Shit! Another X-DAC V3 Problem

[leo]

Ok, just to be sure, this breaking up was only heard after changing the diodes and input cable yes?
What diodes did you use exactly? the part number


Certainly try re fitting the original cable, we need to isolate where the the problem is and start at the basics, fault finding on these sort of things can be long and tedious

[leo]

When you say the opto is not working, what other components is on the opto circuitry?
does this have its own regulation with no other circuitry sharing ?

[rickcr42]

Two thoughts :

1-The optical lens needs a good cleaning or may in fact be marred/scratched (it happens folks )

2-While replacing other on board parts you exposed the optical receiver to heat and it deformed the lens or housing.

If neither of the above look to any capacitors (or any other passive/mechanical parts) between the toslink receiver and the digital receiver chip for your answers

[Deleted]

XTRProf
In some units the IC above the AD898 is a 26LS32AC -Quad differential line receiver . (FWIW !)
Alex

[XTRProf]

sandyk said:

XTRProf
Are you able to try a return to the original cables that were shielded ?
Just to make sure not picking up radiated digital artifacts.I agree, that it does sound unlikely, but you haven't been able to verify that fault wasn't there previously.After all , you thought that Schottkies were a safe bet too !
BTW, I know that Leo will agree that only ONE modification should be attempted at a time, then fully evaluated, before performing the next modification. BTW, short lengths of shielded cable aren't the impedance you think they are ! The true characteristic impedance of a cable is only reached after a certain minimum distance. This will depend on type of cable and it's construction. You will need to research any further info on this subject for yourself. I have forgotten !
Alex
Alex

Hi Alex,

Yes, of course I can reverse engineer to original cable. I will do that for the AUDIO cables as I had not touched the digital cables since I don't have any true loose 75 ohms video lying around. I wouldn't touch that until I have true 75 ohms cable.

Audio cables do not need minimum distance. In fact, shorter the better. Oh dear, you used long audio cables to go for minimum impedance? I will go for 0 impedance for audio cables. Zen and tech tell my senses that.

In fact, noting is safe bet for me as I'm zentech. I will throw out even technically superior things if it doesn't work to my expectation. I bought the schottkies is because I don't have any other more superior diodes on hand. What will you buy if that's the case for about the same price? lesser diodes like hexfred and mur?

I concur mostly with what u had written.

[XTRProf]

leo said:

Ok, just to be sure, this breaking up was only heard after changing the diodes and input cable yes?
What diodes did you use exactly? the part number


Certainly try re fitting the original cable, we need to isolate where the the problem is and start at the basics, fault finding on these sort of things can be long and tedious

I will do that after I had tried the hexfred diodes at reg2 to see the effect. Sorry, part number for schottkies diodes Ican only fill that when I have a look at them again as I'm outside now.

Again, concur with all as written by u.

[XTRProf]

leo said:

When you say the opto is not working, what other components is on the opto circuitry?
does this have its own regulation with no other circuitry sharing ?

I think there are 2 mini pulse trafos there. I will upload a picture to let u see what I mean. Sorry, for only what I think as I had not seen a mini pulse trafo b4.

It should be shared with the other digital circuitry as I believe one 7805 will be for digital and another 7805 for analog with the audio circuits being unregulated but sharing the same 12-0-12 vac line.

[XTRProf]

rickcr42 said:

Two thoughts :

1-The optical lens needs a good cleaning or may in fact be marred/scratched (it happens folks )

2-While replacing other on board parts you exposed the optical receiver to heat and it deformed the lens or housing.

If neither of the above look to any capacitors (or any other passive/mechanical parts) between the toslink receiver and the digital receiver chip for your answers

I concur. but I don't think this is the case as I'm quite meticulous. Now the coaxial is working except the opto. Both of these are on the same circuitry except separted by the pulse impedance matching trafos.

[XTRProf]

sandyk said:

XTRProf
In some units the IC above the AD898 is a 26LS32AC -Quad differential line receiver . (FWIW !)
Alex

Oh, thanks Alex.

[Deleted]

XTRProf and Leo
I had a good look at an underneath photo. The K of both those diodes is joined together. The rectified DC supply then feeds the +5V Analogue, +5V Digital and +3.3V digital electros in parallel. i.e. The inputs of all 3 regulators come directly from the junction of the 2 diodes via top side links.(PCB traces)
Alex

[XTRProf]

Hi Leo,

Luck in on my side as I found the Schottkies SKU Nos in my diary in my car. Might as well post this now.


RS SKU No Description

5450719 MBR2060CTG
5452844 MBR1060G


TIA all advice!

[XTRProf]

sandyk said:

XTRProf and Leo
I had a good look at an underneath photo. The K of both those diodes is joined together. The rectified DC supply then feeds the +5V Analogue, +5V Digital and +3.3V digital electros in parallel. i.e. The inputs of all 3 regulators come directly from the junction of the 2 diodes via top side links.(PCB traces)
Alex

Gee, thanks Alex,

You are faster than me in this.

