Sony XM-754HX bias voltage - diyAudio
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Old 22nd January 2012, 01:22 PM   #1
Mote is offline Mote  Estonia
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Default Sony XM-754HX bias voltage

I got this amp so that it was already taken apart, two PS FETs were removed, I found two output transistors were shorted. Removed shorted outputs, but at first amp did not want to turn on at all. I discovered that +12V and remote terminals did not give contact to PCB. Resoldered the terminals and added some wire, now amp powers up. Bad solder connection seems to be frequent problem with Sony amps, I had another one with bad connection on ground terminal and some other people on this forum have also had similar problems.

Installed IRFIZ44 in place of missing FETs, original part is MTAJ50N05HD but I read from another thread that FKV550 have also been used in this amp.
Gate resistors are 2,2 ohms, should I replace them with 47 ohms and replace the other two 50N05 transistors with IRFIZ44s?
I tested amp with two FETs earlier for audio, all channels produce clean audio but in one channel transistors got so hot that one started smoking and shorted after a minute of working (I did not clamp them).
After installing IRFIZ44s into amp, I also installed missing outputs IRF9540Ns and IRF540Ns.
Amp produces clean audio on all channels and it runs my Sony 8 ohm 150W max speaker to its limits easily.
But I am afraid that it runs a bit too hot.
I tried to adjust the bias voltage, there are testpoints for that on the board, I set my multimeter to 200mV range (lowest DC voltage range) but I only get 0,00 reading with bias pot fully counter clockwise or fully clockwise, there is no change at all.
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Tools: Perrys tutorial, C1-76 o-scope, Weller WHS40, Mastech MAS830L multimeter, Mastech MY6243 LC meter, 16A supply
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Old 22nd January 2012, 05:35 PM   #2
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0.00 isn't good enough resolution. You need a meter that will read to 0.000. The manual says that 3mv is the maximum bias voltage. I'd set it to 1mv.
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Old 23rd January 2012, 08:41 PM   #3
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I can set my scope to 1mV/div, I tried adjusting it with it. I hoped the signal would be a straight line, but it was some gibberish. There was a denser area, which could be called a line so I took that as a landmark, turned the pot so the dense are would move one division, if I turned the pot more, it would move up a few more divisions and then jump 5-6 divisions. Then amp would start to draw current as well.
Can this be called as setting bias voltage or am I going to need to purchase proper multimeter?
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Old 23rd January 2012, 08:54 PM   #4
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Turn it fully counter-clockwise. Watch the current draw from the 12v power supply. Set the bias where the current draw from the 12v power supply just barely begins to increase. Like in this demo.
http://www.bcae1.com/temp/ausettingbias.swf
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Old 25th January 2012, 05:33 PM   #5
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Replaced the remaining PS FETs with IRFIZ44s, gate signals looked good.
I reset the bias like in the demo, installed the heatsink (earlier the transistors were clamped to aluminium strips) ran it for 1,5 hours at high volumes, heatsink did not even get warm.
Seems that amp is OK.

This amp has option to switch it high-current mode so you can use 1 ohm speakers, is the high-current mode more demanding than high-voltage mode?
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Old 26th January 2012, 12:20 AM   #6
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The stress on the amp is probably about the same in either mode.
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Old 26th January 2012, 09:12 AM   #7
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One more question about this amp.
It has a cooling fan, is it supposed to work all the time or is it triggered with a thermistor?
Anyway on mine it does not work and voltage across its wires is 0.
Can I heat the thermistor with solder iron and see if the fan will start working?
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Old 26th January 2012, 04:22 PM   #8
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It's controlled by TH804.
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Old 26th January 2012, 04:41 PM   #9
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Heated TH804 with soldering iron and fan started working, works perfectly. Thanks.
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Old 11th February 2012, 12:54 PM   #10
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I gave this amp to try out but he could not get it working. It goes to thermal protection at once.
Amp is absolutely cold. The moment I switch on my power supply, thermal protection triggers. I started poking around 494, when I measure voltage on pin 16 often the protection comes off. and amp starts to work normally. If I wiggle power connectors amp again starts to work. To rule all of these variables out, I removed power connector, soldered power wires to board, removed thermistor in charge of thermal protection, but still thermal triggers. When I bend the power supply end of board amp starts working.

Transformer?
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