"Arfa Daily" wrote in message
...

"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller
i believe the 32v shorted with 5V line
So....where does one start to diagnose such a *big* problem
?


i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no
component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

Do you mean that the 7805 got hot and shut down ? One of those
should never
"fry", because they are SOA protected. I suspect that something
else
happened there, because if you do the math, it is impossible
for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
that, most variations of the 7805 are only rated to 1 amp,
although there
are exceptions.


Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement to the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.

as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried

this all happened very quickly


When you cobbled together your test supply, did you mount
the regulator on a heatsink,

well the regulator had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm when i pulled it from some other pcb

and most importantly, did you place a 4u7 cap, paralleled
with 0u1 cap directly across the output and ground pins of
the regulator, as
close to the device as you could get them?

no


This is *very* important, to prevent the regulator bursting
into vicious ultrasonic or higher oscillation.

ohhhh

It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it
was measuring, so just displayed some meaningless
nonsense. These regs do get very hot when
they oscillate. Although not strictly necessary,
it's considered good 'belt and braces' to put a 0u1
cap between input and ground pins as well.

i'll try the test again with those cap mods


The more I think about the 'problem' that this board has,
the more non 'real-world' it seems. I can't really see
any reason why three unrelated
fuses, on three unrelated rails, should have blown,
unless there was a serious and easily visible
problem. The only thing
that did occur to me was input voltage.
You're not by any chance firing the thing up
on UK 240v power, and its transformer is actually
rated for 100 or 110v ?That would do
it ...


it is 115v plugged into 115v supply

i wish i knew enough to speculate the connection , for now i
can only report what i see and do. i suppose if i could address
the issue of the seemingly unrelated supply rails i could
probably figure it out

FWIW, if this info helps ....

the machine *was working* when i replaced a VFD display driver
chip. That was an easy problem to fix as it was very specific. I
started with components that were related to the VFD (ie. driver
chip) , i checked pin signals with oscope and i had a datasheet
for guidance. Driver chip had 6 bad lines that would not allow
user button presses to be seen by main board. I replace the DIP40
with a PLCC chip and Everything was working i just needed to
re-assemble and bolt the pieces together.

***BUT***
i stumbled across a "yucky" ring in the peaks of the clock signal
to the VFD driver chip and i thought i should do something to
repair that (ie find the source of ring) again more very easy
specific tasks, just follow traces and check for expected
behavior (in vs outs) what goes in and what comes out

well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little arc snap) and all sorts of jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown. i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298, etc}

i replaced the one fuse, powered on and all three fuses blew

since then i have checked transformer output (ok), desoldered the
L298/L387 ics, continuity tests on various parts of PCB (73 Ohms
between 5v and 0V all around the board), tried to feed 5v into
the 5v line as suggested on the group to feel for warming
components,

now i am here with a very nebulous problem with no specific ideas
about how to repair

and looking for help

thanks again for your help arfa,
robb

BTW there are pictures of all this stuff on
(alt.binaries.schematic.electronics)