Sound Lab ULTIMATE Troubleshooting

Ray Wilson authored this content while he was actively running MFOS as the founder and resident genius.
We retain the content because it reflects a valuable point of view representing that time and place.

Article by Ray Wilson
This is an advanced project and I do not recommend it to beginning electronics or synth-diy enthusiasts. You need advanced trouble shooting skills and equipment. Without a good oscilloscope, DVM, bench power supply with current limited output you will not be able to trouble shoot the circuit should you run into a problem. This project is about 3 times more difficult than the Sound Lab Mini-Synth. If you have successfully built that and found it almost too easy then I would estimate that you have the necessary skills to successfully complete this project. It is assumed that you will know how to provide a bipolar power supply for the project that delivers +/-12 volts with capacity to deliver at least 200 to 300 mA. The circuit draws between 90 and 100 mA but the 200% to 300% current capacity padding is always a good idea. The MFOS Wall Wart Power Supply. using LM7812 and LM7912 would be a good choice but the power supply is left to the discretion of the builder.

Introduction

First of all let me assure you that if you build the project according to the schematics and drawings I provide it will indeed work very nicely as many who have successfully completed the project will attest including myself. I will be describing how I would go about trouble shooting the Sound Lab ULTIMATE. If you already know how to trouble shoot an electronic circuit this will not be necessary for you to read. If you are a little shaky on your trouble shooting skills this should give you some guidance. I have a lot of general trouble shooting advice here: General Trouble Shooting Advice I suggest you familiarize with it as it contains some tried and true pointers and advice.

You will find that after each section where I describe what you should be observing or measuring I pretty much say the same things because... that is where the problem will lie. No offense at all and I'm happy to try and help people succeed on MFOS projects but if I had a dollar for every time someone said "I triple checked the wiring and it's correct and so are all of the component values and it still doesn't work" I would be sailing around the world on my fully staffed yacht.

Unlike the Who I can't see for miles and miles so you have to do the trouble shooting. I can't do it via email either. If you lived near me I would say bring the circuit over and we'll get it working in my lab but obviously that is not practical unless you own a private jet in which case I'm ready to go anytime. So the bottom line is every wire has to be run correctly and every component has to be the correct value and every active and passive component has to be good. Once that is the case your synth is going to impress you and your friends. Another obvious thing you need to do is probe, probe, probe. I've tried to put enough measurements and photos into this guide to help you but not every circuit point is presented nor is every possible photo included. You will have to use Ohm's law sometimes or be a bit Holmesian in your deductions but rest assured that with patience and attention to detail you will find any issue your synth may be facing. Never forget the power of another set of eyes and find a local resource that is interested in electronics to help you. OK enough pontificating and again no offense meant at all.

I use +/-12V in my ULTIMATE so the measurements I cite will reflect that. Your measurements may vary if you are using a different power supply voltage. When I say "about" or "approximately" know that I mean your measurement can vary from what I cite by hundreds of millivolts. Many of the components in the circuit are 5% and higher tolerance (pots can be 20%) so our measurements should be close but will not be exactly the same. When I indicate direction assume I am looking at the back of the front panel and the potentiometers and switches are oriented as shown in the panel wiring diagram.

First I suggest that you familiarize yourself with all of the information available to you on the Sound Lab ULTIMATE project site. This information should be read in conjunction with the associated circuit descriptions on the "Schematics" tab.

The "Schematics" tab is of course your road map to the entire project. I suggest that you print out all of the schematic pages as PDFs (click any schematic to open it as a PDF) so you can make notes or highlight areas on them during your trouble shooting.

The "PCB Info" tab has documentation to help you identify which areas of the PC board contain the various modules, what the component designators are and what the value of each component on the PC board should be.

The "Panel Wiring" tab contains all of the information pertaining to the interconnections both between the panel components themselves and the interconnections between the PC board and the panel components. Every connection is necessary and forgetting one wire can cause problems to occur in several modules. For instance if you forget a ground wire between two panel components that same ground wire may be necessary to continue the ground connection to several other parts of the panel. Close observation and attention to detail are necessary when you are inspecting your wiring. Good soldering is also very important and good solder joints are shiny and silver in appearance. If a solder joint appears cold (dull and gray) it's probably a good idea to reflow that joint.

Good luck in your trouble shooting efforts.

Repeat Gate Generator

The repeat gate generator is a very simple circuit that either oscillates or does not. I have added info to the Schematics tab about using a non-polarized (also called bipolar) aluminum capacitor for C69. Originally there was a tantalum cap but the oscillations on the cap exposed it to reverse bias which could cause the cap to become leaky and prevent the repeat gate generator from working at low settings. If you haven't replaced yours yet now's a good time.

