AION Cygnus - Pete Cornish G-2 troubleshooting

[xtian]

I built a Pete Cornish G-2 clone <https://aionelectronics.com/project/cygnus-distortion-sustainer-cornish-g-2/>.

It's all working, except for the Germanium diodes. See schematic clip. I'm using sockets for D2-5, the four Ge diodes:



If I pull out the four diodes, the pedal works fine, though obviously I'm not getting the diode clipping. If I install D2 and D4, there is no change to the waveform, and signal still passes. However, if I install either D3 or D5 (the reverse biased ones, marked in red on the schematic) the signal gets clamped to near zero. Why?

My Ge diodes all read between 0.34 and 0.4v drop. I've tried all of them in the D3 socket, but they all clap the signal to zero.

Any idea what I'm doing wrong?

[The Dude]

What are the collector voltages on Q4 & Q5 with diodes in vs diodes out?

[xtian]

diodes: none D2/4 all
Q4c 5.08 4.79 4.68
Q5c 3.96 3.79 3.94

not very good table formatting...hope you can read it.

I should also note that signal is a clean sine wave at the left side of R21, but distored on right side of R21.

[xtian]

Still scratching my head. I removed components to simplify, but am still reading 100K resistance between the points marked:

[nickb]

You may not be doing anything wrong! With all or one direction set of diodes of diode removed you just have a high gain amplifier. The maximum output signal will be close the battery voltage, about 9Vpp. When all the diodes are in, the max output is about 0.6Vpp as it's limited by the final set of diodes.

[xtian]

Unfortunately, with D2-5 removed, my nice sine wave becomes very badly distorted at Q4. Listening with guitar thru an amp, the sound is very gated and splatty.

[nickb]

With D2 & D5 removed, feed a 100mVpp 1Khz sine into the input. Put scope on DC coupling with 2V/div, Post a photo of the scope display when monitoring Q4 collector.

Reduce input to 5mVpp and repeat for Q4 and Q5 collector.

Put diodes back, leave input at 5mVpp and repeat the Q4 and Q5 scope shots.

[alexradium]

try with normal 1N4148 diodes and see what you get

[Helmholtz]

diodes: none D2/4 all
Q4c 5.08 4.79 4.68
Q5c 3.96 3.79 3.94

not very good table formatting...hope you can read it.

I should also note that signal is a clean sine wave at the left side of R21, but distored on right side of R21.

If C13/16 are wired according to the schematic, DC collector voltages must not change when connecting the diodes. Even shorting the diodes should not change DC voltages. Please check wiring.

I suspect a DC path between the left side of the diodes and the bases.

[xtian]

Thanks, everyone, for the replies!

@nickb, the sine wave looks perfect at Q4/5 when no diodes D2-5 are placed, but as soon as I add a pair of my Ge diodes, the signal collapses to zero. BUT...

@alexradium, EXCELLENT suggestion! I put in a quad of 1N4148s, and the pedal works and sounds good with guitar!

I don't understand what happened to me earlier, when I wrote, "Unfortunately, with D2-5 removed, my nice sine wave becomes very badly distorted at Q4. Listening with guitar thru an amp, the sound is very gated and splatty." Mystery bullshit made me chase red herrings.

These are the Ge diodes I bought: <https://www.cedist.com/products/diode-1n34a-germanium-sold-individually> Wonder why they aren't working for this application?

[nickb]

Thanks, everyone, for the replies!

@nickb, the sine wave looks perfect at Q4/5 when no diodes D2-5 are placed, but as soon as I add a pair of my Ge diodes, the signal collapses to zero. BUT...

@alexradium, EXCELLENT suggestion! I put in a quad of 1N4148s, and the pedal works and sounds good with guitar!

I don't understand what happened to me earlier, when I wrote, "Unfortunately, with D2-5 removed, my nice sine wave becomes very badly distorted at Q4. Listening with guitar thru an amp, the sound is very gated and splatty." Mystery bullshit made me chase red herrings.

These are the Ge diodes I bought: <https://www.cedist.com/products/diode-1n34a-germanium-sold-individually> Wonder why they aren't working for this application?

In order to diagnose your issue you need to post the scope shots as requested. You are just repeating the description that doesn't tell us what we need to know. The Ge diodes should work just as well as Si with minor only differences in signal level.

[Enzo]

And just to follow up, yes there is still 100k between the two points. You are measuring the path down through 100k R22 and back up through 680 ohm R25.