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DIY Stompboxes => Building your own stompbox => Topic started by: mlhuff on January 29, 2019, 04:19:07 PM

Title: help troubleshooting
Post by: mlhuff on January 29, 2019, 04:19:07 PM
I recently purchased and built a tycho brahe clone from general guitar gadgets. I get sound when the pedal is off. But when i turn it on, nothing. I am new to this and would like some help. I have looked up articles and videos but I havent found anything to help me trace the signal through the pcb and what voltages I should be getting.
Any help with this would be appreciated

Title: Re: help troubleshooting
Post by: GibsonGM on January 29, 2019, 04:53:30 PM
Hi ML, welcome!

Go here, read this, and see if you can provide the info requested back here in your post.  That will get us started   :icon_mrgreen:
These things are usually a builder error, poor soldering, the wrong part put in the wrong place and so on.  Taking it thru the process will help figure out what's going on!

https://www.diystompboxes.com/smfforum/index.php?topic=29816.0 (https://www.diystompboxes.com/smfforum/index.php?topic=29816.0)
Title: Re: help troubleshooting
Post by: thermionix on January 29, 2019, 05:14:08 PM
Just noticed something odd, won't help OP though.  The GGG schematic (http://www.generalguitargadgets.com/pdf/ggg_toct_sc.pdf) has Q1 backwards (it works either way), but it is correct on the actual PCB (http://www.generalguitargadgets.com/wp-content/uploads/1m_toct_pcb.jpg) they sell.
Title: Re: help troubleshooting
Post by: GibsonGM on January 29, 2019, 05:19:45 PM
Good find. 

Might as well toss this in here too, how to make an audio probe to trace the circuit, since OP asked about it.
https://www.diystompboxes.com/pedals/debug.html (https://www.diystompboxes.com/pedals/debug.html)
Title: Re: help troubleshooting
Post by: PRR on January 29, 2019, 07:53:53 PM
...The GGG schematic (http://www.generalguitargadgets.com/pdf/ggg_toct_sc.pdf) has Q1 backwards...

The schematic is correct as drawn. The as-drawn power supply is Negative hot, POSitive ground. The gizmo is a feedback pair, complementary NPN-PNP, with a Darlington in the PNP position. Q1 should be a NPN and should be in normal mode (not inverted).

To show the action I have re-drawn without the AC parts, without the Darlington frill, so reduced resistors around Q1. It biases-up fine. Q1 forces Q2 Collector to follow Q1 Base, nearly. All polarities are correct.

Making the 2nd stage a Darlinton, making a 3-stage amplifier, allows making Q1 a higher impedance, more suited for guitar.

If you are saying Q1 should be "inverted" (C and E swapped), that would negate much of the advantage of a 3-stage amplifier, lower the input impedance. It may not be significant, depending what drives it.
(https://i.postimg.cc/QBKj814Q/GGG-TO.gif) (https://postimg.cc/QBKj814Q)
Title: Re: help troubleshooting
Post by: thermionix on January 29, 2019, 11:28:35 PM
The as-drawn power supply is Negative hot, POSitive ground.

I'm aware, but the actual Tycobrahe Octavias had Q1 (NPN) installed the "wrong" way with emitter toward (+) ground.  The circuit runs either way.  This schematic matches the 70s originals, and the GGG PCB:

(https://i.postimg.cc/q6f39htw/Tycobrahe-Octavia.gif) (https://postimg.cc/q6f39htw)
Title: Re: help troubleshooting
Post by: italianguy63 on January 30, 2019, 05:31:42 AM
The as-drawn power supply is Negative hot, POSitive ground.

I'm aware, but the actual Tycobrahe Octavias had Q1 (NPN) installed the "wrong" way with emitter toward (+) ground.  The circuit runs either way.  This schematic matches the 70s originals, and the GGG PCB:

(https://i.postimg.cc/q6f39htw/Tycobrahe-Octavia.gif) (https://postimg.cc/q6f39htw)

+1

We had a thread on Q1 orientation sometime back..

Yes, the orientation works either way  (verified by me).. for what that is worth!

MC
Title: Re: help troubleshooting
Post by: mlhuff on January 30, 2019, 06:09:06 PM
Thank you for the suggestions. I am reading up on what I need and building an audio probe. Trying to find an electronics store near me. Once I have this info I will post results