Early version Boss CS-3 not working

[beatnik]

Hi there,

I have on my bench a Boss CS-3 compressor which is completely dead.
It's one of the first versions, with the DBX 1252 VCA.
It's supposed to sound very good (actually I've never tried it in person), so I would like to bring it back to life.
Here you can download the original schematic and service manual:

https://www.yousendit.com/download/bFlIRE9sT01wTVUwTVE9PQ

I have started to debug it with a voltmeter and audio probe, and here it's what I've discovered so far:

-  The pedal doesn't pass audio signal, neither the status led powers up

- I've checked the path with the audio probe, starting from the input, and the signal gets lost at IC1 pin 3, just the first one

-  I've started measuring voltage coming out of the DC jack. The +9V rail seems to be ok, but after the voltage divider (R2/R28) I read 5.8V instead of the correct 4.5V.

- Another thing that seems very strange to me, but I don't understand what could be the cause of this problem:

If I measure the voltage between the two poles on the DC jack i read the actual supply voltage, 8.9V.
But if I read between the DC jack positive and the case (which is connected to ground too) i read 6.15V.
More, if I measure voltage between the case and the DC jack negative pole, I read 2.79V

My guess is that the pedal has been feed with a wrong power supply, which has damaged some components, but I don't know which ones.

Of course, I can provide all the voltages measurements you may need for understanding which components are fault. Just ask what you need.

I think that maybe replacing all the ICs, diodes, transistors and electrolytics would solve the problem, but some of the components are hard to find, so I am asking your help to spot on the problem.

Any help is highly appreciated

Ciao!

[Govmnt_Lacky]

Try looking for the power protection diode. It is most often the FIRST diode after the DC jack that goes from the power rail to ground. Use your DMM to see if it has not been shorted.

As a matter of fact, check ALL of the diodes on the board! 

[beatnik]

many thanks for the immediate response.

i checked the power protection diode, 1N4004.

i have done like this, tell me if it's the correct way:

i've pulled one leg out of the pcb. i set my dmm on 2000 ohms / diode mode and measured across the two legs. in one way i read nothing, in another way i read 460. i think the diode is good, but please give me your opinion.

of course i could test all the diodes, but they are very tiny and i don't want to make more problems to the circuit.

are you sure the problem is in the diodes? can't be maybe in other components like electrolytics or ICs?

[ayayay!]

I think here you can test all the diodes in circuit, as long as the power is disconnected.

If you lose everything at pin3 of IC1, then you need to check some things:
-Q5 & D7.  See if it's getting signal along those lines.  You could have a malfunctioning flip-flop (switching) circuit.  
-D2 & D3.  Those protect the inputs of IC1.

Also, you said completely dead, but does it work in bypass?  

i've pulled one leg out of the pcb. i set my dmm on 2000 ohms / diode mode and measured across the two legs. in one way i read nothing, in another way i read 460. i think the diode is good, but please give me your opinion.

Yes, it seems good then.  

Also, plug your power back in for about a minute.  Does anything feel warm to the touch?


Also, Govmnt_Lacky is right.  Check D1.  Check everything around that first IC.  Do you have approx 4.5 volts coming out of pin 7 of IC1?

[twabelljr]

More, if I measure voltage between the case and the DC jack negative pole, I read 2.79V

That is alot of voltage drop for a ground circuit. Looks like pad 17 grounds the board to the case, and 13 the adapter to the board.

[beatnik]

Update:

I've pulled out the polarity protection diode (1N4004) and the pedal started to breath.

Now it passes audio, and quite good (volume and frequency content seem normal), bypass working, potentiometers too. the status led powers up, but the light is very weak.

The problem is that the sound is badly distorted, so I have other fried components somewhere. It's not very much distorted, as you can hear mainly the clean sound, but for sure something is wrong. And seems there is no compression effect.

I tried again with the audio probe, and the signal become distorted after the dbx VCA. Sound is clean on pin 1, but not on the others.

I still have the problem between DC jack ground and case/pcb ground. They are connected (my DMM shows continuity) but make different voltage readings and have 2.79 voltage drop, which is a thing I've never encountered in my previous debugging experiences.

Another thing I don't understand and want to ask about, is the purpose of D10 + R32, they are between the DC jack ground and the Input jack.

At the moment I don't have the components to start replacement, but I'll order them very soon.

For now thanks a lot for your help, I am looking forward to hear some more tips from you for fixing this one.

If voltage readings on ICs and transistors are useful for you, I will provide them. The problem is that I don't know what ground reference I must use, since DC ground and case/circuit ground produce different readings

[juansolo]

I've fixed a dead CS3. No signal after first opamp.
The inline opamps, NJM14558, weren't rated for 9V, they were a 7.5V part, not surprised it died. These were factory soldered.

I swapped them for some  DIL JRC4558s, with a few wires to the board, and it works perfectly.

[ayayay!]

I've fixed a dead CS3. No signal after first opamp.
The inline opamps, NJM14558, weren't rated for 9V, they were a 7.5V part, not surprised it died. These were factory soldered.

