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Old 18th November 2006, 12:28 PM   #31
AndrewT is offline AndrewT  Scotland
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those two aluminium plates and the three transistors sandwiched in between are probably the drivers and Vbe multiplier.

Could the burnt resistor and cap at the bottom be connected to ground? Are they the Zobel network? If so, then oscillation could be the cause.

Try to tie in the schematic to some of the PCB. Give us some clues.

The two 4r7 in the middle are probably the Zobel.

What causes predrivers to blow? Shorted output stage?
Where are the drivers?
regards Andrew T.
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Old 18th November 2006, 12:30 PM   #32
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The burnt bottom resistor is the one that supplies the front end from the main rail, as on the later 8000 edition.
You can see the thick supply tracks shine through the board on both sides of the circuit, and zener diodes connected to it on both sides as well a bit further down the pcb.
It forms a low pass filter with the foil cap. next to it.
If that resistor is burnt, it means the pre-driver is dead.
Pre-driver diseased=> the corresponding driver damaged or dead too.

The lack of bigger drivers makes you wonder whether the output devices are not Darlingtons.
For regular output transistors, the minimum driver size would have to be a TO126 device.
(unless it's a mosfet output stage, which additionally would make the small TO220 devices look more natural)

The ones between the aluminium plates are flat TO92 versions, such as Zetex ZTX devices..
That leaves the outer real-TO92 ones as the drivers, likely MPSA versions as the 2d and 3d edition had.
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Old 18th November 2006, 07:17 PM   #33
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Hi all Its difficult to follow the track but it looks like the burnt out resistors do go to the sandwiched transistors and down through the zener, i have also found another three transistors which were sandwiched by aluminium strips one which has just fell off i dont know where the other is. I will post more photos.

Thanks dave.
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Old 18th November 2006, 07:19 PM   #34
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photo 2
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Old 26th November 2006, 03:28 PM   #35
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ground control to major tom
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Old 26th November 2006, 10:25 PM   #36
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Output devices are sm 8411 does anyone know whether they are transisor or fet, all the left side output devices are shorted plus some of the transistors fron the zoble, 4 resistors, caps are ok and zener diodes.
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