Digital volume control and DACs

[chrispbass]

I’ve been asked to look at a piece of “hifi” by a family member. It’s basically a modern take on the old radiogram with a digital front end comprising cd, Bluetooth, DAB etc with built in speakers/sub etc. This particular model was certainly not cheap (around £2000 about 10 yrs ago) and I’ve drawn a blank with the manufacturer regarding spares/schematics etc.

Symptoms are low distorted output and the digital volume control makes no change to this level. As far as I can tell, all other functions work as they should.

I’m a bit pushed for time and have taken a brief look to check PSU’s which compromise a small smps and a linear supply for the main amps.

I’m not really familiar with the architecture of such a front end, so given the symptoms, is it likely that the fault lies with the main processor (unique to this piece of equipment) and therefore unobtanium and un-fixable, or can anyone guide me to other things to look at please before I waste time on something I have no chance of fixing.
Cheers
Chris

 

[chrispbass]

Just realised this may have been better posted in the “truth table” sub forum so mods, please move if necessary.

 

[sahib]

Troubleshooting naturally starts with checking the supply voltages first. But even if they are ok, without a schematic you will be shooting in the dark particularly on dense surface mount boards. If you have already drawn a blank on schematic and parts with the manufacturer, I would not waste my time on it. You can spend a whole day with it and still get nowhere.

 

[chrispbass]

Troubleshooting naturally starts with checking the supply voltages first. But even if they are ok, without a schematic you will be shooting in the dark particularly on dense surface mount boards. If you have already drawn a blank on schematic and parts with the manufacturer, I would not waste my time on it. You can spend a whole day with it and still get nowhere.

Thanks Cemal. Yes, hence the post. I’ve checked the psu’s and seem ok as far as I can tell.

I’m sure there is a reasonably generic architecture to this type of front end…I’m not sure where/how the volume control fits in with this.

 

[Khron]

I’m not sure where/how the volume control fits in with this.

Well, assuming the numbers haven't been lasered off the bigger chips, you could start there. Is the volume control in the analog domain, but digitally controlled?

 

[sahib]

Thanks Cemal. Yes, hence the post. I’ve checked the psu’s and seem ok as far as I can tell.

I’m sure there is a reasonably generic architecture to this type of front end…I’m not sure where/how the volume control fits in with this.

Thank you too Chris.

What brand is it?

Big brands like Sony won't even respond to your query. Last year I was repairing some broadcast camera mic-preamps which did not even have any problems with electronics. It was all a mechanical failure (horrendous design) and I got nowhere with them.

The other problem you may end up having is that generally the ICs for these big brands have house number. Which means you will be depending on the manufacturer's original part.

 

[chrispbass]

Well, assuming the numbers haven't been lasered off the bigger chips, you could start there. Is the volume control in the analog domain, but digitally controlled?

Thanks Khron. Yes I was going to start looking up the chips…but away at the mo (hopefully back tomorrow) so just trying to get some ideas for when I get back to it

The volume control is not motorised if that’s what you mean by being digitally controlled.

 

[Khron]

The volume control is not motorised if that’s what you mean by being digitally controlled.

No, I did not mean that. Digitally controlled, as in, a digital potentiometer, or volume control chip (analog audio in, analog audio out, digital control of the "pot wiper", via I2C / SPI / whatever digital protocol).

It wouldn't make much sense to control the digital level pre-DAC, because that just wastes bits, and you're left with less level above the noise floor, when coming out of the DAC.

 

[chrispbass]

Thank you too Chris.

What brand is it?

Big brands like Sony won't even respond to your query. Last year I was repairing some broadcast camera mic-preamps which did not even have any problems with electronics. It was all a mechanical failure (horrendous design) and I got nowhere with them.

The other problem you may end up having is that generally the ICs for these big brands have house number. Which means you will be depending on the manufacturer's original part.

It begins with an R and ends with the animals going in two by two

It belongs to my sisters t’other half. He went back to the shop he bought it from who told him they couldn’t repair it (no main boards available) and were also quick to try to sell him the latest model costing circa 3k!

This was also confirmed by a call to the manufacturer. Basically “tough luck”.

At this stage there it’s a bit of a “nothing to loose” scenario and I suppose worst case, it could be used as an active speaker set up with either a different front end shoe horned in or a line in for a crap top etc.

 

[chrispbass]

No, I did not mean that. Digitally controlled, as in, a digital potentiometer, or volume control chip (analog audio in, analog audio out, digital control of the "pot wiper", via I2C / SPI / whatever digital protocol).

