Auna ALP404CH two channels drawing exessive current when speaker connected - diyAudio
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Old 25th March 2011, 09:44 PM   #1
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Default Auna ALP404CH two channels drawing exessive current when speaker connected

Bought this amp as defective. It has 4 channels, output transistors: 2SB688, 2SD718. Power supply MOSFETs - IRFZ44N.
At first it draw too much current, checked output transistors, one in channel 1 was shorted. Removed output transistors from that channel. Then powered amp up, when idling it does not draw current anymore. Tested if channels produce audio, channels 3 and 4 produce audio and everything seems to be OK with these channels. Then tested channel 2, with speaker connected to channel 2, amp starts to draw current, pushes speaker cone out and makes squeaky noises for a few seconds after powering amp up. Channel produces audio, I dont have a scope but seems quite clean audio.

Checked output transistors, soldered them out, to be sure, they are OK. Removed MOSFETs, checked them: OK. Checked rectifiers: OK.
Measured voltage on solder pads of MOSFETs 1: 3.5V, 2:12,2V, 3:0V.
Rectifiers are in correct locations. Then took output transistors from channel 2 and installed them in channel 1, result is the same: amp draws current, pushes speaker cone out.
Measured voltages to ground terminal on legs of output transistors. Working channels: 1: 0V, 2:21V, 3: 0V. Problematic channels: 1:21V, 2:21V, 3:21V.
I dont know what causes this...
Attached Images
File Type: jpg DSC00474.jpg (741.1 KB, 131 views)
File Type: jpg DSC00475.jpg (817.9 KB, 115 views)
File Type: jpg DSC00476.jpg (620.2 KB, 98 views)
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Old 25th March 2011, 09:58 PM   #2
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It appears that you may have a defective driver or output in the channel that pushes the cone out.

Was the voltage read with the outputs in or out of the circuit?

Did you check the outputs for open junctions and for leakage (meter on resistance)?

How can the channel with 21v on it produce clean audio?

Is the DC (pushing the cone out) only present for a few seconds?
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Old 27th March 2011, 11:37 AM   #3
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Voltage on output transistor legs was measured with outputs in.
Now I removed the transistors and measured voltage on their solder pads:
1: 21.1V that should be base?, 2: 21.1V collector? 3: 0.5V

I measured resistance between legs. One transistor was shorted, that pair I threw away already. The pair in the other problematic channel did not show signs of being shorted and there were no small resistances between legs. There was a difference in htfe one transistor had htfe 69, other 120. Datasheet says that minimum htfe should be 50 and maximum 160.

I measured voltages on driver transistor legs. Those are near emitter resistors, one leg connected to outputs base? They have 21V on each leg, on both of the problematic channels, does not matter if outputs are in or out. 21V is present on the next row of small transistors as well. Only on the third row of small transistors (almost in the center of the board) I measured voltages 1:0,0 2: 21V 3:0,0V on transistor legs.

DC is present all the time, not for a few seconds. I dont know how it produced audio. I had only 1 RCA connected to channel 1, one speaker connected to channel 1, cone was pushed out and audio was played, although very quietly.

So, what to do next remove small transistors and check them all?
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Old 27th March 2011, 09:58 PM   #4
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Are both defective channels on the top in the photo?

There are eight 5.1k and eight 2.7k resistors (2 of each each per channel). Compare the voltages on the 2.7k resistors in the good channels to those in the defective channels. Four of the 2.7k resistors should have approximately 15v on one end. The other four should have approximately negative 15v on one end. Is this what you have?
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Old 28th March 2011, 02:39 PM   #5
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Both defective channels are on the top in the photo.

Measured voltages on 2K7 resistors. + voltage is a bit lower than 15V: +14,2V - voltage is almost -15: -14,96V.
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Old 28th March 2011, 04:43 PM   #6
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Have you pulled the driver transistors to see if they're leaking?
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Old 28th March 2011, 06:46 PM   #7
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Pulled driver transistors A1013 and C2383 next to big emitter resistors on both defective channels. Checked for 0 ohms between legs, did not find it. Soldered jumper wire to leg 1 and 3, set meter to diode check, measured resistance between legs 2 and 3 one way and then the other way. Each transistor had open circuit one way and 1500-1600 ohms of resistance the other way. Measured hfe, 2 had about 180, one had 190 and one had 200.
I understand that they are OK?
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Old 29th March 2011, 03:38 AM   #8
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The jumper is only required for enhancement mode FETs. These are BJTs (Bipolar Junction Transistors).

Re-check them with your meter set to ohms (no jumper) and make sure that they read open 1-2 (no matter the orietation of the probes), and one way 2-3 and 1-3. Then set the meter to diode check and confirm that you read open 1-3 and 2-3 and with the probes reversed, you read ~0.6 1-3 and 2-3.

The following is one example. It's important is that you confirm that you have two good junctions (base-collector and base-emitter) that pass current in one direction but block current in the other direction. It's also important that no current flows from collector-emitter in either direction.
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Old 29th March 2011, 03:12 PM   #9
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Checked the 4 driver transistors again as seen on image you attached.
Results:
Numbers (1, 2, 3 etc) are probes positions according to your image. 1 means open circuit.
Transistor 1
1: 1
2: 1,496 (diode check)
3: 1
4: 1
5: 1,481 (diode check)
6: 1
Transistor 2
1: 1
2: 1
3: 1
4: 1,517 (diode check)
5: 1
6: 1,510 (diode check)
Transistor 3
1: 1
2: 1,482 (diode check)
3: 1
4: 1
5: 1,467 (diode check)
6: 1
Transistor 4
1: 1
2: 1
3: 1
4: 1,515 (diode check)
5: 1
6: 1,508 (diode check)

My multimeter goes only up to 2Mohm so with that setting I get open circuit from everywhere.
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Old 29th March 2011, 03:38 PM   #10
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Did you check the outputs that you pulled from the other channel as thoroughly?
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