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

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Diamondj421 said:
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
 
Hairball Audio said:
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!
 
Foots said:
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
 
Hairball Audio said:
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!!
 
Hairball Audio said:
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 said:
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.
 
Hairball Audio said:
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?
 
Foots said:
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
 
Hairball Audio said:
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...
 
Hairball Audio said:
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
 

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