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Old 5th April 2006, 06:20 PM   #1
lopezi is offline lopezi  United States
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Default Amp repair, gurus please read

Hi,

I am repairing a Sony XM1502 2 channel amp. It uses IRF540 N-channel and IRF9540 P-channel FET's in the output stage, 2 pairs each channel (4 total FETs each channel, 2 of each). I found one pair of FET's blown in the left channel. I replaced them with the same part number FET's (an IRF9540 and IRF540).

The amp powers up fine and sounds fine when a music signal is supplied to either input channel. One thing I have noticed is the left channel heat sink consistently gets warm just sitting idle and the right channel heat sink stays cool. The amp also draws approx. 2 amps at idle. What else should I look into as to what might be wrong?

Thanks to anyone that can help.

Ivan
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Old 5th April 2006, 06:21 PM   #2
anatech is offline anatech  Canada
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Hi Ivan,
Check the bias current on both channels. Right might be low, left is too high.

-Chris
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Old 5th April 2006, 06:45 PM   #3
lopezi is offline lopezi  United States
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Yes, I've tried adjusting the bias pots. and the left channel heatsink still gets warm, while the right is still cool. If I adjust the left channel pot to the point where the idle current drop under 1 amp. I can hear a hissing sound, but I am unsure where that is originating from.

I'm assuming there is still some other part that needs to be swapped out, but I am unsure what it is. There is a small transistor that butts up against the heat sink, I think it's the PNP transistor for the output FET's, if that is bad would it cause the left channel output stage to draw unnecessary current at idle?
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Old 6th April 2006, 01:39 PM   #4
lopezi is offline lopezi  United States
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Default Amp repair, any insight?

Quote:
Originally posted by lopezi
Yes, I've tried adjusting the bias pots. and the left channel heatsink still gets warm, while the right is still cool. If I adjust the left channel pot to the point where the idle current drop under 1 amp. I can hear a hissing sound, but I am unsure where that is originating from.

I'm assuming there is still some other part that needs to be swapped out, but I am unsure what it is. There is a small transistor that butts up against the heat sink, I think it's the PNP transistor for the output FET's, if that is bad would it cause the left channel output stage to draw unnecessary current at idle?

Ok, I checked those small PNP transisitors and they check out fine. The resistors, very tiny ones, next to the FET's all check out fine at 47 ohms. Power supply FET's all check out fine as well. At idle, what is the acceptable range of current for an amp to be drawing?
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Old 6th April 2006, 07:16 PM   #5
anatech is offline anatech  Canada
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Hi Ivan,
The hissing is coming from the torriod power transformer.

Early Fosgates adjusted their bias for minimum THD into a 4 ohm load at 10 or 20 KHz. Most just have a target voltage reading across a (source) resistor. Low standing current is what you want for a car application.

-Chris
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Old 6th April 2006, 07:30 PM   #6
lopezi is offline lopezi  United States
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Quote:
Originally posted by anatech
Hi Ivan,
The hissing is coming from the torriod power transformer.

Early Fosgates adjusted their bias for minimum THD into a 4 ohm load at 10 or 20 KHz. Most just have a target voltage reading across a (source) resistor. Low standing current is what you want for a car application.

-Chris

So 1-2 amps is ok for idle current? What would unacceptable? Obviously there should be no hissing coming from the torriod power transformer?
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Old 6th April 2006, 07:39 PM   #7
anatech is offline anatech  Canada
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Hi Ivan,
Sometimes hissing is normal. I can't say as I am not familiar with your amp.

It would be great if someone who repairs these would comment. Same goes for idle current draw. I personally can't say. Can you e-mail the distributor or manufacturer?

-Chris
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Old 6th April 2006, 10:40 PM   #8
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I've never worked on the amp model that you have but generally, you can set the bias current by the voltage across the source resistor (as Chris said). Typically, setting them to ~1mv DC (0.001v DC) will be good enough. When there are multiple transistors that are essentially connected in parallel (in each channel, the 540s are essentially in parallel and the 9540s are essentially in parallel), make sure the voltage across the resistors matches. For example, if you use a resistor for one of the 540s as the test point when setting the bias current, be sure that the voltage across the other source resistor for the other 540 is the same. Ideally, the voltage across all of the emitter resistors for that channel should be the same.

