Tech Notes from the Field

Not to focus on anything like this, but, damn, I have not posted in a while.  Let’s start flooding you with some real boring stuff!  A few weeks back I had the un-welcomed 12:30am phone call from the remote control.  How we love those calls.  The basics were there:  No forward power, off air alarm, etc.  I do the normal, let’s try and turn it back on.  Nothing.  I tried to switch to the Auxiliary transmitter, but that did not happen.  My first thought was “great there goes a UPS” which would explain lack of control, but my remote control was working.  Up on the Aux site I went.  Mind you all this took place quick enough the person in building was never aware of the condition.

Off to the site I go ready to find some melted metal.  First lesson of the evening: Make sure all connections are tight and good even on the remote control!  A wire backed out of the Phoenix connector on my command for the coax switch.  Doh, no wonder it didn’t switch.  Fixed that and put the Aux transmitter on the air and turned off the Aux site.  (Nice to have an actual auxiliary site.)  Crazy how that switch ALWAYS remotely switched until this night.  Now on to the main transmitter.

The HTHD+, as all HT series transmitters, has the mimic panel. I see red LEDs for the Plate and a Fault indicators.  Well, isn’t that nice.  I did just replace the tube a month prior.  Let’s see what happens if I shut everything down and start it up again.  This usually clears a strange fault or situation especially with a power fluctuation.  Wham and Bam.  Nope.  Immediate slam, in your face, time to troubleshoot sound.  I checked the obvious PA cavity and tube.  Always check that plate blocker.  No obvious signs and no arcing.  Jeez, it was clean.  Give the tube a shake.  Not rattle.  Good.  Let’s give it another chance, so I turn on the filament, then plate.  Same result.  Nothing.  At this time I take a step back and glance at the big picture.  Plate and Fault indicators on the mimic panel.  I glance at the High Voltage Power Supply (HVPS) cabinet and there I see it, a MAG OVLD indicator.

How I didn’t notice it when I first walked in I cannot say.  Tired or just fixated on the mimic panel.  Time to focus on the HVPS.  First step is I verified the fault through the multimeter overload faults.  Code 21.  A cross check corresponded with the MAG OVLD which indicates either relays being open or control voltage was not there.  I did all the basic checks:  Input line volage, cabinet connections, and interlocks.  All good.  So, I either had a bad relay or, worst case, a shorted transformer, though I did not smell any magic smoke.  I messaged GatesAir via the customer portal, and then studied the schematic.  As all line voltages were good I was beginning to suspect a set of relays, and after a nice chat with GatesAir, we determined to focus on magnetic relays 3K1 & 3K2.  I set out to do just that.  There happens to be contacts on those relays that are exposed.  I gave them a little burnshing.  I did a quick test into the load.  Everything came up flawless.  As I had the aux transmitter on the air, I decided to proceed with a cleaning of the HVPS cabinet.

After cleaning up I placed the HTHD+ back on the air.  I still find it odd that this failure occurred in the middle of the night.  I also find it all that something got in between the contacts on either or both relay contacts while they are normally closed.  I did go through a cleaning while I did the tube replacement earlier, but it never occurred to me to burnish these contacts.  In addition all was running fine up to that point.  As power at the site can be somewhat inconsistent, I wonder if a phase dropped briefly and upon an automatic restart an arc or some dust affected the relay contacts.  I have an HT-25, I’ve installed and maintained an HT-30 in the past.  I have never had this situation, and they were in environments worse that this.

I love this job as there is always something to learn even if it is a minor, and somewhat obvious item.  We do not “know it all” and I love to learn.  Pack it in your data bank of a brain so next time the repair is a no-brainer.

Cheers!