Thursday, September 20, 2018

Resolving the KAT100 Wake-Up Problem

KAT100 antenna tuner below the K2/100.
My KAT100 has been a little troublesome for a couple of years now. Sometimes, when turning the K2 on, the KAT100 would take a second or more to turn on, and the K2 would act like the KAT100 wasn't present, giving NOT INST when any of the KAT100-related buttons were pressed.

I noticed it sometimes at the Walton County QTH. When it did happen, cycling the power a few times usually got rid of it. Sometimes I had to leave the rig on for a while. Later, when I moved the radio back to Gwinnett, it didn't happen any more.

Then I moved the radio to the new Fulton County QTH. This problem appeared with more frequency. Indeed, it happened nearly always, and warming up the rig for an hour or so the only way to get around it. This sometime problem had become annoying, and I wanted to get to the bottom of it.

At first, I thought it was some kind of power delivery problem in the KAT100. Normally, the KAT100 is supplied continuous12v power on the coaxial jack J1. The K2 supplies a signal 12CTRL on the J3 Control jack that goes to 12v when the K2 is powered on. This signal turns on Q1, and Q2, which supplies 12v power to U8, the regulator chip, which supplies 5v power to the unit and everything comes on.

That theory was shot down quickly. As soon as the K2 was switched on, the 12CTRL voltage would come up immediately, and U8 would deliver 5 volts immediately. That part of the circuit was working perfectly. Power was available to the KAT100 the moment the K2 powered up.

Even so, the KAT100 LEDs would not light up right away. Sometimes, it would be a second or two, sometimes ten or more seconds. When this happened, I'd get the NOT INST behavior. If the KAT100 LEDs lit as soon as I powered on the K2, everything was fine.

My next theory was that must have been something wrong with the AUXBUS connection -- the internal bus that the K2 uses to talk to the micro controller in the KAT100 (as well as the several micro controllers in the K2!). That one was busted as well, as a scope probe revealed very healthy transitions on the AUXBUS right on U1 pin 40.

I tried other remedies. Thinking there might have been something weird about the control cable connecting the K2 and KAT100, I swapped it out for another straight through cable. No change. Maybe U1 was not properly seated in it's socket. I removed it and reseated it. No change.

This problem appeared to the temperature and humidity related. This is why it happened sometimes in Walton county, never in Gwinnett county, and nearly always in Fulton county.

Since power was present, what was delaying the lighting of the LEDs? Looking at the circuit diagram, the LEDs have to be turned on programmatically by U1, the PIC micro controller. Maybe it wasn't resetting properly?

But, power-on reset logic is part of the chip itself. Something else had to be delaying the reset.

I decided the problem might have to do with the U1 clock. If the clock oscillator didn't start up right away, that would delay the reset. Putting scope probe on pins 13 and 14 or U1 deepened the mystery. When the KAT100 would fail to power up, these pins would float up to about 1 volt. When it worked, the pins would be locked at 0 volts. Why 0 volts? Because the chip goes to sleep after initialization! This uses much less power. U1 awakens on transitions on the AUXBUS.

So, it seemed very likely this was the problem. But the clock circuit is dead simple. U1 connects to Z1, a ceramic resonator.

I tried hitting Z1 with a heat gun, but I didn't see any immediate effect. Then I nudged Z1 with a screwdriver, and the KAT100 came up immediately. Maybe I had a bad device, or maybe a bad connection. I reflowed the solder on U1 pins 12-14, plus the three pins on Z1.

This worked right away, and even the next day when I let everything cool down.

Solved! It appears it was a problem with the U1 clock circuit, likely because of a poor connection to Z1. If it fails on me again, I'll likely end up replacing Z1 entirely.

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