Hi Marv...
The exact system you describe - a BIG neon bulb in a relaxation oscillator,
blinking near the rf section of a PP 813, may well have saved my life about
40 years ago while I was W0FID and in high school back in Minnesota.
The PA used plug-in BC-610 tank coils and had an interlocked, hinged top
cover (nearly six feet off the floor) for coil-changing access. I climbed
up on a chair, opened the cover to change bands. Just before I would have
grabbed the (HOT!) tank coil by both ends to pry it out of its very stiff
banana-jack-bar socket (elbows solidly on the aluminum enclosure), I
happened to see that neon lamp blinking. The cover interlock was open, but
the primary relay contacts had welded shut, leaving full ~2500V HV on the
tank coil.
That relay was a husky, very popular Potter-Brumfield PR-type which I'll
bet many home brewers are still using. That kind of experience will make
one a real advocate of using BOTH an a-c interlocks and an HV crowbar --
AND pulling the a-c plug physically before working on any HV equipment!
BTW, one more little insurance policy is a simple step-start system as
often discussed on the reflector. By limiting inrush current to ~10-15A, it
usually will prevent relay contact welding (which typically is caused by
very large initial inrush current transients).
73, Dick W0ID
-----Original Message-----
From: Radio WC6W [SMTP:wc6w@juno.com]
Sent: Tuesday, April 06, 1999 8:58 AM
To: amps@contesting.com
Subject: [AMPS] HV Safety
Hi all,
I just thought I'd share an idea from my own homebrew amplifier
experience along this line.
In my own amp and separate power supply which is built in the bottom
of a rack cabinet I have plastic covers over as many of the exposed HV
points as practical. I do not have any cabinet interlocks since the
first thing you would naturally do is disable them if you were going to
do any work on the unit in operation.
I do have neon indicators both on top of the HV supply and inside the
RF deck. The one in the RF deck is located at / connected to the bottom
of the Plate RF choke and blinks for additional effect. I used a 500M /
10 KV resistor (Victoreen) in conjunction with an NE-2 and a .1 uF
capacitor. It is quite noticeable.
I must credit some long forgotten manufacturer of Apple II accessory
boards for this idea. They used an LED to indicate power present inside
the computer and warn against removing a board while powered since that
was likely to zap a couple of integrated circuits.
73,
Marv WC6W
*
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