Digital volume control and DACs

[Admiral-dk]

You Really need to identify Chips !!!! as the majority of digital designs (and many modern Linear ones too) are done very close to the Application Note for the same IC => a schematic almost identical to what you see ....

Of cause if they are marked with special customer numbers as suggested - that one will not work - but I have found a good number of useful Tidbits that way over the years.

Per

 

[chrispbass]

You Really need to identify Chips !!!! as the majority of digital designs (and many modern Linear ones too) are done very close to the Application Note for the same IC => a schematic almost identical to what you see ....

Of cause if they are marked with special customer numbers as suggested - that one will not work - but I have found a good number of useful Tidbits that way over the years.

Per

Thank you Per for the advice

Hopefully I’ll get chance to have a proper look tomorrow (or v soon).

It’s nice to have some support

Best to you

Chris

 

[chrispbass]

So I've had a bit of time to continue the journey and also help my friend finish off his Elam 251.

This is the PSU and power amp section.





Power amp tests fine after injecting a test tone.

This is the front end and board



I presume the large chip with the white label is the MPU

Here there is a Cirrus Logic 8416 - CZZ



And here is a PT2037 - LQ





Data sheets attached. Both chips are available!

I'll give the board a visual inspection under the microscope first, and to try my best to familiarise myself with it, before moving on to checking anything else i can at this stage!

Cheers for any input.

 

[chrispbass]

Inverted pcb on pin headers.

QS7779CM underneath

https://www.qsound.com/products/analog-processors.htm

 

[JohnRoberts]

Nothing obvious from a quick glance but back to troubleshooting 101 try to zero in on exactly what is and isn't working.

Again perhaps work backwards from the speakers ( it has them right?).

JR

 

[thor.zmt]

Here there is a Cirrus Logic 8416 - CZZ

SPDIF receiver, probably ok.

And here is a PT2037 - LQ

Probably your culprit. Analog Audio processor including Volume control, Loudness and tone control.

Soldering looks ok, but giving it a complete reflow will not hurt.

Thor

 

[Disco Volante]

Chopstick pokeypokey while playing? Power amp works, now work backwards...

 

[chrispbass]

Back on the case! Assembled the front end and and power amp/psu assembly on the bench, as it’s pretty much impossible to work live on the main board in situ in the housing.



Here’s a quick recap.

Digital input selector selects and routes all audio to main outs albeit low fuzzy volume.

Digital volume control works display, but doesn’t change volume.

All menus appear to be working correctly.



Checks I’ve done previously (no schematics)

1.PSU’s voltage checks and given (everything seems to be working (from a control point of view) I’m 99% sure we’re ok there.

2. Ive injected a signal directly into the PWR amps and they’re ok.

Latest checks…

3. Using the data sheet for the analogue controller chip, I’ve injected a signal on the audio output pins of the chip. I’ve got a fully variable signal through the op amp stages on the main board all the way to the output of the PWR amps

4. Injected a signal on the input pins.
Low fuzzy signal at PWR amp output

5. Checked analogue supply voltage to chip, approx 8V

6. Checked digital supply voltage (DVDD) to chip, approx 1.2V.

Data sheet doesn’t seem to give supply voltages, but does the DVDD supply seem low? I presumed around 3 to 5V, but not sure if that’s typical!

I’m still getting to grips with analogue electronics, never mind digital so excuse my layman’s terms.


Given the input selector is routing ALL inputs wether previously digital via a DAC or analogue line ins to the main analogue outs, it does indicate that the main controller is transmitting the correct data to the analogue controller chip but I suppose one still can’t assume that the correct data to vary the volume is not present on the data line, or the analogue controller chip is indeed faulty.

Is it more likely to be corrupt data and therefore a main controller issue, or the analogue controller chip partly faulty?
Dunno is where I’m at!!

At least a bit further down the road I suppose.

 

[chrispbass]

Thinking about my post above, I’ve realised that I’ve made some very wrong (naive part conclusions) that I shall sleep on.

 

[ccaudle]

Checked digital supply voltage (DVDD) to chip, approx 1.2V.

That seems low for commodity digital devices from 10years ago. 3.3V would be a more common supply voltage for standard logic. You could find 2.5V and 1.8V logic then, but that was primarily for interfacing to small geometry devices which were running the core at low voltage, so that you did not have to have voltage translation on die. That should not apply in this case, at most there would be a commodity microcontroller running the interface and display, not some kind of multi-GHz processor that would be running at 1.5V or lower.

 

[chrispbass]

That seems low for commodity digital devices from 10years ago. 3.3V would be a more common supply voltage for standard logic. You could find 2.5V and 1.8V logic then, but that was primarily for interfacing to small geometry devices which were running the core at low voltage, so that you did not have to have voltage translation on die. That should not apply in this case, at most there would be a commodity microcontroller running the interface and display, not some kind of multi-GHz processor that would be running at 1.5V or lower.

Thanks for the help. I’ll recheck in case I’ve mis-measured.

 

[chrispbass]

I’ve rechecked the voltage of the digital supply and it is 1.3V…hmmm!

