Yamaha QY20

After fixing the MS-20 I was having a lot of fun with it, but really wanted some kind of sequencer. I would have preferred not to have to rely on a laptop and was looking for something small and cheap.

I found this Yamaha QY20 on sale from a Japanese eBay seller, listing as non-functional. It was a portable sequencing device from the early 90s, with some workstation-style sounds and MIDI I/O. The service manual was available online, it was very cheap and it looked like a decent form-factor so I took a chance.

Yamaha QY20, with carry case

I applied 12V from a bench power supply to the DC jack - it pulled around 135 mA but didn't show any sign of life. However, when I connected the headphone jack to a speaker and pushed some keys I could hear a piano. I could also connect up the MS-20 by MIDI and trigger notes using the QY20 keys. It looked like the there was just an issue with the display.

Usually LCD displays have a bias voltage supply that is much more positive or negative than the supply for the logic chips. I expected a bad switching power supply. The service manual is very comprehensive and has a complete schematic, it shows a MAX680 inverting charge pump generating a negative supply of -9.3 to -9.6V from the 5V rail. Either the charge pump has failed, or the contrast pot has gone open circuit.

LCD bias supply schematic, from service manual.

It's fairly easy to take apart the QY20, but there is a copper sheet for shielding that needs to be desoldered.

Shielding has to be desoldered at the MIDI and line output connectors.

Digital and audio PCB

The insides are fairly typical surface mount construction for early 90s, particularly for a portable unit.

Funnily enough, the microcontroler is a H8/520, the same family used in the MS-20 20+ years later. The main soundchip is a YMW-258-F "AWM & FM Tone Generator", which is likely to be some variant on the chips Yamaha were doing for synths and video games at the time.

I measured the Vee bias voltage for the LCD at only -5 V. It should be closer to -10V, as the MAX680 charge pump is an inverting and voltage doubling converter. I would guess that one switching stage inside the MAX680 is bad and it is only inverting.

I removed the MAX680 (looks like I forgot to take a picture of this, it's a SOIC 8 chip on the backside of the board) and the 4 22 uF switching capacitors (C26,C27,C28,C29).

C26-C29 removed

After installing new parts, Vee was now -9.2V - a little bit low, but certainly a change. When I re-attached the LCD board, the display worked again.

Reflective LCDs are surprisingly difficult to photograph,

I can change the cheesy electric piano to 100+ other throwback workstation sounds! Time to learn how to use this as a MIDI sequencer.