Kenwood TS-940S

Yes, I bought a TS-940 and have been trying to diagnose hilarity.

Marker

The calibration marker circuit is .. sub-optimal, and picks up a lot of stray, random noise throughout the radio.

There’s a very nice write-up about this, applicable to at least the TS-930 and TS-940:

Power supply issues

The TS-940 power supply is not the best designed and it’ll go pop, taking your hard-to-find driver transistors with it. (k6iok)[https://k6iok.com/] has replacement power supply bits to drop in and it works!

My issues!

K6IOK installed his replacement PSU in my TS-940 and it runs like a charm! The radio runs much cooler and it’s much lighter. However, I’ve been having some RF issues which I bet are due to this rig.

I think the buck/boost board is a bit noisy and it’s being picked up by the wiring looms, ending up on the marker connector on the RF board.
Adding three clip-on ferrites reduced noise on 18MHz from S9+10 down to about S5. So there’s something weird going on there.
Removing the MKR connector (J2) from the RF board fixed the rest of the IMD showing up on 160/80m and the noise floor on 18MHz is ~ S0.k

PLL repair / recalibration

Yes this has sony bond related goop on the VCOs on the RF board and on the PLL board.
From Cliff Holland, re VCO-0 on PLL board (the HET / 36.22MHz Oscillator):

“Melt the wax out of VCO-0 (the one next to IC19). This is to allow easier resoldering of the components. Turn the board over and remove the old solder and resolder all of the parts that are inside the enclosure. Pay close attention to the leads on L35. Clean any potting compound on the bottom of the board especially under the cap tacked on the bottom of the board. Once clean and resoldered install in radio and adjust the coil for 7.5v at the TP. If stable and operating then you may repot if desired but not necessary.”

For reference: https://forums.qrz.com/index.php?threads/ts-940s.448583/

Dry / Wrong Electrolytic Capacitors

This radio runs hot. Very hot. I found two fun issues here:

A lot of the caps are dry/leaked after all these years. So yes, you should replace them.
I found some capacitors that were close to the operating voltage or wildly wrongly specified, which lead to them leaking.
The PLL board has a bunch of electrolytic capacitors within a volt or two of the rail voltage; I replaced them all with 25 or 50v rated capacitors.
My control board had incorrect rated capacitors on the linear regulators! (Like, a 12v cap on an 18v input rail.) It was either a bad repair or a factory messup. In any case, this caused all kinds of PLL stability issues so I just pulled all the large/power rail capacitors off and replaced them.

There are around 190 electrolytic capacitors to replace in this radio and it’s worthwhile doing it.

Service Manual Calibration Errata

Dave Phillips via groups.io to kw-ts940s

The power nob will not be linear, remember, ALC.

If the ALC is calibrated properly, which is difficult to do since the procedure in the Service Manual is seriously flawed, the control will be even less linear. The proper adjustment of the ALC is actually quite simple but requires the ability to insert precise levels of a 1500 Hz tone at the microphone input. Basically, the circuit is first adjusted to establish the ALC base line with a minimal amount of modulation in SSB mode. Then, the modulation level is doubled, and the ALC is adjusted to the high point in its zone.

The procedure in the Service Manual for this adjustment is flawed. The first step indicates you should adjust VR8 on the Control Unit while injecting 1500 Hz at the Mic input in Transmit Mode. This is wrong. VR8 sets the ALC meter mechanical zero point, so the Mic Gain should be Min and no modulation inserted while adjusting this setting. Then, add the 1500 Hz input at 5 mV and advance the Mic Gain to move the ALC meter to the bottom of the ALC Scale. Finally, increase the 1500 Hz modulation input level by 6dB (100% increase to 10 mV, and adjust VR9 on the Control Unit to set the ALC meter to the top of the ALC Scale.

Once calibrated, you will find the Mic Gain will be more linear, at least until the ALC begins to limit the power output.

Operating practices using the ALC Meter Indication.

Obviously, since the ALC reduces transmitter drive to prevent distortion, then the optimum performance of the transmitter will be obtained when the ALC meter deflection is minimal. The higher the ALC indication on the meter, the more the transmitter drive is being reduced. It has been ‘Common Wisdom” amongst Amateur Radio operators for some time that the ACL should be full scale while transmitting. This is absolutely false. Operating with the ALC meter deflection predominantly in the middle to high region of its scale is not efficient and does not improve your signal power or quality.

When operating the TS-940 in SSB/AM/FM or FSK Modes, it is advisable to set the MIC GAIN(FM MIC GAIN) or the PROCESSOR OUT controls to prevent get minimal ALC indication. Adjusting the optimum output in CW Mode is performed using the CAR LEVEL control. The TS-940 has the ability to produce excessive drive to the PA, especially in the CW mode. To prevent this, the CAR LEVEL control must be adjusted to obtain the maximum power output with the least amount of ALC. To achieve this, rotate the PWR control to full CW position, press SEND, then adjust the CAR LEVEL control to obtain maximum power output with minimal ALC.

– 73’s Dave KB7JS