Magnat Phantom Mono-block car amp

[jansiu]

Hi,
Any help with Magnat Phantom Car amplifier?
I bought broken amp. Somebody has shortcutted subwoofer output.
Now I have checked all the mosfets, transistors and diodes. Them are OK.
(Tested with simple mosfet tester circuit, that I found from Google)
When I put the amp on, I hear popping noise from speaker.
And if I turn GAIN (volume) knob, I can hear crakling noise from speaker.
Sound comes out _only_ if I connect some audio device to amplifier RCA output, not when it is connected to amplifier RCA input. That is little bit weird.

What I should check next?

Thanks already!

P.S. Sorry for bad english.
I attach PDF service manual.

[stj]

Re: Magnat Phantom Mono-block car amp

Sound comes out _only_ if I connect some audio device to amplifier RCA output, not when it is connected to amplifier RCA input.

that makes no sense unless the schematic is wrong or maybe the 4.7uf capacitors c107/c207 have gone open-circuit.

[CapLeaker]

Re: Magnat Phantom Mono-block car amp

or IC11 on the Line Out is bad.

[jansiu]

Re: Magnat Phantom Mono-block car amp

that makes no sense unless the schematic is wrong or maybe the 4.7uf capacitors c107/c207 have gone open-circuit.

Schematics are right ones, checked multiple times.

I already changed those capacitors.
Now I ordered all of those ICs (4x NJM4558D).
When I get and change those ICs, I will let you know what happened...

[kaboom]

Re: Magnat Phantom Mono-block car amp

Do you have both +/- 15V opamp/bias supplies?

What are the +/- rail voltages, and are they more-or-less symmetrical?

[Longbow]

Re: Magnat Phantom Mono-block car amp

First things first - what are the power supply voltages?
Would you mind putting up the complete schematics? Where are the FET's???

[stj]

Re: Magnat Phantom Mono-block car amp

he did put up the schems.
it's an input op-amp problem.

[kaboom]

Re: Magnat Phantom Mono-block car amp

it's an input op-amp problem.

He's able to backfeed the line-outs, and pass a signal to the input bus, which isn't possible unless IC11 is also bad.

I'd be breaking the path at SW1B, and isolating the input stage/filter/EQ from the PA, in order to properly isolate and troubleshoot. Loss of +/- 15 will make the PA act strange, since those voltages also bias the PA's input section.

Jansiu, you won't have that crackling when moving the gain pot unless the pot has DC across it. That's a direct-coupled AC opamp circuit; at least when/if proper supply voltages are present and the opamps are good.

All signal pins on those input/filer opamps should be at zero volts DC.

[jansiu]

Re: Magnat Phantom Mono-block car amp

+/- 15V is OK. (measured 14.9VDC)
RAIL voltages are both +/- 54VDC. They are identical.

IC's voltages measured. Negative probe in pin 4 and positive in pins 2,3,5,6.

IC11
2= 1.4VDC
3= 1.4VDC
5= 1.5VDC
6= 1.5VDC

IC12
2= 0.0VDC
3= 0.0VDC
5= 0.0VDC
6= 0.0VDC

IC13
2= 14.8VDC
3= 14.8VDC (1.0 to 14.8VDC changes if I turn volume knob)
5= 15.5VDC
6= 15.5VDC

IC14
2= 15.3VDC
3= 15.3VDC
5= 15.3VDC
6= 15.3VDC

I checked those pins from NJM4558D datasheet.

[stj]

Re: Magnat Phantom Mono-block car amp

ic12 looks um... suspect!

remove ic11 for now, and use it to replace ic12

[jansiu]

Re: Magnat Phantom Mono-block car amp

I changed them and measured voltages.

IC12
2,5,6 = 30VDC
3 = 0VDC

IC11
All pins 0VDC

[stj]

Re: Magnat Phantom Mono-block car amp

so the fault moved?

[jansiu]

Re: Magnat Phantom Mono-block car amp

Yep. Maybe I get next week all of those IC's, that I ordered.
Should I change all of them or just only the IC11?
And should I check some other components or only replace the IC's?

[stj]

Re: Magnat Phantom Mono-block car amp

no idea.

[Th3_uN1Qu3]

Re: Magnat Phantom Mono-block car amp

Replace the opamps and coupling capacitors between them. Should work fine then.

[CapLeaker]

Re: Magnat Phantom Mono-block car amp

Yep. Maybe I get next week all of those IC's, that I ordered.
Should I change all of them or just only the IC11?
And should I check some other components or only replace the IC's?

Since you have plenty of them coming, I would change at least IC11 and IC12.
I wouldn't want to open that thing again in 2 weeks only to find out that the other one is now toasted too. Some amps use one IC per channel, so I SURLY replace BOTH!

[jansiu]

Re: Magnat Phantom Mono-block car amp

I changed IC11 and IC12. Now they all have same voltages. I connect my phone to the input and I get sound out. But if I stop the music, speakers starts keeping humming sound. I dont know if this is caused by bad power supply ( omron 12vdc / 4.2A) or bad rca cable. Or can those coupling capacitors cause this?

[CapLeaker]

Re: Magnat Phantom Mono-block car amp

I first would try it on a better power supply or battery.

[stj]

Re: Magnat Phantom Mono-block car amp

does it hum if you unplug the phono's?

[jansiu]

Re: Magnat Phantom Mono-block car amp

Tested with better power supply, no difference. I have video that you can hear that sound.
Sound does not change if there are phonos connected or not...
https://youtu.be/k9cqQZpS6FI