Electronics > Repair

CMU200 Rx level issue / failed loop test

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_Wim_:
Hi, after installing some firmware updates to allow AMPS functions on my CMU200, the unit asked to perform an FM Modulation calibration. This calibration failed, and upon further testing I discovered that also all loopback tests failed.
This problem existed probably from before the firmware update, but I only noticed it now. 

Issue 1 & 2 description:
-   Rx level is down -10dB between 10 MHz and 1200.1 MHz, -3dB down to 2200.1 MHz, again -10dB down to 2700MHz. (see FreRes plots)
-   Rx level is down exactly the same on RF4, RF2 and RF1 (so not channel dependent)
-   Rx level is down exactly the same on all signal levels (so not attenuator dependent)
-   Tx output level has been checked with an SDR dongle, and seems fairly accurate (so level drop is not related to Tx part)
-   The wideband power meter measures the correct level on RF2 and RF4 (remark: a 2nd separate issue is that the wideband power meter on RF1 does not work at all and that RF1 is +-12dB  more noisy than RF2)

Attached are the plots measured with FreRes sweeping the entire frequency range at different signal levels. At first glance this appears a similar issue as reported here: https://www.eevblog.com/forum/repair/cmu200-failed-loop-tests/msg3810332/#msg3810332

Remark: I have removed of the remaining RF foam absorber (see attachment), but for me it seems unlikely that this little copper corrosion could be the culprit. I did check the conductivity of the corroded top surface, that the copper plating is still completely conductive everywhere on the top surface.

Rootcause 1: level issue on RF1,2 and 4
For the level issue I suspect some damage around IF1 on the RxTx board because:
-   Its identical on all 3 input channels
-   The wide band power meter (which is included in the RF Front end) indicates the correct power
-   From the block diagram I would expect the different IF1 Frequencies do not affect the front end nor the ADC (and the level shift matches exactly with the IF1 switch points)

Rootcause 2: broken RF1 frontend
For the 2nd issue my guess is that RF1 got severely overloaded at one point causing damage to the power meter and creating the additional noise (my unit has the additional RF generator, so RF1 is limited to 2 Watt instead of 50 Watt, although I am unsure when and how I overloaded it).  This seems however not related with the level issue and is for the moment less important for me, as I can use RF2 instead without losing any functionality. I will work further on this once the first issue is solved.

What I have checked so far for issue 1:
-   Checked the RxTx board for visual damage (quite a few bodges on the RxTx board, indicating a previous repair, but no obviously damaged parts)
-   Ran all unit self-tests. Only failures are the  loopback tests (1=>1; 2=>2,1=>4;3=>2) and the FM Modulation calibration.
-   Switched back to the previous firmware configuration (no effect)
-   Reseat all cards, plugin boards and connectors (no effect)

I am however not sure how to debug the RxTx board, because it is not easily accessible for probing, and without the Rf housing efficient cooling could be an issue. The block diagram in the service manual is not very detailed , so a lot of reverse engineering will be required. I have made some detailed pictures for this purpose, these will be attached in the following posts. Any tips how to proceed are certainly welcome!

Temporary Fix:
Currently I have configured a frequency dependent attenuation correction that flattens the response quite perfectly, even when running a sweep with a 0.001MHz step. This off course is not ideal but will do for now.

_Wim_:
Detailed picture of Rx side of the RxTx board

_Wim_:
Detailed picture of Tx side of the RxTx board

_Wim_:
So I would expect this is the main region of interest. Before appears mainly the attenuator section, and further on we are back to a single IF so it must be in between those (I think  :-//)

_Wim_:
I started with a little reverse engineering. What Shahriar probably does in 5 minutes took me the whole afternoon.

What I find strange is that the block diagram in the service manual shows a shared LO (between Rx & Tx) for the 2nd mixer, but as far that I can see, this is not the case (only mixer 1a shares an LO with the Tx side).

Mixer 1 in the block diagram also appears to be made up of 2 stages, where mixer1b is used in both directions (up & down conversion).

Disclaimer: I have almost no knowledge of RF, I only watch "The signal path", so attached could be full with mistakes...

Any hints were the issue might be with this extra information?

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