It wouldn't make much sense to control the digital level pre-DAC, because that just wastes bits, and you're left with less level above the noise floor, when coming out of the DAC.

Ok, this is why I stuck my hand up Khron…I don’t know. Tbh, I was hoping you would chime in.

I think the best thing is for me to get some photos up of the main board and I’ll gen up as well on the chips.

I’ll happily state for the record that I’m a hobbyist tech (what do you mean you already knew! ) but I generally manage to repair most things, but this is a bit (a lot) outside of what I usually get my hands on.

I do have some smd equipment though, if there is a chance of repair.

Thanks again

 

[sahib]

It begins with an R and ends with the animals going in two by two

I am the worst guy for pub quiz

R otel by any chance?

 

[JohnRoberts]

It is difficult with no schematic and having to completely guess about the circuits.

Besides the unresponsive digital volume control, do any other controls work, like the input selector? If the other controls work then dig deeper into the digital volume control (perhaps a rotary encoder). If nothing works that looks like perhaps a faulty MCU central processor.

You didn't mention how old it is? The expected failure mode for old components is dried out electrolytic capacitors, but I wouldn't expect that at only 10 years old. Typical MCUs use a small electrolytic cap with discharge resistor to boot or reset the MCU at power up.

JR

 

[chrispbass]

It is difficult with no schematic and having to completely guess about the circuits.

Besides the unresponsive digital volume control, do any other controls work, like the input selector? If the other controls work then dig deeper into the digital volume control (perhaps a rotary encoder). If nothing works that looks like perhaps a faulty MCU central processor.

You didn't mention how old it is? The expected failure mode for old components is dried out electrolytic capacitors, but I wouldn't expect that at only 10 years old. Typical MCUs use a small electrolytic cap with discharge resistor to boot or reset the MCU at power up.

JR

Thanks John (and for moving this to the correct location)

All digital functions seem to work ok. It connects to WiFi as I was able to put our password in, I can select DAB, CD, line ins etc. There are “up and down” buttons on the front panel for volume which correspond with the display (display shows it’s working) and there is a remote which also works all functions (as far as I can tell).

Approx ten years old give or take a few years. In regular use (ish). Switched back on after a holiday and now has the mentioned symptoms.

 

[chrispbass]

I am the worst guy for pub quiz View attachment 133181

R otel by any chance?

Well you can’t give up now!

It’s not Rotel.

Tbh, it’s a small English brand that I’d never heard of before - made I believe in South End on Sea.

I only got to know it as it’s part of “the after show party” on the odd occasion I stay over when we’ve had a night out.

 

[JohnRoberts]

Thanks John (and for moving this to the correct location)

All digital functions seem to work ok. It connects to WiFi as I was able to put our password in, I can select DAB, CD, line ins etc. There are “up and down” buttons on the front panel for volume which correspond with the display (display shows it’s working) and there is a remote which also works all functions (as far as I can tell).

That's good news it suggests that the MCU (brain) is alive and functioning.

Approx ten years old give or take a few years. In regular use (ish). Switched back on after a holiday and now has the mentioned symptoms.

OK, unlikely to be a dried out electrolytic. I perceive two symptoms but they may be related. No/low audio output and volume control not affecting audio output level (but you said the panel indications suggests it is responding to controls.)

The obvious thing to check is the probably still analog output amplifier. I need to admit I don't know what a "radiogram" is. A quick search reveals a combo console with multiple inputs and loudspeakers.

I would suggest working backwards from the output (speakers). It seems from your description that some sound is coming out just not the correct sound. 10 years ago is not that long for technology. IIRC there are modern audio amps that accept digital input. Some may even accept digital volume controls. If you trace the speaker lines back to an obvious IC, count the number of pins and search for possible candidates.

Good luck

JR

 

[thor.zmt]

I’ve been asked to look at a piece of “hifi” by a family member. It’s basically a modern take on the old radiogram with a digital front end comprising cd, Bluetooth, DAB etc with built in speakers/sub etc. This particular model was certainly not cheap (around £2000 about 10 yrs ago) and I’ve drawn a blank with the manufacturer regarding spares/schematics etc.

Ruark. No chance of a service manual. You can ask the factory.

Consumer digital etc. moves fast. If 10 Years old and made with mainstream IC's in Scheena at the time, it means most parts went EOL 5 Years. Typical expected product lifespan is 3 Years.

