adventures in DIY music

Thursday, 17 October 2019

Yamaha DX7 power issues

This is an original mark one DX7 from about 1984. It began to spontaneously reboot itself, and at times not boot past the initial screen where the LEDS show "88". The problem was traced to a connector on the PSU where there was a cracked solder joint from thermal stress. This pin (pin 8 on header C1) connects a little circuit on the PSU (a zener diode and two transistors, hanging off the 5 volt line) with a similar circuit on the main board (marked on the schematic as "Initial Clear") that controls the CPU reset. A 10uF electrolytic cap charges up from zero to 5 volts over about 2 seconds, which then allows the CPU to operate normally - if this doesn't happen, the DX7 won't boot. The cap tested fine. The headers are very close to the heatsinks for the voltage regulators, and the board was dark from heat effects. Many of the solder joints on this board were cracked. Surprisingly, none of the large electrolytic capacitors on the PSU board were out of spec, as checked with an ESR meter. This unit has been in almost daily use since purchased new in the eighties!

A few months earlier, I did have to replace an X2 mains capacitor on the mains input board, that bypasses the power switch, which had failed as a short. Symptom: power switch had no effect, the unit was always on if the mains was connected.

One thing I have noticed with this recurring problem of cracked solder joints in old units - if the problem is more or less frequent when the unit is in a different orientation to usual, be suspicious of a cracked joint. In the case of this DX, it would boot more often lying flat, but if propped at an angle, it wouldn't boot at all. The effect of gravity on the surrounding hardware can open or close the faulty joint.

Saturday, 24 August 2019

Digitech Studio Quad repair

Problem: the four audio level meters in the LCD display showed irregular full-range signals on power up with no audio input to the unit. Also, the backlight for the large LCD was flickering.

Easy one first: the backlight power comes from a two wire cable connected to header "H4" on the main board. This was loose. A gentle bend to the pins improved the contact.

The main problem was assumed to lie in the analogue side of the power supply, as the logic and functions behaved normally. The Studio Quad requires a 9 volts, 2.2 amp AC (not DC) supply, which uses a 4 pin DIN connector, non-standard. I measured a bit over 10 volts AC here. Input filtering electro caps C87 and C61 had been previously replaced. After these came a diode bridge and four filtering electro caps (470 uF 25 volts). The solder joints were just mildly discoloured from heat, and did not appear cracked. The two +5 volt and one -5 volt regulators measured ok. The +12 volt regulator measured over 13 volts, and the -12 volt regulator measured - 9 volts that gradually increased to over -10 a few minutes after power on. There are associated small electrolytic capacitors  at the outputs of all these regulators (10 uF, 16 volts).
Using an ESR meter, nearly all these caps (except the more recent replacements) measured over 20 ohms, where they should have been around 2 ohms. Similar size and age electrolytics elsewhere on the main board, away from the PSU, measured as they should, so this was presumably the effect of heat stress.
Replacing all electros in the area of the PSU, and reflowing the solder joints to the regulators and diodes, restored a robust +/- 12 volts supply and fixed the problem.
Note: one of the replaced electros was C41, which acts as a timing capacitor for the undervoltage sensor chip next to it, marked MC34064. This was 4.7 uF so I replaced it with the exact value electro.

Below: the front panel needs to be unscrewed from the chassis to release the circuit boards, however their cables can be reconnected without the chassis in place for further testing if needed.

Saturday, 22 June 2019

Rear Panel: Boss RSD-10 digital sampler/delay

From left to right: standard Boss barrel connectors for 9 volts DC, negative tip, in and out; Effect Remote on/off footswitch, Pad control input, Trigger control input, Keyboard control input, Tape control input on RCA phono, Audio Out on phone and RCA phono, Level switch, Audio In on phone and RCA phono.

As in the RDD-20, the Effect Remote switch expects to see a press-to-break, normally closed switch, which grounds the contact when a plug is inserted. The front panel Effect switch must be on for this to work. Pressing the footswitch turns on the effect only while pressed. While this is very handy, for my purposes I wanted control over when the audio was reaching the circuits, so I converted this to an input mute switch, allowing me to drop audio in to the delay or sampler input with a tap of the foot, leaving hands free for time adjustments and keyboard triggering.

The Pad input detects a trigger to start the sample playback, as well as a volume level for that trigger by sending a CV to the compander circuit.

The Trigger input just detects a sample start trigger. Volume is fixed.

