Author Topic: High hFE in regards to Mangler & biasing issues  (Read 2478 times)

Zhirin

  • Diode Destroyer
  • *
  • Posts: 5
    • View Profile
High hFE in regards to Mangler & biasing issues
« on: May 22, 2013, 02:27:32 PM »
Good evening everyone! So, I've decided to be ultra-cool and build some pedals, of course without any knowledge on electronics. "Oh, it'll be easy" I thought.

Anyway, I've built two boards, both not working properly. I shall ignore the first one and concentrate my question on the second one, the Mangler. I am building a silicon NPN-based Mangler. I have followed the schematic/layout religiously, and have swapped the diode and electrolytic capacitors round to suit the NPN transistors.

The problem lies in the biasing. According to the schematics, I should be biasing the transistors to -4.5v (or something like that). However, I'm getting no lower than ~7.41, depending on what transistors I have in (I have 5 BC108s and 5 BC109s, and have tried various combos). Is this voltage reading due to the high hFE of the silicon transistors? And if so, how do I go about lowering said reading?

I haven't tested to see if it sounds ok yet, as I wanted to take this board slowly after totally ruining the other one (which, however, may end up working if I can nail this transistor problem!).

Thank you so much for your help in advance.

midwayfair

  • Electron Doctor
  • *****
  • Posts: 7091
  • Jon P
    • View Profile
    • Midway Fair
Re: High hFE in regards to Mangler & biasing issues
« Reply #1 on: May 22, 2013, 02:30:08 PM »
What's your voltage on Q1?

You should be able to get down to like 2v regardless of the gain. There might be something wrong elsewhere. Also, double check your pinout. A metal can will "look" backwards from a TO-92 soldermask icon.
Myself's music & things I make: http://jonpattonmusic.com. My band: http://midwayfair.org. PCBs of my designs from: http://www.1776Effects.com (Bearhug Comp & Cardinal Harmonic Trem); http://www.jmkpcbs.com (Hamlet+ delay & Blue Warbler envelope vibe); Snow Day OD/Flabulanche: www.madbeanpedals.com

Zhirin

  • Diode Destroyer
  • *
  • Posts: 5
    • View Profile
Re: High hFE in regards to Mangler & biasing issues
« Reply #2 on: May 22, 2013, 02:53:11 PM »
According to the interwebz, the pinout is correct - with the emitter being closest to the little notch (in Q1, this is going to ground, I believe). So the transistor casing matches the board print.

Ahem, how do I check the voltage of the transistors? The internet talks of resistance measurements, etc. but I'm not sure if these are the same thing.

Thank you!

EDIT: as I'm a total noob, I should ask this - would wiring the input and output jacks and/or switch lower the voltage in any way?

Zhirin

  • Diode Destroyer
  • *
  • Posts: 5
    • View Profile
Re: High hFE in regards to Mangler & biasing issues
« Reply #3 on: May 23, 2013, 04:42:33 PM »
So, I have done a bit more research, and I hope these readings are correct. I basically did a diode test on the transistor at Q1.

base/emitter .713
base/collector .709

Is that the correct way to measure voltage? Wow, I have got myself into a whole world of pain, yet I have learnt so much in just a week!

midwayfair

  • Electron Doctor
  • *****
  • Posts: 7091
  • Jon P
    • View Profile
    • Midway Fair
Re: High hFE in regards to Mangler & biasing issues
« Reply #4 on: May 23, 2013, 06:28:32 PM »
So, I have done a bit more research, and I hope these readings are correct. I basically did a diode test on the transistor at Q1.

base/emitter .713
base/collector .709

Is that the correct way to measure voltage? Wow, I have got myself into a whole world of pain, yet I have learnt so much in just a week!

Voltage is measured to ground. The tech help rules thread has good instructions.
Myself's music & things I make: http://jonpattonmusic.com. My band: http://midwayfair.org. PCBs of my designs from: http://www.1776Effects.com (Bearhug Comp & Cardinal Harmonic Trem); http://www.jmkpcbs.com (Hamlet+ delay & Blue Warbler envelope vibe); Snow Day OD/Flabulanche: www.madbeanpedals.com

Zhirin

  • Diode Destroyer
  • *
  • Posts: 5
    • View Profile
Re: High hFE in regards to Mangler & biasing issues
« Reply #5 on: May 24, 2013, 06:28:35 AM »
Thanks! So, here's my readings (did some other tests too). I'm really not sure what I'm looking for, so hope these help. I've tried to be as detailed as possible:

Q1

emitter - Starts ~-0.60, then slowly drops off to 0
base - Starts ~0.60, then slowly drops off to 0
collector - Fluctuates between 6.00 and 7.00 then settles at 6.20

Q2

emitter - Starts at ~-1.00 then slowly drops off to 0
base - Starts ~0.60, then slowly drops off to 0
collector -Starts ~7 then settles at 4.88

9v in at board - ~5.25
Ground in at board - fluctuates then settles at -0.02

Attached are two photos, one of each side of the board. I've disconnected the pots.

midwayfair

  • Electron Doctor
  • *****
  • Posts: 7091
  • Jon P
    • View Profile
    • Midway Fair
Re: High hFE in regards to Mangler & biasing issues
« Reply #6 on: May 24, 2013, 06:38:10 AM »
You have to connect the fuzz pot for one thing ;)

Take a look at the schematic. The fuzz pot is a voltage divider between the emitters, the 22uF cap to ground (that's your AC gain), and ground (creating a constant 1K resistance between the emitters and ground, basically biasing the transistors consistantly regardless of where the fuzz pot is set). If your transistors don't have a connection to ground, how are they going to bias?

Also, your battery appears to be reading low. Not sure if it's just your multimeter acting funny.
Myself's music & things I make: http://jonpattonmusic.com. My band: http://midwayfair.org. PCBs of my designs from: http://www.1776Effects.com (Bearhug Comp & Cardinal Harmonic Trem); http://www.jmkpcbs.com (Hamlet+ delay & Blue Warbler envelope vibe); Snow Day OD/Flabulanche: www.madbeanpedals.com

Thomas_H

  • Electron Doctor
  • *****
  • Posts: 1420
    • View Profile
    • DIY-TH custom effects
Re: High hFE in regards to Mangler & biasing issues
« Reply #7 on: May 26, 2013, 01:36:47 PM »
Also in picture 2 it looks like your transistor sockets are not soldered to the board. This also may cause additional instable voltage readings.
DIY-PCBs and projects:

Zhirin

  • Diode Destroyer
  • *
  • Posts: 5
    • View Profile
Re: High hFE in regards to Mangler & biasing issues
« Reply #8 on: May 29, 2013, 01:23:58 PM »
Apologies about the delay, had a long weekend or recording (which I really would've liked to use this on!)

I've now permanently connected the fuzz and volume pots, and tested the DMM directly on a set of batteries. They seem to read fine (between 8.5 and 9.4).

I've also soldered in the transistor sockets to try to stabilise the readings, and also the input but only to the battery snap and GND) Still no joy.

I want to ask a question in case I've been really stupid - to set the bias, I place a probe in the test hole and the other against the GND hole?

EDIT: The readings I took the other day on the Transistors, are those ok?
« Last Edit: May 29, 2013, 01:33:11 PM by Zhirin »