[BUILD] New FET/RACK Official Help Thread - Please read first post!

[Echo North]

I'm using the Mouser BOM so I believe Q2-Q4 and Q14 were 2N3391A, Q5, Q7-Q10 and Q12-Q13 are 2N3708, Q6 is 2N3053 and Q11 is 2N5457.

Q12/13 (2N3708) need to be matched for hFE. If you got a partial kit you need to that yourself.  Most meters have an hFE function.  If you don't match them there is still like a 60% you'll be fine.  Sounds like you might be in the 40%.

You could try swapping one out with another.

Mike

 

[Foots]

Attack switched off = GR off.

Measure AC not DC volts.

TP1  .1VAC
TP15  .2VAC
TP17  0.0VAC with lots of noise and radio signals
Out Brown  .08VAC

Thanks!

 

[Echo North]

TP1  .1VAC
TP15  .2VAC
TP17  0.0VAC with lots of noise and radio signals
Out Brown  .08VAC

Thanks!

Does rotating the Qbias help the AC V @ pad 15?

oh and have your in/out at 12 noon and ratio at 20:1

 

[Foots]

Does rotating the Qbias help the AC V @ pad 15?

oh and have your in/out at 12 noon and ratio at 20:1

IN/OUT were at 12 noon and ration was 20:1 already . Rotating Qbias does not affect AC V at TP15.  Thanks!!

 

[Echo North]

IN/OUT were at 12 noon and ration was 20:1 already . Rotating Qbias does not affect AC V at TP15.  Thanks!!

What is your DC V at TP18?

 

[Foots]

What is your DC V at TP18?

-1.019 VDC

 

[Echo North]

-1.019 VDC

Can you safely remove Q1?  Does that increase your output?

 

[Diamondj421]

Q12/13 (2N3708) need to be matched for hFE. If you got a partial kit you need to that yourself.  Most meters have an hFE function.  If you don't match them there is still like a 60% you'll be fine.  Sounds like you might be in the 40%.

You could try swapping one out with another.

Mike

Which 2 would you recommend swapping? Are there any that will not affect the other components as much?

 

[Diamondj421]

Actually after further investigation, Q12-13 are both 2N3707.

 

[Echo North]

Actually after further investigation, Q12-13 are both 2N3707.

Doesn't matter.  They still need hFE matching.

 

[Diamondj421]

Ohhhhhh. Ok. Now I understand. How does one do the HFe matching?

 

[Echo North]

Ohhhhhh. Ok. Now I understand. How does one do the HFe matching?

It's covered in the older guide, you need a meter that can do it:
http://www.hairballaudio.com/blog/resources/build-guides/populating-the-pcb-

In the transistor section.

 

[Foots]

Can you safely remove Q1?  Does that increase your output?

Not sure that I will be able to remove Q1. That sucker is in there! Will keep working at it though. Anything else I can check in the mean time?

 

[Echo North]

Not sure that I will be able to remove Q1. That sucker is in there! Will keep working at it though. Anything else I can check in the mean time?

Set your trimmer so you have the highest DC you can get at TP18, then test the DC resistance (Ω) across the S and D of Q1.

Mike

 

[Foots]

Set your trimmer so you have the highest DC you can get at TP18, then test the DC resistance (Ω) across the S and D of Q1.

Mike

Trimmer? Sorry, probably a dumb question, but want to make sure I do what you are asking...

 

[Echo North]

Trimmer? Sorry, probably a dumb question, but want to make sure I do what you are asking...

Sorry qbias trimmer.

 

[Foots]

Set your trimmer so you have the highest DC you can get at TP18, then test the DC resistance (Ω) across the S and D of Q1.

Mike

27.10kΩ

 

[Echo North]

27.10kΩ

Hmmmm. You have something wrong in the "signal preamp" section of you build...it's on the far left of your PCBs.  Check all of those parts and soldering.

Mike

 

[Foots]

Hmmmm. You have something wrong in the "signal preamp" section of you build...it's on the far left of your PCBs.  Check all of those parts and soldering.

Mike

Thanks for your patience and help Mike! I will check that section and get back. BTW, what should the resistance be at Q1 between S and D?

k

 

[Echo North]

Thanks for your patience and help Mike! I will check that section and get back. BTW, what should the resistance be at Q1 between S and D?

k

What you got seems right in circuit.