help diagnose old circuit board, fault

[robb]

wow thanks legg,

i guess alot of detective information is allready there in the fault
description that i ignored.

namely a group of buttons have all failed yet other groups of buttons with
different functions has not failed on the same board should lead one to
suspect a possible common shared connection between these three failed
buttons.

so look to the common connection point for all three failed buttons.

how simple and forthright yet elusive to amateurs like me.

one interesting thing i noticed is one of the alphanumeric LED cells has
artifacts (slight glowing) displayed in it and the artifacts shimmer and
change slightly when a BAD buton is pushed could this indicate a failure of
some component ?

thanks... i will look as you suggested,the Balance key, Stitch length
key and Stitch width key are the only buttons that do not seem to
work.

The programming keys (ie. stitch selector buttons) do work as do all
other buttons including the top right key that shows optimal settings
and recommended foot. The machine will stitch the correct stitch
according to parameters in the display for the selected stitch (eg.
stitch 00 straight stitch at 2.5 length *and* stitch 10 zig zag at
3.0 wide 2.0 length and so on)

*problem* being that i can not change the Balance nor Stitch length
nor Stitch width for any of the stitch selections 00-99 ??? when a
number appears above the keys, then some stitches have the amber
light and does not chnage those as expected.

I tried erasing memory as you said (no luck). I tried removing and
installing new batteries (no luck).


rob

 

[James Sweet]

wow thanks legg,

i guess alot of detective information is allready there in the fault
description that i ignored.

namely a group of buttons have all failed yet other groups of buttons with
different functions has not failed on the same board should lead one to
suspect a possible common shared connection between these three failed
buttons.

so look to the common connection point for all three failed buttons.

how simple and forthright yet elusive to amateurs like me.

one interesting thing i noticed is one of the alphanumeric LED cells has
artifacts (slight glowing) displayed in it and the artifacts shimmer and
change slightly when a BAD buton is pushed could this indicate a failure
of
some component ?

Sounds like there might be a missing ground, which would cause current to
flow down paths it shouldn't. It's common for problems like this to be
caused by cold solder joints or hairline fractures in circuit boards, I'd
start by touching up any suspect joints.

 

[Anthony Fremont]

Isn't the title of this thread a bit misleading and unfair? It's not
"all" brits - just one!

I know that, of course, but I have to play it the "dumb ass" way and lump
everyone into one group. ;-)

 

[Lostgallifreyan]

Might as well throw my bit in too. >

Forget diagnostics, just treat the most likely symptom; it's a harmless
excercise, and if it fails you can start doing tests.

Start by assuming that pressure on switches has caused a stress fracture in
one of the address lines, so find the track common to the failed switch
group, then remove solder from the first switch terminal that track arrives
at, and use a scriber to scratch the solder resist back along the track
about 3 to 5 mm, then reapply solder.

If that doesn't fix it, start testing stuff, starting with a resistance
(not continuiuty) test of that address line's track back to source.

 

[Lostgallifreyan]

Sounds like there might be a missing ground, which would cause current
to flow down paths it shouldn't. It's common for problems like this to
be caused by cold solder joints or hairline fractures in circuit
boards, I'd start by touching up any suspect joints.

Exactly. As well as what I said in my other post, look for other possible
stress points like the one by the failed switch group, there might be more
about to fail, and reworking the connections is far easier and faster than
doing elaborate diagnostics which you might never have to do.

 

[Ron(UK)]

Isn't the title of this thread a bit misleading and unfair? It's not "all"
brits - just one!

That's just typical of Americans, they ALL generalise!
<--- please note


Ron

 

[Radiosrfun]

I know that, of course, but I have to play it the "dumb ass" way and lump
everyone into one group. ;-)

Ah, ok........... I get your point......

 

[robb]

you could make a list of the Ics on the board and download all the
datasheets for them. That should help you.

Hey Don,
I realize i am not even an electronic hobbyist more like a "dangerous
adventurere" at best when it comes to repairing electronics (i have
succesfully stripped 10 guage wire and i can distinguish resistor from
diodes most of the time)

but, how would looking up all the IC datasheets help ?

would it not be more efficient to concentrate only on the ICs directly
connected to the suspect components ?

thanks for your time,
rob

 

[Radiosrfun]

Hey Don,
I realize i am not even an electronic hobbyist more like a "dangerous
adventurere" at best when it comes to repairing electronics (i have
succesfully stripped 10 guage wire and i can distinguish resistor from
diodes most of the time)

but, how would looking up all the IC datasheets help ?

would it not be more efficient to concentrate only on the ICs directly
connected to the suspect components ?

thanks for your time,
rob

If you look up the "Data sheets" of those ICs - you can see what their
actions are "supposed" to be - and see if anything is failing to do it's
particular job. You can analyze the outside parts all you want - if a Chip
is failing - you won't know it without the data sheet and way to check it
for input/output. If you have an "input" signal to one of the chips - but no
output - then it isn't working. Not being able to understand the action of
those chips on that board - can leave you scratching your head a while - and
maybe running in circles looking. Checking all the traces, switches,
whatever - isn't a bad thing - but it won't uncover a dead IC - especially
if you have tunnel vision - not wanting to accept it "may" be a chip. I'm
not suggesting it "is" a chip - but the suggestion offered couldn't hurt.
You will have a much better understanding of the "intended" performance of
the circuit. Chips "can" go bad.

 

[Arfa Daily]

Might as well throw my bit in too. >

Forget diagnostics, just treat the most likely symptom; it's a harmless
excercise, and if it fails you can start doing tests.

