hello,
i have a 40 pin VFD driver chip SN755518N that three of the pins Q20 Q21
appeared to have gone quiet/died . Q24 seems to have its expected output
mangled.

I am trying to figure out wether i need to check out those lines (how and
what to look for) before i put a new chip in so that i do not fry the new
chip .

Could a faulty connection or other component connected to those lines fry
those pins ? or is it possible for the chip to just suddenly go bad ? is
there a typical reason for IC pins to go bad ?

what kind of test can i perform on the board to be certain that it is the
chip and not something connected to the chip ?


i put an ohm meter from grd pin to the dead lines and i get 25 MOhm reading
where other lines get 0 reading but that was not with chip disconnected
ffrom the board so i dont know if there might be a 25 MOhm short somewhere ?

thansk for any Ideas and help,
rob

robb wrote:

hello,
i have a 40 pin VFD driver chip SN755518N that three of the pins Q20 Q21
appeared to have gone quiet/died . Q24 seems to have its expected output
mangled.

I am trying to figure out wether i need to check out those lines (how and
what to look for) before i put a new chip in so that i do not fry the new
chip .

Could a faulty connection or other component connected to those lines fry
those pins ? or is it possible for the chip to just suddenly go bad ? is
there a typical reason for IC pins to go bad ?

what kind of test can i perform on the board to be certain that it is the
chip and not something connected to the chip ?

i put an ohm meter from grd pin to the dead lines and i get 25 MOhm reading
where other lines get 0 reading but that was not with chip disconnected
ffrom the board so i dont know if there might be a 25 MOhm short somewhere ?

Maybe it was killed by a fault in the VFD ?

Graham

robb wrote:
hello,
i have a 40 pin VFD driver chip SN755518N that three of the pins Q20 Q21
appeared to have gone quiet/died . Q24 seems to have its expected output
mangled.

I am trying to figure out wether i need to check out those lines (how and
what to look for) before i put a new chip in so that i do not fry the new
chip .

Could a faulty connection or other component connected to those lines fry
those pins ? or is it possible for the chip to just suddenly go bad ? is
there a typical reason for IC pins to go bad ?

what kind of test can i perform on the board to be certain that it is the
chip and not something connected to the chip ?


i put an ohm meter from grd pin to the dead lines and i get 25 MOhm reading
where other lines get 0 reading but that was not with chip disconnected
ffrom the board so i dont know if there might be a 25 MOhm short somewhere ?

thansk for any Ideas and help,
rob


INside each chip there is "black magic smoke". Each chip smoke enable
the function of the chip. If you mis handle the chip, in any way, the
smoke is release from the chip's body. After that the chip never works
the same again.

IN a perfectly, good and functioning unit, should it suddenly stops
working or perhaps stop over night (when you would not be present to see
the smoke!) :

here are some reasons:

over voltage in transient state(s)
over current during transient states
ESD electrostatic discharge from handling and creating a "spark" over
voltage results.
over heating, over time

some times, power does not come up sequentially, and timing of power
transients must be considered.

Power supply reversal, not tolerate well !

Mechanical degradation of chip lead to substrate connection.

Marc

"LVMarc" wrote in message
news
robb wrote:

I am trying to figure out wether i need to check out those lines (how
and
what to look for) before i put a new chip in so that i do not fry the
new
chip .

what kind of test can i perform on the board to be certain that it is
the
chip and not something connected to the chip ?


over voltage in transient state(s)
over current during transient states
ESD electrostatic discharge from handling and creating a "spark" over
voltage results.
over heating, over time

some times, power does not come up sequentially, and timing of power
transients must be considered.

Power supply reversal, not tolerate well !


thanks for smoke joke :} and especially thanks for reasons

so, what sorts of tests can / should i be doing to find out if there is a
problem in circuit board that may have led to IC pin/line failures ?

chip was in circuit and presumably not handled prior to failure. the only
event that **may** have preceeded failure was is either spilled liquid on
circuit board (cofee or coke) or an alkaline battery leak on main board
(after sitting for a long period) and then it was powered on .

did i mention this is a hobby and so i do not have experience to know all
the sundry conditions that may lead to such failures, and how to look for
the hose conditions.... which is why i am asking ?

thanks again for any useful help youi can offer.
rob

On Sep 13, 3:11 pm, "robb" wrote:
so, what sorts of tests can / should i be doing to find out if there is a
problem in circuit board that may have led to IC pin/line failures ?

