Need help identifying components

I rebuild and repair vintage race car ignitions(magnetos, etc)
I am currently working on a Spaulding BDI system. It uses a
shutter to break a light beam to trigger the spark. I have found
that these parts are bad. After getting them out of the potting
compound I find that there are no identifying marks on them.
In the patent app Spaulding mentions two Fairchild numbers,
FLD-100 and FPD-100 and a Motorola MRD-310. Can find
nothing about these pieces. Any help in identifying or finding
substitute parts would be appreciated. Thanks.


I’m not finding much in the way of documentation either–it can be particularly tough for parts that went obsolete prior to the internet age.

That said, based on your description it seems pretty likely that it’s a simple LED and phototransistor pair, which these days are quite cheap. Finding/making a schematic for the unit or any sort of documentation at all would be a big help in terms of guesstimating what might be worth trying as replacements.

I was able to find two datasheets for you.

MRD300.PDF (161.5 KB)
FPT100.PDF (56.3 KB)

Both are photo transistors. I’ll keep looking to see what I can find on the first part number.

We do not have a cross for both part numbers.

I found the Motorola data book that has the MRD-310 in it online (my copy went to recycling 5 years ago with 100 other data books, no storage space).

The 1978 and 1980 Fairchild data books I found online do not contain “FLD” or “FPD” parts.

I am attaching the schematic that was in the patent application. There are no values
but the voltages out of the box to the distributor are from 5vdc to 6.2vdc. Don’t know
if that helps. Thanks for your help.
Lee Mullin

2024-01-14_133527BDI.pdf (652 KB)

Yep, that’d look like a pretty plain LED/phototransistor pair. There’s a lot of items that could potentially be made to work, but I’d suggest SFH 4550 and SFH 313 FA-3/4 as a set worth trying. These should have fairly tight focus on either side and work in the infrared, so a person might have a decent chance of being able to bench test things without having to turn the lights off…