It’s a common problem: failing ceramic filters, thanks to a design flaw in many transceivers. The FT-857/897 are (in)famous for that, and so are a number of Kenwood models. Recently I repaired and modified four transceivers in short succession. That’s a lot, probably a matter of Murphey’s Law.
The last one on the bench was a Kenwood TS-2000, owned by my neighbor ham Jan PE1LJS. He complained about crackling noises on AM and SSB, even when no antenna was connected. I immediately recognized the problem and offered to fix his rig. In the case of the FT-857/897 only one filter has to replaced, but in case of the Kenwood I didn’t get away with that – I had to replace all three of them in order to get it fixed.
TS-2000 inside, after replacement of the three original filters (Toko).
The flaw
When no DC block is located at both the input and output of these ceramic filters, a strange phenomenon occurs: electro-migration. In the end this process destroys metal, and the filter’s functionality in the process.
I’m still puzzled by the fact that this flaw is still part of many designs, and that it’s up to the ham to fix and modify the PCB’s. I took two of the original filters apart. The picture at the right shows a generous amount of copper oxide where it shouldn’t be, and other parts of the filter didn’t look much better either.
Oh well, it keeps me off the streets.
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