TS-790 TCXO Adjustment

 In the past year or so, I have noticed that the displayed frequency for both the 144 Mhz and 430 Mhz operations of the TS-790 have been considerably off.

The frequency for both units is derived from the 144Mhz PLL unit.  It has a 10.24Mhz TCXO that is multiplied by 10 or 12 (depending on band) and mixed with various other frequencies to arrive at the final target frequency.

The Service Manual suggests monitoring the x10 frequency of 102.4 Mhz at TP5 on the 144Mhz PLL Unit, and adjusting the TCXO there.

 

10.24 Mhz Probe Point

 

I have an HP5334 Counter that is tied to my external GPS locked 10 Mhz standard.  This is a very stable and accurate counter, but it is not able to count above 100Mhz without an option that I do not have installed.

Still, it is the best option I have for setting the PLL frequency of the TS-790.  So rather than adjusting the PLL at the 102.4 Mhz test point, I adjusted it at the 10.24 Mhz output of the unit.

 

10.24 Mhz Oscillator Off Frequency

 

The unit was off by about 42.5 hz at the 10.24Mhz point.  This would translate into an error of 425 hz at the 102.4 Mhz point.

I was able to adjust the 10.24 Mhz signal to within about 2 hz, and after buttoning the radio back up, it appears to be on frequency on both bands once again (or close enough that I cannot tell the difference).

 

Original 102.4 Mhz Test Point and new 10.24 Mhz Point

 

10.24 Mhz Adjustment

I neglected to snap another photo of the frequency counter with the good read after adjustment, but it was within about 2 hz (jumped around a little bit).