Probably none. How expensive is the replacement part?

25 MOhm (presuming M = Meg) is not usually considered a short.
IIRC, VF display voltages aren't high enough to care about that high
a resistance.

The other thing you might check for is a short _between_ the non-
functioning
outputs. Compare those readings to measurements between functioning
outputs, and between non-functioning and functioning. If you observe
any
anomalies, remove the IC from the board and repeat. Or follow the
PCB
traces from the questionable pins and look for a short. If you don't
find
any apparent external cause for blowing up those outputs (and you've
cleaned up the battery contamination from the PCB), try the new part.

TM

wrote in message
ups.com...
On Sep 13, 3:11 pm, "robb" wrote:
so, what sorts of tests can / should i be doing to find out if there is
a
problem in circuit board that may have led to IC pin/line failures ?

Probably none. How expensive is the replacement part?


Not so expensive ($10), just difficult to find anyone who stocks it that
does not have a $300-500 minimum order.

On Thu, 13 Sep 2007 16:11:10 +0100 Eeyore
wrote in Message id:
:



robb wrote:

hello,
i have a 40 pin VFD driver chip SN755518N that three of the pins Q20 Q21
appeared to have gone quiet/died . Q24 seems to have its expected output
mangled.

I am trying to figure out wether i need to check out those lines (how and
what to look for) before i put a new chip in so that i do not fry the new
chip .

Could a faulty connection or other component connected to those lines fry
those pins ? or is it possible for the chip to just suddenly go bad ? is
there a typical reason for IC pins to go bad ?

what kind of test can i perform on the board to be certain that it is the
chip and not something connected to the chip ?

i put an ohm meter from grd pin to the dead lines and i get 25 MOhm reading
where other lines get 0 reading but that was not with chip disconnected
ffrom the board so i dont know if there might be a 25 MOhm short somewhere ?

Maybe it was killed by a fault in the VFD ?

I've tested and serviced several thousand of these, and never have I seen
a VFD kill a driver. A short on the other hand...

Hi!

INside each chip there is "black magic smoke". Each chip smoke enable
the function of the chip. If you mis handle the chip, in any way, the
smoke is release from the chip's body. After that the chip never works
the same again.

Sometimes. It depends upon the value and rarity of said chip! Expensive ones
tend to go "bang" very quickly, let lots of smoke out and don't work at all
afterwards.

More common and less expensive chips can usually take a lot of attempted
smoke releases before giving up. I had a 387 math-co and motherboard with
the pin one portion of the socket installed improperly. I couldn't see the
correct silk-screened orientation under the socket. The co-processor took
three incorrect insertions and power-ons before it "died". :-(

William

On Sep 13, 10:11 am, "robb" wrote:
[VFD driver]

My favorite datasheet of all, the Signetics WOM, has a graph showing
"number of pins remaining" vs "number of socket insertions".

It's not directly applicable here, of course, but I'd extrapolate:

If your VFD display is a module in a larger system, I would look at
power sequencing for the logic vs the drivers and display voltage. If
there's anything pluggable/unpluggable that's an obvious way to
sequence things wrong (for example, having ground become floating
before the signal lines is a bad thing!).

Tim.

"Tim Shoppa" wrote in message
oups.com...
On Sep 13, 10:11 am, "robb" wrote:
[VFD driver]

My favorite datasheet of all, the Signetics WOM, has a graph showing
"number of pins remaining" vs "number of socket insertions".

It's not directly applicable here, of course, but I'd extrapolate:

If your VFD display is a module in a larger system, I would look at
power sequencing for the logic vs the drivers and display voltage. If
there's anything pluggable/unpluggable that's an obvious way to
sequence things wrong (for example, having ground become floating
before the signal lines is a bad thing!).

Tim.


thanks for help Tim,
its possible a previous epairer unpugged something as you state. The circuit
board had obvious solder work done, it wasn't pretty and that is coming from
a mere occasional amateur solderer.

thanks again ,
robb