Reply 40 of 85, by Jepael
I've tried to analyze the pictures to form some kind of mental schematic of the card.
I don't know how much you know about IC pin numbering etc, so here is a crash course. Every chip has pin 1 marked with dot or dent; also every chip on card has the same orientation, i.e. they all happen to have pin 1 on their lower left corner, also marked by square copper pad on PCB. Then they are numbered counter-clockwise.
I can see the resistor color codes and ICs markings from your photos and other photos, but I cannot see any other component values. Can you help me a bit and make a list of what markings are on the components? I am mainly interested in the blue volume pot R14, transistor Q1 (black three-legged device), C18 value (220uF 16V in some other picture?), diode D1 markings if you can find, and all the rest of the smaller capacitor markings (C1 through C26 or how many there are).
So here are some things for you to measure.
1) check if the RC4136 op-amp U8 has power. -12V from ISA bus goes to C21, C5 and to pin 7 of U8. +12V from ISA bus goes to C22, C4 and to pin 11 of U8. Measure the IC pins with multimeter, the black lead going to any nearby ground, even PC case should work.
2) check if the YM3014B DAC U7 and op-amp work together. First measure DAC supply voltage, pin 1 of U7 should have +5V.
3) Pin 7 of U7 is the half-supply voltage that comes out, is filtered with C3 and goes to U8 pin 9 for buffering. This should measure +2.5V.
4) Buffered half-supply voltage comes out of U8 pin 10, goes also to U8 pin 9 for feedback and ends up back to U7 pin 8. This should measure +2.5V as well.
5) U7 pin 2 is DAC output which goes to U8 pin 5 for buffering. When no audio is playing, it should read +2.5V.
6) Buffered audio comes out from U8 pin 4 and goes also to U8 pin 6 for feedback. Should read +2.5V when no audio playing. This pin is where something should be heard if connected to very small speaker or preferably some kind of amplified speaker line input. Connect amplifier ground first to PC, then this point, or potential difference of ungrounded devices may kill the op-amp.
7) Next the audio is filtered with two second-order filters with some gain. If I am not mistaken, the first filter stage has gain of 2.2 so a 2.5V input to U8 pin 2 should make U8 pin 3 have 5.5V, and that fed to second filter stage, U8 pin 13, with gain of 1.15, should provide 6.325V on U8 pin 12. Audio should be heard on a speaker if connected to U8 pin 3 or U8 pin 12. I do not know how loud it is.
8) After the op-amp, there is the mute and volume control stage. C10 removes DC component from filter stage output, so DC voltages are no longer important to measure. Audio should still be there if it works.
9) after volume pot, the audio goes to U9 LM386 power amplifier input pin 3. It should be amplified with a gain of 20, and U9 pin 5 is the output. The big capacitor C18 blocks DC so amplified audio is available after it and on the audio output connector.
Please post your findings so I can comment on them.
There is no visible marks of anything being broken. From what other people have said, the RC4136 op-amp may have died. But it is unknown if it has taken something else with it, like the DAC or power amplifier chip or even the YM3812 OPL2 chip through DAC pins.