Re: Vcc buss rails being pulled down.

William Boucher wrote:
> I think that the odds are that it is a shorted electrolytic or
> tantalum capacitor so maybe check those first.
>
> Usually the shorted component gets warm, at least significantly warmer
> than the others (not counting the shorted regulator). You can use one
> of those handheld gun type infrared temperature measurement tools to
> see which chip is taking all of the current and heating up. It only
> takes an increase of couple of degrees to find it.
>
> Another method, I don't know if they are still available but I have a
> probe called a "short squeak". It looks just like a typical logic
> probe but it is designed specifically for finding shorts like yours.
> It generates a audible tone the goes up in frequency as you get closer
> to the short. It has two probes so all you do is "measure" across any
> two points and listen. The higher the pitch, the closer you are to
> the short. It is amazing how fast you can track down which device is
> faulted.
>
>
> William Boucher
> http://www.biltronix.com
>
>

If Todd does not have he Short Squeak (very handy tool - I have one too)
then the next best way to find shorted ICs is to isolate the rows of TTL
by cutting the +5VDC trace where it feeds each row of TTL and then do a
continuity test, or put 5VDC to the board and check the voltage after
each row is isolated.

I do this by starting with the ohm-meter connected across the 5VDC bus
connections at the edge connector - that gives me the shorted Vcc
reading, usually something like 0.5R or so. Now you cut one row off and
check the Vcc resistance - still 0.5R? Keep cutting Vcc bus lines one at
a time to find the single row that clears the short. Now split that line
in half (measuring the resistance of just that row), then half again to
pin down the likely suspect or pair of suspects and remove just one or
two ICs. You can also clip the Vcc leg of suspect ICs, leaving enough
leg on the IC that you can bend the clipped leg back and solder it on
securely if the IC is not shorted...

John :-#)#
>
> ----- Original Message ----- From: <litterbox99@mchsi.com>
> To: <vectorlist@vectorlist.org>
> Sent: Sunday, October 16, 2011 5:38 PM
> Subject: VECTOR: Vcc buss rails being pulled down.
>
>
>> I'm working on a Slot machine MPU, with
>> similar 74xxx components like our arcade
>> games.
>>
>> The Vcc buss is being pulled down to about 50mv.
>> The LM340T5 runs very warm, so were keeping the
>> power on time very low.
>>
>>
>> Besides removing the IC's one at a time (and
>> of course their not socketed ) is there a way
>> to ID which component is at fault ?
>>
>> We've removed the IC's that are socketed, and
>> checked for warm or hot components.
>>
>>
>> Todd
>>

-- 
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
                 www.flippers.com 
"Old pinballers never die, they just flip out"
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