Troubleshooting a Datong D70 Morse Code tutor from circa 1984 (dated based upon IC manufacturing dates).
These devices are wonderful all-logic circuits that operate without software (compare with modern Arduino-based tutors). Parts are plentiful and cheap (CD4000 series CMOS logic chips). The singular 'tricky' component would be the IC7 32x8 PROM, as one would need both a map of the address contents (the Morse character dit/dah shapes and character length) and a PROM programmer.
A comment on the PROM memory:
At it's inception around 1980, PROMs were clearly somewhat expensive. There was an active market in 32 byte PROMs - that is 32 bytes, i.e.. 32 addresses only! - and 256 bytes (a.k.a. "1/4k byte"), etc. This likely posed a design vs. cost constraint: keep things simple in PROM content with 256 ROM - but it's costly. Else use a 32 byte ROM, but then juggle the 26 letters+10 numbers (=36 characters) in 32 addresses or less. They chose the latter, shoehorning all characters into 26 addresses by leveraging the similarity of some Morse letters to the starting dits & dahs of the numbers.
I leave it as a fun exercise for the viewer to workout a plausible memory map: which letters would you use as truncated numbers, while requiring other letters to have dedicated memory addresses. Your goal is to do it all in 26 addresses; no more, no less. :-)
[There is no unique solution, but there are clear constraints that make most solutions very similar.]
I point this out in case you do need to replace a PROM and you will need to create this map.
The result was added complexity (beauty?) and modest increase in component count to juggle 26 address whilst riffling for letters, which you can deduce from the schematic.
Original schematics and circuit operation is available through a quick internet search, and I use it to test and identify a bad IC.
For example, DL0BN's website, has the service manual - invaluable for troubleshooting: www.dl0bn.de/d...
I replace the bad IC and get the D70 working as new.
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