Sorry, Alex, I remember vageuly there are 2 12-0-12 VAC tracks. One goes to feed a 7805 and unregulated audio OPAs thru the bridged diodes. The next goes to feed the 7805 and LF33CV thru another two diodes. I might be wrong without having a second look. How come only one common feed to all for your dac?

Sorry guys, I need to go off now. Will be back at night, Singapore time, to disturb the peace here again.



Cheers!

[Deleted]

XTRProf
Had a closer look. The +5V Analogue is derived from the bridge rectifier.The +5V and -3.3V Digital supplies are both derived from the 2 separate diodes. You are correct. It's a lot harder trying to work from a photo than actually looking at the PCB !
Alex

[XTRProf]

It's getting more exciting and I passed by my place and did this first b4 going out again. It's better than sex, huh? He, he, he ......

[leo]

Reason being I asked if theres any other circuitry run from the same supply as what feeds the opto is to see if anything may have got damaged further down the chain.

So now the co-axial input is working ok?

[XTRProf]

leo said:

Reason being I asked if theres any other circuitry run from the same supply as what feeds the opto is to see if anything may have got damaged further down the chain.

So now the co-axial input is working ok?

Hi Leo,

Pls do ask anything as I wanted the problem solved, right? How can a guru like you help without having the facts. It reminded me of the engineers and technicians in my former pcba plant as well as my father ways. Real fault verification systematic approach.

Yes, the coaxial is working but with the faint breakup. As the opto is in the same circuitry, chances are the opto had failed. I will fit in the new opto 177 to see how as when I reached farnell they had ran out of stock. ETA either today or 2moro. Chances are high there is nothing wrong with any of the ICs and something else is cauing the faint breakup like the lossy digital internal cable which is not 75 ohms type. I vaguely recall that I don't have that faint breakup thru the opto using a glass toslink. Well, if the new opto fitted in will give more answers for fault verification. If th sound is clear, the coaxial stream will be giving problem.

Ya, btw any good recommendation for a good digital oscilloscope? That will be helpful for these kind of fault diagnosis although I don't know how to use one since I don't work on the electronic technical side.

[Deleted]

Oh dear, you used long audio cables to go for minimum impedance? I will go for 0 impedance for audio cables. Zen and tech tell my senses that.

XTRProf You don't need to be a Zen Master to know that you use the minimum length necessary for audio cables.
A VERY short length of shielded cable will have a higher impedance than it is marked. A coaxial cable drops to it's rated impedance after a short distance, then does not decrease further with length. Zero impedance would be a short circuit !
Alex

BTW, this is what I was taught years ago. This information may not be completely accurate these days. It mainly related to twisted pair telecommunications cables.

[XTRProf]

sandyk said:

Oh dear, you used long audio cables to go for minimum impedance? I will go for 0 impedance for audio cables. Zen and tech tell my senses that.

XTRProf You don't need to be a Zen Master to know that you use the minimum length necessary for audio cables.
I think your understanding of cable impedance is all confused !
A short length of shielded cable will have a higher impedance than it is marked. A coaxial cable drops to it's rated impedance after a certain distance, then does not decrease further with length. Zero impedance would be a short circuit !
Alex

Oh dear,

I'm talking of audio cables and u r talking of video cables. Audio cables should preferably b 0 impedance. Ya, short circuit for the best sound. U mean u want audio cables to have impedance. I know we cannot get away with impedance. Any tech minded will know that.

[Deleted]

"Oh dear,

I'm talking of audio cables and u r talking of video cables. Audio cables should preferably b 0 impedance. Ya, short circuit for the best sound. U mean u want audio cables to have impedance. I know we cannot get away with impedance. Any tech minded will know that. "



XTRProf
You really are mixed up! Audio or Video, same deal.
This time I suggest you talk to your knowledgeable DAD !
By short circuit for the best sound, you really mean a direct connection. No series resistance or inductance, and no parallel capacitance.

Alex

[Deleted]

I had not touched the digital cables since I don't have any true loose 75 ohms video lying around. I wouldn't touch that until I have true 75 ohms cable.

XTRProf
Seeing that you seem so determined to replace those internal SPDIF leads, why don't you get a short length of S-VHS cable ?
The red and the white shielded cables inside, are nice and thin, as well as 75ohms.
Alex

[leo]

xtrprof said:

leo said:

Reason being I asked if theres any other circuitry run from the same supply as what feeds the opto is to see if anything may have got damaged further down the chain.

So now the co-axial input is working ok?

Hi Leo,

Pls do ask anything as I wanted the problem solved, right? How can a guru like you help without having the facts. It reminded me of the engineers and technicians in my former pcba plant as well as my father ways. Real fault verification systematic approach.

Yes, the coaxial is working but with the faint breakup. As the opto is in the same circuitry, chances are the opto had failed. I will fit in the new opto 177 to see how as when I reached farnell they had ran out of stock. ETA either today or 2moro. Chances are high there is nothing wrong with any of the ICs and something else is cauing the faint breakup like the lossy digital internal cable which is not 75 ohms type. I vaguely recall that I don't have that faint breakup thru the opto using a glass toslink. Well, if the new opto fitted in will give more answers for fault verification. If th sound is clear, the coaxial stream will be giving problem.

