Re: Not-so-dead DX7 Mark I - The saga continues...

Hi @smallbluetag,
 

 If the zener diode has failed it can result in spurious writes to the RAM as the power is turned off.
 It won't necessarily matter how long the power is off since the corruption can occur right at power off, even if the supply voltage supplied to the RAM is still good.
 

 On my DX7 it was causing the names and sounds of presets to become corrupted.

 

 While powered up in normal operation, the RAM voltage is supplied from 5V via Tr1 (main board).


 

 It's unlikely to be a supply voltage issue, but you can check. If you attach a DVM to the RAM power pins (24/12) you can see if the voltage is dropping too low when powered off. If the diode from the battery (D4) is good you should still have around 3V there.

 Note: the data retention voltage for the original TC5518CP RAMs is 2.0V.
 

 As a slight aside, the 5V regulator on my DX7 was running really hot so I replaced it with a pin-compatible switching regulator (OKI-78SR-5/1.5).  :-)