[AMRadio] Stock or modify? BC rig "value"


Phil Galasso k2pg at worldnet.att.net
Tue May 30 21:59:33 EDT 2006


----- Original Message -----
From: "VJB"
> Several people have asked me, over the past 10-15
> years, whether it is "better" to preserve an old
> broadcast transmitter as-is or modify it to make it
> more useful in a second life on the ham bands.
>
> I wholeheartedly say modify it !

Whenever I have acquired a broadcast transmitter, I spent many weekends
REMOVING some totally atrocious mods that were installed by "engineers" over
the years. Some of these were done under time pressure in order to get the
transmitter back on the air in a hurry, but most of them seriously
compromised safety. If they were done for that purpose, the transmitter
should have been fixed properly at the earliest opportunity, during an
overnight maintenance shift.

Worse are some of the mods made by hams who think that they know more about
engineering practices than do the people who design transmitters for a
living. While some broadcast transmitters were designed "on the cheap" (the
Gates BC-1T is a prime example), the Collins 20V series and the RCA BTA-1R
series are fine transmitters in their own right. Don't butcher them. Use
them and enjoy them for what they are.

I personally prefer to keep my broadcast rigs stock, using them on 160. The
only mods that I have ever made in these transmitters are slight changes to
tuned circuits to get them to hit 1885 and the installation of slave relays
for turning the filaments and plates on and off by remote control. Many
broadcast transmitters from the 1950s and 1960s use 115 or 230 VAC on the
control ladder. Good engineering practice dictates that control lines
running outside the box should have no more than 28 volts on them. If you
prefer more modern techniques, you could build a logic controller, using
optoisolators and solid state relays to interface to the remote control
terminals inside the transmitter. One other modification would be to replace
any selenium rectifiers (which may be encountered in a bias supply in the
low level stages of the transmitter) with silicon diodes. Selenium
rectifiers deteriorate with age and the fumes emitted by a failing selenium
rectifier are highly toxic. The toxicity rivals that of arsenic! Also, any
paper tubular capacitors, "Black Beauties", or "bumblebees" would be
replaced with Sprague Orange Drops for better reliability.

The worst mod that I often see (and one that would cause me to fire a
subordinate at work, if I caught him or her doing it) is the defeating of
door interlocks. These are there for a reason. The plate transformer in a
typical 1 kW tubed transmitter can deliver sufficient voltage and current to
run an electric chair. YOU COULD GET KILLED! With ONE exception, every
broadcast transmitter that came into my hands had the interlocks jumpered
out. The first thing that I did before connecting power was to restore the
interlocks to their stock condition. Overload relays should also be
preserved, as they can save you some very expensive tube failures.

Granted, these transmitters probably have little intrinsic value. In fact,
the most I ever paid for a broadcast transmitter was $1! But I still like to
restore them to near-pristine condition, much as car collectors enjoy
restoring vintage cars. Some of the broadcast transmitter mods that I saw on
the Internet remind me of installing a Yugo engine in a Lamborghini!

If you obtain a broadcast transmitter, think about SAFETY FIRST. Don't play
games with the power supply, don't jumper out interlocks, and bear in mind
that features that were designed in a broadcast transmitter that some hams
may not like are there for a reason. You could risk a fire, shock, or death!

Phil K2PG




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