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Morrow Micro Decision MD11

D

DeltaDon

Veteran Member
Joined
Oct 26, 2016
Messages
875
Location
Dutchess County, New York, USA
I just started working on a Morrow MD11 with a Rev 2A main board. I measured the resistance on the power input connector pins (pwr supply disconnected) and got about 160 ohms on the +5VDC pin, 1.6 ohms on the +12VDC pin and 0.4 ohms on the -12VDC pin. I think the 160 or so ohms for +5vdc is about right for the 40-50 TTL's on the board. I'm sure there's some pull up resisitors somewhere on the board for RS-232, but think the resistance is way to low. I pulled all the socketed 24 -40 pin chips and the 3 PAL's and will read the 2764 EPROM chip and make sure I have a copy of it. I've got to pull out the schematics, (First page on Dave's site is corrupted) but for sure there's 3 sets of 1488 and 1489 RS-232 chips which are soldering in place that need the 12 volt supplies. The +/- 12VDC power does also go to the hard drive connector. Hard drive and floppy are removed and the rev 2a board is on my workbench so the problem is on the board. Not external. I can't think of any of the other chips that would be using the +/- 12 other than RS-232 port chips. I think the next step will be voltage injection and see which chips get hot. I have only a few NOS 1488 & 1489's so I hope it is only one or two chips.

OBTW, the ADM-20 terminal that came with this system had a broken key switch, now fixed and the CRT power board was broken. I've epoxied the board and bridged all the traces and only have to repair a tuning coil that's snapped at it's base - well below the tuning slug. I'll worry about the ADM terminal after I get the MD11 working since I have a couple of Wyse terminals too.

Any other MD11 owners out there?
 
Status update, two shorted, but not popped, tantalum 22 @ 16V caps on the -12V rail and the board no longer has signs of dead shorts.

So the next step is to test the power supply. Does anyone know the output current ratings for the original supply -- the +5 and/or 12 volt outputs? If the supply is really messed up I might just replace it with a Meanwell. I do have a Meanwell here with 5 volts @ 5 amps & +/- 12volts at a couple of amps each, I think. The five volt side might be a bit low when the floppy and HDD are installed, but should work for testing the circuit board. I believe the supply and circuit board are the same as the later MD-2 and so on stuff. The MD11 just has a hard drive vs. two floppy drives.
 
A recent hospital stay has slowed down my work on these computers. At this point I've determined that one, the one with the blown cap's, also has a dead Astec power supply. So I put that one aside for now while ordering a Mean Well for it. The 2nd MD11's has a Han One switching power supply and it measured about +5, +12 and -11.76 volts when disconnected from the main computer board. So it appears to be working. I measured the main board's resistance on the 3 power rails and it appeared to have a shorted +12v power rail (0.02 ohms). So before connecting the power supply and attempting to power the board I applied 1v @ 4 amp from my bench power source to the 12 volt rail hoping to find the short (IE - a popping tantalum etc). But after a quick burst of high current the current dropped off to a low number. Measuring the rail a second time revealed the resistance was now much higher. So I connected power supply hoping for success and measured voltages. The power supply had shutdown and outputted near zero volts on all rails. So disconnected it and used some jumper leads to connect just the 5 volt output to the 5 volt rail. The rail measured 4.35 vdc. Not so good. So either the Han One supply doesn't like just one output connected or there's a problem with the board pulling down the 5 volt rail. (I didn't try the +/- 12vdc rails since with the hard drive and floppy disconnected about the only loads on them should be the RS-232 chips and should be easy to test later one) I first have to get the main 5 vdc rail problem fixed. That ended my testing for the day.

I don't know if I'll have much time today to work on the board and I'm still thinking about what should be my next step. I'll pull all the socketed IC's and see if the rail responses, but that's only the Z80, eprom and a few others. The memory and TTL logic chips are all soldered to the board. If pulling the few chips doesn't change the rail voltage, which I'm thinking it won't then what's the next step? Since the 5 volt rail isn't dead shorted and the rail voltage is on the low end for TTL logic at 4.35 volts, I'm thinking I'm going to have to attempt to put a higher current supply on the rail and test for hot chips. Or pull all the tantalums on the 5 volt rail and hope to solve the low votage. I don't think looking for clock, CPU or other signals on the address or data buses will lead to any solutions since the low voltage could be giving false issues. Any suggestions?
 
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