TS-790A Frequency Calibration?

[VK2SL]

Hello all.
I have had a TS-790A with the 23cm module fitted to it for quite some time with very little use until last weekends 23 on 23.
After literally throwing a double quad antenna onto the roof of the shack I had a couple of good contacts with Rick VK4HF/VK2RP. I noticed that although the radio was working fine it appears to be quite a way of frequency. Rick was transmitting on 1296.100 indicated and I was resolving him on 1296.103.2 indicated. Obviously there is a bit of a problem here.
I'm wondering if anyone knows anything about calibrating the TS-790 and is it possible to add precision frequency referencing to this radio?

73

De Steve VK2SL

[VK5ZD]

A check of the block diagram indicates that it uses a 10.24MHz reference TCXO.
You may just need to adjust this to the correct frequency. A 23cm GPS locked beacon would be useful for this.

[VK3HZ]

If you're after GPS-locked precision, this is what one guy is doing:

- purchase a VCXO to replace the TCXO in the radio. Here's one on eBay: http://www.ebay.com/itm/171604007573

- use a PLL to lock the VCXO to 10 MHz. The XRef-PLL can be supplied on that frequency: http://www.vk3hz.net/XRef/XRef_Home.html#XRef-PLL

Regards,
Dave
VK3HZ

[VK5KK]

Hi Steve,

Yes confirming all oscillators (apart from FM gen and FM IF ) are locked to the one 10.24 MHz TCXO.
The TCXO doesn't have a VT input so Daves mod to use with XRef is the way to go if you want to lock it to an external reference (thnx Dave for tip on suitable VCXO!)

I've had my TS790A from brand new (25 years old!). I must admit it has been relegated to less critical duties in recent years but gave it a birthday recently and recalibrated it... it was 2.2 khz out on 1296 mhz after all those years. I suspect probably wasnt spot on to start with, that aside it looks like the TCXO had aged about the same as yours. I used the 1296 GPS locked beacon put in service by Iain VK5ZD to calibrate!

BTW: The original TCXO used in the 790 is very good for its vintage once temperature stable (it is better than what is standard in modern transceivers). Not good enough for serious WSPR on 144 but have used mine quite a bit for casual WSPR use albiet with incorrect frequency indicated. Have even managed to decode WSPR on 1296 with Df of 2-3 hz over several cycles

Cheers

David VK5KK

[VK2SL]

Thanks very much for the very helpful comments guys . Looks like I might be able to get the dear old thing on track now.

Steve