There are two pitch control inputs, Keyboard and Tape. Tape won't work without Keyboard also being plugged in - with nothing plugged into the keyboard jack, the pitch of the sample playback is determined by the front panel pot. So how does it detect pitch? There's no midi, and it doesn't use a pitch CV in the sense that a modular or analogue synth does. A chip known as a PLL (phase-locked loop) changes the frequency of its VCO when it detects the frequency of the Keyboard signal. To do this, it needs to see a fairly clean, pitch stable, monophonic waveform with minimal harmonics, preferably a sine wave. The Keyboard input also triggers the start of the sample, the volume, and uniquely, the playback time (gate).

Roland had two purposes in mind for the Tape input. The first is a rather complicated way of using a cassette as a backup device for the sampler and it's settings, by recording the initial key pitch, followed by the sample itself, onto different channels of a standard stereo cassette tape, while switching settings on the front of the machine*.  Secondly, under the heading "As a Sequencer",  there is this single obscure paragraph in the instruction manual:
"By recording the keyboard sound onto the cassette deck and feeding the playback sound to the RSD-10 through the Pitch Control Tape input jack, the sampled sound will be automatically played back".
I'm assuming that the "sequencer" refers to the sequence of pitches on a tape. Of course, any suitable audio source would work here.

This machine, like so many other Roland and Boss effects from this era, is based on the "long chip", the custom RDD63H101 CMOS gate array. But it is the external control circuitry of this machine that  enables the magic to happen. It is a fascinating little bit of eighties technology, another example of Roland ingenuity.

* I've never attempted this, however, there is of course a way of doing the same thing with a modern DAW that works quite reliably. You put the audio tone on one track, and the sample on another, and make sure your triggering audio tone starts bang on with the sample you are loading (this works in Mode B Manual Rec/Play).

Thursday, 20 June 2019

Rear Panel: Boss RDD-20 digital delay

From left to right: DC power input 9 volts standard Boss barrel connector negative tip, DC power output for daisychaining, Modulation Bus polarity switch and phone jack (input/output), Effect Remote on/off footswitch, Delay audio signal output on phone jack and RCA phono, Mixed audio signal ouptut on phone jack and RCA phono, Level (Unigain) switch, Input audio signal on phone jack and RCA phono.

The Boss Micro Studio system of half-rack sized modules from the mid-eighties used the same 9 volt, negative tip, barrel connector as the Boss effect pedals, and here they conveniently gave you a daisychain power output on each unit, with the current draw of that particular unit printed below, so a sufficient supply/current capability could be calculated. Maximum total draw is recommended not to exceed 200 mA.
The Mod Bus allowed a 0 - 5 volt CV to control the VCO that provided the master clock for the main controller chip (the custom Roland/Boss "long chip" used in so many of their effects around this time). Several other units in the series sported a Mod Bus connector - the idea was you could link two units (perhaps more?) and have one as the "master" (Mod depth turned up) and the other as the slave (Mod depth turned down), changing the polarity as necessary, for stereo effects.

The Effect Remote jack expects to see a typical Roland type footswitch, normally closed, press-to-break type switch. While this is handy, I wanted an easy way to implement a "dub delay" function, where the unit is used as a send for special effects, rather than say, corrective or fixed delay uses. The simple but surprisingly effective Tone control in the feedback circuit makes this unit great for this sort of thing. I disconnected this jack from it's stock purpose (by de-soldering R63), and ran a wire from the tip to the audio input. Thus, when a footswitch is plugged in here, the audio input is grounded. Pressing the switch breaks the contact allowing you to briefly drop in sounds at a touch, for those classic dub effects.

Tuesday, 18 June 2019

Rear Panel: Korg SDD-3000 digital delay

This is the original rack unit from 1982.
From left to right: ground terminal, ID plate, captive mains lead, Direct output jack, "+"Mix/Mono output, "-"Mix output, Delay Mod CV input 0 - 5 volts, Bypass footswitch, Hold footswitch, Prog Up and Prog Down footswitches, audio input jack.

The +Mix jack can output a mix of direct and delayed signal, the mix determined by the front panel Level Balance pot, and the phase of the delayed signal can be inverted to out-of-phase also by a front panel switch. The - Mix output can have a similar mix, however the phase of the delay is always inverted. This allows appropriate stereo set-ups for mono compatible recording, or live PA scenarios.

The Delay Mod input has a dramatic effect on the delay time. This can be scaled using the front panel Intensity pot. This gives scope for creative external control e.g. using your midi-CV converter to send quantized  5 volt changes can allow beat-matched pitch-shifting type effects on melodic material, or create interesting polyrhythms or fills from a drum input.

