EHX Small Stone Version 2/Issue J Troubleshooting

[Major Sparky]

Hey, I just bought an original Version 2 small stone.  I bought it in non-working condition hoping to be able to fix it.  The issue with the pedal is that it has no phasing effect when activated.  When the pedal is engaged the volume drops and the only change in sound is a slight motorboating effect that gets louder and faster as you turn the rate knob up. You can hear the motorboating even when the pedal is bypassed when the rate knob is all the way up, but it goes away when the knob is turned down.

I've opened it up and examined the traces and solder joints. Everything looks pretty decent so far, I can't see any shorts and the solder joints all look good.

Hoping someone can give me an idea of where to start.  Any help is greatly appreciated!

Here's a link to the schematic for this pedal, http://www.generalguitargadgets.com/pdf/ggg_smallstj_sc.pdf No modifications seem to have been made to the pedal.

[Scruffie]

Sounds like a possible LFO issue but voltages would be useful to tell for sure.

But my instinct from working on lots of Small Stones is the 33uF cap in the LFO has gone bad, C12 on that schematic.

[Major Sparky]

I've replaced the electrolytics and the problem still persists.  I took voltages.

Battery    8.6v

IC1   
1   4.3
2   3.2
3   3.2
4   0
5   0.62
6   3.3
7   7.6
8   7.6

IC2   
1   4.3
2   3.15
3   3.15
4   0
5   0.62
6   3.1
7   7.4
8   7.4

IC3   
1   4.2
2   3.1
3   3.1
4   0
5   0.63
6   3.1
7   7.2
8   7.2

IC4   
1   5.9
2   3.05
3   3.02
4   0
5   0.63
6   4.67
7   7.1
8   7.1

IC5
1   1.71 - 1.89
2   3.1 - 5.5
3   3.1 - 5.6
4   0
5   0.64
6   .62 - .69
7   6.7 - 6.9
8   2 - 5.5

Q1   
e   1.6
b   0.334
c   .5
   
Q2   
e   4.14
b   6.01
c   6.6
   
Q3   
e   1.9
b   1.27
c   1.6
   
Q4   
e   2.12
b   2.15
c   1.6

IC5 was the only IC that had oscillating voltages.  I couldn't quite tell if the voltages on the transistors were oscillating or not.  They were definitely moving, but I couldn't really tell what the ranges were, they seemed to be moving around kind of erratically, but slowly.  They also don't seem to be the 2N5087 and 2N5088 transistors. Q2-4 have the markings BCC 309 on the back, and Q1 has the marking BCC 239. All voltages were taken with the rate knob turned all the way up, and the color switch in the up position.

[ElectricDruid]

ICs 1 to 4 all have basically the same voltages, so that's a good start. It looks to me like the phase shift stages are probably ok, so I'd also suspect the LFO.

Are the ICs socketed or soldered in?

[Scruffie]

ICs 1 to 4 all have basically the same voltages

I thought the same at first but look again at IC4 pin 1 & 6.

I would audio probe IC3 pin 6 where if the LFO was working you'd hear vibrato and then IC4 pin 6.

[Major Sparky]

The IC's are soldered in.  I get vibrato coming out of pin 6 on IC3 but nothing out of pin 6 on IC4.

[ElectricDruid]

Pity. I was going to suggest swapping the chips around.

If you've got vibrato up to stage 3, but nothing after stage 4, that means the LFO is working (otherwise there'd be no vibrato), and it's looking a lot like stage 4 is dead for some reason. A dead chip is possible (and would be easy to test if you could swap the chips around - does the fault move, or does it always affect the chip in position 4?) . But it's also possible there's a faulty component around that chip somewhere.

[Major Sparky]

Does it make sense that the transistor voltages moved around so much? It seemed like every time I probed them they read a different voltage.  I'm also not sure I got the pinouts right for them.

[Scruffie]

Regarding the transistors, EHX regularly switched between the 2N5088/5087 & BC309/239 so no concern there.

The two in the LFO (Q3 & 4 on the schematic you posted) I believe form a schmitt trigger for the LFO so yeah moving voltages there would be expected. The two that form the preamp shouldn't move about.

I notice on the dead phase stage that the only voltage issue appears to be the output and transistor pair on the CA3094 (EH1048) so it's worth continuity checking the parts around it before tracking down a CA3094.

[Major Sparky]

So I've rewired the pedal, cleaned up the pads on the PCB and reflowed all the solder. I've also checked the value of most of the resistors around IC4 and they all check out.

Right now when I use an audio probe on pin 6 of IC4 I can hear a faint vibrato.  And when the pedal is plugged into the amp normally I can very faintly hear the effect.  But I'm still getting the same clicking sound I was getting before, which gets louder and more rapid when I turn up the rate knob.

So is this a definite sign that the IC is fried, or could it still be the transistors or capacitors around it?

[Major Sparky]

I replaced IC4 and now the pedal works. Sounds fantastic really.

One last question though. Is it normal to hear the effect faintly when it's bypassed? I don't have it wired for true bypass, it still has the original foot switch. When it's in bypass I can hear the effect very faintly when I'm not playing anything.

[Scruffie]

Congrats! And yes that is 100% normal, I have 3 vintage Small Stones from different era's and they all exhibit that behaviour.

[ElectricDruid]

Excellent work! It's nice when you work through the process and it all comes out like it's supposed to, isn't it?!