Frequency adjustment on TS-870

The standard Kenwood TS-870S employs a 20 MHz quartz crystal oscillator to generate the reference frequency f_std. All the synthesized frequencies are derived from this one reference oscillator. Performance is reasonable, and the oscillator can be trimmed exactly onto 20.000 MHz using trimmer TC800. Access to the trimmer is relatively simple – Kenwood provides a hole in "Cover-B" over the PLL board to allow easy adjustment. There are full instructions on page 70 of the TS-870S Instruction Manual explaining how to reach the trimmer and how to adjust it while tuned to a standard frequency station such as WWV on 10 MHz.

According to the specifications, the standard TS-870S has a frequency stability of +/- 10 ppm from -10 to +50 deg C. Installation of the optional SO-2 temperature-compensated crystal oscillator (TCXO) improves the stability by a factor of 20 times to +/- 0.5 ppm from -10 to +50 deg C. In practice, if you noticed a frequency drift from cold switch-on to fully warmed-up of 40 Hz, this would be reduced to 2 Hz with the SO-2.

As mentioned in the "Raspy TS-870" section, there are instructions for installing the SO-2 module on page 76 of the TS-870S Instruction Manual. Unfortunately, once the SO-2 is installed, there is apparently no way to trim the 20.000 MHz reference oscillator exactly onto frequency. In my case, the tuning error eventually reached 10 Hz at 10 MHz. One solution is to switch back to the original 20 MHz crystal – there’s a miniature slide switch to select the output – but then the temperature drift returns.

It turns out that the Kenwood SO-2 TCXO does have its own frequency trimmer, but it is concealed by the Kenwood sticky label on the top surface. By feeling around with a miniature screwdriver, I found there was a hole underneath the "N" of "KENWOOD".

If you need to adjust the SO-2 TCXO in your TS-870, proceed as follows:

1. Switch off external power, remove the top case, remove the speaker bracket then lift up "Cover-B" as described in sections 1 and 2 of "Raspy TS-870" and on page 76 of the Instruction Manual.

4. Turn on power, let the transceiver warm up for fifteen minutes or more then gently adjust the newly revealed trimmer with a trimming tool or miniature screwdriver for exactly 20.000 000 MHz output from the TCXO.

There are several ways to adjust the reference oscillator onto frequency. If you have an accurate frequency counter, you can measure the SO-2’s output directly at connector CN510. You can also tune the transceiver to a standard frequency station such as WWV on exactly 10, 15 or 20 MHz, select CW mode to listen to the carrier, then use the technique in the Instruction Manual to beat the transceiver’s 800 Hz sidetone oscillator against the received signal from WWV. Choose the highest frequency that gives good reception of WWV for the greatest accuracy, then adjust the trimmer for zero-beat.

For the final adjustment, I employed a slightly different technique. During daylight, when reception of WWV on 20 MHz is good, it is possible to observe the beat note between WWV’s incoming carrier and the 20 MHz reference oscillator radiated by the TS-870 with its covers off. For best results, I would recommend using a second receiver — preferably with an analog S-meter — to observe those beats. I put my Kenwood TS-430S to good use as the second receiver, switching the TS-430 to AM and then to FM to monitor the slow beats between the two 20 MHz signals. You should be able to adjust the TCXO trimmer until the S-meter is pulsing at less than one beat per second.

There’s a third technique described on the "CR-282" page. This involves monitoring of WWV during one of the periods when it is modulated with a steady audio tone. With the TS-870 receiving WWV on a frequency of exactly 10, 15 or 20 MHz, switch the Mode back and forth between USB and LSB – you will probably notice a small change in audio frequency during this change. When the SO-2 reference oscillator is adjusted to exactly 20.000 MHz, the modulation should not change pitch during the switches from USB to LSB.

5. When you are quite sure that the SO-2 trimmer is accurately adjusted, prepare to replace cover B and the case. Hint: before you do this, cover the new hole in the SO-2 with a small piece of sticky tape to prevent convection currents. Switch off external power then replace all covers.

6. Switch on the TS-870 then recheck the frequency accuracy against a standard frequency station immediately after power on and one hour later when the transceiver has thoroughly warmed up.

Afterthought: the SO-2 TCXO is an option for other Kenwood transceivers including the TS-50S, TS-570S, TS-850S and TS-950S. If you need to adjust the SO-2 for exact frequency operation in these radios, you should be able to use a similar technique.

-de NM9J, 24 Feb 2004

G3VNQ-NM9J amateur radio site, 17-Apr-2015