Help repairing a board with SN74LS123N

[Dave, I can't do that]

Hi people,

OK, out of my depth here so be gentle.

I have a 6" x 4" PCB with a display and other stuff on it and it is
supposed also output a variable PWM signal of 5vdc at 5KHz. Tracing
back from the output connector, the first chip is a SN74LS123N,
Monostable Multivibrator.

It is putting out a square wave but at only around 460mv, it
fluctuates between 440mv and 480mv.

Using a Tektronix TDS-210 scope, I am getting a wildly drifting
frequency ranging from about 3.8KHz to a little over 8KHz.

Would this indicate the chip is faulty and that by replacing it I can
solve this problem?

OR

Could it be something before that chip?

The chip has 5 caps in it's vicinity and 2 resistors, I am guessing
that some or all set the 5KHz frequency. Not all of those may be
connected to the chip but they are close by. It is difficult to follow
the traces as it is a multi layer board. One of the small blue caps
with 473M as the only marking looks like it has been replaced as there
is resin marks around the soldering.

I know it is probably a question like, "how long is a piece of string,
" But I am totally dumb when it comes to digital stuff but quite
capable of swapping out the chip. I live out in the boonies and it
takes about a week for stuff to get here so if I have to order I'd
like to get the right thing.

Thanks for any crumbs you guys can sprinkle.

Dave

 

[John Popelish]

Dave, I can't do that wrote:

Hi people,

OK, out of my depth here so be gentle.

I have a 6" x 4" PCB with a display and other stuff on it and it is
supposed also output a variable PWM signal of 5vdc at 5KHz. Tracing
back from the output connector, the first chip is a SN74LS123N,
Monostable Multivibrator.

It is putting out a square wave but at only around 460mv, it
fluctuates between 440mv and 480mv.

Using a Tektronix TDS-210 scope, I am getting a wildly drifting
frequency ranging from about 3.8KHz to a little over 8KHz.

Would this indicate the chip is faulty and that by replacing it I can
solve this problem?

OR

Could it be something before that chip?

The chip has 5 caps in it's vicinity and 2 resistors, I am guessing
that some or all set the 5KHz frequency. Not all of those may be
connected to the chip but they are close by. It is difficult to follow
the traces as it is a multi layer board. One of the small blue caps
with 473M as the only marking looks like it has been replaced as there
is resin marks around the soldering.

I know it is probably a question like, "how long is a piece of string,
" But I am totally dumb when it comes to digital stuff but quite
capable of swapping out the chip. I live out in the boonies and it
takes about a week for stuff to get here so if I have to order I'd
like to get the right thing.

If the output swing from the chip is not at least 3.5 volts

or so, then either the chip is bad, or there is something
overloading it. Are there any other components between the
signal and ground or the supply? Or is it just a single
conductor from the chip output to the board connector or cable?

It is pretty unlikely that anything upstream of the chip
(except the supply voltage) could cause it to produce this
reduced swing. That stuff could cause lots of other
problems, but not very likely this one.

--
Regards,

John Popelish

 

[Dave, I can't do that]

On Sep 3, 4:18 pm, John Popelish <jpopel...@rica.net> wrote:

Dave, I can't do that wrote:

Hi John,

Are there any other components between the
signal and ground or the supply? Or is it just a single
conductor from the chip output to the board connector or cable?

A single line to the connector.

(except the supply voltage) could cause it to produce this

As suggested by BW, I will check the 5v supply.

Thanks a million guys,

Dave

 

[Dave, I can't do that]

On Sep 3, 11:54 pm, "bw" <bweg...@hotmail.com> wrote:

Hi BW,

Standard troubleshooting. Is V+ steady at pin 16??

Yes, 4.92vdc

Is is a DIP or flat pack, etc, etc

It is a DIP-16

OK, I took this to a friend of mine who is younger and has better eyes
and steadier hand. {grin} I am 65 and trying to get this thing going
as a hobby.

It turns out the track scooted under and parallel to a resistor with
several other tracks and I picked up the the wrong one coming out. It
appears the lead from the connector goes to pin 19 of a GAL16V8AS
(E2PROM CMOS PROGRAMMABLE LOGIC DEVICE).

I think this is getting too complicated as the tracks from the GAL16
go to a large Intel P-80C32 so I think I may have to scrap the idea of
getting this working as there are no schematics available for it. Now
that I have discovered programmable chips, if any of that is fried
then there is definitely no chance of getting a replacement as this
was designed in 1992 and the company no longer has information on it.

The board is a Universal Laser Systems P/N 12-900-D just on the off
chance that it spikes a memory in someone who may have a schematic
lying around. Ha haa.

Thanks for trying and sorry for the misleading track following.

Dave