Author Topic: Chunk chunk q3 problem  (Read 1097 times)

zilla

  • Stompbox Star
  • ***
  • Posts: 134
    • View Profile
Chunk chunk q3 problem
« on: February 01, 2015, 12:14:23 AM »
Hey all. I'm building a chunk chunk for a friend and I am having a strange issue.

I cannot get q3 to bias properly. It always strays around 8.95-9v no matter what. Q1, 2, and 4 bias fine.

I've tried replacing the jfet. I've tried a new trimmer. I've replaced c11 and c12. I've re flowed the solder joints of components in the area. I've checked for solder bridges but I can't seem to find the smoking gun.

Does anyone have any suggestions got what to try?

Thanks

Tim

TGP39

  • Electron Doctor
  • *****
  • Posts: 783
  • Steven
    • View Profile
Re: Chunk chunk q3 problem
« Reply #1 on: February 01, 2015, 01:02:51 AM »
Double check R9, R10, R11 are the correct values. Make sure T3 is a 100k trimmer.
Let us know what you find.

Steve.
Follow me on Instagram under PharmerFx.

zilla

  • Stompbox Star
  • ***
  • Posts: 134
    • View Profile
Re: Chunk chunk q3 problem
« Reply #2 on: February 01, 2015, 12:22:58 PM »
T3 is a 100k. I've tried a few different ones. I'll try checking those resistors also. Thanks.

zilla

  • Stompbox Star
  • ***
  • Posts: 134
    • View Profile
Re: Chunk chunk q3 problem
« Reply #3 on: February 01, 2015, 02:06:34 PM »
The colour codes on the resistors seem ok. I'll try replacing them and see what happens.

TGP39

  • Electron Doctor
  • *****
  • Posts: 783
  • Steven
    • View Profile
Re: Chunk chunk q3 problem
« Reply #4 on: February 01, 2015, 10:58:31 PM »
Ok. Let's dig deeper.....
1.  What are your voltages on the drain, gate, source of your Q3 JFET?
2.  Set T3 at noon and take a voltage reading on pin 3 of that trimmer?
3.  It's actually a good idea to list the voltage readings from all your transistors....

When you post your readings, I'll pop my chunk chunk open and compare.

Steve.
Follow me on Instagram under PharmerFx.

zilla

  • Stompbox Star
  • ***
  • Posts: 134
    • View Profile
Re: Chunk chunk q3 problem
« Reply #5 on: February 08, 2015, 01:34:40 AM »
Q3
D 8.95V
S 1.385V
G 0V

T3 trimmer
pin 1 9V
pin 2 9V
pin 3 8.95V


Same j201 on Q1:

3.26 V
.26 V
0 V

t1 trimmer
pin 1 8.98 V
pin 2 8.98 V
pin 3 3.26 V



there's obviously something wrong since pin 3 on T3 does not change. this is my 3rd trimmer with the same results. i'm at a loss for what the problem is.. it's like something is shorting across the circuit board....

zilla

  • Stompbox Star
  • ***
  • Posts: 134
    • View Profile
Re: Chunk chunk q3 problem
« Reply #6 on: February 08, 2015, 01:57:13 AM »
being the stubborn person that i am, i ended up taking all the pots off the board and checking everything.

Turns out the pin on the socket for the jfet drain had sheared off right at the board (no idea how this happened!). I replaced the socket and voila! it biases! Thanks for your time TGP39.

TGP39

  • Electron Doctor
  • *****
  • Posts: 783
  • Steven
    • View Profile
Re: Chunk chunk q3 problem
« Reply #7 on: February 08, 2015, 02:19:35 AM »
Hey Zilla.....so glad you got it sorted out.  I don't think I would ever have thought of that solution. Cheers

Steve.
Follow me on Instagram under PharmerFx.