Zen 9 - Troubleshoot Bad Channel - diyAudio
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Old 7th April 2012, 08:24 PM   #1
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Default Zen 9 - Troubleshoot Bad Channel

Hi, have problem with one channel on Zen 9. Other channel works; plays music.

No burned parts in bad channel
Tried replacing jfet and mosfets, but did not fix issue.
Did not replace ZTX450 in CCS yet.
Good channel warms up, but bad channel does not warm up at all.

Grounding is a star ground to chassis ground via thermistor, then to earth ground. Each channel has its own star ground.
Both channel go to ground through its own thermistor. Running 1 ground connection from board to star ground. Did not use grd for input. Chassis input grd goes to star ground. Board is single-sided.

When applying power, bad channel ramps up in voltage, but eventually voltage drops back to zero. Only happened on first start up and when replacing all fets. After, first startup, the voltage appears as shown below.
Q2 is 0v.

Voltage at power supply (dual mono) before connecting to the amp circuit- 51.8v
Voltage at power supply with bad channel 51.8v
Voltage at power supply with good channel 45.1v

Good channel voltages:

Q1:

G- .2v
D- 5.5v
S- 1.5v

Q2:

G- 9.3v
D- 23v
S- 5.4v

Q3:

G- 44.4v
D- 45.0v
S- 41.0v

Q4:

G- 27.4v
D- 41.1v
S- 23.9v

Bad Channel Voltages:

Q1:

G- 0v
D- 0v
S- 0v

Q2:

G- 0v
D- 0v
S- 0v

Q3:

G- 0v
D- 49.4v
S- 0v

Q4:

G- 51.2v
D- 51.8v
S- 49.2v

Wanted to get some advice before removing bad channel from amp.
Like to know if there are other test I can run.

Thanks in advance!

Vince
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File Type: jpg zen9_2.JPG (582.8 KB, 130 views)
File Type: jpg zen9_3.JPG (504.0 KB, 124 views)
File Type: jpg zen9_4.JPG (556.6 KB, 118 views)
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Last edited by vdi_nenna; 7th April 2012 at 08:29 PM. Reason: Added ground info
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Old 8th April 2012, 12:29 AM   #2
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Got it working. Must have been Q5 and a resistor next to it.
Reflowed the solder join for the resistors. Listening now on test speakers. :-)
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Old 8th April 2012, 12:33 AM   #3
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Glad you found the problem. Happy listening.
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Old 8th April 2012, 12:39 AM   #4
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Thank! Very happy right now!

Sounding good even through a mini set of speakers.
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Old 19th April 2012, 07:18 PM   #5
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Does anyone know why the power supply sags to 46v at the power supply output, instead of 50v? With no amp circuit, after the PS caps, it's closer to 51v, but when the circuit is attached, it drops to 46v.

Had to adjust P1 for around 20-21v at drain of Q2. Also, the pot is down to its lowest setting. It will not drop the voltage lower. As you can see in the photos, each channel has its own transfromer and caps.

Any ideas? I'm sure the two issues are somehow related.

Thanks,

Vince
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Old 20th April 2012, 06:02 AM   #6
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What are the transformers VA rating?

Gyuri
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Old 20th April 2012, 10:55 AM   #7
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Antek AS3218, 300va, 8.3A, 18+18v
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Old 20th April 2012, 11:26 AM   #8
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Vince,

Looking at the datasheet for your transformer ( http://www.antekinc.com/pdf/AS-3218.pdf ), the voltage seems a bit low to me, especially under load.

Are you using CRC in your power supply? If so, perhaps you can measure
the drop across R to see if it might be a significant amount.

Dennis
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Old 20th April 2012, 11:54 AM   #9
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what??? around 2V at full power. is that a high voltage drop?
or was it the 5V drop you ment was a ittle high?

Last edited by AudioSan; 20th April 2012 at 12:03 PM.
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Old 20th April 2012, 12:57 PM   #10
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Dennis, you might be on to something there. I used a single 1 ohm resistor between 30k uF caps. I'll check the voltage drop tonight. I followed a CRC power supply designed for another amp. Probably should have stuck with the three .47 ohm in parallel, that's only .156 ohms.

Could get approximately the same value by adding a .33 and .47 ohm resistors in parallel with the 1 ohm for .162 ohms. Sound possible?

The transformers are each rated almost 4x the current draw of 2.2A.
Not saying it's impossible, but unlikely that they can't supply the voltage.

Thanks,

Vince
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