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Hi guys,
Just following up a week later as I was away after doing the CR2032 coin battery holder installation. Thanks for your patience!
I figured I'd get you guys some revised values before doing anything else.
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So, the DX7 has sat unplugged and unused for over a week in its case. I turned it on, and surprisingly enough, there were no E! RAM error messages this time! (However, I'm really waiting for the shoe to drop and to lose all the RAM data all over again.) Maybe I'm lucky and the battery holder fixed it?
Here are the revised values (leaving E! in place, which means I can't access diode D4). CR2032 Battery voltage was measured while plugged into the motherboard.
Off for 1+ week, unplugged.
Batt: 3.08v
E! U4 RAM (pins 14 and 28): 2.99v
Turned it on. ***No error message this time.***
Batt: 3.04v
E! U4 RAM (pins 14 and 28): 4.92v
Approximately 1 minute after turning it off.
Batt: 3.04v
E! U4 RAM (pins 14 and 28): 2.97v
Approximately 30 minutes after turning it back off.
Batt: 3.03v
E! U4 RAM (pins 14 and 28): 2.97v
Turned it on again. ***Still no error message this time.***
Batt: 3.04v
E! U4 RAM (pins 14 and 28): 4.92v
Approximately 1 minute after turning it off again.
Batt: 3.04v
E! U4 RAM (pins 14 and 28): 2.98v
I don't know if the U4 contacts would suffice to measure the voltage drop after the diode, but if so -- we're talking of a net change of 0.07v to 0.09v instead of the expected 0.2v to 0.3v. Would that indicate a leaky diode? If so, I'm guessing the next step would be to order an 1N34A germanium diode - as that seems both affordable and quickly obtained online as opposed to an OA95?
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Beyond this, if it fails again, I'll take some re-measurements before doing an E-ectomy - which seems to be the next test?
Basically, I'd need to source a Mitsubishi M5M5118P-15L for the SRAM off of eBay. Not sure how many SRAM chips I'd need in that case, but I guess I'd see after (and if) I need remove the E! board. (What a PITA to reinstall and get those E! pins reseated correctly!) I could quickly source and drop in an SER-7 EPROM, so that's not an issue. I'd still love to use the E! board if that's not the problem, particularly for full MIDI implementation and other features.
Thanks everyone!