Alt-BEAM Archive

Message #13567



To: beam@sgiblab.sgi.com
From: Evan Dudzik evandude@yahoo.com
Date: Mon, 17 Apr 2000 10:57:51 -0700 (PDT)
Subject: Re: 247 thingy


its a 74***240 (replace *** with either HC, HCT, or
ACT, act is best, but not cheapest) and you can get it
at digikey (digikey.com) or even radio shack

--- SkavenArmy@cs.com wrote:
> where do i get the 240746 or whatever it is?? the
> thing thats in all the
> bicore circuits? btw: some new robotics hax:
> macdonalds around here is selling furby's that
> say one of two phrases
> when you push a button......this would make for an
> interesting selection if
> you were to use it as a tactile , and then whatever
> after that , or put 2 on
> diferent circuits so that if one is pushed and not
> the other it would send
> only 1 of the 4 options.....idea 1
>
>
> more to come...
>
>
> FrOm ThE SiCk AnD TwIsTeD....
>
> Jay z
>

=====
+----------------------------------+
|http://members.xoom.com/Star_BEAM |
|Star BEAM!!! |
|Evandude Dudzik |
+----------------------------------+

__________________________________________________
Do You Yahoo!?
Send online invitations with Yahoo! Invites.
http://invites.yahoo.com



13568 Mon, 17 Apr 2000 11:59:30 -0600 Yet More Info (Re: Deadman's stick (was: RE: Monitor Salvage)) beam@sgiblab.sgi.com "John A. deVries II" Probably better is this article I just found=20
(http://www.pacwest.net/byron13/sam/crtfaq.htm). On the other hand I don't=
=20
know what is in a normaloid TV set that you'd want to salvage.


>Safe Discharging of Capacitors in TVs and Video Monitors
>It is essential - for your safety and to prevent damage to the device=20
>under test as well as your test equipment - that large or high voltage=20
>capacitors be fully discharged before measurements are made, soldering is=
=20
>attempted, or the circuitry is touched in any way. Some of the large=20
>filter capacitors commonly found in line operated equipment store a=20
>potentially lethal charge.
>This doesn't mean that every one of the 250 capacitors in your TV need to=
=20
>be discharged every time you power off and want to make a measurement.=20
>However, the large main filter capacitors and other capacitors in the=20
>power supplies should be checked and discharged if any significant voltage=
=20
>is found after powering off (or before any testing - some capacitors (like=
=20
>the high voltage of the CRT in a TV or video monitor) will retain a=20
>dangerous or at least painful charge for days or longer!)
>The technique I recommend is to use a high wattage resistor of about 100=20
>ohms/V of the working voltage of the capacitor. This will prevent the=20
>arc-welding associated with screwdriver discharge but will have a short=20
>enough time constant so that the capacitor will drop to a low voltage in=20
>at most a few seconds (dependent of course on the RC time constant and its=
=20
>original voltage).
>Then check with a voltmeter to be double sure. Better yet, monitor while=20
>discharging (not needed for the CRT - discharge is nearly instantaneous=20
>even with multi-M ohm resistor).
>Obviously, make sure that you are well insulated!
>=B7 For the main capacitors in a switching power supply which might=
be=20
>100 uF at 350 V this would mean a 5K 10W resistor. RC=3D.5 second. 5RC=3D2.=
5=20
>seconds. A lower wattage resistor can be used since the total energy in=20
>not that great. If you want to be more high tech, you can build the=20
>capacitor discharge circuit outlined in the companion document:=20
>=B7 Capacitor Testing, Safe Discharging, and Other Related=20
>Information. This provides a visible indication of remaining charge and=20
>polarity.
>=B7 For the CRT, use a high wattage (not for power but to hold off=
the=20
>high voltage which could jump across a tiny 1/4 watt job) resistor of a=20
>few M ohms discharged to the chassis ground connected to the outside of=20
>the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive=
=20
>circuitry. The time constant is very short - a ms or so. However, repeat a=
=20
>few times to be sure. (Using a shorting clip lead may not be a bad idea as=
=20
>well while working on the equipment - there have been too many stories of=
=20
>painful experiences from charge developing for whatever reasons ready to=20
>bite when the HV lead is reconnected.) Note that if you are touching the=20
>little board on the neck of the CRT, you may want to discharge the HV even=
=20
>if you are not disconnecting the fat red wire - the focus and screen (G2)=
=20
>voltages on that board are derived from the CRT HV.
>WARNING: Most common resistors - even 5 W jobs - are rated for only a few=
=20
>hundred volts and are not suitable for the 25kV or more found in modern=20
>TVs and monitors. Alternatives to a long string of regular resistors are a=
=20
>high voltage probe or a known good focus/screen divider network. However,=
=20
>note that the discharge time constant with these may be a few seconds.=20
>Also see the section: Additional Information on Discharging CRTs.
>If you are not going to be removing the CRT anode connection, replacing=20
>the flyback, or going near the components on the little board on the neck=
=20
>of the CRT, I would just stay away from the fat red wire and what it is=20
>connected to including the focus and screen wires. Repeatedly shoving a=20
>screwdriver under the anode cap risks scratching the CRT envelope which is=
=20
>something you really do not want to do.
>
>Again, always double check with a reliable voltmeter!T
>Reasons to use a resistor and not a screwdriver to discharge capacitors:
>It will not destroy screwdrivers and capacitor terminals.
>It will not damage the capacitor (due to the current pulse).
>It will reduce your spouse's stress level in not having to hear those=20
>scary snaps and crackles.
>
>Additional Information on Discharging CRTs
>You may hear that it is only safe to discharge from the Ultor to the Dag.=
=20
>So, what the @#$% are they talking about? :-).
>BTW, don't wash your CRTs even if the Maid complains about the filth until=
=20
>you have confirmed that your 'Dag isn't water soluble (maybe that's why it=
=20
>has 'aqua' in the name!). It may all come off! Wipe off the dirt and dust=
=20
>with a cloth (and stay away from the HV connector or make sure it is=20
>discharged first!).
>(From: Asimov (mike.ross@juxta.mnet.pubnix.ten).)
>'Dag' is short for Aquadag. It is a type of paint made of a graphite=20
>pigment which is conductive. It is painted onto the inside and outside of=
=20
>picture tubes to form the 2 plates of a high voltage filter capacitor=20
>using the glass in between as dielectric. This capacitor is between .005uF=
=20
>and .01uF in value. This seems like very little capacity but it can store=
=20
>a substantial charge with 25,000 volts applied.
>The outside "dag" is always connected to the circuit chassis ground via a=
=20
>series of springs, clips, and wires around the picture tube. The high=20
>voltage or "Ultor" terminal must be discharged to chassis ground before=20
>working on the circuit especially with older TV's which didn't use a=20
>voltage divider to derive the focus potential or newer TV's with a=20
>defective open divider.

Home