Regarding Internal Receiver Noise in the Kenwood TS-2000

I have taken a bit of interest in recent reports of internally generated noise, described as static crashes, occurring in the TS-2000 receiver. This problem has been reported in transceivers purchased in the last few years (around the time I purchased one) and has been attributed to CF1, CF2, and CF4, the 455 kHz ceramic IF filters. I would like to summarize this issue and share my thoughts.

It has been reported on various user groups and elsewhere on the web that the ceramic filters, manufactured by Toko, have been failing due to a manufacturing flaw in recent production run. Owners have reported that, after replacement with new stock filters, the symptomatic static crashes disappear and the problem appears to be fixed.

Others have noted that such ceramic filters require DC blocking in an active circuit, as indicated by the manufacturer. In the TS-2000, these filters encounter a DC voltage as they are switched with diodes (D68-71), but no DC blocking exists. There is evidence that adding DC blocking capacitors to the filter outputs also cures this noise problem.

On a related note, many TS-2000 users have reported other cases of receiver noise. Some have sited the switching diodes on the BPF board as the source. A popular modification has been to replace these with Schottky diodes, but the reported real world results seem subjective and inconsistent. In the first TS-2000 I owned, I recall a perceptible change in the main receivers noise floor depending on whether the sub receiver was powered on or off, even if the sub receiver was muted.

I hypothesize that these problems result from the absence of a DC blocking capacitor at the outputs of various ceramic filters. Replacing the filter may only be a temporary fix unless DC blocking is incorporated. I also suspect that this problem may relate to other reports of internally generated noise in other production runs of the TS-2000.

What does DC voltage do to ceramic filters? When a DC voltage is present across the plates of the filter, particles of dust, oxidation, etc., collect on the surface and begin to form a (poorly) conducting bridge. This becomes a source of internal noise (liken it to a dirty AF gain pot or headphone jack but in the IF stage). SV8YM reports this concern in other transceivers on his website: http://sv8ym.blogspot.com/2010/07/mysterious-case-of-withering-filters.html

I suspect that Toko filters may not be electrically faulty, but imperfections in their epoxy resin seal are allowing airflow and foreign contaminants to reach the internal components. This would result in oxidation and the formation of conducting bridges described above. It may very well be that recently produced filters are more leaky and result in the sudden onset of receiver noise being reported. This would also suggest that other batches are sealed more tightly, but may still become problematic over time, increasing receiver noise at a rate less noticeable my the user (I wonder how many reports of a high noise floor in the TS-2000 are from radios purchased used, and several years old). This is similar to the change in performance characteristics that can occur in imperfectly sealed vacuum tubes.

This is my suggestion for owners of the TS-2000 who experience internally generated noise in the receiver: Replace the ceramic filters that have been implicated in this problem AND have them installed with DC blocking capacitors (10nf has reportedly been used). This should solve the sudden onset of static crash type noise that has been reported as well as the possibility of noise developing overtime. I plan to perform this modification prophylactically my TS-2000 I currently own and will post result. If anyone else tries it please send a comment and report your experience.