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Old 3rd January 2005, 08:32 AM   #11
Eva is offline Eva  Spain
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Sometimes the power supply blows just due to exagerated power consumption. Check the output devices of each channel. Check loudspeakers and wiring against any short, also to the chassis or to +12V. Check also for punctured sil-pads [the ones between the transistors and the heatsionk], for solderballs shorting pads in the PCB and for bad solder joints in the entire PCB. Half of the car-audio amplifiers I repair suffer from bad solder joints

These two A1266 transistors are probably the ones that discharge the gates of the switching MOSFETs when the conduction cycle ends and the TL494 stops suppling current. Each one should be placed near 1N4148 or similar diode and a 1K or 1K5 resistor and the diodes may be shorted. In this arrangement the TL494 charges directly the gates from it's emitter outputs [pins 9 & 10] so it may be also blown

The first step is to repair the oscillator and gate drive circuit. To test it remove all the switching MOSFETs and temporarily solder some 4.7nF capacitors between gate and source pads of each set of MOSFETs to emulate gate capacitances. Then check gate waveforms with oscilloscope. The frequency should be between 30Khz [60Khz osc] and 50Khz [100Khz osc]. The fall time should be short <500ns and the Vgs should drop to 1V or less. The rise period may be a bit longer <1.5us and the Vgs should rise to approx. 2V below the supply voltage. There must be some clearance period [dead time] of about 1us between the fall of the Vgs of each set and the rise of the Vgs of other set, the waveforms must not superpose, otherwise there will be cross-conduction and transformer saturation [and blown MOSFETs in unexplainable circumstances]

When you get the gate drive circuit to work properly install the switching MOSFETs and thest the amplifier with a 5A to 15A 12V power supply. Idle power consumption should be below 1A [250mA typ.] and the switching MOSFETs and diodes should be cool even without heatsink, otherwise something is wrong. Be careful since the output devices [or the +-15V regulators in some models] may require heatsinking for idle testing if the amplifier is class AB, if they get too hot mount the board in the heatsink for testing [most are class B and test fine without heatsink]

Once you have the PSU working mount the board on the heatsink and check gate waveforms whith some current consumption [connect some load to the amplifier and play some signal at increasing volumes]. The fall period may get longer with load and the dead time will be reduced as the load increases, but there should be still no superposition

Note that if you replace the MOSFETs by another model you have to compare input capacitances, if the new ones have higher capacitances you may have to reduce the value of gate resistors to keep a reasonable dead time. Also, if the new devices have substantially lower Rds-on and higher input capacitance, you may use less of them with smaller gate resistors [ie: 3 instead of 4 or 5] to avoid overloading the gate drive circuit. [ie: I have a lot of IRFZ48V and allways use them to repair car-audio amplifiers. Sometimes I've replaced up to three IRF530 by one IRFZ48V or three P50N06 by two IRFZ48V with suitable gate resistor to keep switching times, it works fine]

As same-pinout replacements for A1266 in gate drive applications you may use BC640 or BD140, both have ECB pinout [and are the ones I use]
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Old 3rd January 2005, 12:11 PM   #12
seahag is offline seahag  Canada
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amazing post.
Too bad its a little over my head. :/
I get the gist of what needs to be done.
In the past I`ve just replaced burnt out parts, never really went as far as that.


Is an osc all that I need to test for what your outlining?

Chris
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Old 5th January 2005, 12:25 PM   #13
seahag is offline seahag  Canada
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Finally back up

questions about fixing this gate drive circut.

1. The 4.7nf Caps? is this important to match the specs of my fets?
as well what type can I use here?


2. I havent used my osc before and not sure on the scale of divisions
it will provide. Its a An LG OS 5020 dual.
Am I just looing for a general shape to the cycle times.. and assuring there is no over lap?

I probably need 2 probes for this I assume.

3. Replacing devices with other types always seemed like black magic to me. You mad it sound so simple.. and I guess it is.

How on earth would you replace 4 fets with 2 or 3 when there
are spots for them on the board? I know they are paralelled but come on

Very excited abut fixing this PS.
Really appreciate your help.

Chris
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