MGL V6 frequency stability problem

[Matti Adolfsen]

A local pilot contacted me, as his radio did not work properly. We checked the symptoms:

It transmits on the frequency it was set to transmit, and this transmission was checked to be ok. on a hand- held radio
but it did not receive the signal from the other radio, unless tuned on 25 kHz lower frequency.

I removed the unit and brought it into my workshop. Using a signal generator, I found the receiver was at the most sensitive on a bit higher frequecy, this was about 13 to kHz higher. the situation was similar on the low end of the scale as on the higher end

This fault is not stable, vibration makes it come and go.

Any suggestions? I think I heed a service manual to proceed.

[rainier]

Sadly I do not have the circuit diagram of the V6 - while it was based on a early prototype radio I designed around 2004 but never put into production it has changed a bit. The design was taken over by another company that then created the V10 and later the V6 and we sold it as a MGL radio for a while (it is now sold as a South-West electronics radio and is still available as far as I know).
Anyway - I can take an educated guess as to what is going on with your radio.
The V6 is a double-superhet receiver. The second IF is created around the TDA1072 receiver chip and this runs its own oscillator using a 44.454 Mhz crystal finally resulting in a 455Khz low IF. This oscillator is finicky with respect to the crystal and I suspect your crystals aging is starting to get marginal for this circuit. My first step would be to replace that crystal - you can get them from Digikey etc.

[Matti Adolfsen]

Thank you.

I'll check that chrystal. is there any test point I can probe the local oscillator with frequency counter or spectrum analyzer?

And... i found the fault disappeaars when bending the RF circuit board, so I suspect a bad solder joint somewhere there.

Matti

[rainier]

Thank you.

I'll check that chrystal. is there any test point I can probe the local oscillator with frequency counter or spectrum analyzer?

And... i found the fault disappeaars when bending the RF circuit board, so I suspect a bad solder joint somewhere there.

Matti

You can probe the crystal directly.
It may be a bad joint - but if te frequency shifts it has to do with either the 1st IF (45Mhz) or the second IF. There is a traditional PLL circuit with mixer to do the first IF so you could probe the input of the mixer with a frequency counter. The mixer is the tiny 8 lead chip in about the center of the RF board - you can see the feed from the PLL.