Ya, btw any good recommendation for a good digital oscilloscope? That will be helpful for these kind of fault diagnosis although I don't know how to use one since I don't work on the electronic technical side.

A 100mhz scope would be useful, some of the TEK scopes are quite decent.
With a high range scope you could have a look at the SPDIF signal

Does the X-dac have any adjustable attenuation for SPDIF input? this distortion problem you are getting is proving tricky.
If I had a problem like this I'd try tapping in another dac circuit after the X-dac V3 SPDIF circuitry to eliminate the possibilities the problem is further down stream but this isn't easy to do.
We need to isolate where the problem is first.

I know these units have input DC blocking caps but check to be sure no DC is getting to the primary of the pulse traffo's.

Are you sure the chips in the SPDIF circuit are ok, if you eliminate the circuitry up to the input receiver your not really left with much circuitry to test

[XTRProf]

Good night gurus ,

Reporting time. I had recieved the TORX177 and did some preliminary measurement of the TORX176 against the TORX177 before deciding on the fault corrective action as that would involved some time consuming modifications to the PCBA and 177. There was an open circuit between the output 1 and ground 2 of the 176. The 177 showed close circuit at these same taps. So the chances are high this is really the problem supported by previous observations. So I went ahead to do the mode and fitted in the technically more superior 177. Surprisingly, the mode is quite simple and I took only about 1/2 hour to do it. So all staying by the sideline and looking to change the 176, carry on. It's not difficult. However be careful wih your hole marking and drilling as you don't want to be out for such a precisely molded 177. Any questions as to how to do it, please feel free to pm or put your questions here. I'm always available to help you guys and gals in this forum provided I know them.

After putting in the 177, I connected up the glass toslink and started the usual tracks of Amanda McBroom with finger cross. Ha, ha, I'm very glad the sound came and the dac lock-in. So fault verification was correct with some help from your guys here. Now both coaxial and opto working. Danke. However, there is still the same faint breakup on heavily modulated vocal.

Leo, after sitting down to think further, the problem may not be as complicated as first though. I think there is some external influences distorting the sound. On depeche mode of synthesizer type of music, I can hardly note the breakup and the music was smooth and distortionless. Only on heavily modulated music like vocal baritone will this breakup be noticeable. It kind of reminded me of FM with strong signal but poorly directed anttena, there those reflected signal causing multiplex distortion. Going backwards with an very open mind, I think there can be some basis of changing to the unscreened audio cables due to no time to do a double screen with a copper foil and finishing with a copper braid. Is the digital emission in a dac that strong as to be able to cause those "multiplex" distortion? I'm really noob in this. Please elaborate as I'm learning. In the meantime, I will prepare myself to change back to the screened audio cable and if successful will buy the copper screen since I have the copper foil. If that's the case, I think screening all the OPAs and other analog circuitries will really provide a big improvement in sound.

Anyway, I had been looking to invest in a scope for sometime if I'm to advance more into this AV hobby. I wanted to be able to account for what I hear as well although sometime we could not account for it technically. Well, we can be just that much closer. Ok, I will check up the Tek 100MHz scope. Is that a digital scope as nowadays the trend is towards digital scope and digital scope software. Is those digital scope software high rez enough for the AV job.

Nope, the x-dac SPDIF is not adjustable. I wish I'm as advance as you and know where to tap another dac after the spdif to check for fault. Come to the worst, I will just buy another dac chip and replace the present one as calculated lucky dip and nothing technical after much fault evaluation and thoughts.

Hmm, checking for dc. That's a good idea. Must be less then 50mV, right? I will do that on Sat as tomorrow, I will be fully occupied. I'm really not sure whether those AD898 and the TI chips are ok. However, I will try the screened wires first before changing that. I will report back on Sat as I will be getting the screened braid on Sat.

Thanks all gurus for your kind advice. Hail!

[XTRProf]

Hi Alex,

I don't think it will be appropriate to change any more things now until I can get things back on track first. Yes, I'm still determine to change the internal digital cables until I can find a true 75ohms quality one. Those good quality loose ones are mostly audio cables, btw.

Although there are these problems, I had learned alot with the help of all you gurus here. Anyway, how to learn without problems, right?

[XTRProf]

Hi all gurus again,

As the sky had darkened and raining heavily now, I'm stuck at home as I was getting out to buy some copper braid for the audio cable screen. Time to get back some senses after the whole day occupation of driving about yesterday. So I opened up the x-dac to look closely at the spdif section for some educated fault verification.

As both the coaxial and the opto are working now, it seems there is no problem from the input RCAs or opto up to before the 2 digital receiver SOICs. This is because the same faint breakup was being faced by both and the coaxial is connected to the SOICs only after the impedance matching pulse trafo and the opto is connected directly to the SOICs, bypassing the pulse trafo. So have to go further downstream from the digital SOICs onwards for problem verification and educated reasoning. I will try screening the audio cables after I had got the copper screen.

Aha, coast clear again and now I can go out now. Will report back tomorrow for all those interested.

[Deleted]

Please do !