and power supply current are normal before and after the breaker trip
  3. No fault lights
  4. The service panel breaker, which was tripping, is rated for 70 amps, and the transmitter front panel breaker which did not trip, is 50 amps.

The Nautel factory rep was thinking either breaker fatigue or the big transformer in the base of the transmitter had gone bad.  According to him, no one had ever heard of a transformer going bad in these transmitters, which makes a certain amount of sense.  Unlike a tube transmitter, which steps the B+ voltage up several times, these transmitters reduce the B- voltage by about 2/3rds or so.  With a step-up situation, a surge would be multiplied many times and could very easily punch a hole in the transformer’s secondary winding insulation.  I have, in fact, experienced this on at least two occasions.

That leaves the wiring between the transmitter and the service panel.  I double-checked the panel breaker with my volt meter to ensure that the voltage was indeed off.  Then I removed each phase from the connection lugs in the transmitter and tested the wire to ground with my Fluke 77 DVM.  Sure enough, two of the phases showed resistance of 1.2 and 1.7 MΩ to ground where it should have been infinite.  Further, when I took the cover off of the service panel, I found a dead mouse.  Unfortunately, I didn’t have any #4 THHN, and all the home improvement stores were closed by that time, so it had to wait until morning.

The thunderstorm seems to be a coincidence.

After we pulled the wire out of the conduit, we found this:

mouse chewed feces encrusted electrical cable
Mouse chewed feces encrusted electrical cable

It is a little hard to see, but that shiny spot is copper.  The cable jacket is chewed back quite a ways and the entire thing is encrusted in mouse feces and urine.  I love to work on stuff like this.  LOVE IT!  Hantavirus, here we come!  That reminds me, I need to get some of those blue latex exam gloves and throw them in the truck…  Anyway, far back in the conduit running through the concrete floor where it bends to go up to the service panel, the mice apparently had a nest.  They got into the conduit under the transmitter, where it transitioned from 3-inch rigid to 1 1/4-inch flexible metal without the benefit of a junction box or proper fitting.

We pulled new copper conductors in and installed a proper junction/transition between the 3-inch and 1 1/4-inch conduit.  The service panel was also missing several knockouts of various sizes, which were sealed with knockout seals.  The transmitter was back on the air at full power about 16 hours after it went off.  Unfortunately, the station has no back up transmitter, so they were off for that period of time.  Perhaps now they will look into a backup transmitter or at least an exterminator, but probably not.

WCKY transmitter site tour

This is of interest because of the GE BT-25-A transmitter footage.  I do not know the serial number of the WCKY BT-25-A transmitter, but it looks identical to the old WPTR BT-25-A unit which can be seen in this post.  As I stated in that missive, I have not heard any transmitter before or since, that sounded as good as this unit.  They were really engineering marvels, even in 1999 when this video was shot.

No doubt the MW-50 (no letter) and particularly the DX-50 transmitters are more efficient. In this day and age when many AM stations are just scraping by, overpaying for utilities is not an option. I noticed the Harris MW-50 transmitter with the PDM drawer open. That brings back memories too, those PDM boards were a pain in the rear, as I recall.

Donating old equipment

There is a propensity among radio engineers to save old equipment. Sometimes I look at something and think, “Man, that cost a lot of money ten or twenty years ago.”  Truth be told, much of what is saved will never be used again.  This equipment should be scraped or donated to someone who might find it useful.  One thing that is most appreciated by Amateur Radio (AKA Ham) operators is old 1 KW tube type AM transmitters.  Ham operators love these things and with good reason.

A fair amount of repair work, some cleaning, and a bit of reworking will turn what might have been a useless dust collector into a 160 or 80-meter AM rig and with a good story to boot.

Personally, I’d rather see a Gates BC1T or RCA BTA1R off to a new home than off to the scrap yard.  To that end, today we unloaded the BC1T at WLNA to a willing ham.  This particular transmitter had last run in 2001 or so and was used as a spare parts supply for other BC1T transmitters owned by the same company.  There was no way it would ever work again and truth be told, it really wasn’t needed any longer anyway.  Since the Harris MW5B was replaced as the main transmitter by a BE AM6A, the backup transmitter was never used.

Gates BC1T transmitter
Gates BC1T transmitter

John Aegerter, a frequent commenter on this blog, drove all the way from Madison, Wisconsin to pick it up.  Prior to picking up, I removed all of the tubes, transformers, crystals, and glass envelope time delay relays.  I packed up the glass objects in a box.

Gates BC1T tubes, transformers and spares
Gates BC1T tubes, transformers and spares

There were several spare tubes and parts which are no longer needed.  These went with the rig, along with whatever manuals I could find.

Gates BC1T loaded into pickup truck
Gates BC1T loaded into pickup truck

The transmitter was then loaded into the back of a Dodge Ram 2500 pickup truck and tarped for its trip back to Wisconsin.

The burned contactor fingers

This is a set of burned contactor fingers on a Harris HS-4P 30 amp RF contactor:

Harris HS-4P RF contactor
Harris HS-4P RF contactor with burned finger stock

The back story is this:

The contactor in question is at the base of Tower #3 of the WBNR (1260 KHz, Beacon, NY) antenna array.  This is the tallest of all the towers, at 405 feet.  As such, it gets struck by lightning often.  There was at least one occasion where one of the inductors in the ATU got “sucked in” due to the huge magnetic field of a high current strike.  It is not at all surprising to me to find other component issues in this ATU.  Because of the burned contacts, I’d suspect that the station was switching modes under power, but I didn’t see that happening today.

The problem manifested itself in very high SWR after changing over from day pattern to night pattern.  This did not occur every time, in fact, it only occurred once in a great while at first.  Then, over the last couple of months, it began occurring more and more often.  Since the snow drifts are now down to a manageable six to eight inches, it was a good day to go out and do some exploring.

First of all, I put the station into nighttime mode just to confirm that there is still an issue.  The transmitter, a Broadcast Electronics AM1A showed very high SWR and carrier fold back.  Left it in night pattern, but turned it off and took a walk, not a drive, to Tower #4 which is all the way at the bottom of a hill, near the old City of Beacon landfill.  I figured that I would check that one first, then look at Tower #3 on the way back.  When I got to Tower #3, I found the issue right away.

Fortunately, I was able to salvage a set of contact and contactor bar from another relay in the same ATU that was not using them.

Burned RF contactor bar
Burned RF contactor bar

The night pattern is only 400 watts, but these are tall towers, 225 degrees, therefore current and voltage are high at the base.  In fact, the slightest change at the base of the nighttime towers will greatly upset things.

Burned RF contactor fingers
Burned RF contactor fingers
Harris HS-4P contactor repaired
Harris HS-4P contactor repaired

This is the repaired contactor.  I will say, the EF Johnson RF contactors are easier to work on.  Those are the ones with the big rocker bar across the top and two solenoids on either side.  All of the wiring, status switches and contacts are exposed and easy to get to.  This one, not so much.  This is the BE AM1A transmitter

Broadcast Electronics AM1A transmitter
Broadcast Electronics AM1A transmitter

It is not a bad unit, compact, sounds good, is reliable, etc.  In order to work on the power supply or anything in that top cabinet, the whole thing needs to be removed from the rack and taken down.  I suppose that is my only gripe about the thing.