Author Topic: Big Muff Pi Rams Head transistor voltage problem  (Read 266 times)

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jjg

Big Muff Pi Rams Head transistor voltage problem
« on: September 12, 2017, 04:51:22 PM »
Hi, I'm building a BMP Rams Head from the vero layout at http://tagboardeffects.blogspot.co.uk/2012/06/ehx-73-rams-head-big-muff.html.
I'm left with the transistor Q1 (in that layout) with voltages C=8.52, B=1.40, E=1.08.
It should be more like C=4ish, B=0.6ish, E=0.05ish, I believe. The voltages of the other three transistors are spot on.
I've checked and double-checked resistor values and cannot see a problem.
Any advice about what COULD be causing this problem, from the layout above, would be very much appreciated!

GibsonGM

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #1 on: September 12, 2017, 05:38:51 PM »
Too bad they don't give the schematic too, as that is where all the real info is!  That aside...if your voltages there are off, it really has to be due to a wrong part value (mixup), a short somewhere in the general area...perhaps a track not cut that needs to be cut, or a 'solder bridge' between tracks.

These kinds of problems are mostly our own fault, so we have to go back and look, look, and look more, again and again until we find where we went wrong!   Look around the 470k, 100k resistors right there...a trouble in that area, I suspect.  They are what sets your bias, along with the 12k.


Welcome to the forum, JJG   :) 
« Last Edit: September 12, 2017, 05:46:08 PM by GibsonGM »
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

Tony Forestiere

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #2 on: September 12, 2017, 05:59:46 PM »
Duck would ask for clear pictures of the top, bottom, and off-board wiring of the PCB/Vero.
Welcome.
Duct tape is like the Force. It has a light side and a dark side, and it holds the universe together. Carl Zwanzig
Whoso neglects learning in his youth, loses the past and is dead for the future. Euripides

jjg

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #3 on: September 12, 2017, 06:53:52 PM »
Too bad they don't give the schematic too, as that is where all the real info is!  That aside...if your voltages there are off, it really has to be due to a wrong part value (mixup), a short somewhere in the general area...perhaps a track not cut that needs to be cut, or a 'solder bridge' between tracks.

These kinds of problems are mostly our own fault, so we have to go back and look, look, and look more, again and again until we find where we went wrong!   Look around the 470k, 100k resistors right there...a trouble in that area, I suspect.  They are what sets your bias, along with the 12k.


Welcome to the forum, JJG   :)
Thanks so much for the pointers - in checking the soldering with a magnifying glass, I found that the emitter pin on the socket I used for Q1 was not properly soldered in, so it was not actually connected to the rail even though a blob of solder was on the pin! So subtle I couldn't tell without magnification.

Fixed and tested - the circuit sounds great now! Thank you again for taking the time to write your advice.

I'm making the pedal for my dad (old rocker), for his 70th birthday next week, and I was starting to lose hope!


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GibsonGM

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #4 on: September 13, 2017, 07:15:03 AM »
Glad it worked out, JJG! It's very often just something we did (or didn't do...), you get used to it eventually, ha ha...happy b day to your dad, I hope he enjoys the BMP!!!
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

duck_arse

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #5 on: September 13, 2017, 10:50:23 AM »
Duck would ask for clear pictures of the top, bottom, and off-board wiring of the PCB/Vero.
Welcome.

he would. and if the circuit was fixxed before he got here, he'd then ask for the transistor voltages on the now working board, just because. and he'd still say post the pics, just so's we can have a geek.

and welcome.
"I think the slipproof bottom is the Victoria’s Secret of pedal underwear" - unsolicited testimonial from PH
"Hello Steven, it's Clem Fandango - can you hear me?" - Clem Fandango
"We jam with room full onion, but happy" - 287m

jjg

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #6 on: September 13, 2017, 02:40:09 PM »
Ha! I'll do those things when I get a moment, cheers. J


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jjg

Re: Big Muff Pi Rams Head transistor voltage problem
« Reply #7 on: September 13, 2017, 02:41:37 PM »
Duck would ask for clear pictures of the top, bottom, and off-board wiring of the PCB/Vero.
Welcome.

he would. and if the circuit was fixxed before he got here, he'd then ask for the transistor voltages on the now working board, just because. and he'd still say post the pics, just so's we can have a geek.

and welcome.
*yikes* they want to see my soldering...


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