I swapped them for some  DIL JRC4558s, with a few wires to the board, and it works perfectly.

What, wait... No kidding?  I've got a dead one too and IIRC it had NJM14558's as well.  Those are really only rated up to 7.5v? 

[juansolo]

According to datasheet max supply voltage is 7.5v.
Crazy.

[beatnik]

that's freaky!

actually it's not the case of my pedal, wich uses M5218L. the datasheet rates them at + / - 16V operating voltage

i am still getting crazy about the ground thing, and why i have this voltage drop between grounds.

someone can spread the light?

and if i have distortion after the VCA, is sure that the problem comes from the VCA, or maybe could be another component which is next in the signal path?

i can make all the measurements you may need (in the limits of my equipment), for helping me to solve the problem. my knowledge in electronics is mostly limited to just build and use devices, and not knowing how they work. so i like the debugging part (when it gets solved :-D) because always has something to teach me, more than just copying an existent design. plus, the sactisfaction of recycling something, and thus do not contribute to the consumerism so pleasing to the capitalist system, is incomparable

[beatnik]

anyone?

[Govmnt_Lacky]

anyone?

Follow the instructions in this link and repost.

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

It will help the forumites understand what is going on with your circuit.


Good Luck 

[beatnik]

I have read that thread of course, and the only thing i'm missing is all the voltages.

i have started doing the measurements, but it's very strange that:

if i measure the voltage between the negative pin of the DC jack, and the various points on the circuits, all the voltages are around 9V.

if i measure between the circuit ground and the various points on the circuit, all the readings are 0V.

i can post all the detailed voltages if you want, but i don't understand how useful they can be.

please let me know something about this grounds issue because it's driving me crazy.

the pedal works, bypass and led work, potentiometers work, but the sound has distortion and no compression effect.

thanks in advance

[Govmnt_Lacky]

if i measure the voltage between the negative pin of the DC jack, and the various points on the circuits, all the voltages are around 9V.
if i measure between the circuit ground and the various points on the circuit, all the readings are 0V.

You are correct.... this is strange. 
IF.... the circuit is a negative ground then, the circuit ground SHOULD BE connected to the DC jack negative pin.

Looks like a good place to look! 

Good Luck 

[beatnik]

the circuit ground SHOULD BE connected to the DC jack negative pin.

That would be normal, and I always seen like this in all schematics or pedal wich I've built.

But in the schematic of this CS-3 the negative pin of the DC jack is connected to D10 and R32, and then to the input jack.

I really can't understand this arrangement.

I've pulled out D10 and it looks ok.

What else can be?

[Govmnt_Lacky]

But in the schematic of this CS-3.....

Sorry, but I cannot view the schematic link that you posted above from my computer. Might be my firewall.  

Can you repost the schematic with a different link?

EDIT: Found this with the "SEARCH" function. Might be helpful. Has a schematic + pictures of a working CS-3.

http://www.diystompboxes.com/smfforum/index.php?topic=63909.0

EDIT2: I found the original Boss CS-3 service notes online (you can too if you look) and it appears that D10 and R32 ARE IN FACT connected between the Input sleeve and the DC jack negative post on the NEWER models.

[ayayay!]

This may sound like a really weird question, but here goes:  Look around your PCB.  Are there ANY diodes on there that should not be?  Like somewhere that has a resistor and diode in series, where there should only be a resistor?  About 3 years ago, I ran across two CS-3's like that, but they were in different spots.  They seemed to come from the factory that way.  I know it sounds nuts, just check. 

[beatnik]

EDIT2: I found the original Boss CS-3 service notes online (you can too if you look) and it appears that D10 and R32 ARE IN FACT connected between the Input sleeve and the DC jack negative post on the NEWER models.

I've re-uploaded the schematic + layout of my CS-3 version.

EDIT: Found this with the "SEARCH" function. Might be helpful. Has a schematic + pictures of a working CS-3.

Looking at that picture (it's a newer unit, with THAT vca) D10 and R32 are jumpered.

Maybe I have to try to jumper them in my unit too?

I know it sounds nuts, just check. 

I checked the pcb against the original schematic and there are no differences, all the components seem to be in the right place.

But I still don't understand if this D10 + R32 is correct or is a manufacturer error.

[twabelljr]

But I still don't understand if this D10 + R32 is correct or is a manufacturer error.

I don't understand them either. They are jumpered in my CS-3 which is 5 or 6 years old. A jumper would ground the input sleeve to the adapter jack and circuit with no resistance. Unless they are there to protect your guitar from reverse polarity??

[Govmnt_Lacky]

But I still don't understand if this D10 + R32 is correct or is a manufacturer error.

I don't understand them either. They are jumpered in my CS-3 which is 5 or 6 years old. A jumper would ground the input sleeve to the adapter jack and circuit with no resistance. Unless they are there to protect your guitar from reverse polarity??

Why couldn't you just remove D10 and R32 completely and just Jumper from Pad 13 to Pad 17 on the board?

That would solve that issue