The Program shift Up and Down inputs also allow creative mangling e.g. synchronising different flange settings on different points of the rhythm.

The audio input jack has a convenient feature. It can stay patched in, but when a plug is inserted into the front panel input, this signal is switched out.

Saturday, 15 June 2019

Rear Panel: Roland SDE-1000 digital delay

From left to right: ground terminal, captive mains lead, ID plate, Delay time pot x1 to x1.5, Modulation CV input jack 0 - 5 volts, Preset shift control input jack, "Play Mate" control input jack, Hold on/off control input jack, Delay on/off control input jack, Delay audio output jack, Mixed audio output jack, "Unigain" switch for input and output levels, Audio input jack.

The Delay Time pot directly affects the VCO in charge of the master clock pulses to the main controller. For some reason, turning this up will increase the unit's tendency to go into nasty high frequency oscillation at high feedback levels. Also, the quality of the delay signal is noticeably better at lower settings. Moving this pot will affect the time of the 4 programs.

The Modulation Foot Control has 5 volts available to directly control delay time via a pot or a volume pedal, or you can send it 0 - 5 volts from another source. This can be scaled from the front panel Mod pot. Unfortunately the range of the external control is somewhat underwhelming.

The Preset Shift jack takes a momentary switch/footswitch to step through the four programmable presets. This would seem to give scope for some creative program changing. Say for example, to enhance a rhythm track, you could have different preset resonant flange effects synchronised with the beat. Unfortunately, Roland put a spanner in the works there, by getting the CPU to mute the delay output every time a preset is changed (this muting also occurs when the delay time is changed from the front panel, and is there to suppress glitches. The glitches, for me, are often where the happy accidents occur). Looking at the schematic, I identified the line from the CPU to the logic that controlled this muting. I disconnected the CPU pin, and tied the logic side to 5 volts, disabling this "feature" (without disabling the power-on mute etc).

The "Play Mate" footswitch function, on first tap, resets the display to all zeroes, then the following two taps allows the CPU to calculate the time between them and set that as the delay time, i.e. tap tempo.

The Hold jack takes a footswitch, and plays the contents of the memory over and over, uninfluenced by front panel control except, happily, the Timex2 switch.

The Delay on/off switch is very handy for external sequencer control, especially in high feedback scenarios - you can chop it rhythmically! Note there is no corresponding front panel control for this.

The Delay output jack obviously just has the delay signal on it. When a plug is inserted here, the delay signal is switched out from the Mixed output, leaving just the dry signal, for those live "stereo" configurations.

The audio input can take line or guitar level signals. The pre-amp is a push-pull emitter follower type with a gain of about x24.

Sunday, 19 May 2019

Farfisa "Compact"

The Farfisa "Compact" organ line was manufactured from 1964 to 1970, and there were many variations. Their unique sound stems from the way the transistor divider circuits act on the high frequency "square" wave oscillator, resulting in narrow, rounded, negative-going pulse waves.
The model I have here is probably from around 1966 (hard to say for sure as the serial numbers aren't consistent). Although they made a big deal about it's "electronic transistor"  circuits, it also contains two twin-triode valves. Of the four valve stages available, one is for the preamp, two for the reverb driver and one for the reverb recovery.
To service one of these beasts, the legs need to be in the open position, to allow access to the 5 wing nuts in the wooden base, so that the internal frame that holds the keys and the electronics can be lifted up out of the flightcase.
!!!Now's the time to re-check you unplugged it from the mains!!!
The trick here is to find the four screws holding down the PSU chassis, with it's gigantic transformer  and two valves sitting atop. Just loosen these enough to allow them to slide in their slots, moving the PSU towards the front of the machine so that the pots on the rear side clear the body cut-out. Then you can lift the internal frame up and rest it on the rear edge of the case, allowing access to the voice boards and (sorta) the key contacts. You can't really put this thing on your bench unless you do a much more comprehensive  dismantle, so you'll need plenty of room to get it as close as possible to an area you can work off.
Above: with the internal frame resting on the rear edge of the case, you can see the 3 groups of 4 voice boards ("master oscillators"), one for each of the twelve notes, and the filter boards to the right of these.

Above: the large transformer and the two ECC83 valves sitting atop the PSU chassis, on the left with the bass (pedal) trimpots board just in front of this.

