Yamaha SPX900 Refurb

Yamaha SPX900

Introduction I bought a used Yamaha SPX900 multi-effects unit in December 2021 to complete the FX rack for my main analog console, the Soundcraft 22MTK which has five AUX sends. The plan is to refurbish the SPX, and use it for delays with pitch shifting and the Symphonic preset. Launched in 1988 (at over£700) the SPX900 was Yamahas first full bandwidth effects processor with 16-bits at 44.1Khz.

With only a mono analog input and multiplexed stereo output it was eclipsed by the more powerful SPX1000 in 1989, which has 4x the sound memory and digital I/O but at twice the second hand price. The SPX990 is a 20-bit version of the SPX900, with the same DSP chip but more memory and effect types.

The SPX900 arrived without much packaging and the rack ears were bent back. It is always important to protect rack ears when packing a unit to avoid this.  I managed to straighten them out but they have cracks in the metal now.

Technical Design The SPX900 uses an 8-bit microprocessor the HD6306, which is a derivative of the Motorola 6800 running at 4Mhz and with a 32kbyte ROM  chip holding the main OS. There is a large main custom DSP chip, as well as custom chips for EQ and modulation, this is when Yamaha were designed their own chips. The main sound memory is 7x 256k bit DRAM chips, with equates to 1.3 seconds of stereo audio signal at 16-bits and 44.1KHz. Delays times of up to 1.48s are possible, with Stereo Delay providing 740ms. In Freeze mode up to 1.35 seconds of mono audio can be sampled and replayed.

The ADC chip is the PCM78 and the DAC the often used PCM56, which deliver the 16-bit performance. The Op Amps are medium quality bi-polar NJM4556/4558’s, and the ones in the audio path could be upgraded with care to faster and lower noise versions.

Effect Types The SPX900 has 13 different effect types, with the spatial effect generated by the DSP chip, whilst others involve the EQ and Modulation chips.

• Basic Reverbs, Detailed Reverbs, Early Reflection Reverbs
• Delay, Stereo Echo, Modulation
• Noise Gate, Pitch Change, Freeze, Pan
• Compressor, Distortion, Aural Exciter

The majority of the 50 factory presets are SINGLE effects (all with digital EQ), but there are of course some MULTI effects where the different types are combined in series, and DUAL effects where two are combined in parallel (limited to reverbs and delays). There are 49 user memory slots which I will soon fill up!

Custom Early Reflections Preset 17 (Programmable ER) enables access to four sets of user modified early reflection parameters, so you can design your own reverb space. They are edited using the USER ED EDIT function. The parameters are; room size, liveness, diffusion, initial delay, high and low pass filters. This feature is often overlooked as it buried in menus but it is covered in the user manual and is a great way to create new spaces.

New LED Display

New Display The original LCD backlight had faded, and I was keen to replace or even upgrade to an OLED. However the LCD is a narrow version, which reduces the potential replacements, and rules out an OLED. I have used a LED display with white text on blue (the Midas MC21605J6W-BNMLW-V2), just £11 from Farnell.  The height and width of the new display are the same as the original, and the new display fits snuggly against the front panel bezel.

The LCD mounting is a simple snap fit into the false panel, with Yamaha deciding not to use any machine screws. The LCD is connected to the SPX900 main PCB with a 14-way ribbon cable with a captive IDC header on the LCD and a plug/socket on the main PCB.

The new display uses the same cable and pin out but with 2 extra pins (15 and 16) for the power to the LED backlight. I desoldered the header from the old LCD and soldered it into the new display leaving Pin 15 and 16 unconnected. I then wired a 220R resistor from Pin 2 to Pin 15 (+5V) and connected Pin 1 (GND) to Pin 16.

Contrast Trimmer

The contrast circuit at R226/R227 needs replacing as it does not provide the right voltage to VO at Pin 3, the LCD shows no characters. I desoldered the resistors and replaced them with a small 10K trimmer that adjusts the contrast for the display.

