[ortek_service]

Originally Posted by SiriusHardware

Thanks for trying the 6520 swap. It was worth a shot.

If you try the same thing with UC6 pin 11 / UC6 pin 16...

...After 0 is poked
...After 255 is poked

What sort of change do you see on those pins of UC6?

Originally Posted by ScottishColin

UC6/11 and UC6/16 before any POKE - 3.85V

UC6/11 and UC6/16 after POKE 59426,0 - 0.4V

UC6/11 and UC6/16 after POKE 59426,255 - 3.87V

Colin.

I've just been catching-up with this, and at first couldn't understand why there was a 'Logic-High' voltage on the Least & Most significant bits of the port, but these had previously been read-back as 0, when set high.

But after taking another look at the schematic, it seems I accidentally got the ports swapped over. And UC6(6520) Port-A (Pins 2 to 9) is actually the inputs from buffers, with Port-B (Pins 10 to 17) actually the outputs to the buffers.

So what I really meant to suggest was measuring the voltage on UC6(6520) Pin 2 and Pin 9 (With the POKE the same as before to set all of Port B (Outputs) High).

If you haven't removed UA7 & UA8 yet, you could do this just to really confirm 100% that a receiver buffer in each really is faulty (But that does seem most likely, from other people's experience of these).


Whilst searching for equivalents of the MC3446 (Only finding the HP in-house part numbered one - although the SN75136 & SN75138 / AM26S11C maybe similar, also being listed alongside the MC3446 in the TI 1977 Line drivers Databook - but must have been different enough for TI to also make the MC3446 and keep the Motorola part number).

I have now found the full Motorola MC3446(P=Plastic DIL) datasheet in the Motorola Semiconductor-Data-1976 book on bitsavers, so will extract the datasheet for it from the (20MB) full book, being as it wasn't on the usual datasheets websites - Where I'd only found the (later?) Motorola MC3446AP suffix version and the TI MC3446(D,J or N = Different material DIL packages in Jan'77 / Rev.1986 datasheet)


I did also come across the previous thread SiriusHardware mentioned, where there had also been many tests (Inc. ctrl lines?) on the IEEE interface, to establish the MC3446 buffers were faulty. So there may be something useful there, regarding testing that part of it without having to work it all out again: https://www.vintage-radio.net/forum/...163589&page=13