Set the Repeat Gate Rate control to about 2 oclock.

You should observe a pseudo triangle looking wave on U21 pin 2. It should be about 2.74V peak to peak oscillating about ground.

You should observe a square wave on U21 pin 3 at the same frequency. It should be about 2.76V peak to peak oscillating about ground.

You should observe a square wave on U21 pin 6 at the same frequency. It should be about 20V peak to peak oscillating about ground.

You should observe a square wave at the kathode of D9 at the same frequency. It should be oscillating between ground and about 10V.

The LED should be flashing at the same frequency as the repeat gates are occurring.

Adjusting the Repeat Gate Rate control from fully counter clockwise to fully clockwise should change the frequency of the Repeat Gate output from very slow to about 30 or so hertz.

If the Repeat Gate Generator is not operating as described several problems may exist in the Repeat Gate Generator circuitry:

Attack Release Envelope Generator

The following assumes that the Repeat Gate Generator (RGG) is working properly.

Set the Attack and Release controls fully counter clockwise. Set Mode to trig and set Range to short.

Connect the output of the RGG to the Gate input of the Attack Release Envelope Generator (AREG).

Turn the Repeat Gate Rate fully clockwise.

The output of U22-B pin 7 should be a repeating envelope with about a 2 mS risetime and a 2 to 3 mS fall time occurring at the same rate as the RGG. The voltage of the envelope should be starting at about ground and rising to about 8 or 9 volts.

U22-B pin 7 Ground To About 9V Envelopes

The output of U22-C pin 8 should be a pulse which rises from about -10 to about 10V occurring at the same rate as the RGG. In gated mode the pulse stays high during the gate period.

U22-C pin 8 Neg. Sat. to Pos. Sat. pulse.

The output of the AREG (U22-D pin 14) should be a repeating envelope with about a 2 mS risetime and a 2 to 3 mS fall time occurring at the same rate as the RGG. The same signal should be observed on the right hand terminals of both R164 (VCF AR MOD control) and R191 (VCA AR MOD control). The voltage of the envelope should be starting at about -5.2V and rising to about 4V.

AREG Output When Gated By RGG (Trig Mode)

Change the AREG Mode to Gate.

The output of the AREG (U22-D pin 14) should be a repeating envelope with about a 2 mS risetime, a hold time that is about the same as the high time of the RGG and a 2 to 3 mS fall time occurring at the same rate as the RGG. The same signal should be observed on the right hand terminals of both R164 (VCF AR MOD control) and R191 (VCA AR MOD control).

AREG Output When Gated By RGG (Gate Mode)

Change the AREG Mode to Trig and the Range to Long.

The times of both the attack and release phases of the envelope should lengthen to about 8 or so milliseconds. Changing the rate of the RGG should cause the envelopes to follow the rate of the RGG. Changing the Attack and/or Release times should be reflected in the observed repeating envelopes.

AREG Output When Gated By RGG (Trig Mode and Long Range)

Change the AREG Mode to Gate.

You should observe a flat spot at the top of the envelopes because the gate holds it high as long as it is present. Changing the rate of the RGG should cause the envelopes to follow the rate of the RGG. Changing the Attack and/or Release times should be reflected in the observed repeating envelopes.

AREG Output When Gated By RGG (Gate Mode and Long Range)

Remove the Repeat Gate from the AREG gate input and set the AREG to Triggered Mode.

Depressing the Manual Gate button should cause an AR cycle to occur. The output of U22-A should go from about -10V (Manual Gate NOT depressed) to about 10V (Manual Gate depressed).

The attack and decay times should lengthen or shorten depending on the settings of the Attack and Release controls.

If the Attack Release Envelope Generator is not operating as described several problems may exist in the Attack Release Envelope Generator circuitry:

Low Frequency Oscillator 1

I am going to describe what you should see while ringing out LFO 1. Use the same techniques to check out LFO-2 if it is not working properly. You just need to find the designators that apply to LFO-2 by looking over the schematics and PC board drawings and then probe or test for signals on the leads of the corresponding component in LFO-2.

Set the Range switch to high. Set the output to rectangle wave. Observe LFO-1's output at the LFO-1 Output banana jack. The LFO-1 output should also be observed on the right hand terminal of R159 (VCF LFO-1 Mod).

LFO-1 has two ranges.

In high range when the Rate pot is turned fully clockwise LFO-1 should oscillate at about 250 Hz. When the Rate pot is turned fully counter-clockwise LFO-1 oscillates very slowly and becomes a bit skewed. My LFO-1 stays high for about 24 seconds and low for about 6 seconds. As you increase the frequency slightly from fully counter-clockwise the waveform becomes symmetrical.

