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.

Yamaha DDS-20M Delay/Sampler

I wouldn't really think of Yamaha as a guitar pedal company. They have made a lot of classical instruments, drum machines, e-drums and studio gear (I have an SPX90 I really need to repair) so I guess the pedigree is there. This is a mid-80s 2 second digital delay/sampler, from that period when putting "sampler" on products must have excited guitar players.

The mechanical design is a little different - all of the controls are recessed at a height below the footswitch and there is a "unibrow" along the top with the logos and LED.

There are no schematics and not a huge amount of info on these that I could find online, other than there was a range of pedals in this form factor. No signs of life.

After looking on the inside, I got the impression that someone else had attempted a fix. There was a resistor between the +9V and ground pads of the DC jack (?), the ground trace had been cut in two places for some reason and a wire had been soldered in to replace it. I can't see why the resistor between 9V and ground would be needed for anything other than wasting power, so I removed it. I also found that the ground from the DC jack was not connected to circuit ground. Adding a jumper was enough to get this to fire up with a DC supply, but not with a battery. It's possible that the resistor was originally connected between jack ground and circuit ground but as it was several kilo-ohms a wire would be a better solution.

The pedal uses a 2 board design, connected together with wires. Both are single-sided PCBs that look like a phenolic paper material and no through-hole plating anywhere. This is pretty typical for 80s Japanese electronics but it can mean that any board-mounted hardware can be fragile as there is only solder on one side of the board anchoring it place. In this delay, the output jack had taken some force (probably a cable was stepped on) and the pad to the jack tip had broken, this had to be re-soldered to get an output signal. One of the PCBs has most of the analog parts, the inner one has the digital controller and the pots and rotary switch.

2 wires added and a diode replaced

It still didn't work from a battery. Following the (broken) ground trace to the other side of the board I could see that a diode was used to switch out the battery when a power supply was used, replacing this diode fixed everything.

Analog board, component side

The analog board has a 4066 for bypass/effected switching, 5534 opamps, a Motorola M5218 audio pre-amp (I have only seen this part in Japanese pedals, I think the original Boss DS-1 used something like this), μpc1571 compander (NEC's NE571 clone) and the chip in the bottom left corner is an LM311 comparator. The comparator is forming part of a sigma-delta ADC with the digital controller, much like the Boss DD-2 and the Digitech PDS series of delays.

Bizarre vertical component assemblies

I thought the parts marked "< RIVER < RIVER" were SIP ICs, but they are actually components standing vertically! They seem to be clamped together at the top. I don't know if there was automated machines to produce these assemblies or if they were re-used in other designs but it's surprising that this was cheap enough to be worth the effort.

Digital board

The digital controller is an "M654131". No info on this, probably a Yamaha custom. Maybe they used it somewhere else in a different product. I don't have any pictures of the back side of this board so I don't know what type of memory is used. It looks like there are two 9-pin SIPs connected straight to the controller so I would guess 2 64k DRAMs, assuming this is an 8-bit delay.

It sounds great. I like that these early digital designs use companders and have simple master clock signals. It's like a holdover from the days of analog delay designs. This one can pitch-shift the delay signal smoothly, unlike some more modern delays.