Troubleshooting a Boss DS-1 Keeley Mod

[shiner_man]

Hey guys.

I attempted to do the Keeley Mod on a Boss DS-1 over the weekend and I've run into all sorts of strange issues.  I can't believe something so simple has gone so bananas.

Anyway, I'm at the point now where I'm 99% certain I have a bad component or bad solder joint somewhere on the board.  Here are the symptoms:

• The pedal works normally in bypass mode.
• When the pedal is engaged, the LED lights up.
• There is no sound when the pedal is engaged unless you hit the strings of the guitar very hard at which point it makes a very strange fuzz/computer type sound.  It sounds like a Devi Ever pedal or something.

I am working off the following schematic for the mod:



After fighting with the pedal for some time, I took out the LEDs in the clipping section and replaced them with the original diodes that are stock with the DS-1 just to make sure the issue wasn't with the LEDs.  I also removed the SPDT switch and the 47pf cap in that section as well.

So now the pedal is completely stock except for the capacitors and resistors with different values that I changed out for the mod. 

I think it's time I start probing this thing for voltages but I've never had to do that before so I have a few questions:

1.  Where exactly should I be measuring voltages in this circuit to troubleshoot the issue?
2.  How do I know when one of these voltage readings indicates an issue in the circuit?  For instance, if I measure pin 7 of the opamp, what should I be looking for? 

Any advice would be appreciated.

Thank you.

[bluebunny]

You'd expect the inputs and outputs of the opamp to be mid-supply, so around 4.5V.  Check pins 1, 2, 3, 5, 6, 7.  The sounds you describe sound like the biasing has gone bananas, which might be indicated by off-readings on these pins.  Same goes for the bases of the three BJTs, the emitters of Q1 and Q3, and the collector of Q2.  It's where your audio signal lives, and it needs room to "wiggle", hence around 4.5V.

[duck_arse]

and, photos of your board as it now sits, please. what type of caps did you use as replacements?

[shiner_man]

You'd expect the inputs and outputs of the opamp to be mid-supply, so around 4.5V.  Check pins 1, 2, 3, 5, 6, 7.  The sounds you describe sound like the biasing has gone bananas, which might be indicated by off-readings on these pins.  Same goes for the bases of the three BJTs, the emitters of Q1 and Q3, and the collector of Q2.  It's where your audio signal lives, and it needs room to "wiggle", hence around 4.5V.

Thank you.  I will take some readings later on today and see what I can find.

Hypothetically speaking, let's say I get a reading that isn't close to 4.5V on the collector of Q2 or something like that.  Would the conclusion then be:

1.  Q2 is bad
2.  Something before Q2 is bad?  Perhaps C3?

Thanks for your help.

[shiner_man]

and, photos of your board as it now sits, please. what type of caps did you use as replacements?

I will post photos later.  I used polyster film caps for everything I replaced. 

[duck_arse]

voltage on collector of Q2 is determined by the resistors around it. measure the voltages at base, collector and emitter, then work out the current flows using Ohm's Law. that will tell you if there is something wrong with some part of that stage.

[shiner_man]

Okay, here are the voltages I got:

Q1 E-3.57 C-9.32 B-4.01
Q2 E-.01   C-4.27 B-.63
Q3 E-3.17 C-9.32 B-3.5
Q4 E-0      C-.04  B-.63
Q5 E-0      C-0     B-.64
Q6 S-4.6   G-.6    D-4.6
Q7 S-4.6   G-.09  D-4.6
Q8 S-4.6   G-.08  D-4.6

OpAmp:
Pin 1-4.6
Pin 2-4.6
Pin 3-4.6
Pin 4-0
Pin 5-4.6
Pin 6- (Starts off measuring around 8 volts then drops down to almost 0 volts)
Pin 7-1.8
Pin 8-9.33

Any help would be appreciated.

Thanks.

[duck_arse]

s o m e w h e r e - antonis has posted a pic of that stage [Q2], showing where all the currents go and how they add up, so's the user might work [and practise] ohm's law on hizz circuit. but the voltages seem ok from where I sit. and if the check light goes on and off, we can say that the flip flop section is also working, and bypassing proper.

but - if you put 4V6 on pin 5 of the opamp, it should show very similar at pin 7, and pin 6, because it's also connected. [quite what/how the effect of R40 is/should be, you'll need a smarter than I to tell. but, it should at least pull the output pin upwards, towards the 9V it's connecting.]

photos?

power off, and pull the opamp if socketed. then measure the resistance between pins 7 and 6, as you vary the distortion pot.

[shiner_man]

photos?

I'm really about to throw in the towel on this thing. 

[PRR]

> photos?

Red-dot solder joint looks bad to me. Yellow-dot joints are dubious.

If you didn't do these joints, then the factory was sloppy and the joints held-in by a hair for final test, but let-go when handled for modding.