The contrast can now be set to have a clear blue background with no ghosting of characters. A perfect blue display with light white lettering. Much better! The contrast adjustment means you can set it up for different positions in your rack, an external trimmer pot fitted at the rear would be a great upgrade.

Recapped PSU

Power Supply Recap The SPX900 has a standard linear power supply rather than a switched variety (as used in the SPX90). I replaced the main reservoir capacitors at R503/R504 (2,200uF 35V) and R505/R511 (2200uf 16v), along with the three 7XXX voltage regulators and all the 100nF bypass capacitors which did look worn.

• Nichicon 2,200uf 35V
• Nichicon 2,200uF 16V

I also replaced all the local 10uF/35V capacitors near the Op Amps, 1/2.2/3.3uF near the ADC/DAC chips and 10/33uF BP capacitors in the audio signal path. 100nF ceramic capacitors were also replaced through out the main and power supply boards.

Yamaha SPX990 I bought a cheap SPX990 soon after the SPX900 purchase, which needed no refurbishment as it was in better condition and had less “miles”. The SPX990 lives in the left of my studio in a 12U 19″ rack where it is the fifth FX unit connected to the Soundcraft 22MTK, this enables the 18-bit quality and wide range of effects to be used by my collection of poly synths.

The SPX900 is mounted in a 19″ rack to the right of my studio, where it is used on the FX sends of my Behringer RX1602 mixer. This provides a synth sub mix into the Soundcraft for my Guin Guin MME, Model D, Pro-One and Korg Odyssey (Mk3).

Alesis Midiverb II Repair

Overview I bought a couple of MidiVerb II’s in May 2020, both were listed for repair and at a super low cost of £30. Although there is no service manual or schematics online, some information can be gleaned from the MidiVerb III service manual, and I was hopeful that they would be simple repairs. Typical issues are voltage regulators going out of spec, or the crystal oscillators fail. The rest of the analog and digital circuits seem reliable and robust.

Midiverb II – S/N 91606 The first one I bought has seen a lot of use, the rack ears have been cut off and the lettering was rubbing off. Powering it up initially proved it was working and the effects were really quite good and the noise floor low (its uses TL084 chips, the LF354’s in early units are noisy). I went ahead and recapped the power supply and replaced the voltage regulators. The PCB is a later version and has separate oscillators for the 8031 microprocessor and the DSP chip. The OS is MVII.OBJ dated 4/10/87.

Midiverb II – S/N 39303 The second one I bought was sold as broken and the seller stated it was non functional with all LED’s on at power up. Once in the studio I switched it on and it was working ok, so I went ahead and recapped the power supply and replaced the voltage regulators (as they were below specification). I noticed there were some PCB cuts and the timing capacitor for the 16 MHz oscillator was not connected. Powering back on resulted in failure, the most common error code was all LED’s on, sometimes OVLD off. This error code means the microprocessor has not started.

Further investigation shows a 12 MHz crystal and timing capacitor added onto the rear of the PCB for the 8031 microprocessor and the PCB cuts were to disable the divide down clock from the 16 MHz oscillator. This looks like a factory modification to increase the clock speed for the microprocessor from 8 to 12 MHz, as newer models had two clocks. I checked that both clocks were working and found the RESET signal for the 8031 was high. This is why it was bricked, it should be zero.

The reset circuit for the MV2 is very simple; a resistor and capacitor charge up after power on. This is not 100% reliable and the Midiverb III changed it during production to a more reliable transistor based circuit. When I replaced the 4.7uF capacitor I had shorted it out on top of the PCB, so hidden from view, this shorted the RESET pin to +5V. I re-soldered a new cap and added in a missing 10k resistor. Power back on and the MV2 now works perfectly.

Power On Patch One of the differences I noticed between the two MidVerbs is the patch that they initialized to on power up is different. With no battery backed RAM it wasn’t until I looked at the Xicor X2444P chip near the 8031 that I realised there was a very small amount of on board NVRAM. Very small is just 32 bytes, but it does last 100 years! This is where the MIDI program change message (01-32) mapping to the patch number (00-99) is stored and retained during power off. It also stores the MIDI control channel number (0-16) and at power on the first program (01) in the table is loaded.

