Tonebender MKI: Trouble with Transistors

dudecoolhat · Post by **dudecoolhat** » 08 Nov 2022, 03:30

I was wondering if anyone could help me diagnose my issue with the Tonebender MKI that I'm building. I am able to get sound through the bypass with the pedal off but there is no sound the the pedal on. I made an audio probe and the sound cuts out at the emitter to Q1. I am using 2N404 Germanium transistors and have tried a couple to no avail. Any insight would be much appreciated, this is only my second build and my first on vero.

Link to schematic:
http://tagboardeffects.blogspot.com/201 ... r-mki.html

Photos of build.:

mauman · Post by **mauman** » 08 Nov 2022, 04:33

Welcome! Vero builds can be a little tricker than PCBs. I'm posting a schematic that I think matches your build, if not let us know.

: SolaSound TB MKI

For starters, make sure there are no solder bridges between any rows on the solder side. Easiest way is to run a sharp knife from end to end down the space between every two copper traces. Or you can do the same with a hot soldering iron.

Good work with the audio probe. If Q1 isn't putting out anything on the emitter, check the power. Remove Q1 and verify you have -9V (not +9V) on the collector socket. Then remove power and measure for 1 meg of resistance from base to ground, and 8.2k resistance from emitter to ground. If all those are good, try another germanium PNP transistor in the Q1 socket, ensuring it's oriented C-B-E from top to bottom. Then let us know.

dudecoolhat · Post by **dudecoolhat** » 08 Nov 2022, 05:07

Thanks for the reply! That looks like the schematic I'm building from, I also went through and made sure there wasn't any continuity between rows. I am getting -9V at the collector socket

and 1M from base to ground with the power disconnected

. I have tried a couple other transistors (also 2N404) to no avail. Are these the wrong type of transistors to be using?

mauman · Post by **mauman** » 08 Nov 2022, 14:45

You have a good signal from input to Q1 base but not on emitter, right?

Do you have 8.2k from emitter socket to ground with Q1 removed and power off?

One more thing to check on the solder side, make sure your cuts are fully cut, a sliver of copper can maintain continuity across a hole. You shouldn't have a dead short across any of the cut rows.

2N404 should work fine, their specs are close to the original Mullard OC-75. The emitter is the closest leg to the tab.
Are your transistors from a reliable source?

: 2N404 pinout.JPG (18.75 KiB) Viewed 328 times

dudecoolhat · Post by **dudecoolhat** » 08 Nov 2022, 17:25

I'm currently getting 8.06k from emitter pin to ground. I've checked and double checked for continuity across my cuts but haven't found any. Fair to say I am stumped.

The transistors are just what I found on Amazon:
https://www.amazon.com/dp/B011OACJCQ?ps ... ct_details

dudecoolhat · Post by **dudecoolhat** » 08 Nov 2022, 22:22

I just checked the voltages at each transistor pin, from what I can tell, Q1 seems to be functioning properly so I am not sure why my signal goes dead at Q1e.
Just check the voltages, see below:

Q1c: -9.13V
Q1b: -0.45V
Q1e: -3.20V

Q2c: -7.74V
Q2b: -0.16V
Q2e: 0V

Q3c: 0V
Q3b: 0V
Q3e: 0V

mozz · Post by **mozz** » 09 Nov 2022, 00:37

2N404 may be too good of a transistor. You need some leakage from c-b.

dudecoolhat · Post by **dudecoolhat** » 09 Nov 2022, 15:48

Update: I got it working! Thank you for helping out a newbie I would've probably given up on this if it wasn't for the help I received online.