This baby had been 'in the family' since the late seventies, and had given trouble-free service all that time. However now it wasn't making any sound except buzzing. Inspection immediately revealed a few electrolytic capacitors had given up the ghost, and lifting up the PSU revealed the charred remains of what I think was a spark suppression capacitor near the power switch.

Replacing these, and a few other electros in the PSU, and we had organ sounds again, but they weren't quite right...

When diagnosing faults with the sound of the organ voices, it helps to isolate the oscillator divisions. Start with all tabs off, except one of the 4' voices. Play all the notes chromatically.
There might be an obvious crackle suggesting the contacts need cleaning. Or, there might be a dead key altogether. Is it the contact, or the voice electronics? Switch the 4' tab off and the 8' tab on. Did the dead key stay dead, or did it just move up an octave? If it moved, it could be that specific divider output on the master oscillator boards for that note. I had a dead "A" that moved with the footage selection. This was traced to the 1uF electrolytic cap at the output of that division, on the master oscillator board for "A". Each division of each of the twelve notes has such a cap on the output. 
Shortly after fixing that, all my "D" notes went wonky, sounded horrible across all octaves. Getting the scope onto the boards I found the shape of ALL of the D divisions looked like this

Whereas the shape of the healthy notes looked like this

The top frequency of the D oscillator, the square wave from which all the lower notes are divided, looked OK, much like the other notes. So whatever was wrong, it had to affect ALL the divider circuits. I couldn't see, from looking at the schematic of the master oscillator board, what that one component could be. 

But inspecting the voice board itself revealed a cap that wasn't marked on the schematic, a 50uF 12 volt electrolytic that was bypassing the 8 volt rail (top line of the schematic above). This rail is a common connection to ALL the emitters of the divider transistors. Ah ha! Sure enough, replacing this fixed the sound, and the pretty pulse waves were back again.

Above: the "D" oscillator board. The blue electro was the unmarked cap.

Key contacts will likely need cleaning. Each key (except for the bass octave) has a contact for 4', 8', and 16' footages. Cleaning these contacts will test your patience, I assure you. Make sure you allow a LOT of time. Beforehand, you will need to fashion some kind of tool. Now, this could be something as simple as an old-school pipe-cleaner with a couple of judicious bends made in it. That didn't work for me, I needed something more rigid. But it needs to be very narrow, to get past the crowded linkages at the rear of the key assembly. Yes, 3 contacts per key. And you can't really see what you're doing. Be very careful, if you damage one of the delicate springs (that live in that tiny space where you can't see) that actually make the contact, it will be a major nightmare to replace!           
If anyone knows of a shortcut...?

Above: by carefully sliding out the PCBs on their harnesses, you can gain slightly easier access to the rear of the keys to clean the contacts... if you can make it past the white plastic linkages.

So having fixed the oscillators and cleaned the key contacts, I then realise the reverb has gone. The reverb in this machine is a source of great character. The springs are suspended inside a box that itself is suspended below the base of the flightcase, beneath the keys. Hitting the box produced the usual "reverb explosion", however no organ sound was making it to the springs. I deduced from this that something on the driver side must have failed, while the pick-up circuitry was still functional. Two halves of valve 2 are involved with the driver amplification, and half of valve 1 with the pickup recovery amp (see below). There was a seemingly very healthy 100 volt signal at the driver side output to the transducer. Still, I tried replacing the valves - no change. The driver and pickup transducers are actually ceramic gramophone cartridges. In the place of the stylus, is a wire that is soldered to the springs.

These sort of cartridges appear to have some sort of piezo material inside them, that decays over time. I couldn't find anything like them for sale anywhere.However, Jaycar sold "buzzers" that contained piezo elements, so I cracked one of these open and experimented with it, after bringing out some new connections from the PSU so that I could have the box on the bench. (About here is where you realise it's going to be much easier if you adapt the reverb wiring to accomodate some Molex connectors, rather than crouching under the machine with a hot soldering iron in one hand..) There were definitely some "sweet spots" for the transducer, and the wire needed to be tight against it. I tried it both in driver and pickup positions.

I settled on using the disc as the new driver. It sounds to me like there is some "plate" effect as well as a fair bit of distorted "direct" sound going on with this arrangement, but in any case, it sounds great, certainly just as characterful as the original!  In fact, I liked it so much I converted the output jack in the PSU chassis that had been dedicated to the bass or pedal octave, to an audio input jack for the pre-amp and reverb, summing it with the internal organ sounds, making that dirty, springy, valve-y goodness available to all!

I'm grateful for the all the assistance I received with these repairs, from the heavy hitters on the Sound On Sound DIY forum!