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GPSDO

Introduction 

It Electron magazine of September 2023, Jan PA3FEX published a article about a GPSDO they made with Veron department A32 (Meppel). Wanting to have a good stable 10MHz source for a while, after reading the article I emailed Jan if they would sell the PCB's to other OM's. He wrote back that Veron department A32 (Meppel) website would have contact information how to sign-up for the PCB's. So I ordered one.

While waiting I download (safe for off-line usage) all the information I could get from the A32 website:

• A32 Presentation construction GPSDO
• A32 GPSDO Partlist
• A32 GPSDO chematic
• GPSDO 158.hex (MCU firmware)
• Partly translated of Yannick article`
• Yannick orignal schematic GPSDO
• Yannick component placement

After a few weeks, I got a envelop within that the bare PCB of the GPSDO.

Collecting the parts and starting the assembly.

Most of the parts incl, the OCXO I already had in stock and what I didn't have, I bought at my local electronics store.

And after a few evenings using my spare hobby time, it was starting to get to it's final stage. 

I didn't use the onboard transformer because it was not available at first and I had a good noise free external 9V power supply. 

"First time" boot

Then the moment was there, time for the smoke test. I setup my bench top power supply to 9V and a current limit of 250mA and powered the GPSDO up. 

Because there is no config it starts it default to PWM of 50%.

The display went on and almost and almost instantly flew into the current overload because I forgot than when power-up current of the OCXO heater will rice above that level.

So after increasing the current limit to 600mA, it booted up correctly and no magic smoke appeared. And it started to heat up.

TODO add more text.

 

 

 

 

 

  

 

 

  

 

Findings, fixes and mods

When you are a professional or a home brewer, there always is a change that somewhere you slip up and also in the A32 there are a few things incorrect.

The following differences were found between the parts list and the diagram:

• “Elco 220h / 10V” seems incorrect. The order number “rad fr 220/16” and diagram indicate an “Elco 220u / 16V”.
• The parts list states “20K potentiometer” with order number “64w-20k”. However, the schedule states “P1 10K”. 10K is fine in practice, but 20K is also sufficient.

The following findings can be noted for the scheme:

• Various 100nF capacitors are not mentioned on the diagram although they do need to be mounted on the PCB.
• S1 is called Reset, but this is not a reset but the switch for the “push button” function (bypassing the start-up, changing display info, etc.) as described in the original diagram and Yannick Turcotte's manual. .
• The ATMEGA328 lacks a reset circuit, which means that in practice it sometimes does not start up properly. Placing added a 100nF between pin 1 and GND and 10K between pin 1 and +5V (pin 7. This ensures a neat external reset and start-up problems will be a thing of the past. Both adjustments can easily be made at the bottom of the PCB (see Fix 2 below)

The follwing findings can be noted on the PCB:

• The 100nF decoupling capacitor at N2 is not connected to GND (solder pad is open). This can be easily fixed (see Fix 1 below)

Fix 1

The 100nF See photos below. This can be repaired by scraping away some of the paint on the underside next to the island and laying a tin bridge.

100nF not connected to GND (bottom of PCB)

100nF now connected to GND (bottom of PCB)

Fix 2

To fixed the  AVR MCU from not booting sometimes correctly, you can added a 100nF between pin 1 and GND and 10K between pin 1 and +5V (pin 7)

 

Boxing it up.

A still had one of the nice Italian construction cases laying around which could hold the GPSDO.

 

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