Thursday, February 4, 2010

How to Tune a Kenwood Hybrid from K4EAA

Here are some instructions for how to properly tune your Kenwood hybrid transmitter. It takes longer to describe than it does to actually tune the rig! When you have done it a few times, you can tune up in about 5 seconds total, faster than it took you to read this paragraph!

* Begin tuning with the Mode Switch in the Tune position. That reduces the screen voltage, and mis-tuning won't harm the rig. Start with a dummy load if possible.
* Set the Load Control to Minimum, and dial in the band (approximately) on the Plate control
* Set the Meter Switch to ALC
* Turn the CAR Control (carrier) up 1/2-3/4
* Place one hand on the Send switch and the other on the Drive control
* Flip the Send switch and peak the Drive control. If the meter pegs, turn down the CAR control and peak Drive again.
* Switch Meter to Ip (plate current)
* Flip Send switch and tune the Plate tuning to dip the Meter (minimum reading). This is usually the max power output point as well.

Now you're ready to go for some power! As you perform the steps below, don't leave the rig in Send more than 5 seconds at a time. Give it a rest now & again as you get familiar with this. Most of us can tune a rig in about 5 seconds total, you will too when you've done it a few times!

* Switch Mode to CW
* Flip the Send switch and re-dip the Meter with the Plate control
* Note the Meter reading - You are looking for about 225ma, depending on your power needs - About 1/3 to 1/2 less for PSK or RTTY
* It will probably be low. Turn up the Load control about 10 degrees, and re-dip the Plate control. Continue doing this until you get about 225ma of plate current when dipped. Remember not to leave the rig in Send too long while you're learning, give it a 5 second rest every 5 seconds.

You're almost finished! Transmitting at this point would be fine, but you may be able to tune for a bit more power. The next adjustments should be small adjustments, you are "fine tuning" things at this point.

* Switch your Meter to RF, or use an external power meter for reference.
* Tune the Plate control for maximum power output. You should see about 100W if you have a power meter.
* Try adjusting the Load up or down a bit, re-peaking with the Plate tuning after every adjustment.
* When you're satisfied with your tune-up, check Ip, the plate current, and make sure you're not too far from the 225ma target, and note the position of the controls to use this as a starting point whenever you return to this band.
* For CW, use the CAR control, and for SSB, use the MIC control to keep the ALC within the range shown on the Meter. This will insure maximum power output without excessive compression.

Things to Watch for While Tuning

Tune-up is a good time to check your rig for maintenance requirements. Your Kenwood hybrid is an excellent tube tester, better than commercially made ones, in fact. It exercises the tubes under actual working conditions, and meters the results for you to observe.

1. Notice how much drive is available. A fresh 12BY7A will provide more drive than necessary to swing the ALC Meter through its range, even on 10M. Low drive on any band shows that the driver tube is probably falling off in emission. It will continue to function for a while, but you might consider replacing it in the near future.
2. Watch for falling drive level as the rig is keyed for a few seconds. A 12BY7A near the end of its life will fall off in drive within a few seconds or so of key-down. If you see your drive fall as you are watching, that tube is positively ready for retirement.
3. Check the sharpness of the dip in CW Mode. A broad, shallow dip means the finals are nearing the end. You will notice reduced power output as well, most noticeable on the higher bands.
4. Check for falling plate current under key down conditions. Just like the driver tube, final tubes at the end of their useful life will jump to full Ip and then quickly start falling off. When you spot this, they are ready for recycling.

A Few Notes Related to Output Power

The difference between 50W and 100W is 3db. An S-unit is 6db. Consequently, the difference between a 50W rig and a 100W rig at the receiving end is about 1/2 S-unit. The difference between 90W and 100W rigs is not even discernable at the other end. Likewise between 100W and 110W. Resist the urge to load up your rig "To The Max," as all it does is shorten the life of most everything in the final section of your hybrid.

A reasonable increase in power, the first step really worth taking, is times ten. This holds for audio amps, HF amps, heck, for most all amps! The difference between 100W and 1,000W is 10db. That is about 1-1/2+ S-units at the receiving end, and is indeed noticeable. About the same as going from a poor antenna to a good antenna.

The HV position of the meter reads the plate voltage applied to the finals. The Kenwood hybrids utilize a voltage doubler circuit in the HV power supply, and aging High Voltage capacitors can result in low output. To check for this possibility, monitor the HV as you key the rig. It is normal to see perhaps 10% drop in HV when keyed. Anything much in excess of this might indicate failing capacitors.

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