Author Topic: Yamaha TX81Z Power Supply  (Read 689 times)

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Offline RJDog

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Yamaha TX81Z Power Supply
« on: January 07, 2019, 01:47:18 am »
I have a Yamaha TX81Z, circa 1987 -- to save a Google, it is a 1980s MIDI sound module; that is, MIDI in, FM synthesized audio out, so it has both analog and digital circuitry.  Anyway, it works fine, but the transformer in the internal power supply is starting to make a godawful hum.  I haven't really looked but I'm guessing finding a replacement transformer is going to be nigh impossible, so I'd like to replace it with a DC-DC converter with an external power brick, or similar approach.  So, a couple of questions:

- The original power supply outputs +5V, +15V, and -15V (7805, 7815, and 7915 in the original power supply).  The +/-15V is for the analog output circuitry -- is there a problem if I use a power supply which outputs +5, and +/-12V?  I have a DC-DC converter with these outputs already...
- The original power supply seems to maintain different/separate grounds for "digital" (0V, +5V) and for analog (-15V, 0V, +15V) -- is this critical to keep separate or can "digital" 0V == "analog" 0V as would be the case for probably any power supply I might replace it with?

  Any insight would be greatly appreciated!
« Last Edit: January 07, 2019, 02:52:53 am by RJDog »
 

Offline GregDunn

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Re: Yamaha TX81Z Power Supply
« Reply #1 on: January 07, 2019, 03:08:34 am »
Have you verified that the transformer secondary isn't being loaded by a failing cap or part of the regulator circuit?  You usually don't see low power transformers  like those failing without some kind of external cause.  Is the power supply still within spec?

I would expect that the analog portion is being run at ±15V to maximize S/N and voltage swing out of the unit - taking it too low might compromise the performance of the op amps, but without knowing more about the circuitry it's just a guess.

You definitely want the analog and digital grounds kept separate throughout the unit to avoid crosstalk and possible digital noise in the outputs.  They should be joined at exactly one location inside the module (a star ground to the power supply common if it was designed right) and separate everywhere else.
 


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