Engineer Killer

That was the title of the email with this photo attached:

Disabled high voltage shorting bar, Collins 820D-2
Disabled high voltage shorting bar, Collins 820D-2 AM transmitter.  Courtesy Pete Partinio

That seems about right.

For many, many reasons, this is a bad thing to do.  First of all, the shorting bar is the last point of discharge for the high-voltage power supply.  When all else fails, this is designed to route the 3,500-volt plate supply safely to ground.  Having a stray 3,500 volts floating around inside a transmitter is never a good idea.  Fortunately, it was spotted and removed before anything bad happened.

Secondly, it looks like somebody used a 12 VDC cigarette lighter plug as an insulating device.  Wow, did they get lucky.  This could have started a fire.

As to exactly why it was there in the first place, I cannot rightly say.

And this is why only properly trained people should be working on transmitters, especially tube-type ones.

Print Friendly, PDF & Email

13 thoughts on “Engineer Killer”

  1. Brings to mind a quote from an unknown person that I’ve had hanging in the WKIP transmitter building for many years……
    “It is impossible to make anything foolproof, because fools are so ingenious.”

  2. Why on earth would anyone do this?

    I’ve also seen interlocks disabled with all manner of “systems”. Again, very stupid!

  3. While I’ve seen my share of jury rigging in studio equipment I consider myself fortunate I’ve never run into anything as dangerous at a transmitter site. That one is just asking for trouble.

  4. Interlocks defeated, cabinet doors removed, Plate relays jumpered so you can’t turn the HV off…. once saw an RCA Am chugging along completely rewired with white zip cord. Stuff like you’d find on household table lamps. Nothing surprises me anymore. I bet they saved a buck though, so…..

  5. Don’t remember the transmitter but the shorting bars (like in the picture) would occasionally stick in when the covers were removed. Working them every time the box is opened is just something I do. Every time!

  6. Ouch. –I am, however, reminded of a line of Collins/Continental FM rigs with tube drivers and tube final (4CX250s into some power grid tube) in which a routine but infrequent adjustment — driver tuning? PA/driver coupling? — requred removing a panel, defeating the interlock and reaching in above HV filter caps while holding a screwdriver to turn the variable element. Don’t drop it! When the station I worked for bought another same-model transmitter, I asked the salesman, “Surely they don’t tune ’em that way in the factory?” …He had to check, but it turned out no, they used a special Plexiglas jig to make the task safe. I asked him to sell us one, which he did. My predecessors had been using a (clean) foam coffee cup to hold the shorting bar open. Dumb design and they fixed it in the next year or two.

    RCA F-series TV rigs had LV interlock switches with plungers could be pulled outwards to a detent position that would defeat them. Closing the door reset the switch to normal operation. Yeah, not great (I once chinned myself on a General Manager’s gesturing arm when he was walking around at the transmitter site being congratulatory after a disaster recovery: we still had doors open and interlocks defeated, and there went his arm towards the hot stuff!) but you needed it occasonally for troubleshooting.

  7. Roberta, you are thinking of the Continental 816R2 transmitter with a pair of 4CX250B drivers. The cathode currents needed to be balanced as part of the driver tube replacement process. That required taking the front panel off the transmitter and defeating the interlocks. I never liked that procedure, having done it countless times. Once done, all the interlocks are restored. You are correct, this design was changed in later models.

  8. It is true that the newer 816 rigs with solid state drivers didn’t need the pots to adjust static cathode current…but the newer ones I have worked on still had a couple of knobs down there that needed once-in-awhile touchup. There was a grid tuning control that worked in conjunction with the one on the front of the transmitter, and there was the so-called “efficiency control” that IIRC, adjusted an air variable across the PA filament.

  9. At a site you presently administrate,an IT guy was once sent to replace a 5 Kw AM’s final,alone, for the first time!
    Oh,did I mention that he was sent alone? Good…
    Thankfully,this was over 10 years ago,mentioned to me by a venerable staff member during a social visit.
    I hope this fella survived!

  10. Allen: wanna bet you can still buy a version of the safety rig? –IIRC, it couldn’t be left in place, too disruptive to cooling air flow, but it made a touchy, two-person job into something one person could do alone.

    Paul: Yes, that’s the one. The orginal rig was a Collins and the new one was a Continental — same transmitter (mostly), louder paint job. The new one came along as part of a transmitter site move and we were finally able to use the HV regulator (pretty much a massive light dimmer) properly: the old site was shared with a TV station and they objected to the noise (electrical and audible) if the regulator was doing much work. …Made tube changes fun, as we had to tweak screen volts to set power at “minimum noise” and the Collins required tap-changing.

    Did you ever encounter what they would do if the “Plate” pilot light shorted? It was…interesting.

  11. Faceplam…let me guess, some “University of Phoneix online” IT graduate was hired on to be the new engineer. This is what we get nowadays. Everyone who can setup a WiFi access point is a qualified RF engineer. The process of natural selection will commence with stupidity like this.

  12. Some times it is necessary to defeat the interlocks to find a problem but never leave it that way, you or worse someone else is looking at sudden death.

Leave a Reply

Your email address will not be published. Required fields are marked *