Are you a medical doctor by any chance ... ? ;~)

Arfa

 

[Arfa Daily]

If you look up the "Data sheets" of those ICs - you can see what their
actions are "supposed" to be - and see if anything is failing to do it's
particular job. You can analyze the outside parts all you want - if a Chip
is failing - you won't know it without the data sheet and way to check it
for input/output. If you have an "input" signal to one of the chips - but
no output - then it isn't working. Not being able to understand the action
of those chips on that board - can leave you scratching your head a
while - and maybe running in circles looking. Checking all the traces,
switches, whatever - isn't a bad thing - but it won't uncover a dead IC -
especially if you have tunnel vision - not wanting to accept it "may" be a
chip. I'm not suggesting it "is" a chip - but the suggestion offered
couldn't hurt. You will have a much better understanding of the "intended"
performance of the circuit. Chips "can" go bad.

Here's a good one that has stood me in good stead for years. The more pins a
chip has, the less likely it is to be faulty ... !!

Arfa

 

[Radiosrfun]

Here's a good one that has stood me in good stead for years. The more pins
a chip has, the less likely it is to be faulty ... !!

Arfa

Don't think I ever heard that one!

 

[robb]

The way I would attack this problem would be:
Use your ohmmeter to trace the common connections from the switches to the
ribbon cable, then make sure that you have continuity through the ribbon cable.
From there, trace the circuit to whatever component(s) they connect to.
As another poster mentioned, they might be strobed using a CAS/RAS technique.
If either signal is missing, then the circuit won't respond to set of switches.
It's pretty much useless to attempt to troubleshoot your problem using the
pictures you posted. You really need to get into the circuit with your ohmmeter
and trace the circuitry. Draw a rudimentary diagram of your tracing. See
You can also power up the boards and look at the switches with a scope. You
will probably see pulses on one side of the switches. Press one of them and
trace the resulting signal back through the circuit and see where it goes away.
You'll have to find data sheets on the ICs involved, so plan on doing some
Googling. Search for the part number of the part, and datasheet. Like this:
7401 datasheet

That should get you in the ballpark.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra

Thanks Dave,

I appreciate your helpful answer.

I have a tough enough time with single layer trace board this is 2 layers
zig zagging through the board all over ( i know a joke to most with 6 -8
layers now common place) and well i just nowstumbled onto holding the board
up to a strong light to see both traces at same time.

I now know some of the switches feed into a DM7496N a "5 bit Parallel in
Parallel out shift register" but that does not really help me as i am not
sure of functional purpose of switches feeding a shift register other than
to count pulses maybe ?

and button lines also feed into the SN75518N a "VFD driver chip" which
does not make alot of sense to me either ?

originally i was just hoping it was a simple obvious failed component the
tracing stuff is more of a struggle
anyways thanks for the help i plan to struggle through it a bit more

thanks,
rob

 

[robb]

What else is on the display board (part numbers)? Ca. 1987, it's
probably not a dedicated micro, but there's something else active
there to keep the number of wires down. I'm guessing it's probably a
mechanical issue on the display/key PCB (broken trace, broken wire in
the flat cable). I'd pay particular attention to the flat cable wires
(test them) and the traces going to the switches.

PCB technology isn't/wasn't always 100% reliable, especially back
then, and it's also possible that a via has cracked and is
electrically open.

Thanks for help,

that is what i was hoping for as well but it is not easy for this amateur to
find.

i noticed alot of the larger traces have bukkled up or wrinkled if that
makes sense. i am now wondering if those may lead to the problem ?

i guess i will just plod through it all

thanks again for your advice and ideas,
rob

 

[robb]

Sounds like there might be a missing ground, which would cause current to
flow down paths it shouldn't. It's common for problems like this to be
caused by cold solder joints or hairline fractures in circuit boards, I'd
start by touching up any suspect joints.

thanks for helping,
that seems to be the general consensus i am just not so good at tracking
that problem i guess.

but i have not given up yet.
thanks again for help,
rob

 

[robb]

Might as well throw my bit in too. >

Forget diagnostics, just treat the most likely symptom; it's a harmless
excercise, and if it fails you can start doing tests.

Start by assuming that pressure on switches has caused a stress fracture in
one of the address lines, so find the track common to the failed switch
group, then remove solder from the first switch terminal that track arrives
at, and use a scriber to scratch the solder resist back along the track
about 3 to 5 mm, then reapply solder.

If that doesn't fix it, start testing stuff, starting with a resistance
(not continuiuty) test of that address line's track back to source.

thanks Lost,

i like to try simple solutions first myself. lots of good ideas and here
from everyone.

thanks again for your time,
rob

 

[robb]

hello,

i have posted more pics "alt.binaries,schematics.electronic" of my progress.
the front cover was held on by the 16 cell VFD so had to desolder to get
front black panel off to see the traces and other components ICs etc

thanks for looking,
rob

 

[Lostgallifreyan]

Are you a medical doctor by any chance ... ? ;~)

Arfa

Nope, I am the greatest impostor, but I try to be convincing. >

 

[Lostgallifreyan]

i noticed alot of the larger traces have bukkled up or wrinkled if that
makes sense. i am now wondering if those may lead to the problem ?

Poke gently but firmly with a pin, you'll soon know if they left the deck.
That appearance is probably original though, building up the surface is one
way to get larger currents through tracks. If the resist layer (green,
usually) is entirely intact it's probably fine.

 

[Don Bowey]

Hey Don,
I realize i am not even an electronic hobbyist more like a "dangerous
adventurere" at best when it comes to repairing electronics (i have
succesfully stripped 10 guage wire and i can distinguish resistor from
diodes most of the time)

but, how would looking up all the IC datasheets help ?

would it not be more efficient to concentrate only on the ICs directly
connected to the suspect components ?

If knowing a lead goes to an IC, without knowing what the IC is and does,
tells you what you need to know, then you do not need the datasheets.
Personally, I'd be looking at and around the keyboard multiplexer.