[Resolved] My first preenFM2 metal case .. trouble shooting no audio

[horridus]

Hello,

I put the first kit together last week. It appears to really "want" to work. It boots as expected, but no chime. Although there are a series of fast audio "pops" as the blue leds flash rapidly. There is no audio when attempting to play the instrument. At first I assumed that I have a cold solder joint on the board some place, but I have inspected them with a magnifying glass and a multimeter / continuity tester from the top of the pins or leads on a component to the edge of the solder pads on the other side of the board and get strong signal everywhere. The preenFM2 navigates as expected, USB drive is easy to access to load patches etc. Power seems clean on both USB connector as well as when I use an external power supply. I bought 3 kits, so I swapped out all of the chips in pairs and have gotten the exact same results each time. The Oled screen works great. I'm stumped. I ran continuity on all the pins of each socket to the edge of the solder pads as well, also clean signal. I double and triple checked the resistor values, polarity of the Electrolytic Caps and diodes, all seem ok.

Any ideas? Maybe I have missed a crucial step in the build? Pics are attached.

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[horridus]

Another pic ... other side

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[Xavier]

I don't see any obvious error and everything looks nice (although a bit too much soldering )

How do you test audio ?
Do you see a note blinking on the display when you send midi notes ?

Have you tried holding down "MENU" button and press one of the five left buttons ?
What happens then ?

[horridus]

Hi Xavier, thank you for your reply

I don't see any obvious error and everything looks nice (although a bit too much soldering )

Yeah, I'm pretty rusty on the soldering ... will be less zealous on the next!

How do you test audio ?

I've tested by loading DX7 patches / instruments and then pressing the Inst button and one of the buttons on the left side. The note lights up, but still no sound.

Do you see a note blinking on the display when you send midi notes ?

Yes. The instrument appears to play, but still no audio. Mysterious!

[Xavier]

The pop you hear is the empty audio signal around the op amp reaching 2.5v.

As you have both audio channels not working, it must be the connection between the preenF405 and the DACs.
I'll be back tonight with some aditional testing you could do.

Xavier

[horridus]

That would be great .... It is late morning here now, and I am awake and working. I can easily take a break and try your tests.

[Xavier]

I attached 2 pictures.

1. First you can check all 5v, ground and 2.5v voltage. (2.5v only if the preenfm2 is powered 0v otherwise).

2. on the other picture, I colored the wires that the preenF405 uses to send audio samples to the DACs. Check the solder points and the connections (null resistor).

Xavier

[horridus]

Ok thank you, I'll take a look.

[horridus]

Hi Xavier,

I checked all of the power 5v, 3.3v, 2.5v .. and ground lines to each chip and the OLED etc ... Voltage and ground continuity appear everywhere they are supposed to. Although, the 2.5v lines were reading 4.02v. All others read perfectly. Nothing is hot or smells strange.

The DAC lines read clear continuity from the PreenF405 on both DACs ... Voltage and ground around the board appear good on both sides, pins to pins solder joints to solder joints.

[Xavier]

I worry about the 4.02v.
That should really be VCC/2. So 2.5 if you get 5v on VCC or 2.4 if your USB power is 4.8.
Seems like there is something wrong in the audio path after the DACs.

Please, follow the following picture.
Remove your op amp, and try to connect directly with a metal wire the 2 blue points (or the 2 green points).
Do you hear something when you play midi notes while the note are connected ?

[horridus]

I removed the op amp and connected the 2 points while playing a note, still no audio heard.

Also, I triple checked the points that should 2.5v ... The 2 resistor points (68 ohm) read 4.02 ... The 2 DAC points read 3.85v each.

I tested this using a ground connection from the small 5v holes on the bottom left, also from the top of the 5v regulator. Hopefully this was a correct move for an accurate reading. I tried also using the top of the 3.3v regulator as well, which reads .7v on the DACs and .72v on the 68 ohm resistors.

[Xavier]

Have you tried to swap the preenF405 with an other one one ?

[horridus]

I did ... and it gave the exact same results, which was somewhat a relief :>)

I just ordered all of the other chips from Mouser and they showed today ... I think I may change out the DACs and see if maybe there was just a bad run of DACs? The 2.5v on VCC is regulated internally on the DACs?

[Xavier]

The 2.5v on VCC is regulated internally on the DACs?

No, this is the zero of the preenF405 audio output signal. It sends 2048 on the 4096 DAC range.

So another test, put back the opamp, remove the preenF405 and test the 2.5v points. You should get 0v.

[horridus]

Yes, zero as expected.