TCXO on Chinese MMDVM boards

[G4AON]

I recently bought a cheap Chinese MMDVM board and struggled to get it up and running.

By a process of elimination, I was able to determine that the board was significantly off frequency, so far off that it was not possible to transmit or receive with it on the indicated channel. The method to find the actual frequency was to change reflectors/unlink them on D-Star and observe the transmission on the spectrum display of my Icom IC-9700.

The frequency seemed to jump around, adding some soldering flux and re-flowing the solder under the TCXO "chip" seems to have cured the jumping in frequency, however it was still around 5 KHz off frequency. The TX and RX offset entries in Pi-Star would normally be around +/-200 or 500, in my case the offset needed was around 5000.

Having determined the offset and checked that it really did transmit on the indicated frequency courtesy of the Icom spectrum display, the board seemed to work reliably on D-Star. However it was intermittent on DMR, sometimes it worked perfectly, sometimes I could receive but not transmit, then not at all. By adjusting the final 70cms frequency up/down in 1 KHz increments, it appeared that the board was again off frequency by around 1 KHz. Back to tweaking the offsets... and once again the board works on DMR. On observing the transmitted frequency it was noticed that the oscillator is drifting up to 1 KHz from the indicated frequency as it warms up.

The bottom line is that a 22 GBP MMDVM board has a 10 pence TCXO that is nowhere near good enough to hold a frequency stable on 70 cms. Whether some are better than others is a matter of guesswork. Maybe the solution is to replace the TCXO with one of a higher grade, although if the board has an OLED display it means removing that to gain access.

Anyone else noticed similar problems with these boards?

[G4AON]

I decided that the original TCXO was just hopeless. The size and basic specification of the original Chinese 14.7456 MHz TCXO by SCTF looks to be the same as an ECS-TXO-3225MV-147.4-TR. However, the ECS one is specified as +/- 2.5ppm over -30C to +85C, as opposed to 20ppm for the Chinese one (assuming the Chinese one even meets their claimed spec).

Mouser (https://www.mouser.co.uk) stock the ECS TCXO at £2.05 plus delivery, which is free if you order items totalling £33 (current at Feb 2023). For those who haven't used Mouser from the UK, they ship from the USA and pre-pay the vat, items arrive in 2 or 3 days by courier.

Swapping the part is straightforward as it is just a matter of heating the original with an SMD hot air blower and lifting the part with tweezers. Add a little solder paste or flux, place the new TCXO on the board and heat it up. Obviously check the orientation before fitting, the data sheet is available from the Mouser site.

The new offset is -100 for TX and RX, as opposed to between +4000 and +5000. Drift looks to be zero.

73 Dave