It measured fine off the board. A new 6488 didn’t help. I also replace the 10uf50v cap behind it. Still low amplitude drive signal.
Sorry I wrote the number down incorrectly before:
The 2n6488 near the middle of the board referencing amp negative wire
1; 0.530v (Originally wrote down 0.053)
2; 3.2v
3; 0.002v
The 2n6488 near the middle of the board referencing amp negative wire
1; 0.530v (Originally wrote down 0.053)
2; 3.2v
3; 0.002v
I put the rects back in to help answer that. With the rects in, the amp powers up and will pass audio as long as the input voltage is <12vDC. The drive to the PS FETs looks OK up until that point with amplitude over 4v as long as the input voltage is low. Over 12.5vDC input voltage, the drive amplitude is reduced to about 3.5v and the same bank of fets behind the speaker plug get hot and draw as input voltage is raised further.
The PS doesn't act as normal when input voltage is high enough for the PS to switch to regulated PWM.
The PS doesn't act as normal when input voltage is high enough for the PS to switch to regulated PWM.
It's not PWM. It's PAM.
As long as the rail voltage is approximately the same as the Zener driving the base of the 2N6488, the drive voltage will be reduced. When the voltage on the rails drops, the drive voltage will increase.
It's somewhat normal for the FETs to run hot when the drive is approximately the same as the threshold voltage of the FETs.
With the rectifiers in the circuit, does the drive still have that noise between pulses?
Does twisting the transformer make a difference with the current draw?
How hot, how fast?
Do they seem to be heating up when tightly clamped to the heatsink?
As long as the rail voltage is approximately the same as the Zener driving the base of the 2N6488, the drive voltage will be reduced. When the voltage on the rails drops, the drive voltage will increase.
It's somewhat normal for the FETs to run hot when the drive is approximately the same as the threshold voltage of the FETs.
With the rectifiers in the circuit, does the drive still have that noise between pulses?
Does twisting the transformer make a difference with the current draw?
How hot, how fast?
Do they seem to be heating up when tightly clamped to the heatsink?
The rail voltage is +-21vDC.
Yes there is still noise on the drive pulses with all parts in circuit. Its maybe a little better but still not too clean.
Twisting the transformer doesn't change anything.
Some of the FETs will heat up to 170F in a matter of about 10 seconds.
The amp draws 0.7A at 12vDC power, and 1.75A at 14.4vDC power. Sometimes while varying the supply voltage the PS will draw to the bench supply soft limit of its settings such as 4A current limit (Gophert 1620).
I have not tried clamping them.
The amp came in with this side of the PS fets blown exactly behind the speaker terminal - the same ones getting hot above 12.5vDC power
Yes there is still noise on the drive pulses with all parts in circuit. Its maybe a little better but still not too clean.
Twisting the transformer doesn't change anything.
Some of the FETs will heat up to 170F in a matter of about 10 seconds.
The amp draws 0.7A at 12vDC power, and 1.75A at 14.4vDC power. Sometimes while varying the supply voltage the PS will draw to the bench supply soft limit of its settings such as 4A current limit (Gophert 1620).
I have not tried clamping them.
The amp came in with this side of the PS fets blown exactly behind the speaker terminal - the same ones getting hot above 12.5vDC power
Clamp them down and check for excessive heating.
Are all of the gate resistors within tolerance?
Are all of the gate resistors within tolerance?
With the FETs clamped, I thought this amp was repaired after testing; sent to customer. A few months later amp came back with the same exact problem. Not really sure what to look at next?
One gate resistor failed nearest the 2n6488, that FET also shorted.
Its back to limping on the bench.
This amp came back with 1/2 the PS fets blown. I replace the bad bank of FETs, the driver MPSA06/56, and 10ohm gate resistors. Pull down resistors are good measuring 820ohms. Drive signal to the PS FETs looks OK, but the fets run hot at greater than 13vDC at B+.
I replaced the 1n4742 diodes, checked the 10ohm resistors to PNP collectors.
The amp appears to be able to pass audio under mild bench load just fine.
As soon as more than 13vDC enters the amp, the FETs heat.
Removing the rects and the PD Fets never heat (Though fets gets driven to 100%)
If I mount the amp to a sync, it'll hold for a bit; but I'm afraid it'll come back (again) once strained.
One gate resistor failed nearest the 2n6488, that FET also shorted.
Its back to limping on the bench.
This amp came back with 1/2 the PS fets blown. I replace the bad bank of FETs, the driver MPSA06/56, and 10ohm gate resistors. Pull down resistors are good measuring 820ohms. Drive signal to the PS FETs looks OK, but the fets run hot at greater than 13vDC at B+.
I replaced the 1n4742 diodes, checked the 10ohm resistors to PNP collectors.
The amp appears to be able to pass audio under mild bench load just fine.
As soon as more than 13vDC enters the amp, the FETs heat.
Removing the rects and the PD Fets never heat (Though fets gets driven to 100%)
If I mount the amp to a sync, it'll hold for a bit; but I'm afraid it'll come back (again) once strained.