complicated question about audio dac

[paulhm81]

hi everyone.
 I have a digital sony amplifier with a problem. The volume doesn't work. The volume moves on the display but the audio volume remains the same. The amplifier was in a shop for repair but the guy that gave it to me said they asked for too much. I forgot about it for two years and when  I looked at it an IC was missing. It was the PCM1602A which is a digital to analog converter. I bought a new one and tried to get it working to no end. Also there is noise on any input that i select. I tried the mute on the remote and the display says muting but nothing happens. I tried the FM input and it work's at a loud volume so i guess the analog part is ok.
   I read the datasheet and it seems there is a serial connection between this dac and the main IC but I don;t know how to verify if they are communicating because I don;t have access to the IC-s while it's powered on and I don't know what to look for. The ic=s are on a vertical board and I can;t get the probes in there.
      Is there any preprogramming needed for the DAC?
      How do you probe in situations like this one where you don't have access?

[theatrus]

If the DAC was missing I would expect no audio, not just a lack of volume.

Volume can be implemented either via digital attenuation (somewhere on the digital I2S path, likely in an all-in-one DSP or specialized processor), or on the analog path via a digital stepped attenuator ("digital pot"). I don't see anything on the block diagram for an analog component, so its in the digital path.

As for probing while inaccessible, what is limiting access? Can't you just run the board (watch out for thermal limits!) out of the chassis?

[paulhm81]

the dac was missing but I put a new one. In the datasheet for the dac it says it supports soft mute and digital attenuation from 0 to - 63 db. I don't know much about the digital part of electronics but if I understand correctly the dac gets it's commands for the volume from a 5 wire serial command from ic1101 (MDO, MDI, MC, ML, RST).
The digital board is connected to the main board with two slot connectors and there are a few ribbon cables from the frontal keypad and screen and from the power supply. I tried to get it started without being sloted in the main board and it tries to power up and than shuts down instantly. Maybe i will try to get a wire soldered to the MDI pin and probe that with the scope to see if it tries to write the commands to the dac but I only have an analog scope.
    My worst fear is that the dac requires preprogramming of some sorts. Maybe someone knows this pcm1602 or take a look at the datasheet to tell me not to waste my time if it shouldn't work "out of the box".
Thanks!

[Rerouter]

There is no eeprom on the chip so it should run out of the box as long as its the correct chip,

Equally the volume control would likely use the DAC's volume opposed to the DSP, as its already there,

Perhaps post the audio output section just to make sure there not using analog volume control, but i cannot see why they would,

[theatrus]

Agree, there would be no programming of the DAC - its all configured over the SPI bus.

It looks like the next course of action is to check the SPI bus lines (MDI, MDO, MC, ML on the datasheet) to see that the attenuation control register is getting programmed.

[AndyC_772]

Guys, there's a major red flag here. Components don't just disappear. If there's a part missing, then that's either intentional (ie. DNF from the factory), or someone's removed it.

If somebody's removed acomponent, they've done it for a reason, which could be anywhere between "urgent need for that component in another project" all the way down to "idiot with soldering iron".

You need to find out what else has been done to the unit, by whom, and why.

[paulhm81]

I think I'm getting somewhere.

I probed the serial pins and there was a signal but only in the milivolt range (like 20mv p-p). I'm assuming that's noise, right? I started to look at the ic that's commanding the dac and i noticed it has an oscillator (x1101 - 24 mhz) and in the service manual it shows that on pin 82 I should see a 2.2Vp-p  signal at that 24mhz and there's nothing. I'm attaching the waveform page of the service manual where at (7) is shows the signal and I'm attaching schematic for this IC. 
What should I do next? What could be the problem? The oscillator ? the IC?

[theatrus]

I'm with AndyC, the first mystery is why was there no DAC

[paulhm81]

I tried to explain in my first post. The unit was in a shop for repair and they asked for too much and my friend who gave it to me refused the repair. They gave it back to him with the dac taped on the case (i forgot about this as it's been a long time since i looked at it). I soldered it back in to see what's happening and I saw the volume didn't respond so I bought a new dac a week ago. With the new one the problem remains. Probably the repair shop wanted to replace the whole digital board. They asked for about 100 euros. I found one in working condition on ebay for ~ 80 usd. I'm not interested in buying one but rather curious what's wrong with it. Learning purposes. If I get it running of course I will use it, but if not so be it!

[theatrus]

Aha, I somehow missed that. Oscillators don't often fail, and again I'd expect no output from the system if no clocks were running. Have you taken stock of the general health of the unit? Measure all rails, check for ripple, and check all other clocks as you find them.

If you have a signal generator, you can feed the output of that into the oscillator input of the main IC, even if the frequency is too low. *something* should happen if its alive.

[paulhm81]

I'm very new to scopes so I'm not very sure if I'm getting it in terms of ripple and noise. I checked all the voltages on the board and except a 7.6 instead of 9v from the psu everything seems ok. I didn't worry about the 7.6 v because it goes in a 7805 and the output is ok.
I don't have a signal generator , sorry. That oscillator is a piezoelectric oscillator. Do they fail? 
Being new to scopes tell me something. I'm attaching two photos of the screen of my scope. The sine wave is on 50 ns at 20mv. The other photo is at 20 us if I remeber correctly at 20 mv. What could cause that? I haven't seen a serial signal on my scope before  but I don;t think it would be at just 20 mv, would it? I was expecting at least 1v p-p. That's why I said there is no signal on MDI, MDO, MC, ML. When measuring the piezo oscillator I got nothing at all.

[GiskardReventlov]

The repair shop must have provided a diagnosis or did they just provide the repair cost? A little surprised to hear that they gave him back the unit with the chip removed. Maybe that's considered helpful as-in that's a big hint to what's wrong.

You should post some pics of the unit.

[paulhm81]

it's this one - http://esupport.sony.com/US/p/model-home.pl?mdl=STRDG510&LOC=3&session_id=d5eca0267eceb82f86039cb0d135cc96#/howtoTab

I searched a bit how to check an oscillator and it is said that by probing it I'm adding capacitance and that might stop the oscillation. Maybe use the 10x? Any other way to check if it's ok???
Please tell me what peak to peak voltage should I see when probing the spi between the chips. The only passives between the spi pins are 100 ohm resistors. Sould I see the bits on an analog osc? I have a philips 100 Mhz osc.

[theatrus]

You should see a logic level square wave on the clock line when there are commands to send. Twiddle the volume knob and check.

[paulhm81]

I got it half way working. The spi signal was there but very faint on my screen. I only noticed it with the intensity at maximum and playing with the volume. The problem was (is ) elsewhere. There is a auto calibration board which they call dcac  with a microphone jack. On that board there is a double opamp. I found a capacitor with 8 uf instead of 10 and replaced it and the volume started working. It;s very odd. The problem now is that with a flat eq (everything at 0) everything works but as soon as I for example add 1 to the trebble or any other sound correction, the sound disappears. I will try to put a mic in that jack to see if it has anything to do with it. At one point I reset the unit to factory defaults hoping it would come to life. Maybe I need to let it calibrate now. Who knows!? I'm happy for now that it works as it does.