Steven N. Hirsch wrote:
mdj wrote:
I've built some interesting audio equipment in my time too - one
particular amplifier that has 40,000uF of filter capacitors on it's
supply rails that I very thoroughly discharge after lifting the lid. To
me, the filters on the secondaries of power supplies are a *lot*
scarier than the HT of a CRT.
Tell me about it. I used to work on Fender and Marshall guitar amps.
Nothing like a 750V 0.25A B+ to make one cautious. They are built with
a bleeder resistor, but no sane technician would stick their hands into
a powered-down chassis before applying a handy screwdriver between power
supply output and ground.
I was going to write this reply to Matt's post, but after reading yours,
*this* is where it belongs...
By the time I was a sophomore in highschool, I had accumulated a pretty
complete electronics "lab". Back then, that meant vacuum tubes, and
B+ voltages routinely in the 200v-400v range (except for higher-powered
applications like Steven's).
I had a few "utility" power supplies that I used for powering breadboard
designs, and one very simple, handy one supplied 6.3vac and +280vdc for
typical tube circuits.
The supply was built on a simple folded "channel" aluminum chassis, with
open ends. It had a simple capacitor-input filter, with two 40uF
sections. It also had a 100K bleeder resistor across the output so that
when it was turned off, the caps were discharged in a couple of seconds.
Of course, the bleeder serves no function other than to discharge the
capacitors, and so, when one of the leads broke off one day (too much
bending), I didn't rush to replace it...
A couple of weeks later (I really wasn't in a rush ;-), I had been using
the supply when I decided to move it to the other side of the bench to
make some room.
I reached over, picked up the supply by the *open* ends, and had lifted
the 10+ pound power supply about 4", so I had a firm grip on it. As
I lifted, my fingers curled under the chassis just enough to contact
the positive terminal of the filter capacitor.
What happened in the next 10 milliseconds still amazes me! My arm
and chest muscles were seized by a "higher power" and the heavy supply
was instantly launched with toward the other side of the basement as if
shot from a cannon!
It hit the wall 25 feet away on the other side of the basement,
transformer first, and fell to the concrete floor...
After straightening the end of the chassis and replacing the 5Y3
rectifier tube, the damage to the power supply proved quite temporary
(and it sported a new bleeder resistor ;-).
But the vibrant memory of the day I "shot putted" my power supply
harder than any throw I've made before or since remains as permanent
as the gouge in the concrete block wall.
Needless to say, I've taken bleeder resistors and charged capacitors
more seriously since! And many of my screwdrivers have "welding marks"
from where they were used to discharge caps.
It in this context that I conclude that the HV supply of a standard
monitor is only a minor annoyance--aside from the startle reaction. ;-)