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Old 25th August 2006, 12:56 AM   #1
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Default DBX BX-3 Amplifer Woes...What is going on?

This is a four-channel amp I'm fixing for a buddy. One channel has no positive swing on the output. The bases of all the output devices have about -0.220V on them.

The dual FET at the input is a µPA68HA. The drain of 'FET #1' on the bad channel has 5.1V on it, the good channel has 8.05V. Q6 has 6.9V on the emitter (the good channel has 7.3V here), and thus the B-E junction is reverse biased. No current flows through R16 (on the collector of Q6) or through R18 (on the emitter of Q6). Current IS flowing through Q7 and R17 and R19.

I didn't mark it on the drawing, but the regulator formed by DZ2/Q4 is working and outputting about -11V.

I've stolen the FET from a working channel with no positive results, as well as swapped Q1, Q2, Q5 and Q6.

Anyone have any ideas on what I'm missing?

(caution...big pic!)

Good voltages shown in black, and match the good channels, bad voltages shown in red)

Click the image to open in full size.
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Old 25th August 2006, 02:04 AM   #2
clem_o is offline clem_o  Philippines
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Voltages would have turned off Q6. Input FET must be turned on pretty hard to pull the voltage at its drain to 5.1

What't the voltage at the feedback resistor to ground?

Is the amp's quiescent output 0V?
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Old 25th August 2006, 02:27 AM   #3
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Amp output is sitting at about -30mV. I can force it further negative with the offset pot, but -30mV is about as positive as it will go with the current issues.
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Old 25th August 2006, 02:49 AM   #4
clem_o is offline clem_o  Philippines
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Makes sense. Input pair is working to pull the offset to zero, but can't.

Check Q9 collector voltage, followed by base voltages of Q12 and Q14.

Q14 base should be near 1.4V, Q12 base near 2.1; therefore Q9 collector perhaps a bit higher than that i.e 2.5?

One of those voltages has to be off to get your output transistor to be biased off, causing the negative offset...

Cheers
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Old 25th August 2006, 03:37 AM   #5
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very hard to read, poor image quality of schematic. Q9 should be on hard. Check Q9's emitter resistor voltage drop (120 ohm). After the collector of Q9 there's a 390 ohm resistor feeding pre-drivers' Check that and those clamping transistors that shunt the signal to the ouput line. (Q1/Q3). If Q1 isn't shorted, check Q16's emitter resistor for open. If it's open, it'll always turn on that voltage clamp (Q1)
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Old 25th August 2006, 04:37 AM   #6
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Quote:
Originally posted by clem_o
Makes sense. Input pair is working to pull the offset to zero, but can't.

Check Q9 collector voltage, followed by base voltages of Q12 and Q14.

Q14 base should be near 1.4V, Q12 base near 2.1; therefore Q9 collector perhaps a bit higher than that i.e 2.5?

One of those voltages has to be off to get your output transistor to be biased off, causing the negative offset...

Cheers
Q9 is as off as a transistor can be. No current at all through R23 (its 120 ohm emitter resistor). Emitter is sitting at the positive supply voltage, and the collector is at 0.00V. The base of both Q12 and Q14 is sitting at 0.00V (plus/minus a millivolt or two).

Quote:
Originally posted by mrshow4u
very hard to read, poor image quality of schematic. Q9 should be on hard. Check Q9's emitter resistor voltage drop (120 ohm). After the collector of Q9 there's a 390 ohm resistor feeding pre-drivers' Check that and those clamping transistors that shunt the signal to the ouput line. (Q1/Q3). If Q1 isn't shorted, check Q16's emitter resistor for open. If it's open, it'll always turn on that voltage clamp (Q1)
Yes, it's a crappy schematic. All I can find, and part of the reason that the scan is so large, and I chose to use png...no sense making it look worse. But make sure you are clicking on the image so it will go full size. It is pretty large (if it isn't cascading off your screen, it ain't full size yet. You may have to click on the image twice).

The clamping/limiter transistors (Q1~Q4) were removed from the board early on to make sure they weren't screwing things up. Forgot to mention that.

R33 is the 1K shared emitter resistor for Q12/Q13. Measures fine. R34 is the 150 ohm shared emitter resistor for Q14/Q15. Also looks good.
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Old 25th August 2006, 05:01 AM   #7
clem_o is offline clem_o  Philippines
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Try checking the voltages on Q7 then. With Q6 off, Q7 should be pulling hard on Q9, but if there's no emitter current on Q9 then maybe Q9 is shorted (i.e. b - e) or Q7 isn't doing its job...

Cheers!


oops - that should be "Q9 is open" (b to e)... :-)

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Old 25th August 2006, 05:11 AM   #8
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Q7 IS on, but Q9 wasn't doing anything.

I tested Q9 earlier, and it measured OK, but I just now replaced it anyway, and it looks like that might have fixed the whole thing (I haven't passed a signal yet, but I'm getting current flow in the proper places now).

I got fooled into thinking that because a transistor works in the tester that it MUST be good. I know better...

I'll report back once I see it pass a signal.
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Old 25th August 2006, 02:42 PM   #9
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Looks like that took care of it. Running fine.

I'm not completely impressed with the build quality, but I gotta admit, it DOES crank out the watts...in 2-channel mode into an 8 ohm load, I just measured output at 540WPC at clipping.

Appreciate your help guys! Thanks!
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Old 25th August 2006, 10:55 PM   #10
clem_o is offline clem_o  Philippines
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That is SOME power! Great work!

Cheers
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