D'oh! Yes, pin 13.

The datasheet is very unhelpful with regard to the loudness circuit. I don't know if loudness circuits are still common on audio amps, but in auld lang syne they were. Essentially they provide a bit of frequency response shaping (some bass boost, but I don't remember details) that was intended to make music played a low volume sound more "true" and typically could be turned on and off with a switch on the front panel.

I have no idea if the resistor value is critical. It may be since 3.9k is slightly oddball, even if it is a standard E12 value. I'd try the closest available value on the high side as an experiment. There is no reason not to use a through hole part for the final fix as long as fits when the board is mounted.

Surface mount resistors are pretty robust, but is is possible to crack them without it being obvious. When they are numerically marked, you read the markings just as you did. Some are marked with a three character alphanumeric code that is essentially a sequence code, requiring a table to map the code to actual value. The trend, unfortunately, is to put no marking at all on surface mount resistors. Most SM ceramic caps have been unmarked for many years. I have seen a few with markings.

Anyway, happy to help. You did an excellent job with your original post by providing good photos, datasheet links and a good description of what you had tried. Sometimes when people ask for help it takes a dozen back and forth requests and replies to pry out the info you provided right at the start. Well done!

 

Anyway, happy to help. You did an excellent job with your original post by providing good photos, datasheet links and a good description of what you had tried. Sometimes when people ask for help it takes a dozen back and forth requests and replies to pry out the info you provided right at the start. Well done!

Thanks!!!

The datasheet is very unhelpful with regard to the loudness circuit.

I think this datasheet (attached: "TC9235P - datasheet.pdf" ) has more information. I think the 3.9K ohm resistor is used when loudness function is not required. There is a table for attenuations if 3.9K is used but it doesn't specify what happens if you use another value.

If you have any sort of capacitor somewhere in the range of 30 to 100 or even 220 nanofards (0.03 - 0.22 µF) on hand, you could use it as a test replacement. The sound might not be "right" but it would verify your hypothesis.

Another a question on the previous problem. This board used two 68nF capacitors in series and a 820 Ohm resistor in what looks to be a highpass filter (5708.57Hz cut off) before the U10 OpAmp. Since I am using 2x 100nF capacitors the cut off is now 3881.828Hz. What does this mean exactly for my audio quality? Sounds ok to me but should I look into changing out those 100nF caps to the original values?

Also I think the output of U10 goes to Power Amp input pins 11,12 (as you mentioned earlier) and the corresponding Power amp output pins are 14,15 which got to the left right external speakers (tweeters). So it seems U10 doesn't supply the sub woofer. Correct? which would mean the highpass filter mentioned above sends everything above the cut-off frequency to the external speakers (tweeters). It would seem that the sub-woofer is powered through LM324 getting a signal from earlier on in the preamp stage.

 

Ok Second Problem with the volume control FIXED!

Any chance that resistor might have been damaged or desoldered at one end in changing the IC?

@ebp it turned out that your intuition was spot on while replacing the other volume control IC I created a tiny solder bridge (or maybe just some conductive dirt) got between pins 12 (LT2) and 13 (AGND) effectively shorting it. I was nearly going to remove the resistor to test and decided to reflow and clean that area first. Tested with Multimeter and it was measuring correctly again!!!

So all fixed!!! Couldn't have done it without you guys! That thing has been lying in my basement waiting to be fixed for months. Glad I found this forum. I will definitely be back. Special thanks to @ebp ...your judgement and intuition were invaluable!

 

Excellent!

I don't imagine the cutoff frequency of the filter is terribly critical, though lowing it probably does put a bit more power into the tweeters that they can't efficiently turn into sound.