EHX Small Stone (RUS) debug - help please

[zedsnotdead]

Hello!

A friend of mine just asked me for help on his small stone. It isn't working as it should of course.... the problem is similar to http://www.diystompboxes.com/smfforum/index.php?topic=86067.0.
What happens is that there is no phasing effect.
Here is the filled "debugging form":

1.What does it do, not do, and sound like?
- Sound goes through with the effect on, but no phasing effect at all.
- The "colour" switch is working. It makes a noticeable difference in sound (darker).


2.Name of the circuit
- Electro Harmonix Small Stone Russian (can't understand ciliric, the version is stamped on PCB)

3.Source of the circuit (URL of schematic or project)
I think it's this one:


4.Any modifications to the circuit? Y or N
I don't know, but when I opened it up I noticed a 100nF cap connecting C+B of Q4, like the picture below:


Here are some other pictures I took (click to enlarge):




5.Any parts substitutions? If yes, list them.
I can't seem to find any... I don't think so.

6.Positive ground to negative ground conversion? Y or N
No.

7.Voltages

What is the out of circuit battery voltage? = 9,20V (PSU connected, not battery)
Voltage at the circuit board end of the black battery lead = 0,00V


Q1 = 2N5088
C = 8.42V
B = 3.36V
E = 2,97V

Q2 = 2N5087
C = 5.51V
B = 8.42V
E = 9.05V

Q3 = 2N5087
C = 6.93V
B = 8.42V
E = 9.05V

Q4 = 2N5087
C = 6.89V
B = 6.89V
E = 7.50V

IC1 = CA3094
P1 = 4.73V
P2 = 3.58V
P3 = 3,58V
P4 = 0,00V
P5 = 0,60V
P6 = 3,60V
P7 = 9.05V
P8 = 9.05V

IC2 = CA3094
P1 = 4.64V
P2 = 3,58V
P3 = 3,58V
P4 = 0,00V
P5 = 0,60V
P6 = 3.53V
P7 = 9.05V
P8 = 9.05V

IC3 = CA3094
P1 = 4.75V
P2 = 3.58V
P3 = 3,58V
P4 = 0,00V
P5 = 0,60V
P6 = 3.63V
P7 = 9.05V
P8 = 9.05V

IC4 = CA3094
P1 = 4.65V
P2 = 3.58V
P3 = 3,58V
P4 = 0,00V
P5 = 0,60V
P6 = 3.53V
P7 = 9.05V
P8 = 9.05V

IC5 = CA3094
P1 = 1.73V
P2 = 6.88V
P3 = 6.88V
P4 = 0,00V
P5 = 0,56V
P6 = 0,60V
P7 = 9.05V
P8 = 6.88V

Now for some notes I gathered:
1- no voltage is oscillating. None!
2- using audio probe I can hear the signal coming out of pin 6 of all IC's (except IC5, i think it's the LFO signal generator?)
3- checked for any shorts, everything is OK it seems....
4- checked all electrolytic caps, they read OK with the right values.
5- this pedal used to work and then just stopped working, according to the owner.


It's the first time I am asked to debuug a phaser, I am trying to understand how this one works, already read some articles on how OTA works and also RG Keens superb article (http://www.geofex.com/article_folders/phasers/phase.html). But I don't know if I understood correctly and need your help and advise.


Can you please help me?


Thank you very much!
Cheers!

[zedsnotdead]

No help.... 

[mojokorn]

I'd say all of your output stages have healthy voltages so the op amp chips appear to good.  No phasing effect would lead me to believe that the LFO is not working.  They don't show the LFO as a single IC, but I think it is (one of the 8 pin dip packages maybe) - a part search will sus that out.  A quick swap of the LFO would either fix it or rule it out.

"It's the first time I am asked to debug a phaser, I am trying to understand how this one works,"

Seems to be 4 cascaded active Sallen-Key type LPFs (3rd order, 9dB/octave each) with LFOs slowly moving the cutoff frequency in unison on all stages - a bit like a wah with 36 db/octave LPFs moving through cold syrup. 

[mojokorn]

Might be 2nd order filters.

[garcho]

Seems to be 4 cascaded active Sallen-Key type LPFs

those are all-pass filters, like all phasers

can't understand ciliric, the version is stamped on PCB

i read cyrillic, take a picture and post it

except IC5, i think it's the LFO signal generator?

judging by the schematic, that seems right to me, too.

I noticed a 100nF cap connecting C+B of Q4, like the picture below:

by Q4 (regarding the picture with the 100nF cap) do you mean one of the 2N5087s in the LFO section? because if the LFO isn't working, maybe someone had tried to fix that problem previously with that cap.

[mojokorn]

those are all-pass filters, like all phasers

The feedback loop has a cap (albeit very small), so wouldn't it be a LPF with a very high cut off frequency.  In the realm of guitar frequency spectrum that would look like an All Pass Filter.

[anotherjim]

The LFO is at bottom of schematic - another OTA chip.
Start assuming (hoping) nothing  is fried but something broken from wear & tear...
Color switch - continuity test contacts/wires to pcb.
Speed (?) pot 1Meg -  resistance test pot and wires to pcb.
You have 0v pin 4 on the LFO OTA - maybe that should vary a bit with the 1Meg pot and there should be something close to 1V at the far end of the pot.

[zedsnotdead]

by Q4 (regarding the picture with the 100nF cap) do you mean one of the 2N5087s in the LFO section? because if the LFO isn't working, maybe someone had tried to fix that problem previously with that cap.

Yes, Q3 and Q4 are 2N5087 on the LFO section. The 100nF is connecting B and C of Q4, like a feedback cap.
Don't know what's there for...

The LFO is at bottom of schematic - another OTA chip.
Start assuming (hoping) nothing  is fried but something broken from wear & tear...
Color switch - continuity test contacts/wires to pcb.
Speed (?) pot 1Meg -  resistance test pot and wires to pcb.
You have 0v pin 4 on the LFO OTA - maybe that should vary a bit with the 1Meg pot and there should be something close to 1V at the far end of the pot.

I will test all and report back!


Just bought a couple of 3094  from ebay, just in case... if the IC is not fried, I might as well buy some more and build another small stone from scratch myself...

Thanks a lot for your all your help guys!! This thing is really driving me nuts!!

[anotherjim]

Probably closer to 2 volt on the 1M pot at the junction of 27k+7.5k resistors - not 1.