In Low range when the Rate pot is turned fully clockwise LFO-1 should oscillate at about 8 Hz. When the Rate pot is turned fully counter-clockwise LFO-1 oscillates very slowly and may even stop. This is so you can get the oscillator as slow as possible when you want a REALLY low frequency oscillator. If you never want the oscillator to stop even at low range and Rate pot turned fully counter-clockwise you can increase the value of R255 up to about 1K if you like. This will affect both the high and low frequency ranges somewhat.

The output oscillates about ground at about 5V peak to peak.

LFO-1 Square Wave Output

Triangle Square Wave Output

LFO-1 Square Wave And Triangle Wave Timing

When the wave shape is set to ramp or sawtooth the output frequency will double (this is expected).

LFO-1 Ramp Wave And Rectangle Wave Timing

LFO-1 Sawtooth Wave And Rectangle Wave Timing

If LFO-1 is not operating as described several problems may exist in the LFO-1 circuitry:

Sample And Hold

The sample rate can be observed at the Trig output of the sample and hold.

Adjust the Sample Rate to maximum (fully clockwise). The maximum sample rate should be about 46 Hz. The voltage on C64 should be a pseudo triangle wave oscillating about ground at approximately 6.5V peak to peak.

Voltage on C64 With High Sample Rate

Adjust the Sample Rate to minimum (fully counter-clockwise). The minimum sample rate should be about 0.7 Hz. The voltage on C64 should be a pseudo sawtooth wave oscillating about ground at approximately 6.5V peak to peak.

Voltage on C64 With Low Sample Rate

Adjust the Sample Rate to maximum (fully clockwise) and the Glide to minimum (fully counter-clockwise).

Set LFO-1's output to a triangle wave of approximately 4 Hz.

Connect the output of LFO-1 to the input of the Sample & Hold (S&H).

LFO-1 Output Fed To S&H Input

You should observe the level shifted and attenuated triangle wave on the output of U19-A pin 1.

Level Shifted And Attenuated Signal At U19-A pin 1

You should observe the step like sampled level shifted and attenuated triangle wave on the output of U19-B pin 7.

Level Shifted And Attenuated Sampled Signal At U19-B pin 7

You should observe the step like sampled level and amplitude restored triangle wave on the output of U19-B pin 7.

Level And Amplitude Restored Sampled Signal At U19-C pin 8

Increase and decrease the level of Glide. If you increase the Glide slightly the steps should begin to slew to each new level instead of step. If you increase it enough you can eventually reduce the steps until the output looks almost triangular and become attenuated.

You should observe a pulse wave on U20's output (pin 6) which goes from about -10V to about 10V.

Output Of U20

You should observe a pulse wave on the cathode of D7 which goes from ground to about 10V.

The LED should be briefly flashing at the same rate as the pulse wave you observed.

You should observe a pulse wave at the junction of R212 and R203 that occurs at the same rate as the pulse wave you observed.

Pulse Wave At Junction Of R212 And R203

If the Sample & Hold Circuit is not operating as described several problems may exist in the Sample & Hold Circuit circuitry:

Voltage Controlled Oscillators

I will describe the procedure for checking out the operation of VCO 1. Use the same techniques to check out the other oscillators if they are not working properly. You just need to find the designators that apply to the other oscillators by looking over the schematics and PC board drawings and then probe or test for signals on the leads of the corresponding component in the other VCOs.

You can see where VCO-1 is on the module map located on the "PCB Info" tab.

As you adjust the Coarse Freq Adj for VCO-1 (R3) you should see the voltage vary on the wiper of R3 from about -9.5V on the low side to about 2.8V on the high side. You should see the same varying voltage on the PC board at point X7 as you adjust the control. The frequency of VCO-1 observed at points X8, the center pole of S10 and the right hand terminal of R127 on the front panel should be varying from very low to very high as you adjust the control.

If this is not what you observe check the wiring against the panel drawing and schematic to make sure things are connected correctly and that the components are the correct values. That is the last time I'm going to write that so just take for granted that if a measurement isn't as I describe it you need to do it... every time.

As you adjust the Fine Freq Adj for VCO-1 (R6) you should see the voltage vary on the wiper of R6 from about -1.8V on the low side to about 1.8V on the high side. You should see the same varying voltage on the PC board at point X5 as you adjust the control. The frequency of VCO-1 observed at points X8, the center pole of S10 and the right hand terminal of R127 on the front panel should be varying over a small range as you adjust the control.