You can look for signs of poor soldering, paste that did not flow right and try to give everything a "re-heat" with the SMD rework heat-gun.

Look for bad electrolytic or ceramic capacitors (cracked - needs microscope).

often such extremely complex devices have surprisingly simple failure modes (as opposed to simple devices that invariably have highly complex failure modes).

If the volume control is called LM1972, give up. There was a bad batch and no good ones could be found. Expensive recall at our end, thanx Nat-Semi/TI (naturally nobody took responsibility).

If you are not not good with SMD rework and set up to do it, give up.

If you are not not good tracking complex digital signals, give up.

Thor

 

[chrispbass]

That's good news it suggests that the MCU (brain) is alive and functioning.

OK, unlikely to be a dried out electrolytic. I perceive two symptoms but they may be related. No/low audio output and volume control not affecting audio output level (but you said the panel indications suggests it is responding to controls.)

The obvious thing to check is the probably still analog output amplifier. I need to admit I don't know what a "radiogram" is. A quick search reveals a combo console with multiple inputs and loudspeakers.

I would suggest working backwards from the output (speakers). It seems from your description that some sound is coming out just not the correct sound. 10 years ago is not that long for technology. IIRC there are modern audio amps that accept digital input. Some may even accept digital volume controls. If you trace the speaker lines back to an obvious IC, count the number of pins and search for possible candidates.

Good luck

JR

Cheers John

Yes it’s a combo affair. A wide ish nicely made wooden cabinet (on legs) with built in L/R spkrs. Aesthetically pleasing (depending on tastes) and well made.

Been very busy lately so had an hour or so to go through some preliminary checks reg the power supply’s.

From what I can ascertain, there is a small SMPS, which switches on from the main power switch (on the rear of the unit) When the unit is “powered on” from the front, this powers up a relay on the SMPS board which provides line voltage to the large toroidal tx and linear psu for the power amps. Also on this board are other voltage taps to the front end and power amp boards (driver transistors/spkr relays?) but I haven’t had chance to determine exactly what is provided (if any) from the smps or derived/dropped from the linear supply.

Because of the way the unit is constructed, you can’t really get the front end out far enough to test it in situ due to the length of the ribbon cables, so I disconnected everything to get it out for a visual and to fathom out which ribbon cables did what.

My next session was going to check the main amps with my signal gen to rule that out.

 

[chrispbass]

Ruark. No chance of a service manual. You can ask the factory.

Consumer digital etc. moves fast. If 10 Years old and made with mainstream IC's in Scheena at the time, it means most parts went EOL 5 Years. Typical expected product lifespan is 3 Years.

You can look for signs of poor soldering, paste that did not flow right and try to give everything a "re-heat" with the SMD rework heat-gun.

Look for bad electrolytic or ceramic capacitors (cracked - needs microscope).

often such extremely complex devices have surprisingly simple failure modes (as opposed to simple devices that invariably have highly complex failure modes).

If the volume control is called LM1972, give up. There was a bad batch and no good ones could be found. Expensive recall at our end, thanx Nat-Semi/TI (naturally nobody took responsibility).

If you are not not good with SMD rework and set up to do it, give up.

If you are not not good tracking complex digital signals, give up.

Thor

Thanks Thor

Some good info/advice

I’m under no illusion that I’ll be the hero of the day, but if nothing else, I’ll learn something and as stated earlier it’s a bit of a “nothing to loose scenario”.

Cheers

Chris

 

[Disco Volante]

I second John: As the MCU seems ok, waste no more time on the smps, start from speaker end and work backwards.
Check for bad solder joints (strong magnifying glass is your friend), any tants playing up, inject a sine wave close to the input of the amp, see what comes out. I bet there's a monolithic output amp in there? The cheapo 8-pin black square on a heatsink?
Good luck,

 

[chrispbass]

I second John: As the MCU seems ok, waste no more time on the smps, start from speaker end and work backwards.
Check for bad solder joints (strong magnifying glass is your friend), any tants playing up, inject a sine wave close to the input of the amp, see what comes out. I bet there's a monolithic output amp in there? The cheapo 8-pin black square on a heatsink?
Good luck,

Thanks Disco

I concur.

Yes, PWR amp is next on the list.

As far as I could tell psu’s were ok and functioning, given the general functionality of the unit. Some schematics would have been nice to bolster my initial testing though.

Appreciate your input and everyone else’s of course.

Such a great forum