1. Home
  2. Hobby Electronics Basics
  3. Help repairing a board with SN74LS123N

Help repairing a board with SN74LS123N

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

Reply to
Dave, I can't do that
Loading thread data ...

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
Reply to
John Popelish

Standard troubleshooting. Is V+ steady at pin 16?? Flex the board a little and look carefully at the solder joints. Put finger pressure on the chip and look for changes. Is is a DIP or flat pack, etc, etc

Reply to
bw

Hi John,

A single line to the connector.

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

Thanks a million guys,

Dave

Reply to
Dave, I can't do that

Hi BW,

Yes, 4.92vdc

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

Reply to
Dave, I can't do that

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.