This one came in with half the PS blown, and half of each channel BJTs. I'm troubleshooting the power supply with rectifiers removed, and I cant get the original blown fets now replaced with IRF3205 to not over heat. I also replaced the other side's PS fets which were not blown so all matching IRF3205 now. I found a 12v ZD near the corner of the board just near the inside speaker terminal had also blown short. The PS fets which get hot are the ones also near the speaker terminal. Drive signal to the gates without fets looks strong, but when installed the drive signal gets a bit wavy even when fully driven. I also replaced all MPSA06/56 drivers in the PS. Clean square wave is coming off the TL494. When I apply 14.4v, the amp powers with rects removed and only draws about 0.3A. When I drop input voltage to about 10vDC that side of the PS FETs will overheat and draw excessively. Its defiantly a weird one IMO. With rects installed that same side of the PS overheats easily at any voltage. I've serviced a bunch of these 225HCCA amps but this one is getting me.
The drive signal looks OK, maybe a bit poor on both sides of the PS for my liking but both sides are matching and the other side stays absolutely stone cold.
I think the muting circuit may operate on the faulty side of this amp.
This amp was severely over-loaded, and possibly over-volt. I found both rail caps partially detached and replaced both and the main PS filter caps.
Why does this amp draw excessively when input voltage is dropped?
The drive signal looks OK, maybe a bit poor on both sides of the PS for my liking but both sides are matching and the other side stays absolutely stone cold.
I think the muting circuit may operate on the faulty side of this amp.
This amp was severely over-loaded, and possibly over-volt. I found both rail caps partially detached and replaced both and the main PS filter caps.
Why does this amp draw excessively when input voltage is dropped?
Photo of board?
Are the 10 ohm resistors in series with the collector of the PNP PS drivers within tolerance?
Are the 10 ohm resistors in series with the collector of the PNP PS drivers within tolerance?
When the amp is drawing current, how do the drive signals on the emitters of the drivers compare, from one bank to the other?
They’re about the same. Actually I’m seeing the signal after the ps drivers dip below 0vdc, about -.75vdc actually. I’m not sure what would be causing this but that’s probably the issue
That ringing is definitely not right.
Is the signal on the bases and emitters of the drivers clean?
Is the signal on the bases and emitters of the drivers clean?
No the emitters and bases are signaling similarly, unclean and ringing. I replaced the 494 and no difference.
At the 494 pins 9/10, the drive amplitude is about 12v on the scope. Its clean there no ringing.
What's the DC voltage on the collector of the MPSA06 PS driver transistor?
With no RCAs plugged in, what's the resistance between the primary ground and the RCA shields?
With no RCAs plugged in, what's the resistance between the primary ground and the RCA shields?
The 2n6488 near the middle of the board referencing amp negative wire
1; 0.053v
2; 3.2v
3; 0.002v
When I first apply rem, 2; briefly shows 12v
1; 0.053v
2; 3.2v
3; 0.002v
When I first apply rem, 2; briefly shows 12v
Is that transistor defective?
It shouldn't have been pulling the voltage down with the base voltage you posted.
It shouldn't have been pulling the voltage down with the base voltage you posted.