Re: Vcc buss rails being pulled down.

A quick way of finding a short is to use a bench supply set to constant current mode. Set the current to around 1/2 Amp or 1Amp then use a millivoltmeter to trace along the supply rails until you find the short. Check across Vcc and gnd pins of each IC to find to find the dead chip. Also, Tantalum bead caps are also good candidates for shorts.

Chris

On 16 Oct 2011, at 23:48, litterbox99@mchsi.com wrote:

> The caps were my first thought, however, the board has
> has a hang tag that states "caps replaced, board comes up
> 04-20-1998" The caps look fine.
>
> What is this 'milliohm meter' method you
> speak of ? I have a digital bench meter
> but I'm not sure of it's sensitivity.
>
>
>
>
>
> ----- Original Message -----
> From: "Rodger Boots" <rlboots2@gmail.com>
> To: vectorlist@vectorlist.org
> Sent: Sunday, October 16, 2011 4:04:20 PM GMT -06:00 Central America
> Subject: Re: VECTOR: Vcc buss rails being pulled down.
>
>
>
> First step, disconnect electrolytic capacitors and tantalum capacitors conected Vcc. If you have a very good ohmmeter you may be able to trace it to an area.
> On Oct 16, 2011 4:38 PM, < litterbox99@mchsi.com > wrote:
>
>
> 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
>
>
>
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Received on Mon Oct 17 17:42:51 2011