Hifonics Nemesis NX-400 low rail voltage - diyAudio
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Old 20th July 2011, 11:43 PM   #1
Mote is offline Mote  Estonia
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Default Hifonics Nemesis NX-400 low rail voltage

As if one power supply mystery wasnt enough I have another one which I cant figure out.
This is my first Hifonics amp. Looks very good, compared to the budget stuff I fix most of the time
At first I powered the amp up, it did not draw current, did not go into protection only the sound was distorted on both channels. Dont have a scope so I can only tell what I hear. Seems that there is no noise, higer frequencies sound right but bass is bad on both channels. Sounds like when I use a current limiter and turn the volume so far up that limiter starts to actually limit and lower frequencies get "clipped".

So I measured voltages between ground and:
Output transistors: 0V, 12V, 0V or 0V, -15V, 0V.
PS FETs: 1,8V, 10,6V, 0V
Rectifiers: 22V, -15V, 19V or 19V, 12V, 22V

I assume that rail voltage is not supposed to be 12 to 15V?

PS Chip is KA7500 BD, so I dont know if 1,8V is correct on gates of FETs.
Datasheet says that Common Collector output Vcc is 1,5 to 2,5V. Is Vcc what is sent to gates of FETs?.
10,6V on drain is also low. Same voltage between GND and + terminals
Without remote I got 0V, 12,5V, 0V on FETs

Arent rectifiers supposed to have 0V on two of their legs?

What I tried:
Removed rectifiers to see if voltage between + and GND still drops to 10, yes it does. Rectifiers test out OK. Highest voltage on drain I got when I left in only T1 side FETs. Then drain had 11V on it.

PS FETs: two gave funny readings, I measured and measured again out of the board and finally one gave out the secrets of its soul and shorted. I swear it was not shorted before. Again I replaced all temporarily with IRFZ44Ns. Before source resistors got quite hot, now R72 gets hot (see pictures).
Voltages on rectifiers changed a bit after replacing FETs, now they have: 8V, 12V(-15V), 9V on their legs.

Can not measure voltages on PS chip because I cant get my probe there, transformer is in the way.
Can I remove the driver board and run wires from pins to main pcb?
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Old 21st July 2011, 01:43 AM   #2
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Recheck the voltages but this time. place the black probe on one of the non-bridging speaker terminals.
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Old 21st July 2011, 04:29 PM   #3
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New results on FETs. Black probe on non-bridging speaker terminal.
2,25V
10,33V
0,3V
or
2,6V
10,33V
0,3V

Rectifiers, black probe on non-bridging speaker terminal:
1,6V
-15V
1,8V

1,9V
14,5V
1,3V

2,2V
14,3V
2,2V

2,2V
-13,9V
2,2V

All outputs are out, amp powers up, does not draw current.

I heard some faint sizzling noise coming from T2 transformer, after twisting and turning it I got 0V, -17V, 0V on one of the rectifiers.
After some more twisting amp started to draw some current. Twisted transformer back and current draw went to minimum.
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Old 21st July 2011, 06:00 PM   #4
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It sounds like you may have an intermittently shorted transformer. They most commonly short where the terminal windings meet the rest of the windings on the core. It's most often the one that exits from under the core.
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Old 21st July 2011, 11:03 PM   #5
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By terminal windings you mean on the terminals side, yes?

Because I removed both of them and added insulation to the other side. :S
That did not help. At first it seemed it did because there was 0, -17 and 0V on diodes, but afterwards it had again 2, -16 and 2.

Do find the exact spot i have to set MM to continuity test, one probe to the end of winding and with the other I have to search where the insulation is bad?

-17 and 17 is correct rail voltage for this amp?
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Old 22nd July 2011, 12:02 AM   #6
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The terminal windings are the terminals of the transformer that solder to the board.

I don't know what the rail voltage should be.
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Old 22nd July 2011, 08:33 PM   #7
Mote is offline Mote  Estonia
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I have this lacquer for fixing leaks on transformers but I am not quite sure how to use it.
Writing on the can in german is:
Klarer Schutzlack zum versiegeln und isolieren von gedruckten schaltungen, spulen und sonstigen elektronischen bauelementen, elektrischen anlagen wie verteilern, batterien, motoren. Verhindert kriechströme, beseitigt feinschlüsse an spulen und transformatoren.
Google translate:
Clear protective lacquer to seal and isolate them from printed circuits, coils and other electronic components plants, such as electrical distribution boards, batteries, motors. Prevent leakage, eliminates fine connections to coils and transformers.

I understand that the lacquer is exactly for this purpose?

Do I remove the transformer, find the leaking spot and spray that spot?

Or can I just go nuts and spray the entire transformer and hope that does the trick?
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Old 22nd July 2011, 09:15 PM   #8
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You don't want to blindly apply anything because that may make it unrepairable.

With no remote voltage applied (only B+ and ground connected), place the black probe on the amp ground and the red probe on any of the secondary windings. Twist the transformer back and forth while pushing and pulling on it. At any time do you read anything near 12v? Ideally, you should never see any voltage on the secondary windings when doing this.
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Old 24th July 2011, 11:31 AM   #9
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After insulating the transformers terminal windins I get no noise from transformers any more.
With no remote voltage I get 0 ohms from all the secondary windings.
I replaced temporarily installed IRFZ44Ns with original replacements: 50N06
On Q43 FET I have 0.08V on first leg. Other three PS FETs have 0V on their first leg and -0.03V and 0.06 on center legs of D11 and D12 rectifiers.
That is still without remote voltage.

With remote voltage applied I stll get 2.1-2.8 V on rectifiers 1st and 3rd leg.
When I removed T1 for closer inspection and powered amp up without it, it had 0V on 1st and 3rd leg of rectifiers. T1 in place and T2s secondary windings disconnected from board it still had 2V on rectifiers 1st and 3rd leg.

I dont understand, is it the transformer or not?
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Old 24th July 2011, 01:49 PM   #10
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I meant 0 volts, not ohms, on the secondary windings.
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