Oscillating BSIAB II

[StefanMoring]

I've just finished my BSIAB II, but when I turn the trim pot CW, it starts oscillating, sounding like a motorboat. Funny to play with,
but it wasn't my intention to create a motorboat emulator. With the trim pot all the way CCW it isn't oscillating, the sound is slightly
overdriven, but very thin.

Here are the transistor voltages I measured:

   Q1      Q2    Q3       Q4       Q5
G   0.00   2.84   0.00   2.82   0.00
S   0.82   4.81   0.07   4.16   1.19
D   4.81   8.53   4.16   8.53   8.51

The bold numbers are the odd values.


Any ideas on what the problem might be? Could it just be a messed up transistor? I haven't tried
other transistors yet, don't have them on stock right now 

Any help would be appreciated, cause this effect rocks

Stefan

[petemoore]

  Q1      Q2    Q3       Q4       Q5
G   0.00   2.84   0.00   2.82   0.00
S   0.82   4.81   0.07   4.16   1.19
D   4.81   8.53   4.16   8.53   8.51
  I forget which Q number is which in the MuAmp, there's a top tranny [which sets bias for the bottom one] and a bottom tranny to each Mu Amp.
  Use of schematic and Mu Amp biasing info [GEO] allows Mu bias to be analyzed.
  The bottom tranny D and top tranny S of each mu amp should be 'mid-bias' [~1/2v].
  Mu Amps are self biasing. The gain transistor of the Mu needs it's gate voltage above the source voltage.
 
 

[dxm1]

Q3 and Q4 are the mU. Q3-Source is at ground, and Q4-Drain is Vcc. Q3-Drain and Q4-Source are 1/2Vcc, so it looks like the mU is working...
Q3-Gate should only be at ground when R19 is full CCW.

Q5-Gate at ground is a bad thing - have you double checked the pinout? Solder Bridge?

Were these measurements made with an input signal applied?

[StefanMoring]

I measured it with my guitar and amp plugged in, wasn't hitting the strings though.

[StefanMoring]

R19 and R20 were turned fully CW. I would think Q3-Gate is at ground when R19 is turned fully CW..
thanks for the quick replies btw

[slacker]

Q5 Drain is too high it should be about 4.5 - 5.00 volts, you should be able to adjust it into that range with R16. If you can't then there's a problem round there some where.

[StefanMoring]

i'll try to replace R16 then, but it'll take a while since I'm quite out of money

[dxm1]

Cool - you've got time then... Put the Drive, Tone, and Volume pots in their center positions. Take three more sets of measurements: R16 full CCW, R16 centered, and R16 full CW.  This would point out potential problems with the last stage... Also, double check the values of R14 and R15.