This explains why the different MidiVerbs power on with different patches. It is because the mapping tables are different, they can of course be configured to get any patch (00-99) at power on. Mystery solved!

Mixer Pots The front panel potentiometers get a lot of use over the years especially the MIX. This is a dual 10kB PCB mounted potentiometer, looks like an easy to find Alpha pot.

Outcomes  I sold the Midiverb II’s in 2021 as I have too many FX!  I bought some Behringer 100M replica modules instead.

Lexicon PCM80 On Fire!

LPT45 Power Supply

Overview One of the downsides of owning vintage synths and FX’s from the 1980’s is that there is a risk of power supply failure, with capacitors emitting magic smoke or worse still bursting into flames. I bought an Emulator I many years ago for just £50 because when we switched it on it created a huge plume of smoke as various capacitors failed.

My approach over the last few years has been to refurbish the power supplies as I acquire the item, because they are usually 30 years old. In my new studio I also have a smoke alarm fitted and never leave items powered on overnight. I have fitted 1U Samson Power Strip PS10’s to supply all the gear, this enables me to turn off the power to all the gear quickly and simply.

Blown Capacitor

A Cautionary Tale  I recently installed my PCM80 into the main studio and powered it on, and it worked fine. However 30 minutes later magic smoke came out of the rack and I traced it to the PCM80 power supply and immediately switched it off. Looking at the top of the old switched mode power supply you can just see a capacitor top pushing up, which is a common failure mode for these old units.

But once I had taken out the power supply module you can see the failing capacitor has cause other components on the bottom of the PCB to burn out and worse still the plastic shield between the PSU and chassis has started to melt. Within a few minutes this could have become a major fire with plastic dripping onto the units below and smoke filling the room.

Make sure your studio has a Smoke Alarm!!

Fire Damage!

Lexicon PCM80 PSU Replacement With the PCM80 (1993) Lexicon moved from fully linear to switched mode power supplies. They also added subsequent linear regulators for the analog rails, to ensure low noise. They used a standard third party switching power module, which is easy to replace with a modern version with the same voltages, current supply and fixing holes.

The Artesyn LPT45 is the model to go for, and it can be bought for £50. This is a lot cheaper than sellers on eBay want for a PCM80 PSU, which is simply a LPT45. The service manual states the power rail voltages and current demand, so it was easy to check this was the correct part. PCM80 Max Ratings are:

• +5V is 5A
• +15V is 2A
• -15V is 300mA

The LPT45 delivers 4A on the +5V rail but the consensus is that is sufficient to safely power the PCM80. Lexicon did a great job on the power supply design with carefully designed analog power conditioning, using linear regulators, inductors and electrolytic capacitors. This circuitry looks to be in good condition, as it is not stressed as much as the switched power supply. However I will replace the power capacitors to be safe.

Tufnol Protector

Fitting the LPT45 The new power module is an exact fit, same connections and mounting holes. The only problem I had was the plastic shield had been melted by the power failure. I replaced it with a 1mm piece of Tufnol which provides the correct electrical insulation and is not going to melt or catch on fire.

The LPT45 is grounded to the chassis ground by two of the fixing holes which have earth pads, I double checked the earth continuity from chassis to the LPT45 ground connector to be sure. I returned the PCM80 to its rack and powered up – everything works and no smoke!

Learning Lesson Whilst I have refurbished the linear power supplies of the vintage gear I own, I have left the switched mode power supplies alone. That strategy has now changed, and I am replacing them ASAP. This means replacing the Emax power supply, where I will design a mounting  PCB for the Meanwell RPT-60B, and the Wavestation where an LPT42 may fit (again with a mounting PCB). I will also complete the design of a switched mode power supply and PCB for my Roland MKS-80.

Roland SDE1000 Repair

Roland SDE1000

Overview I bought a broken Roland SDE1000 Digital Delay in November 2020 for just £40, but with the known fault of it not powering on. I already have a mint and boxed SDE1000 which I find to be a fantastic delay unit for using with analog mono synths. The plan is to repair this one and use it with my “Big Moog”, instead of the Revox A77 tape delays that Klaus used.

