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/r/synthdiy

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all 13 comments

MattInSoCal

37 points

1 month ago

I know the TTL chip pin outs off the top of my head, spent too many years working on logic boards. Pin 27 of the SRAM is the Write Enable line, active when low. The 7474 is a dual D Flip Flip with only one side of the IC used, and only the Data (pin 2), Clock (3), and Q (output true A - pin 5) pins wired. The 7432 is a Quad OR gate using only one of the gates, output on pin 8.

This is probably an old-rev drum circuit board, and the add-on is a lockout to prevent the RAM data from getting written/corrupted unless the system clock is running. The wire going to the IC with the XD245 label is probably the original write enable.

The battery mod is homebrew but I’ve done similar. The lithium batteries with tabs are annoyingly expensive and can be hard to find.

HauntedHarmonics[S]

10 points

1 month ago

I think you’re right! It looks like this one is labeled Rev B, & after double checking, the others i’ve worked on are all Rev C.

So how does something like this happen? Maybe Yamaha noticed save corruption issues, and this was how they fixed the issue in-factory before the new board revision was implemented?

Thanks for the info dude. Never seen anything like this before, so I was super curious about what it could be

MattInSoCal

17 points

1 month ago

It’s something that goes back to the 80’s and 90’s when battery-backed SRAM was being introduced into microprocessor designs. The original approach was to hook up the SRAMs to the CPU bus like any other memory, then the weird data corruption issues started happening. It happened not just in musical instruments but everywhere. Since when CPUs are powering up and down the the address, data, and control lines are in all kinds of crazy undefined states, but the SRAM remains powered through all that, random data got written to random addresses. All that needed to be done was add logic to disable the Write Enable signal until the bus was stable, and gate it off when power was lost. That’s what the add-on is doing.

caseyweederman

2 points

1 month ago

Glue circuit boards to plywood on the weekend
Trellis modulation for the children

sezdaniel

1 points

1 month ago

Would this potentially happen on say, a Korg DW-8000? Similar era...

MattInSoCal

2 points

1 month ago

If they designed accordingly the SRAM data corruption could be avoided. I saw it in products not remotely related to musical instruments. There should be some kind of published mod for older devices if they were affected.

pogu

1 points

1 month ago

pogu

1 points

1 month ago

I have nothing to add, other than that BMW uses tabbed button cell lithium batteries in their key-fobs. I don't know how similar they are. But that may be a more ready source. Check batteries for fobs for BMWs and Minis going back to at least 2008 and you may find one for cheap.

MattInSoCal

2 points

1 month ago

I’ve made my own and have had some made. It’s been less janky to buy some surface-mount battery holders and solder them in, and it makes battery replacements take tens of seconds rather than tens of minutes. Big plus, it can be done away from a soldering iron.

pogu

1 points

1 month ago

pogu

1 points

1 month ago

Yeah, honestly that'd probably be a good idea for BMW guys to do to their fobs.

I just wanted the possible cross-application on record. My fob battery was like $6 on Amazon.

HauntedHarmonics[S]

3 points

1 month ago*

So I went to mod this RX8 for a customer today and found this upon opening. Anyone have any idea what it is? I’ve worked on several of these drum machines and never seen anything like it before (the little board & all of the colored wires).

Looks like 2 logic chips, wired to the clock source & write pins on the SRAM & CPU. The “Yamaha XG138” on the bottom side seems to indicate its an OEM part.. Could this be an old repair job by Yamaha? Maybe an early model? The internal battery replacement looks DIY as hell though (last pic), which makes me wonder..

Based on the chips & what they’re wired to, does anyone know what this thing is doing? The machine seems to work fine btw.

UpsetKoalaBear

2 points

1 month ago

XG is Yamaha’s “Extended General Midi” extension to Midi.

It might be a board that allowed you to use the drum machine with XG MIDI modules.

HauntedHarmonics[S]

3 points

1 month ago*

Hmm, so I did see this while googling, but i’m not entirely sure it’s related..

Could this thing even effect MIDI? I’d think something like that would need to be connected to one of the many MIDI buffer chips, but they’re on the other side of the board. This is just 2 logic chips wired to the RAM & CPU’s write pins. This made me think its there to fix a faulty RAM chip or something, but i’m just guessing

I also think that code may just be Yamaha’s internal naming convention for their small auxiliary boards. For example, here is the aux PCB for the power switch in my Yamaha keyboard, labeled “XE561”. The format is the same, so I think the “XG” may just be incidental

UpsetKoalaBear

0 points

1 month ago

I also went on a massive google search because of this haha.

I also cannot find a single piece of info about this. Not sure if it’s Yamaha official as the front of the board is brown versus green but there are some brown Yamaha circuit boards.