[BC] BT-20-A

[gRAdy Moates]

Just a quick note. . .

I saw a man come within 2 inches of dying because of a similar
relay on a BTA-50G 50,000 Watt transmitter.  We were trying to
find and fix a problem, and had cycled the transmitter completely
off, filaments and all.  We had been working for hours, and the
guy had skipped the shorting stick procedure this one time.

He touched the control grid ring on one of the PA tubes and
felt a "tingle", so he grabbed the shorting stick and brushed
the tube with it. . .

. . .and drew a 2" arc from the anode!!

Even though the entire transmitter was shut down, the
"ReClosing Relay", which actually connected primary 480V
3-phase power to the HV supply, was physically stuck in the
"on" position, and the tubes had full plate voltage on them!

That relay was so unreliable, even after rebuilding by the
Boston GE Apparatus Service Shop, that we had to add a
step in the shutdown procedure in the remote control that
would energize the overload relay to force the ReClosing
Relay to open.

Grady




----- Original Message ----- 
From: "Rick Brashear" <rickbras at airmail.net>
To: "'Broadcasters' Mailing List'" <broadcast at radiolists.net>
Sent: Thursday, March 29, 2007 10:32 PM
Subject: RE: [BC] BT-20-A


> Thanks Ron.  I took the old one out and cleaned it up.  In the process I
> discovered how it works and what I think was wrong.  It seems there should
> be a small spring on the solenoid plunger to assist the mechanism during 
> the
> delay portion of the cycle.  I put a rubber band on it as a test and all
> worked great!  I'll get the proper spring tomorrow.  It's amazing how much
> engineering there is to this relay to achieve an approximate 3 second 
> delay.
> All mechanical.  Like you said, there is a small adjustment on the side 
> for
> the timing.  Since this is normally used to prevent the transmitter from
> being knocked off the air during short power outages like in a thunder 
> storm
> I really don't need it for my application.  However, I like keeping things
> as original as possible, so I'll adjust it to delay a second or two and
> leave it in the circuit.
>
> Thanks again for the info.
>
> Rick
>
>
>   I don't know the exact relay, but in an odd place there
> should be a screwdriver driven adjustment to set the time,
> about 3/16" in diameter, a shaft with a slot like a screw
> head.
>   I was told these are pneumatic, but I never tore one apart,
> so that is hearsay.  If so the "rubber" parts may be rotten
> or have been torn.  (I have seen lightning do this!)
>
>   These can be replaced with a modern R-C timer relay or you
> can make one with a DC relay and capicator/s driven with a DC
> power supply.   (big electrolytic)
> -- 
>    Ron  KA4INM - My Florida property tax is 7 times what it was
>                  3 years ago with no change on my part. OUCH!
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