This particular SDE1000 was manufactured in March 1985. This was two years before I rekindled my interest in synths and a secondhand ARP Odyssey Mk3.  Although the Roland was a lot cheaper than studio FX at the time, it was an expensive luxury for me at the time.

The Roland SDE-1000 was one of the Japanese company’s first effects unit, launched in July 1983 at £399 alongside the more powerful “studio version” SDE3000 which was double the price. The delay unit is quick & simple to use, with a very straightforward front panel & interface.  Sound quality is surprisingly good with a smooth, very analogue sound. This is in part due to the companding circuits to get the analog data into just 12-bits.

Maximum delay time is 375ms in standard mode (750 ms in x2 mode) and 605/1210 ms using the x1.5 rear panel control.  The sound of the SDE in X2 mode is quite reminiscent of a slow tape delay, and increasing feedback results in a gradually decaying, dulling repeat – again, like tape.

Delay time is displayed on a 4-digit blue fluorescent display with an Up – Down rocker switch to alter delay settings. The LFO has speed & depth controls, which can give deep chorusing effects or a gently shifting delay with phasing.  There are only 4 memory presets and no MIDI but the SDE1000 was a sales success with over 7,000 made, and it remained popular before Alesis entered the market with custom DSP chips.

SDE1000 Advert

Technology The SDE1000 is an early digital delay which uses a Gate Array chip as the main controller rather than a DSP chip, and a 12-bit R2R DAC rather than a dedicated DAC chip. The microprocessor is the familiar 8049 which Roland used in many products during the early 1980’s. The analog signal is compressed into a 12 bit data word with three 64k bit RAM chips used to store the digital data.

Roland used good quality NE5532 Op Amps in the output circuits and a dual transistor input buffer

Changes The SDE100 went through a number of circuit changes in 1983 to improve the headroom and HF response. They are documented in the service manual. This particular unit in from 1985 so it has these changes implemented and a V3 PCB which is not mentioned in the service manual.

The Repair This SDE1000 is in reasonably good external condition and a sound interior, which has possibly had a small amount of repair work (like the rear pot for time adjustment). It is a robust and reliable design, so I am not expecting chip failure but a power supply problem.

The power supply provides many different voltage rails;

• +/-15V rails for the analog circuits using a discrete voltage regulator
• +5V for the digital chips using a 7805
• +12V rail (7812) for the front panel LED level indicators
• +12V for the DAC voltage reference
• +20V for the LCD driver chips
• +1.7V for the LCD itself

Any part of these circuits could be where the short circuit is, so this is going to take some time! I checked the X2 safety capacitor on the mains side of the transformer and it was ok but I have replaced it. I disconnected the power connectors until I was left with the +1.7V rail which was the source of a short circuit.

I disconnected the LCD and switch PCB from the main PCB to eliminate it as a source of a short, however there was an additional short in the 15V rails. In the end I replaced the W02 regulators and power diodes, all the power supply capacitors, power transistors and power capacitors. Even though none tested as failed, this complete overhaul solved the problem and the SDE1000 powered up perfectly and works a treat.

Sounds The original factory SDE1000 presets, which can be overwritten, are:

• Long Delay: 750ms with feedback and light modulation
• Doubler: 30ms delay with feedback and no modulation
• Chorus: 50ms of delay with light modulation and feedback
• Flanger: 15ms of delay with deep modulation

The sound quality does deteriorate when in x2 mode with a limited bandwidth of just 8kHz. So I keep the delay in x1 mode with the rear trimmer set to 1x or 1.3x.

Outcomes A nice warm digital delay line for £40 and another £40 on component replacements, that will maintain the SDE1000 for another 35 years! I replaced the original old battery while I had the delay apart and I will re-calibrate using the notes in the service manual before it finally goes in the studio. The slight change in power supply voltage rails may not be exactly as it came in the factory back in 1985.