Suddenly I found my Kenwood TS-570D's internal antenna tuner reacting strange. The tuner didn't sounds like the days before and it couldn't tune at bands it could many times before. I inspected my antennas and found everything fine. By connecting the Kenwood TS-570D (ANT1) to my 50 ohm dummy load and pressed the 'AT TUNE' button, for more then about 1 sec., the internal antenna tuner couldn't tune at 12 and 20 meter - the automatic tuning continues again and again and 'error beeps' appears from the speaker... When deactivating the internal antenna tuner, by a quick press at ‘AT TUNE’, the SWR was 1:1 at all bands.
I removed the top of my Kenwood TS-570D and the metal shield around the internal antenna tuner section. Unfortunately I couldn't found anything strange in the section. By doing some very simple DC resistance measurements I found capacitor C816 (18pF/2kV) having a leak of about 250 ohm. I removed the capacitor by cutting it away – do not try to solder out the capacitor since the PCB is easy to damage!
I contacted Surplus Sales of Nebraska and bought some 6kV type capacitors of 18pF and other capacitors matching the other original capacitors in the internal antenna tuner but with higher voltage.
The dimension of the 6kV capacitor is lager then the 2kV original but when I removed the piece of rubber, placed between the relays and the metal shield around the internal antenna tuner, there was plenty of space for the new 6 kV capacitor. I soldered the new 6kV capacitor legs to the two pads where I was cutting the old 2kV C816 capacitor away a few days before.
After assembling my Kenwood TS-570D all works really great! I can now tune all bands on the 50 ohm dummy and my HF antenna - the repair has worked great for more then 8 months now (jan. 2005)!