MM11-L / G231 repair advice?

Mattis Lind mattislind at
Sun Jul 24 02:48:08 CDT 2016

> Speaking generally, without specific familiarity with these dec modules . .
> Writing is performed by driving the X/Y lines in the opposite polarity
> (current direction) than for reading.

OK. That explains why the read signal is routed to the driver selection
circuitry. A couple of inverter was connected in parallel here from the
READ signal coming form the G110. But they were all right.

> Have you checked whether it affects the entire memory module or some block
> or selection of addresses within the module?

It was the entire memory.

> If it's a limited set of addresses it may just be an X/Y driver transistor
> for the polarity appropriate to writing.
> If it's the entire module, you might look at how the drive polarity
> selection is done for the X/Y drivers,
> somewhere it should trace back to the R/W/restore state sequencing for a
> memory cycle.
> The problem may then be in that polarity selection or the state sequencing.
> The inhibit circuitry does just that: inhibits writing (inhibits setting
> the cores to the 'set' or 'written' state),
> so it doesn't sound as much like an inhibit issue, unless it's something
> like the inhibits always being enabled.

With a scope I checked the outputs of E2 which is involved in the sequence
logic. And yes, two outputs were floating. Not normal since this was a
74H00. Replacing it with a 74S00 (no 74H00 available) actually made the
module working again! Thanks for the advice!

Yet again it was a plastic NS manufactured chip from 1973 that had failed.

So now the entire 11/05 is working just fine. Next step is the TC11


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