I think I know what might be wrong.

I now believe that the "2.05" supply should actually be +2.5V.

Here is my analysis of a similar board:
http://www.users.on.net/~fzabkar/HDD/Tu ... 0411N.html

The supply voltages on that board were +3.3V, +2.5V, and +1.37V. The Vcore supply was derived from the +2.5V supply.

Therefore ISTM that the circuit I have identified as "Vneg" is not in fact the negative supply for the preamp. Instead it is the +2.5V supply. The Vneg supply is actually derived from a "flying capacitor" circuit near the motor controller.

I would measure the voltages across each of the capacitors (C1 and C2) at the HDA pins. One should be +5V, the other -5V.

With the power switched off, I would also measure the resistance (on the 200 ohms range) between the top two pins (B & C) of the bottom transistor in the second photo clip (SP1203_regs). The reading should not be low.

Then set your meter on the diode range and measure between B & C and between B & E. If the transistor is good, then you should see approximately 0.6V on your meter for each reading.

_________________
A backup a day keeps DR away.

First reading is +5.05 and -5.05.
Second reading gave nothing.
And the third is 0.625 for both.

The fault appears to be in the 2.5V regulator circuitry.

I have attached a scan from a similar board, plus an application diagram from an L5973A datasheet.

Could you measure the three voltages I have identified?

L5973A, ST Microelectronics, 2A step down switching regulator:
http://www.st.com/web/en/resource/techn ... 003370.pdf
http://www.st.com/web/en/resource/techn ... 004439.pdf
http://www.mrq-ml.com/pdf/L5973A.pdf

_________________
A backup a day keeps DR away.

Left to right 2.05, 1.226 and 5.05.

I found my board and measured it. It does in fact measure 2.06V. Sorry for leading you astray. I don't understand why my other board measures 2.5V, though.

_________________
A backup a day keeps DR away.

That's alrgiht.