SD-1 clone debug - Pot's don't affect signal

[deke99]

Hi all,

I recently built a SD-1 clone using the GGG tubescreamer layout and the following schematic (minus the switching circuitry):
http://www.hobby-hour.com/electronics/s/sd1-super-overdrive.php

I've build many pedals but this one is stumping me.  It passes signal, but the volume, gain, and tone pots don't change the sound.  The sound is very similar to the bypassed signal except when the gain and volume pots are maxed a pulsing sound is introduced.  Any insight would be greatly appreciated.  Here are my DC voltages:

IN  0V
OUT 0V
9V+  9.26
Q5:
C 9.26
B 3.62
E 3.18
IC:
1 4.57
2 4.57
3 4.52
4 0
5 4.56
6 4.57
7 4.56
8 9.26
Q6:
C 9.26
B 2.95
E 2.54

I have measured all of the pots and they are operational.  When power is supplied to the circuit, the tone pot resistance gets weird.  Without power I measure ~25K across the pot.  When power is supplied I measure -1.3K across.  Any insight from the gurus would be greatly appreciated.

Thanks!
Derek

[Pojo]

My guess would we there's an error for the wiring of the pots. An experience I had building an overdrive on a stripboard layout gave me similar results because I had all the pots wiring on one side of the board shifted by one row which obviously threw everything off.

I'd triple check that and go from there.

[deke99]

Thanks for the suggestion Pojo.  I just triple checked the pot wiring (and other off board connections) and everything checks out there.  I'm going to continue checking voltage swings and such.  I have confirmed that the resistance increases between pins 1 and 2 of the IC as the gain increases.  Resistance is 33K with gain at 0 and it goes up to 780K at max gain (low for the 1M pot, but that is just the tolerances of the pot).  With the volume pot at 0, lugs 1 and 2 are both at VR.  I'll keep digging.  Please let me know if you have any other suggestions or target areas of the circuit to check.

Thanks!!

[PRR]

> The sound is very similar to the bypassed signal

Maybe the bypass isn't unbypassing?

Lift one leg of R14.

Tack 1K to 10K across Q1 D and S pins.

Now it is stuck in "effect".

Does it do what it should do?

If not, signal tracing may be a good path.

> except when the gain and volume pots are maxed a pulsing sound

That's supersonic oscillation at MAX gain. Suggests IN and OUT wires are too near each other. (Maybe Drive and Level pot wires.) It feeds-back so violently that the asymmetric diodes charge-up C2, slam the bias way off center. Now it has no gain, no oscillation, it gradually recovers. C2 R4 suggest a low audio rate, "pulse". > The sound is very similar to the bypassed signal

Maybe the bypass isn't unbypassing?

Lift one leg of R14.

Tack 1K across Q1 D and S pins.

Now it is stuck in "effect".

Does it do what it should do?

If not, signal tracing may be a good path.

> except when the gain and volume pots are maxed a pulsing sound

That's supersonic oscillation at MAX gain. Suggests IN and OUT wires are too near each other. (Maybe Drive and Level pot wires.) It feeds-back so violently that the asymmetric diodes charge-up C2, slam the bias way off center. Now it has no gain, no oscillation, it gradually recovers. C2 R4 suggest a low audio rate, "pulse".

[Pojo]

> The sound is very similar to the bypassed signal

Maybe the bypass isn't unbypassing?

Based on the original post, I think the fet switching circuitry was left out.


Deke, I'm curious how your making an SD-1 using GGG's TS layout. Did you just purchase a board and are adapting the circuit to transform it into the SD-1? I know the circuits are similar, but maybe some of the differences between the 2 (like the low end filtering in the 1st opamp's NFB loop absent in the linked schem) are causing the issues. Or are you just sticking with the form of a TS and only swapping certain component values to make it sound like an SD-1?

[deke99]

Thanks PRR.  Pojo is right, I left out the bypass portion of the circuit.  That includes the track to C9, R13, R14 and everything below that.  I have confirmed that the input is not directly connected to the output.  It goes into the circuit and the output comes out of the circuit.  When I hand probe the circuit board it introduces static and I can make the pulsing stop by touching certain parts of the board.

Pojo, to answer the question about the layout, I slightly tweaked the GGG layout.  I apologize for the lack of professional tools to put this together, but here is a link:

http://misconductband.com/random/sd-1.pdf

Thanks!

[duck_arse]

trying to measure resistance with power applied will always result in nonsense readings. it would also wreck meters in the older, darker times. power off, then measure resistance.

[deke99]

Update on this:

I tied V1 to ground instead of VR and took the output off of the wiper (V2) and I have now have a pedal where the volume and tone controls work.  Gain is adding more bass than anything and gets motor boaty at max.  I'm still debugging that but getting closer.

[deke99]

Fully operational now.  The .018uf cap from pin 5 of IC to VRef is incorrect.  I connected it to ground instead of VRef and the pedal now works.  I got the idea from JD at GGG.  His ZW-44 (which is just a modded SD-1) on the ITS8 board info has that cap going to VRef on the schematic, but the layout shows it going to ground.  FINALLY, I have a pedal that is operational! 

[PRR]

> The .018uf cap from pin 5 of IC to VRef is incorrect.

The 0.018 does not go to Vref. (That would short-out all signal.)

......OIC. I *hate* when they do that.



Maybe on the original you could tell where the crossing-dots are. After it has been faxed and scanned and JPEGed and color-shrunk, you can't.

A line-gap would be clearer. I like the line-jump symbol, but it is hard to draw, and some stylists think it is ugly.

I also hate when they omit the leading-Zero on fractional values. Same reason: it goes-away on copies. (Here, not so bad, because the ".0" before the "18" alerts you; but 5 and .5 sure look the same a few copies down.)

[deke99]

Hi PRR, thanks for your insight.  So on my layout (if you look a few posts up at the attachment that I linked to), I switched the IC around.  On the original schematic that you included a snippet of the pin that I was referring to was pin 3.  That 0.018uf cap to VRef made the gain control not work.  When I re-routed that cap to ground instead of VRef, the pedal worked as expected.  Perhaps that was required because I also switched the volume pot lug 1 to ground instead of VRef?