8 May
2007
8 May
'07
3:19 a.m.
I almost posted a lengthly explanation on the march group list before I
noticed you posted the pic on cctech. The power supply in that circuit
is a -12V not +12V. The voltage regulator part # on there is a 7912
which outputs -12V. The part that is blown on there is an electrolytic
cap, not a zener (note the + sign on each end of the cap). Both of those
parts below the 7912 are the same type of part.
Caps always dry up and blowout--they don't last forever. That's why it's
good to have a Variac when powering equipment that hasn't been on for a
long time--a homemade current limiter helps too if no variac is available.
But I'm not sure of the value, it can't be very high in value, possibly
only 3.3uf or 4.7uf considering the size, this isn't very critical , so
long as you keep it in the ballpark. The caps for the 7805 regulator on
there beside it are only 2.7uf. I would suggest changing both of them,
for sanity's sake--they both might be damaged.
The other important issue is to check the traces for damages when you
remove the bad parts. They could have burned through creating an open
circuit. So you'll have to splice the traces back together if this is
the case. Some skinny telephone copper wire always does the trick. And
last but not least, is the 7912 voltage regulator. You should check the
-12V power before you plug in any cards.
=Dan
[ My Corner of Cyberspace http://ragooman.home.comcast.net/ ]
B. Degnan wrote:
Herb/Anyone else..
I have a 1977 Morrow Wunderbuss "Thinker Toys" S-100 Motherboard with a
blown Zener diode near a 12+V position at the back of the board. Anyone
have a schematic? I checked Herb Johnson's site, there is nothing for
this
board listed. I would like to determine the exact type of diode so I can
replace it.
What would cause a Zener diode to pop?
Thanks.
Bill
_
8 May
8 May
2:55 p.m.
From: Dan <ragooman at comcast.net>
I almost posted a lengthly explanation on the march group list before I
noticed you posted the pic on cctech. The power supply in that circuit is a
-12V not +12V. The voltage regulator part # on there is a 7912 which
outputs -12V. The part that is blown on there is an electrolytic cap, not a
zener (note the + sign on each end of the cap). Both of those parts below
the 7912 are the same type of part.
Hi
It may be that the zener is the regulator and not a 7912.
Zeners die from old age as well as over rating. Check to see what the
current draw is for the circuit that would be powered by one of these
shunt type regulators. This can most easily be done with a variable
bench supply.
Break the connection to the unregulated input( a piece of tape on
the input is fine ). After replacing the zener, connect the bench supply.
Slowly bring the voltage to the input resistor up until the zener just
starts
limiting the voltage across the zener.
Measure the voltage across the resistor and after powering down, measure
the resistance of the resistor ( not always what the color bands say ).
Use this to calculate the current. This is the current of the load.
Now, measure the unregulated voltage of the S-100 frame. The difference
is how much drop you must provide by the resistor. Take that drop
and calculate the current that would be through the resistor.
Subtract the load current. This is the current that the zener must
absorb. A simple power calculation will give you the rating needed.
I've seen several times on S-100 boards where the zeners were over
powered in a particular s-100 frame because of high unregulated voltages.
The resistor really should be selected for each frame the board is palced
in.
Dwight
Caps always dry up and blowout--they don't last forever. That's why it's
good to have a Variac when powering equipment that hasn't been on for a
long time--a homemade current limiter helps too if no variac is available.
But I'm not sure of the value, it can't be very high in value, possibly
only 3.3uf or 4.7uf considering the size, this isn't very critical , so
long as you keep it in the ballpark. The caps for the 7805 regulator on
there beside it are only 2.7uf. I would suggest changing both of them, for
sanity's sake--they both might be damaged.
The other important issue is to check the traces for damages when you
remove the bad parts. They could have burned through creating an open
circuit. So you'll have to splice the traces back together if this is the
case. Some skinny telephone copper wire always does the trick. And last but
not least, is the 7912 voltage regulator. You should check the -12V power
before you plug in any cards.
=Dan
[ My Corner of Cyberspace http://ragooman.home.comcast.net/ ]
B. Degnan wrote:
>
>Herb/Anyone else..
>
>I have a 1977 Morrow Wunderbuss "Thinker Toys" S-100 Motherboard with a
>blown Zener diode near a 12+V position at the back of the board. Anyone
>have a schematic? I checked Herb Johnson's site, there is nothing for this
>board listed. I would like to determine the exact type of diode so I can
>replace it.
>
>What would cause a Zener diode to pop?
>
>Thanks.
>Bill
>
>_
>
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10:11 p.m.
New subject: [midatlanticretro] WTD: Morrow Wunderbuss Thinker Toys
Caps always dry up and blowout--they don't last
forever. That's why it's
You are thinking of 2 different failure modes here.
In an electorlytic capacitor, the dielectric (insulator) is a thin
(oxide?) coating on the surface of the positive electrode. The
electrolyte forms the engative electrode.
If the electrolyte dries up, you get what's called a high ESR (effective
series resistacnce). The capacitor appears as though it's got a high-ish
value resisotr in sereis with it, and fails to, for example, smooth out
ripple very well.
The problem you're thinking of is when the dielectric film re-disolves
into the electrolyte. The capacitor then appears as a short-circuit (2
electrodes connected by an electrolyte that's a good conductor). A high
current can pass, the capacitor will overheat and blow itself apart.
good to have a Variac when powering equipment that
hasn't been on for a
long time--a homemade current limiter helps too if no variac is available.
I've never found a variac very useful. In fact for some SMPSUs, which
appear as constant power loads and draw more current as the input voltage
drops, a variac can actually do quite a bit of damage to the primary side
compoents. You'rs much better off removing the capacitors and re-forming
them on a bench supply with a suitable series resistor to limit the current.
-tony
6449
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