Alpine 3545 power supply noise

[Dr Zeus]

This amp is the twin to the other one I'm working on, only this one has a completely different issue.

The amp powers and plays audio well enough, only the power supply near the transformer area has like a white noise sound - almost like theres a small computer speaker just dishing it out in that area. The amp is being powered off a 10A supply without strain. I cant quite pinpoint the noisy area; however if I touch and move the transformer the noise seems to change. The design of this amp allows the transformer to be easily removed with just 10 screws. i was thinking of possibly swapping the transformer in this amp with the other Alpine 3545 but I really dont want to introduce any different problems atm and wanted to get some opinions on doing this first.

If I disconnect the transfomer wires leading to the FMG-33x rectifiers, the white noise goes away, likely because the power supply having 0 load. I removed the amp from the heat-sync and cannot seem to find much in the way of anything obvious (resistors, caps, solder joints) which would cause the problem.

[807tubenut]

Inductors/transformers can make all sorts of noise, you might mix up some shellac, and give it a good soaking over night, and then let her drip clean, if it has wax residue on it, you might need to clean/warm the tranny first.

[Perry Babin]

Check the DC on all of the various filter caps. Does the DC remain clean or does the DC become dirty (very high frequency noise, not ripple related to the audio signal)?

When checking the voltage on the secondary, you may need to connect your scope ground to one of the non-bridging speaker terminals.

If everything remains clean (and the amp is producing clean audio), it's likely that the noise is due to loose windings and is not a significant problem.

[Dr Zeus]

I swapped transformers with the other Alpine 3545 and the probably does not seem to be related to the transformer windings as this amp still makes the hiss/white noise.

I'll start poking around the caps and see what I can discover and if I have any questions. Thanks!

[Dr Zeus]

What does VR601 do? Its a potentiometer near the 494. When I touch/move it slightly the noise changes a good bit but never fixes; and always comes back after I stop moving it. I've only twisted is ever so slightly...

Also, referencing amp ground, the scope shows excessive noise on most of the screw connections leading fro mthe transformer.

[Dr Zeus]

I replaced these two caps but the noise didnt change.



Here is what is coming off the power fet center legs with the scope set to 5V/Div



Again with 5V/Div, only time set to 2uS

[Dr Zeus]

If I remove the wires 9/10/12/13 going to the rectifiers then the same leg of the power transistors is a clean square wave:

[Perry Babin]

The first waveforms look noisy because your scope isn't triggering properly.

The waveform is probably normal. This is a regulated power supply and will operate at reduced duty cycle which causes the waveform to be different from the normal square wave. See item #25 on the Power Supply Troubleshooting page of the DVD.

The amp doesn't use FETs in the power supply. They are bipolar transistors.

[Dr Zeus]

I think I found a bad transistor on the driver board. Q618, part number is A114YF-G. its one of those small rectangular transistors. I pulled it off the board and confirmed its not measuring so well, however I compared it to the other amp and they are either both bad or both good. The one on the other amp looks to have been replaced or removed previously. Diode check fails.

Legs
1-2: 4.3M Ohms
1-2:Open
1-3: 62k Ohms
1-3: 62k Ohms
2-3: Open
2-3: 4.1M Ohms

BOLD for COM

[Perry Babin]

Those are digital transistors in the muting circuit. Section #29 on the audio troubleshooting page.