With both coarse and fine controls turned fully counter clockwise you should see about 194 mV on the output of U1-A pin 1 and -400 mV on the output of U1-B pin 7.

With both coarse and fine controls turned fully clockwise you should see about -61 mV on the output of U1-A pin 1 and -1.84V on the output of U1-B pin 7.

The ramp core of the oscillator should be putting out a ramp wave, on pin 1 of U2-A, that goes from ground to about 2.2V. The frequency should vary as you adjust the VCO-1 frequency controls.

You should observe a narrow approximately 2 to 3 uS wide pulse on the output of U2-B pin 7 that corresponds to the oscillator's frequency.

VCO Ramp Core (U2-A pin 1)

VCO Sawtooth Output (U4-A pin 1)

VCO Ramp Reset Pulse (U2-B pin 7)

When adjusting the pulse width for VCO-1 you should observe that the rectangle waveform has a high time (also called duty cycle) of about 20% of VCO-1's period when the control is fully counter-clockwise. The rectangle waveform has a high time of about 90% of VCO-1's period when the control is fully clockwise.

If the oscillator is not operating as described several problems may exist in the VCO-1 circuitry:

White Noise Generator

The White Noise generator either makes white noise or it doesn't. It is a very simple circuit and depends mostly on the quality (or would it be lack of quality) of the noise transistor. If you see noise at the outputs of the gain stages but at greatly reduced levels you need to keep testing 2N3904s until you find a nice noisy one. I see as much as 500% variation in 2N3904 noise between the quiet ones and noisy ones so you will know when you find a noisy one. You should observe about 320 mV peak to peak of noise at U14 pin 7 and about 3.92V peak to peak of noise at U14 pin 1. More is better. If you buy a noise transistor and find that there is just too much gain and the second op amp is clipping then reduce the value of R145 to 47K or even less if necessary to reduce the second stage gain.

White Noise Generator First Gain Stage Output (U14 pin 7)

White Noise Generator Second Gain Stage Output (U14 pin 1)

If the White Noise Generator is not operating as described several problems may exist in the White Noise Generator circuitry:

Voltage Controlled Low Pass Filter

Turn all of the mixer controls fully counter clockwise.

Turn all of the VCF controls fully counter clockwise.

Pins 5 and 12 of U16 should be within 100 mV of ground when measured with a DVM. For instance mine measured 88mV and 17 mV respectively.

With the Initial Cutoff frequency control fully counter clockwise you should measure about 167 mV on the output of U15-B pin 7, about 600 mV on the output of U15-A pin 1 and about -11V on pins 1 & 16 of U16.

With the Initial Cutoff frequency control fully clockwise you should measure about 48 mV on the output of U15-B pin 7, about 1.85V on the output of U15-A pin 1 and about -10.8V on pins 1 & 16 of U16.

Turn the VCF resonance control fully clockwise. The output of the VCF at pin 9 of U16 should output a sine wave of about 7 volts peak to peak. With the Initial Cutoff frequency control fully counter clockwise the frequency of the oscillation should be about 3 or so hertz. With the Initial Cutoff frequency control fully clockwise the frequency of the oscillation should be about 12KHZ or so.

Turn the VCF's Resonance control to about 2/3 clockwise and the Initial Cutoff frequency control to about 2/3 clockwise. Set VCO-1 to a rectangle wave whose frequency is about two hundred hertz. Turn the mixer's VCO-1 volume knob fully clocckwise. The output of the VCF pin 9 of U16 should be outputting a ringing rectangle wave.

Turn the VCF's Resonance control fully counter clockwise and the Initial Cutoff frequency control fully counter clockwise. Set VCO-1 to a rectangle wave whose frequency is about two hundred hertz. Turn the mixer's VCO-1 volume knob fully clocckwise. The output of the VCF pin 9 of U16 should be at about ground and the square wave should be completely filtered out. Advance the Initial Cutoff frequency control slowly clockwise while observing the output of the VCF pin 9 of U16. The VCF's output should change from a heavily filtered version of the rectangle wave to a sharp edged version of the rectangle wave. There will be a bit of differentiation due to the coupling caps in the mixer section.

The LFO-1 Mod control when advanced should cause the VCF's cutoff frequency to be modulated at the LFO-1 frequency and selected waveform.

The AR Mod control when advanced should cause the VCF to be modulated by the selected AR envelope shape. You can use the Manual Gate button to trigger or gate the AR Envelope generator during testing.

VCF Oscillation Low Setting (U16 pin 9)

VCF Oscillation High Setting (U16 pin 9)

VCF With Rectangle Ringing (U16 pin 9)

If the VCF is not operating as described several problems may exist in the VCF circuitry:

Voltage Controlled Amplifier

Turn all of the mixer controls fully counter clockwise.

