Last time I looked in this area, pre-amp serial control ports were not I2C (actually more like half-duplex SPI), and recent ones may not be 5V-tolerant. For these reasons, I wouldn't attempt to use that specific circuit for anything except accessing 5V I2C EEPROMs and similar. Just MHO...

Thanks for the correction.

Some datasheets describe the interface is "3-wire serial" - Data, Clock, Enable. Most just have little more than a block diagram.

The L7250 SMOOTH motor controller appears to use the same interface. There's a reasonably detailed description in the datasheet.

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A backup a day keeps DR away.

Thank you "pcimage" and "pruggero", i tried both your rom files but the problem is still the same.
I can't access to SA MODs.

Probably the pre-amp is faulty...

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My firmware database:
https://mega.nz/folder/O01DkBRI

I've been looking at preamp datasheets and I notice that the following IC, while being powered from +5V and -5V, has an internal 3V supply. Several of the pins are pulled up to 3V via internal resistors.

Your preamp would undoubtedly be different, but I would look for 3V at the preamp connector.

BTW, the following datasheets discuss the 3-wire serial interface in detail:

VM5141, VTC Inc, Magneto-Resistive Head, Programmable Read/Write Preamplifier, +8V, +5V:
http://shostatsky.narod.ru/rem_comp/pdf/vm5141.pdf

TDA5360, Philips, Pre-Amplifier for Hard Disk Drive with MR-Read / Inductive write heads:
http://www.datasheetcatalog.org/datashe ... 5360UH.pdf

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A backup a day keeps DR away.

This are the voltages on the pcb head pins (with pcb physically disconnected from the drive)

PIN 1: 0
PIN 2: 0,15V
PIN 3: 3,3V
PIN 4: 0,08V
PIN 5: 3,15V
PIN 6: 5V
PIN 7: 3,15V
PIN 8: 2,4V
PIN 9: 0
PIN 10: 0
PIN 11: 0,15V
PIN 12: 0
PIN 13: 0,15V
PIN 14: 0
PIN 15: 0
PIN 16: 0
PIN 17: 0
PIN 18: 0
PIN 19: 0
PIN 20: 3,36V

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My firmware database:
https://mega.nz/folder/O01DkBRI

Pins 11 & 13 and 5 & 7 are the read/write differential pairs, respectively. The write pair will be driven by the read channel within the MCU, the read pair by the preamp. I would expect that the signal levels should be symmetrical, in which case I would expect that the read pair should sit at 3V when the preamp is connected. That's what I'd look for.

There should also be a Fault signal generated by the preamp. If we could identify it, then this would be another thing to check.

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A backup a day keeps DR away.