Re: Monitor Problems

["Steven N. Hirsch" <shirsch@adelphia.net>]

Michael J. Mahon wrote:
> 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.  ;-)

Heh.  My late father was a Signal Corp. radar officer in WWII. When I
first started working with (tube) electronics in my teenage years, he
often dragged out horror stories about men in his unit bypassing
interlock switches when servicing the final RF stage.  While I was never
quite sure whether these were gospel or urban legend, it made a distinct
impression.

Steve