This is the pin out/application notes of the analogue controller chip.



I’ve got no schematics, but being governed by this, it’s not making sense as I’ve got 8V on pin 42 and 1.3 on pin 41…hmmm again!

There is a 5V and 3.3V regulator on the board which are giving out the correct voltages.

 

[Disco Volante]

Is the 22uF a shorted tantalum? or other type of failed cap? That burned a trace somewhere? Can you reverse-engineer the area between the regulators and pins 41-42?
Follow the voltage...

 

[chrispbass]

Pin 40 is showing about 4.5 V though.

This is a close up of this area.



Someone has sneaked in and stolen R172!

 

[chrispbass]

Is the 22uF a shorted tantalum? or other type of failed cap? That burned a trace somewhere? Can you reverse-engineer the area between the regulators and pins 41-42?
Follow the voltage...

Thanks Disco

I’ve got 4.4V on the 22uf going to pin 40 (see above)

 

[Disco Volante]

Does indeed seem to be stolen?! Love your magnifier solution. I have one of those olde-schoole large lenses with flourescent light around it, dentist-style. It's a bit unwieldy..

Those voltages are all over the place if going by the application note. They should be rock solid and probably 3.3V? or 5V?
So either they wired it differently or there's something fishy going on.
Is the regulator output connected?
Can the regulator deliver enough power?

That weak distorted sound you described makes me think starved amp...

 

[chrispbass]

Does indeed seem to be stolen?! Love your magnifier solution. I have one of those olde-schoole large lenses with flourescent light around it, dentist-style. It's a bit unwieldy..

Those voltages are all over the place if going by the application note. They should be rock solid and probably 3.3V? or 5V?
So either they wired it differently or there's something fishy going on.
Is the regulator output connected?
Can the regulator deliver enough power?

That weak distorted sound you described makes me think starved amp...

Thanks. I’ve got ‘the dentist’ magnifier too, but found the microscope at a good price in the small ads from a guy who was packing in mobile phone repairs. I’d bought a newish broken crown power amp with the intention of repairing it and this prompted the up grade. It had a low voltage power supply fault and a partially burnt out FFC clam shell connector on the main board which went to the display. That amp is responsible for the purchase and setting me on a merry little dance. It was 50 pins at 0.5mm pitch! I sat on it for awhile and got some cheap breakout boards with the same connectors to practice on before going for it. It was great to see the display come back to life and the amp fully functioning.

I’ll continue the checks

 

[chrispbass]

Thinking about my post above, I’ve realised that I’ve made some very wrong (naive part conclusions) that I shall sleep on.

Although promising that all functions can be selected and routed to the main analogue outs I’d forgotten the obvious (now a bit further down the road).

The analogue controller has a digitally controlled multiplexer for its analogue inputs. There are 4 stereo pairs and 2 diff inputs. I’m not exactly sure how these are all utilised (if they all are), but as there are 2 stereo line ins, that takes up 2. There is an FM/Bluetooth receiver so perhaps that takes up another. CD post DAC perhaps another. Everything else is selected in the digital domain (there is a SPDIF input as well opt/coax). I’m not sure how these would be separated/converted to use the the other inputs on the analogue controller. Doesn’t really matter I suppose and for now I’ll just say it’s one other stereo pair.

What I hadn’t checked was to see if I had 2 or more analogue outputs at the same time. I’ve just confirmed this by playing a CD and injecting a signal into one of the line inputs.

A slightly long winded way of getting back to square 1 in trying to confirm either faulty analogue processor, MCU, or perhaps a PSU issue.

The investigation continues.

 

[Disco Volante]

It's fun to get stuff to work! I have a Luxman L80 on the bench exhibiting loud crashing sounds in one channel. Turns out a little old transistor in the first stage of the power amp had half-died and the bias was jumping wildly. Found out with a chopstick prod after a full recap (several leaky ones)..

So what you're saying is that playing 2 sources you get a distorted low *mix* of the two sources?

I'd still think you need to look at the voltage weirdness. If I were a transistor switch, I'd require proper voltage to switch properly or get stuck in some half-switched state... Does that make any sense? transistors aren't (entirely) linear..

Cheers

 

[chrispbass]

It's fun to get stuff to work! I have a Luxman L80 on the bench exhibiting loud crashing sounds in one channel. Turns out a little old transistor in the first stage of the power amp had half-died and the bias was jumping wildly. Found out with a chopstick prod after a full recap (several leaky ones)..

So what you're saying is that playing 2 sources you get a distorted low *mix* of the two sources?

I'd still think you need to look at the voltage weirdness. If I were a transistor switch, I'd require proper voltage to switch properly or get stuck in some half-switched state... Does that make any sense? transistors aren't (entirely) linear..

Cheers

Glad you’ve got it sorted and yes very satisfying.


Yes it does make sense thanks. Up until a few days ago I never really had a block diagram of such a unit in my head (still needs refining!), and the type of chips available/used.

It’s just very frustrating with no schematic to confirm how it aligns/deviates from the application notes.

Thanks for the suggestions