Turn the VCF Initial Cutoff frequency control fully clockwise and the VCF Resonance control fully counter clockwise. This assumes the VCF is working properly if it is not trouble shoot and fix it before these tests.

Turn all of the VCA controls fully counter clockwise.

The voltage at the wiper of R177 should vary between about -9.7V and 2.75V as you adjust the Initial Level control from fully counter clockwise to fully clockwise.

The voltage at the wiper of U17-B pin 7 should vary between about -200mV and 15mV as you adjust the Initial Level control from fully counter clockwise to fully clockwise.

The voltage at the wiper of U17-A pin 1 should vary between about 558mV and 1V as you adjust the Initial Level control from fully counter clockwise to fully clockwise.

Turn the VCA's Initial Level control fully counter clockwise. Set VCO-1 to a rectangle wave whose frequency is about two hundred hertz. Turn the mixer's VCO-1 volume knob fully clockwise. As you advance the VCA's Initial Level control clockwise the output of the VCA point X67 on the PC board should go from no signal to about a 1V peak to peak rectangle wave. There will be a bit of differentiation due to the coupling caps in the mixer section.

The LFO-2 Mod control when advanced should cause the VCA's amplitude to be modulated at the LFO-2 frequency and selected waveform.

The AR Mod control when advanced should cause the VCA to be modulated by the selected AR envelope shape. You can use the Manual Gate button to trigger/gate the AR Envelope generator during testing.

If the VCA is not operating as described several problems may exist in the VCA circuitry:

Audio Mixer

This assumes that you have tested the VCF, VCA, Noise Generator and VCOs and they are working appropriately.

Turn all Audio Mixer knobs completely counter-clockwise.

Turn all VCA and VCF controls completely counter clockwise.

Turn VCA Initial Level and VCF Initial Cutoff controls completely clockwise.

Turn the Output Level control completely clockwise.

Connect the Out jack to an amplifier turned to a reasonable listening level.

****************************************

When S10 is set to SawTooth wave you should observe the sawtooth wave of VCO-1 on the right hand side terminal of R127.

Adjust the frequency of VCO-1 and the observed sawtooth wave should change frequency appropriately.

When S10 is set to Rectangle wave you should observe the rectangle wave of VCO-1 on the right hand side terminal of R127.

Adjust the pulsewidth of VCO-1 and the observed rectangle wave should change width apropriately.

Turn up the volume control for VCO-1 (R127). You should hear VCO-1 coming through the amplifier. Adjust VCO-1's controls to verify that it is the VCO you are hearing. If you are hearing VCO-1 you are good to go.

Turn all Audio Mixer knobs completely counter-clockwise.

****************************************

When S11 is set to SawTooth wave you should observe the sawtooth wave of VCO-2 on the right hand side terminal of R129.

Adjust the frequency of VCO-2 and the observed sawtooth wave should change frequency appropriately.

When S11 is set to Rectangle wave you should observe the rectangle wave of VCO-2 on the right hand side terminal of R129.

Adjust the pulsewidth of VCO-2 and the observed rectangle wave should change width apropriately.

Turn up the volume control for VCO-2 (R129). You should hear VCO-2 coming through the amplifier. Adjust VCO-2's controls to verify that it is the VCO you are hearing. If you are hearing VCO-2 you are good to go.

Turn all Audio Mixer knobs completely counter-clockwise.

****************************************

When S12 is set to SawTooth wave you should observe the sawtooth wave of VCO-3 on the right hand side terminal of R269.

Adjust the frequency of VCO-3 and the observed sawtooth wave should change frequency appropriately.

When S12 is set to Rectangle wave you should observe the rectangle wave of VCO-3 on the right hand side terminal of R269.

Adjust the pulsewidth of VCO-3 and the observed rectangle wave should change width apropriately.

Turn up the volume control for VCO-3 (R269). You should hear VCO-3 coming through the amplifier. Adjust VCO-3's controls to verify that it is the VCO you are hearing. If you are hearing VCO-3 you are good to go.

Turn all Audio Mixer knobs completely counter-clockwise.

****************************************

Turn up the volume control for White Noise (R132). You should hear White Noise coming through the amplifier. If you are hearing White Noise you are good to go.

Turn all Audio Mixer knobs completely counter-clockwise.

****************************************

Apply a signal generator with a 1 kHz 200 mV triangle wave to the external input jack (jack connected to MXD or right hand terminal of R134).

Turn up the volume control for External Input. You should hear the 1 kHz triangle wave coming through the amplifier. If you are hearing the 1 kHz triangle wave you are good to go.