I have a couple of circuit boards that I want to repair. They are two
versions of the same thing, although one is slightly newer by maybe a year.
There are some diodes that are toast although there is enough left to read
the markings.

The markings on board #1 diodes a (as you roll the diode around)

M (motorola logo)
48
S33
324

The markings on board #2 diodes a

M (motorola logo)
48
S33
936

Don't know if these are zeners or what. Sorry I'm not a whiz at this, but I
have almost learned which end of the soldering iron is hot. The boards are
circa late 1970's...

Any help would be mucho appreciated, email if you like, after correcting the
obvious anti-spam.

-Dave

Don't know if these are zeners or what. Sorry I'm not a whiz at this, but I
have almost learned which end of the soldering iron is hot. The boards are
circa late 1970's.

Dave-

Do wild guesses count?

First, the "324" and "936" are most likely date codes or production lot
numbers.

The other numbers do not look familiar unless the "S33" happens to be shorthand
for a 1S33, which I doubt. These numbers are likely to be proprietary numbers
used by the manufacturer of the circuit board. Unless you know someone who
works there, it will be difficult to cross reference them.

Now for the wild part. Since the diodes are toast, I would guess that they are
at an interface between the card and the outside world, such as diodes at the
input of a telephone. In that case, a diode bridge is used so the telephone
company's battery polarity won't be backwards for the phone's circuits. Once
the phone is connected, two of the diodes will always be forward biased, and
the other two will always be reverse biased, resulting in a series connection
with correct polarity.

Does this sound familiar? Are the diodes in sets of four? If so, then
ordinary rectifier diodes should work. In the case of a telephone, the line
voltage is most likely obtained from a 48 volt (positive ground!) battery
plant, so diodes should be rated for at least 100 volts. Higher would be
better to prevent future breakdowns from lightning strikes.

If they are not in sets of four, might they be connected back-to-back in
parallel? If so, they may be zener diodes or other protection devices,
intended to absorb spikes from the connecting circuit. A voltage rating would
be determined by maximum tolerable voltage on the card.

Have you checked ALL of the like diodes? If any one of them happens to still
be good, you could check it for a low zener voltage using a voltmeter, a power
supply and a 10,000 Ohm series resistor.

Fred