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PeeCeeBee Preamplifier GB

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shaan

shaan

Member
Joined 2009
Output stage quiescent current or bias is to be measured through R59 and R60 for each channel. TP3 and TP5 are connected to one leg of R59 and R60, and TP4 and TP6 to the other leg. So when measuring bias, you need to probe across these resistors only (or their negative mirrors R61/R62). During bias measurement we only need the absolute value of the measured voltage (and divide by R59/R60 value to get current in milliamps), polarity is not important. So you can connect the probes in any way you want.

TP4 and TP6 are output nodes, just on the left of C27/28. And during offset measurement connecting the black probe to ground and red probe to TP4 or TP6 gives us the exact polarity of the output offset voltage. Reversing the probe polarity is not an issue at all, you will only need to turn the trimmers the opposite way than what is suggested to trim the offset close to zero.
 
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NealJ

Member
Joined 2017
Hi Shaan,

No luck I'm afraid.
I cleaned the underside of the board again and gave it a good close inspection, no shorts that I can see.

  • All capacitors are correct for polarity
  • All diodes are correct for polarity
  • The small transistors are correct in orientation and placement
  • BD139 and B140 are correct in orientation and placement
What should I look for now?
 
S

Sadik

Member
Joined 2015
Just finished assembly of components on PCB, may be in a day or two I will test it :note: :note: :note:
IMG_20201027_230354.jpg

IMG_20201027_230414.jpg


Sadik
 
N

NealJ

Member
Joined 2017
Finally had time to look at this again.

Testing a BD139, no power applied.
Positive probe to B gives 0.7V at C and E. Negative probe to B gives 1.8V to C and 0.7V to E.

I know the negative to B should give OL readings when you measure them out of the circuit, does that still apply in circuit?
 
shaan

shaan

Member
Joined 2009
Negative probe to B + positive probe to C shows Q13's base-collector voltage plus the drop around R17 and R43 in series. Negative probe to B + positive probe to E just shows the drop around R47.

If it is showing 0.7V B-E and B-C with positive probe to B then the BJT is okay.

edit: short base to emitter with a wire and probe collector-emitter voltage drop, should show infinite if it is really okay.
 
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shaan

shaan

Member
Joined 2009
As far as I can see, nothing stands out. If a junction was open or short it would show up in here. And to me it looks like all the BJTs are okay!

Have you measured all the resistor? Also check whether the trimmers' resistance is responding to turning the screws.

Btw, did you turn the trimmers right to maximum clockwise turns and still found no reading across R59/60? Depending on BJT hFE the trimmers sometimes need more turns to turn on the bias.
 
N

NealJ

Member
Joined 2017
Well, I thought I'd turned the trimmers most of the way out, but I hadn't.:eek:

This time I counted, at 30 x 1/2 turns - nothing. 40 1/2 turns and I got a reading of c.70mV. I'm not sure how many turns it was after that to get to 200mV, I'd stopped counting because I was so relieved, but it wasn't many.

Offset tuned to 1mV so it's ready for testing, unfortunately I don't think I'm going to get a chance to hook it up and test for a few days now.

Thank you Shaan, once again your patience with this idiot is very much appreciated.
 
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