Re: GS Monitor keeps getting brighter

["Michael J. Mahon" <mjmahon@aol.com>]

Warren 'Llama' Ernst wrote:
> On Aug 13, 4:44 am, Steven Hirsch <snhir...@gmail.com> wrote:
> > mdj wrote:
> > > My worst experience comes from helping a friend replace a hard drive
> > > in an older AT style PC. The shoddy assemblers had put the switch
> > > assembly together without including the rubber sheaths over the spade
> > > connectors. Trust me, you do not want to be a part of the conductive
> > > path between an active pin and the ground :-(
> > Heh.  One of the most potentially lethal units out there is a Fender Twin
> > guitar amplifier.  B+ supply is 675v at about 1/4 amp.  Power supply filtering
> > is handled by a row of axial-lead electrolytic caps soldered to a circuit
> > board in a can on top of the chassis.  As shipped, a bleeder resistor is the
> > end component on the board and, when working, it drains things down very
> > quickly after power-off.  Problem was, they didn't use any glue or silicone
> > rubber to cushion the resistor on early units and vibration invariably cracked
> > off one or both of the leads.
> >
> > After getting nailed with that baby once (and fortunately living to tell about
> > it) I got in the habit of shorting the output tube plate pin to ground with an
> > insulated screwdriver before delving in :-).  Lost all feeling in my right arm
> > for about 10 minutes after the initial event.
> >
> > Steve
>
> Discharging a CRT it not that difficult a skill to learn (you pick it
> up when you own and repair vintage arcade games):
>
> Wearing rubber gloves (just in case - this probably isn't really even
> necessary), I take a length of cut extension cord with alligator clips
> soldered on each end, and clip one end to the exposed metal on the
> monitor chassis, and the other to a long flat-bladed screwdriver. With
> the monitor off, unplugged, and opened up, you stick the flat part of
> the screwdriver underneath the "suction cup with a cable" that's
> attached to the top tube until it touches the central metal core.
>
> If the monitor has been on recently, you'll hear a "pop." Then I wait
> 5 minutes and do it again.

Good advice.  Better even to leave a ground lead clipped to the
anode while working on it.  (The ceramic HV filter capacitor dielectric
often has remanent polarization which will cause it to "recover" a
charge even after being shorted repeatedly.)

> Looking for a photo, I found a quick website that documents the
> process much better, and with photos:
> http://www.instantarcade.com/discharge.php
>
> They sell "Cap Kits" for old arcade monitors, and you'd be amazing how
> an old broken monitor can sometimes look as good as new after just
> replacing the caps. It's basically just a baggie of 20 capacitors of
> various sizes and ratings, along with a sheet of paper that says what
> caps go where. You just disconnect the various PCB boards, clip off
> the old cap leads, desolder the cap leads and the solder in the new
> cap. Repeat 20 times.

This is a "shotgun" approach that is usually quite wasteful of labor
and components.  It is certainly likely to work, but just the process
of replacing things that aren't failed is a risk.

It's also good training to isolate and replace what is failed, rather
than just shotgunning.  This will cause one to associate the symptom
with the cause, thus learning how to better fix things in the future.

Aluminum electrolytics are notoriously bad "agers", and so replacing
them may be justifiable--but even then, only a small fraction of them
usually need replacement, and the rest may last as long as a new cap!

Non-electrolytic capacitors seldom fail, and you'll practically never
encounter more than one failure (usually current leakage) in a system.

-michael

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