power indicator LED which was supposed to be feed with negative and positive to ground. This is the board with problem. Would the problem be the the 170/74 pair ??
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I spent the whole morning to read through the whole thread so to find the answer to my problem, but there seems no one encounter the same problem as mine. I might need a new pair of 170/74, I have the feeling I tossed the pair the first time I turned on. Is it possible ????
What is the voltage across R1? What is the voltage across R2? We need to figure out if there is current flowing through Q1 and Q2. The end target is 1.3 amps (or so) through each of those resistors. When the currents exactly match on top and bottom half, the voltage seen at the speaker terminals will be 0.0 volts (well within a few millivolts and stable after warm-up).
So assuming your bias circuit is working properly, turning the pot clockwise should gradually increase the current through that half. But go back and forth between the two, "walking" it up slowly, until the target is reached. You have to take your time as things will keep moving around as it warms up.
Folks that understand what's going on just put a meter across R2 and the speaker terminals (you really don't need to "see" the R1 current). You are just looking for 0V of offset at the speaker terminals with 600mV on R2 after all this tweaking.
So assuming your bias circuit is working properly, turning the pot clockwise should gradually increase the current through that half. But go back and forth between the two, "walking" it up slowly, until the target is reached. You have to take your time as things will keep moving around as it warms up.
Folks that understand what's going on just put a meter across R2 and the speaker terminals (you really don't need to "see" the R1 current). You are just looking for 0V of offset at the speaker terminals with 600mV on R2 after all this tweaking.
Always start those pots on the low side so you can see the amp gently "wake up" as you begin the whole process of allowing more and more current to flow through the outputs.
The rail voltage are fine at 24.74v and -24.74v respectively. There is no voltage across 0.47, but the voltage on output keeps climbing no matter how hard I turn the pots just can't turn it down to 0. I thought I have to fine out why there is no voltage on 0.47. The other side works perfectly..
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Remember that the upper pot is for adjusting the offset, and the lower pot is for the output device current.
If there is no voltage across the 0.47 resistor, that means there is no current in the output transistors,
and likely not enough gate-source bias voltage.
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I make change as some members recommended, 6.8v zener and 4.7k.
The other channel is with the same change and work at first time.
Good, is it possible that the Zener is in backwards?
Can you measure the DC voltage across both of the Zeners, which should be 6.8V each?
I had very similar "problem" with "unimpressive" bass when I first built F5 - wanted "more" bass and less treble. Then I began to search for solution, more power, thought about F5Turbo etc., And while I searched...it magically fixed itself over time.
I traced the problem to the PSU capacitors, the strange phenomenon people call "burn-in". I know because when I switched PSU board with another one with a fresh new capacitors - story repeated itself. I immediately tried older PSU and heard the difference. Then after about 2 weeks i had hard time telling the difference between the 2. So maybe simply letting it play may help.
Yes, Even new capacitors need to reform after sitting on the shelf for months or longer.
The ESR will then go down considerably, to spec.
Ya there you go. Q2 has no current flowing through it. I would put clip leads on Q2 gate and drain, and verify that I can adjust the gate voltage with the pot. At 4 or 4.5 volts, the transistor should turn on. Watch current through the .47 with clip leads on each side of it. Current=voltage divided by resistance.
You said to put a clip lead at the point after R12 to the point Out. I now have the two power tube away from the board. I could adjust the voltage of Q1/Q2 gate to source to any voltages I want. I now temporary set both at around 2.5v, but before I put the power tubes back on. What else I could check so to make sure the other parts are working OK? I did this trick once before but with no avail. Would it be the Q2 is gone.
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Sorry, should have said G to S. I'm a dummy that was typing way too fast at the end of lunch break. Q2 may be defective, I don't know. Assuming good rail voltages and working bias, adequate Vgs should get current flowing through the thing. Again, I would have clip leads on each side of the .47 and watch for voltage there as I increase Vgs to get the transistor to start flowing current. 2.5V is a good starting point for gate to source.
William,
Thanks. The channel is on with the clip lead G to D.
Will report after stable.