XP-50 MIDI OUT damaged. A question for the electronic savvy. (with diagram)

[Alexander333]

My beloved XP-50 has a fault in the MIDI OUT . The rest of the workstation works ok. The MIDI IN works fine.
I had a storm with lightning that hit my house. ALL MY STUDIO was unplugged and completely isolated from the electrical, telephone, network, etc supply. I suppose there was some kind of electromagnetic pulse that resulted in two devices with fried MIDI OUTS and a USB port in the computer. The midi interface was not damaged (Roland UM-2G)
I downloaded the diagram of the expansion board that I suppose has the problem (hope so) and I have a question If anyone can help me.
I'm able to solder but I do not know how to investigate the failure.
I do not see any component burned or damaged.
It looks that the HD74LS08P is kind of an amplifier for the UART data coming from the motherboard. Maybe is one of the transistors?

Does anybody have any suspect component that I can try to replace?

Thanks a lot

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[Foksadure]

I don't have a clue in electronics but the Service Manual says to "check IC3,4 on EXB" when [6] MIDI test fails.

IC3 seems to be a TC74HC139P (decoder/demux), which is weird since there's another one as IC5, and they seem to manage the SR-JV80 expansion boards memory addressing. Not sure they have anything to do with the MIDI I/O (which are connected to the main CPU according to the diagram block).

IC4 is the HD74LS08P (quad gate). Since it's socketed, replacing it would be a easy job before seeking professionnal advice.

[MatrixRat]

IC4a and IC4b look like output buffers for Midi Out and Thru respectively.

Q10 and Q9 look like a slightly fancy switch to turn on +5v to R41 and R45. Presumably this is turned off at powerup, and is turned on when the CPU is ready to think about sending Midi.

I'd start by replacing Q10, the 74LS08 and R45 and R44 and checking L23, L24 and L25 for continuity, then cross fingers and test.

The gear may have been unplugged from services but cabling can act like an antenna and pick up ESD and deliver it right where you don't need it.

[Alexander333]

Thank you very much for the comments.
I will try and will post the result here. Thanks again.
Alex

[Alp]

While some IC's are sensitive to static electricity,... if "ALL MY STUDIO was unplugged and completely isolated from the electrical, telephone, network, etc supply." then I doubt the lightning strike would have affected your equipment. It would have taken a path of least resistance to the ground through external metal, wiring, plumbing....

i would first look at trying a different midi cable. Maybe buy a new one. I had some extremely stubborn intermitant midi connection between my keyboard and PC and it turned out to be a faulty cable that I swore was in excellent condition. A new cable proved me wrong.

[jerrybakermr]

IC4a and IC4b appear to be Midi Out and Thru output buffers, respectively. ram
Q10 and Q9 appear to be a fancy switch for turning on +5v to R41 and R45. This is probably turned off at startup and turned on when the CPU is ready to consider sending Midi. I'd start by replacing Q10, the 74LS08, and R45 and R44, as well as checking for continuity in L23, L24, and L25, before crossing my fingers and testing. Although the equipment may have been disconnected from services, cabling can act as an antenna, picking up ESD and directing it to where it isn't needed.

[jerrybakermr]

IC4a and IC4b appear to be Midi Out and Thru output buffers, respectively. ram
Q10 and Q9 appear to be a fancy switch for turning on +5v to R41 and R45. This is probably turned off at startup and turned on when the CPU is ready to consider sending Midi. I'd start by replacing Q10, the 74LS08, and R45 and R44, as well as checking for continuity in L23, L24, and L25, before crossing my fingers and testing. Although the equipment may have been disconnected from services, cabling can act as an antenna, picking up ESD and directing it to where it isn't needed.

no response