Electronics > Repair
CMU200 Rx level issue / failed loop test
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DavidAzulay5000:
Today I checked all frequencies between 1200 and 2200 MHz
are bad
Example with I fed from RF3 frequency generator
In the example -20DB
The signal is low -50DB
Up to frequency 1200 and above frequency 2210
The device works perfectly
I don't have another spectrum analyzer
for this moment i give up  |O

When I ordered the filters
I ordered 3 types
the 836.5MHZ B122
and 2 types of 1842.5MHZ

the
[50pcs] B69813-N1847-X475 PCN RX Filter 1842MHz SMD
It was really rubbish
installed was no signal at all frequencies
that I installed the
B39182-B4139-U410 1842.5MHz SAW Filter 50 Ohm SMD
All frequencies up to 1200 and above 2200 were perfectly fine again.

With the new filter of 1842.5 The Internal loop test
range of 10 MHz and 1200MHz perfect

range of 2205-2700MHz perfect
succeeded
Better than the original.

The frequency group 1200 to 2200 is really bad
the fm modulation calibration pass
But still failed L1 CO PRO LEVEL CAL

The level is too low. Check the if signal connection
_Wim_:

--- Quote from: DavidAzulay5000 on February 16, 2023, 09:37:12 pm ---I don't have another spectrum analyzer
for this moment i give up  |O

--- End quote ---

As an alternative, you could just bridge the filter (IN directly connected to OUT). This will allow the spurs to pass also, but should give you a slightly too strong signal (original filter had a 3dB insertion loss, so I expect the signal to be +3db of what was expected).

If the signal is still to low with a shorted filter, the issue is not the filter but somewhere else. The signal path is relatively easy to follow on these boards (see also my post https://www.eevblog.com/forum/repair/cmu200-rx-level-issue-failed-loop-test/msg3916070/#msg3916070), so than it is a matter to track down where the signal stops (but this does require a way of measuring an RF signal, like a spectrum analyzer or RF-power meter)
Andrejka:
Hello,

Sorry for offtopic. Maybe someone have additional 1842.5 MHz filters to sell?
 
Thanks
Andrej
st0ne:
Hi all  :)

i fixed my CMU200 by cleaning the filters and covering them with conductive paint which is used for repairing keyboard traces!
Maybe this information is helpful  :)

https://www.amazon.de/gp/product/B07WJTYXPH/

this conductive paint is very fluid and gets hard with a hot air gun and about 100C temperature.

all tests passed :)

Philipp
_Wim_:

--- Quote from: st0ne on July 25, 2024, 05:59:44 am ---Hi all  :)

i fixed my CMU200 by cleaning the filters and covering them with conductive paint which is used for repairing keyboard traces!
Maybe this information is helpful  :)

https://www.amazon.de/gp/product/B07WJTYXPH/

this conductive paint is very fluid and gets hard with a hot air gun and about 100C temperature.

all tests passed :)

Philipp

--- End quote ---

That seems like a very good way to fix this. Much easier than locating the required parts. Thanks for posting!
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