Modified Yamaha TX81Z for DC Input

[SynthWizard]

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Modified Yamaha TX81Z for DC Input

So my TX81Z has been having a minor power supply issue... increasingly loud hum from the transformer, so it is clearly becoming delaminated. That, combined with the fact that my model is from Japan so is rated for 100V (I'm in North America, my mains measures 121V, so I'm using an external step-down transformer with it), and I really hate equipment that have permanent attached cables on them, I decided it was time for a change.

https://imgur.com/gallery/w9dDeGi

I decided to use switching DC-DC converters... got from AliExpress for a couple bucks. Knowing I wanted to use a 9-12VDC external power brick, I need +5V buck converter for digital logic, +/-15V boost converter for analog, and +26V boost converter. Why +26? No idea.. but it's in the original power supply, so I need it. Maybe it's for the headphone amp (more on this later)? I found a 24V boost converter so that's close enough.

Simple wire-up on a small piece of perf board using the original power harness I desoldered from the original power supply, and it tests ok. Mount it using some standoffs (yes, I drilled holes in the original power supply board), and wire it into the original power switch so that the front power button can still be used, and all good. Getting the jack into the hole for the old power cord proved to be the most fiddly... the hole wasn't quite big enough... had to do some filing.

Unit tests good, but I get some very noticable noise through the headphones, regardless of the set volume. Noise like on cheap computer onboard audio. My guess (as someone who is NOT intimately familiar with the electrical design of the TX81Z) is that the 24V is for the headphone amp, and the switching frequency of the +/-15V DC-DC module and frequency of the 24V DC-DC module are beating against each other creating the noise I hear. The line level output on the rear, though? Perfectly pristine... no detectable noise at all. Maybe even better than before.

Now, the LCD... I understand this is a pretty common problem on TX81Zs... the contrast is terrible, the background is much too bright to read anywhere other than straight on. So, knowing it uses a bog-standard "1602" style LCD, there is a voltage-divider driven contrast pin. Sure enough, briefly looking at the schematics in the service manual (god, I wish this was still a thing), right beside the LCD connector are a 270R and 4.7k resistor making a voltage divider for pin 3 (the expected pin). Weird values in my opinion... very skewed towards 0V, measures 0.3V, but whatever. I replace the 270R with 220R + 470R (690R total... values I had on hand), and it's much better. Probably would be even better with 470R+470R (940R) but, whatever, I'm happy with the result, and don't feel like doing more soldering.

So, apart from the noisy headphone output, I'm pretty happy with the change.

Edit TL;DR: OP replaces old linear power supply with new-fangled DC-DC modules, and fixes LCD contrast. OP happy.

submitted by /u/rjchute
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