Digital volume control and DACs

[ccaudle]

I’ve got 4.4V on the 22uf going to pin 40

Which should also go to pin 42. So where is the 8V coming from?

 

[chrispbass]

Which should also go to pin 42. So where is the 8V coming from?

My mistake.
I forgot to say that the 22uf cap although shown in proximity is not connected to pin 42 as per application notes.

Updated voltages and an additional discovery!

44 - 4.45v (this is the discovery and makes no sense)

43 - zero (gnd)
42 - 8.9V (makes sense as there is 9V dual rail supply on board)
41 - 1.3V
40 - 0.3V
39 - 4.45V (which is connected to the 22uf cap)

Thanks for pointing this out

 

[chrispbass]

I did have another thought which is regarding the inverted pcb shown further back. Underneath there is a chip for the ‘3D sound mode). This would be on the main analogue outs and on the inverted board are 2 op amps and a relay which I presume is to bypass the 3D mode.

Relay is clicking in/out when operated by the sub menus. I need to get a data sheet to see how that control is implemented though.

 

[chrispbass]

I probably should just sit on my findings for a bit rather than trying to relay they them in real time.

Anyway, I need to put this down and get on with other things, but here’s an interesting one.

Apart from the voltages previously listed and the diff inputs nearly every input/output pin is reading about 4.45V!

 

[ccaudle]

nearly every input/output pin is reading about 4.45V!

The PT2037 does not have a bipolar supply, so likely the inputs and outputs have to be biased to half of the positive supply.

 

[chrispbass]

The PT2037 does not have a bipolar supply, so likely the inputs and outputs have to be biased to half of the positive supply.

Good point CC…thanks.

I’m still in the dark as to why the DVDD is so low at 1.3V. I pulled off the FM module and found another 3.3V regulator underneath but it’s working ok. I re-checked the DVDD voltage to the analogue controller with it removed and it had gone up to about 1.43V.

As Thor mentioned previously, this chip is the likely culprit, but if faulty, would it likely pull down DVDD to such a low level?

I wouldn’t have thought there is much too a 3.3V psu, especially if using a regulator.

 

[JohnRoberts]

Good point CC…thanks.

I’m still in the dark as to why the DVDD is so low at 1.3V. I pulled off the FM module and found another 3.3V regulator underneath but it’s working ok. I re-checked the DVDD voltage to the analogue controller with it removed and it had gone up to about 1.43V.

As Thor mentioned previously, this chip is the likely culprit, but if faulty, would it likely pull down DVDD to such a low level?

It could, or it could suggest a bad solder connection.

Can you trace the VDD back to a 3.3V regulator? A) What is the voltage at that regulator? B) with power off what is the DC resistance between the 3.3V regulator output and the VDD power input pin?

I just looked at the data sheet and the damn thing does not say what the DVDD voltage should be?

JR

I wouldn’t have thought there is much too a 3.3V psu, especially if using a regulator.

 

[chrispbass]

It could, or it could suggest a bad solder connection.

Can you trace the VDD back to a 3.3V regulator? A) What is the voltage at that regulator? B) with power off what is the DC resistance between the 3.3V regulator output and the VDD power input pin?

I just looked at the data sheet and the damn thing does not say what the DVDD voltage should be?

JR

Thanks John

I have done what you’ve suggested earlier today.

I can do a simple continuity test between the VDD on the SPDIF chip and the closest 3.3V reg no problem.

Can’t do that with the analogue controller to either of the two 3.3V regs.

I’ve tried to trace the VDD supply, and can so far as it is connected to a bank of 4 ceramic caps, then I lose the trace under an electrolytic cap, but it’s not connected to it and I did probe around the area. I did a resistance check to ground and to the regs (in case) and everything was in the Meg ohms range.

Like you say, could be a bad solder joint, or perhaps the chip just requires a low voltage. Very little info online and all the data sheets are identical so no further supply info.

A few further observations regarding the logic control.

Option in sub menu allows the operation of a relay (bypass or sub on/off I presume) to the 3D sound chip which would be on the main analogue outs.

Inserting headphones changes the display with regards to showing a ‘H’ next to the volume bar and limits the number of steps.
Also toggles the spkr relays

Hopefully more positive signs.

 

[ccaudle]

I just looked at the data sheet

I think you are too generous with the term. That is a brochure, and I'm sure originally had a description to the effect that if you are interested contact your local rep for full NDA documentation.

 

[chrispbass]

Got back home from work tonight and into the lab. Decided to trace out the DVDD supply, but lost the trace under an electrolytic cap as previously mentioned. Decided to un-solder the cap as it looked like it really should connect to it.

Well well well, what do we have here?!!





I’ve now traced the bottom side to the 9V rail, which tally’s with the data sheet suggesting DVDD and AVDD are sitting at the same potential.



A bit more tracing to do as there is a via feeding off elsewhere.

Doesn’t look like a burnt trace to me, rather a dissolved one as it coincides with the via.

Thoughts?

 

[chrispbass]

Here’s another pic showing the other end of the via.


Think a couple of carefully soldered in insulated wires maybe the best approach (pad to pad).

 

[Disco Volante]

That does indeed look suspect. A burnt via or trace dissolved (by what? leaky cap?)
Fixable, but what caused it, dear Watson, what?

 

[chrispbass]

That does indeed look suspect. A burnt via or trace dissolved (by what? leaky cap?)
Fixable, but what caused it, dear Watson, what?

Exactly. I’ve checked the cap value and seems ok. The break is right on the corner of the plastic base of the cap.

I’ve soldered in a couple of jumper wires after much probing. Tested supplies to ground and ok.



Suppose it’s time to see if the magic smoke comes out!!

 

[JohnRoberts]

good luck

JR

 

[chrispbass]

So did the magic smoke come out?

Did Chris P Bass take a large hammer to the pcb?


Tune in next week folks!









No magic smoke!







No hammer!








It is working!!





But!

 

[scott2000]

 

[Disco Volante]

Raahahaa! Outstanding! Good work!

 

[chrispbass]

Raahahaa! Outstanding! Good work!

Thanks Disco.

 

[thor.zmt]

It is working!!

Lucky but good job.

Thor

 

[chrispbass]

Lucky but good job.

Thor

Thanks Thor. Yes, certainly lucky and a good dose of stubbornness for good measure