|[AMRadio] Stock or modify? BC rig "value"|
lhwill at verizon.net
Tue May 30 22:01:31 EDT 2006
Good advice Phil!
At 09:59 PM 5/30/2006, you wrote:
>----- Original Message -----
> > 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!
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