Electronics > Repair
CMU200 Rx level issue / failed loop test
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_Wim_:
Thanks for all the excellent contributions so far!  :-+

For the sake of forum knowledge (and to satisfy my curiosity), I went a little overboard in testing.  ::)

First I tried so remove the glue from the copper tape, and then solder it to the surface of the filters. Results were worse than with the copper tape applied directly, both for the 1842 as for the 836 filter.  Probably due to the fact the tape is to bendy and I could not get it to solder perfectly flat.

Then I removed the soldered copper tape again (which probably overstressed the 836 filter, see below), and solder a tiny copper plate on top  (plate pressed flat on the surface filter, only soldered the sides while pressing down). This improved the result on the 1842 filter to only -3dB (was -10dB before), my best result so far. Unfortunately no more output on the 836...

So, it is possible to restore them to some degree, but not without any risk. Corrosion on my filters was probably not the worst to begin with, so this also could be a reason for a partial success.

(pictures were taken before clean up, so some debris is still visible in the pictures)
Michael-VK2BEA:

--- Quote from: Sas on January 05, 2022, 07:27:24 pm ---Re-alignment of the filter may be possible after supplementing the copper layer, taking into account the imperfections of the ceramic core.

--- End quote ---

From the explaination in the IEEE paper and by examining the 'gaps' in the filter posts, it seems that the entire filter is plated and then the gap created by grinding off the copper. I'm amazed that this can be done in a cost effective way, considering how cheap these originally were. I find it hard to believe that they were individually tuned; more likely to be mass produced from calibrated tools (and testing samples).
Sas:

--- Quote from: Michael-VK2BEA on January 06, 2022, 06:02:16 pm ---I find it hard to believe that they were individually tuned; more likely to be mass produced from calibrated tools (and testing samples).

--- End quote ---

Probably you are right. They produced it in a large number of pieces at the factory. Do you think they cut the filters and sold it like that, or rather they cut, measured and marked the group, what they got from the measurement?

I don't know what the difference between the two new filters might be. The manufacturer states a loss of 2.6dB in BW for the 836MHz BPF (2.8dB for the 1842MHz BPF). No two filters are alike, and the entire device is calibrated for differences in filters (and all the weight of other parts). Replacing the filter with the same second filter will not recalibrate the entire device. Maybe it will be a small brick to improve your results. The fact is that even minor damage to the surface causes a significant loss of signal. Hence my suggestion about trying to save the original, especially the 836MHz BPF filter, that you can't buy it anywhere.
Michael-VK2BEA:

--- Quote from: Sas on January 06, 2022, 07:25:53 pm --- The manufacturer states a loss of 2.6dB in BW for the 836MHz BPF (2.8dB for the 1842MHz BPF). No two filters are alike, and the entire device is calibrated for differences in filters (and all the weight of other parts).

--- End quote ---
Yes, the whole chain is used for the factory calibration. The unit I repaired was within 0.1dB so there was little difference in the filters I swapped.
Has anyonw cracked the code on the calibration file? (where is it stored and what is it's structure?)

Michael
RF_fanatic:
Nice work and good research.

I gave up on repairing CMU;s after countless units checked with reference units,and cleaning up my shack now (see for sale part of this forum) this specific filter is indeed a culprit, cleaning or better finding a donor filter is best option.

Most of the time a bad overal respons can be pinpointed to the digital units but the specific band drop more point to the rxtx or frontend, in this case the rxtx was a good conclusion, well done!
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