If the hissing is coming from the transformer...
Many times, when you set the bias current too low, there is not enough load on the power supply and the regulator becomes somewhat unstable. The noise you hear is possibly the pulse width changing rapidly as the regulator tries to maintain the target voltage. If the noise goes away when the bias voltage is set properly, the supply is likely OK.

If the noise is coming from a speaker connected to an underbiased channel it could be from the amplifier trying to maintain zero DC offset (with no signal). When the bias is properly set, the output transistors are on the threshold of turning on and the drive circuit only needs to swing a fraction of a volt to control the output. When underbiased, the drive circuit has to swing more to maintain zero DC offset and the noise is sometimes audible.

For relatively small amps, the idle current should be ~1-1.5 amps. For larger class AB amps with a large number of outputs, the idle current could be as high as 3 amps. Unless the biasing is set very high, these numbers are very common.

The small transistor between the outputs on the heatsink is the bias compensating transistor. It helps to maintain bias current over a wide temperature range.
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Old 7th April 2006, 06:22 PM   #9
lopezi is offline lopezi  United States
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Quote:
Originally posted by anatech
Hi Ivan,
Sometimes hissing is normal. I can't say as I am not familiar with your amp.

It would be great if someone who repairs these would comment. Same goes for idle current draw. I personally can't say. Can you e-mail the distributor or manufacturer?

-Chris

A few weeks ago I emailed Sony for a copy but never received a reply back. But last night while I was searching for some replacement transistors for a different amp, I stumbled across a website that had schematics for some of the Sony amps. In particular they happened to have the model I was working on.
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Old 7th April 2006, 06:31 PM   #10
lopezi is offline lopezi  United States
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Quote:
Originally posted by Perry Babin
I've never worked on the amp model that you have but generally, you can set the bias current by the voltage across the source resistor (as Chris said). Typically, setting them to ~1mv DC (0.001v DC) will be good enough. When there are multiple transistors that are essentially connected in parallel (in each channel, the 540s are essentially in parallel and the 9540s are essentially in parallel), make sure the voltage across the resistors matches. For example, if you use a resistor for one of the 540s as the test point when setting the bias current, be sure that the voltage across the other source resistor for the other 540 is the same. Ideally, the voltage across all of the emitter resistors for that channel should be the same.

If the hissing is coming from the transformer...
Many times, when you set the bias current too low, there is not enough load on the power supply and the regulator becomes somewhat unstable. The noise you hear is possibly the pulse width changing rapidly as the regulator tries to maintain the target voltage. If the noise goes away when the bias voltage is set properly, the supply is likely OK.

If the noise is coming from a speaker connected to an underbiased channel it could be from the amplifier trying to maintain zero DC offset (with no signal). When the bias is properly set, the output transistors are on the threshold of turning on and the drive circuit only needs to swing a fraction of a volt to control the output. When underbiased, the drive circuit has to swing more to maintain zero DC offset and the noise is sometimes audible.

For relatively small amps, the idle current should be ~1-1.5 amps. For larger class AB amps with a large number of outputs, the idle current could be as high as 3 amps. Unless the biasing is set very high, these numbers are very common.

The small transistor between the outputs on the heatsink is the bias compensating transistor. It helps to maintain bias current over a wide temperature range.

Perry, thanks for the information. As mentioned in my previous post I happened to stumble across a schematic for the amp in question after I had read your post. Your information was spot on with the schematic. They even had a section of the document directly related to the bias adjustment and the idle current is ~2amps. So I guess I have successfully repaired it.

For others who may be searching for similar info on Sony amps, see the previous post for the link for schematics and here is the Sony recommended bias adjustment

Procedure :
1. Turn the variable resistors RV105 (L-CH) and RV205 (R-CH) full clockwise as seen from the component side to minimize the bias current.
2. The input signal is to be no signal.
3. Apply the voltage to the B+ and REM terminals from the stabilized power supply and gradually increase it up to 14.4 V while checking for any unusual current.
4. For the XM-1502SX, adjust each of RV105 (L-CH) and RV205 (R-CH) so that the power current of the stabilized power supply is increased in steps of 500 mA (total of 1 A). For the XM-1902GX, adjust each of RV105 (L-CH) and RV205 (R-CH) so that the power current is increased in steps of 600 mA (total of 1.2 A).
5. After adjustment, check that the power current is at 1.3 to 2.0 A.
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