16-Bit PSN-ADC-USB, Crystal and Scale Issues

[carlbe...@hotmail.co.uk]

Hello All.

I don't know if anyone can help, but I recently finished building the 16-Bit PSN-ADC-USB and I'm having some trouble adjusting the crystal and Scale.
 
Crystal: In the build_info it says to use a 74HC14 Hex Schmitt-trigger to measure the frequency, which i have done. When adjusting C27 (9-50pf ceramic trimmer) nothing happens, it just jumps from 7.98Mhz to 8.02Mhz regardless if i alter the pot or not. When measuring the frequency without the 74HC14, just measuring pin 14 i get the same.
 
Scale:  When the board is connected to my PC running WinSDR, looking at the ADC counts, providing 10v into pin 1 with the jumper shortened and adjusting Scale, again nothing happens. The ADC counts stay the same.
 
Could i have flashed the wrong Hex data? In the hexfiles i uploaded 'SdrBoot3014_Usb.hex' and using command prompt ran 'dsdr26_usb.hex', both where successful.

I have checked all my solder joints and everything looks good to me. When powering the board for the first time there was no smoke or heating of the IC's.

Also, when tapping the inputs with my finger i see small jumps/spikes on the graph on WinSDR and i have no issue adjusting the Offset.

Thank You.

[Larry Cochrane]

See comments below...

On 4/22/2026 1:34 PM, carlbe...@hotmail.co.uk wrote:
> Hello All.
>
> I don't know if anyone can help, but I recently finished building the 16-Bit PSN-ADC-USB and I'm having some trouble
> adjusting the crystal and Scale.
>

> _Crystal_: In the build_info it says to use a 74HC14 Hex Schmitt-trigger to measure the frequency, which i have done.

> When adjusting C27 (9-50pf ceramic trimmer) nothing happens, it just jumps from 7.98Mhz to 8.02Mhz regardless if i alter
> the pot or not. When measuring the frequency without the 74HC14, just measuring pin 14 i get the same.

You might be overloading the crystal. Adding a FET amplifier/buffer, with low input capacitance, instead of 74HC14
between the crystal and the frequency counter may stabilize things.

>
> _Scale_:  When the board is connected to my PC running WinSDR, looking at the ADC counts, providing 10v into pin 1 with

> the jumper shortened and adjusting Scale, again nothing happens. The ADC counts stay the same.

I ran into this problem now and then when I was producing the board. To fix the problem I would lower the 1 meg resistor
going to the scale pot to around 800k to 900k.

Regards,
Larry Cochrane
Redwood City, PSN

[Carl]

Hi  Larry,

Thank You for your reply.

I will give this ago, I just need to order a  FET amplifier. I'm going to use a MPF102 and will report back here if it worked or not.

On the seismicnet site it shows C52 (68uf) as unpopulated, is this correct?

Thanks Again for the help.

[Larry Cochrane]

Hi Carl,

> I will give this ago, I just need to order a FET amplifier. I'm going to use a MPF102 and will report back here if it
> worked or not.

When you make the measurement be sure to connect the amp/counter to the crystal output driver on the DspPic IC. The
effect of the extra capacitance is less if you use the crystal driver output not the crystal input pins.

>
> On the seismicnet site it shows C52 (68uf) as unpopulated, is this correct?

I'm leaving town for a few days so I don't have time to look this up. What page/document do you see this note on?

-Larry

[Carl]

C52. It was just in the picture (https://www.seismicnet.com/serialatod.html) and i dont see it in the BOM, so i assumed it was not populated.

[Carl]

I have tried different resistors (680k, 750k, 820k, and 910k) and non made any change. During each test i applied 10v into input 1 and had the jumper closed. Adjusting the Scale all the way in each direction made no change.

I also tried the MPF102 to measure the crystal and nothing, i still get the same thing. I removed the ceramic cap to see if it was damaged and it works perfectly. Also, as suggested by Google Gemini removed C28, it though that the capacitance may be to high. It did nothing. 

I'm stumped as what to do next. I don't think any of the IC's are damaged, as adjusting 'Offset' works and when i tap the inputs with my finger i see change on the graph on WinSDR. Also, when i apply 10v the graph shoots up and i get an alarm.