Good advice Phil! Larry W3LW
At 09:59 PM 5/30/2006, you wrote:
----- 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 ______________________________________________________________ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb
