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Lexicon MRC schematics

Anyone got a schematic on a Lexicon MRC Midi controller? I have got two dead ones i have been trying to get going for customers for awhile. Kind of hard with no prints. Essentially a one board computer with Z80, 4 rom, static ram, uart,analog input chip, and display controller.

Unit #1 will go though self test until it get to the Duart test and them the display goes to gibberish and unit locks up. Subbed the CPU, RAM and Duart from another working unit and all the symptoms stayed the same.

Unit #2 comes up with display solid, no activity. Swapped CPU,RAM,DUART and still the same. Now, this unit has the CPU reset line staying low. I think this is related to the problem but need to find the circuit that controls the CPU reset on power up. I am hunting down some generic Z80 circuits for a guide.

Both have the EL panels out, but that's a moot point with no brain activity........

Bob

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Reply to
Bob Urz
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This one will have to wait since i robbed the max 690 out of it. But i am still looking for theories. Since it goes through self test fine until it hits the DUART, its got to be some problem there. The Duart is socketed, and i swapped it with a good one an there is no change. So it cannot be the chip. But, i could be some issue with the addressing of the chip. Maybe an address decoder? If i had a Z80 Micro emulator to hook to it i could easily track it down. But that's not a tool i have laying around

Well, i got unit #2 back on line. Downloaded some generic z80 block diagrams. I figured out that the reset pin on the Z80 has some sort of drive. Traced it down to a MAX690 chip. I compared voltages to the good and bad unit and all was the same EXCEPT the reset pin. So i figured this ruled out a power or low battery problem. Swapped the chips between the units and it brought the unit back to life.

I am also investigating some LED fiber optic replacements for the EL panel on the unit.

Bob

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Reply to
BOB URZ

In my experience, if you have a working one to compare it with you're laughing. Well, maybe a slight chuckle. Scope all the pins of the working UART and you may be able to see roughly what kind of signals should be there and those that are missing or corrupted on the duff one.

Or you could try removing both UARTS and then compare signals - maybe a more obvious boot up difference/failiure mode may become apparent. (You would have to constantly reset each machine to see these boot up procedures working)

Good luck,

Gareth.

Reply to
Gareth Magennis

That's how i confirmed the problem with the reset pin driver chip on the other unit. All was the same except one pin, the reset pin.

The one that goes into la la land i can make some assumptions. I swapped CPU, UART, RAM, ROM from a working unit and the symptoms were the same. And the swapped chips worked in the working unit. SO, i would assume its NOT a problem with any of the socketed chips. The unit will go into self test and gets up to the DUART test before it goes south. SO i would assume the basic data and address lines are intact. Power seems to be OK, but further tests are always in order.

I think it some sort of chip addressing issue. Since i have no print and its NLA from Lexicon, i will have to wait until i can trace the chips down and draw one out. The one unit that did this i modified with the V4 roms, but all seems to be OK with that.

Bob

Well, maybe a slight chuckle. Scope all the pins of the working

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Reply to
Bob Urz

Have you checked the clock into the UART?

Gareth.

Reply to
Gareth Magennis

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