CMU200 Rx level issue / failed loop test

[_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?

[_Wim_]

For those who do not know: if you right click the above image (when it is large) and select "open image in new tab" you can zoom in to great detail. I have tried to make the max resolution I could (original image size was almost 80Mb)

[Michael-VK2BEA]

Wim,
   I did respond to your question here https://www.eevblog.com/forum/repair/cmu200-failed-loop-tests/
It looks remarkably like the problem I fixed with my CMU200.

To debug this I used an RF probe (one of the cheap ones from eBAY) and a spectrum analyzer, since you don't care so much about the actual level ... just the difference between the input and output of the filter.
Remove the heat sinks and you can do some probing toward the top half of the board (it's awkard but possible). For sections toward the bottom you might have to solder wires from the board to the probe.
The filters are at the top, so I would try and check that. Since I found this problem, I have heard from three other people who have the exact same failing filters. I think eventually they will all suffer the same fate.
I replaced the IF filters and the unit is restored.

Michael

[_Wim_]

Wim,
   I did respond to your question here https://www.eevblog.com/forum/repair/cmu200-failed-loop-tests/
It looks remarkably like the problem I fixed with my CMU200.

To debug this I used an RF probe (one of the cheap ones from eBAY) and a spectrum analyzer, since you don't care so much about the actual level ... just the difference between the input and output of the filter.
Remove the heat sinks and you can do some probing toward the top half of the board (it's awkard but possible). For sections toward the bottom you might have to solder wires from the board to the probe.
The filters are at the top, so I would try and check that. Since I found this problem, I have heard from three other people who have the exact same failing filters. I think eventually they will all suffer the same fate.
I replaced the IF filters and the unit is restored.

Michael

Thanks. Will try to measure them first. Good point about about the RF probe, I remember I bought one of those a while back when they were discussed here on the forum.

Did you end up buying the replacement filter here?
https://www.ectransistors.com/10-PCS-INDUCTOR-TRANSFORMER-MMCL-DFC3R836P025BHD-3R836P025-DFC3R836P025BH.html

[_Wim_]

Thanks. Will try to measure them first.

I managed to measure the 836MHz bandpass filter, and you are indeed correct, loss is much higher than it should be (8.55dbm instead of something around 3dbm). The 1842MHz filter I did not check , as the RTL-SDR dongle that I am using tops out at 1766MHz (but I am now sure it will be similar )

I must say I am very surprised by this, as the corrosion did not look so bad (see picture from after they were cleaned), and the copper plating is still conductive over the entire surface of the filter. It seems like some corrosion must have migrated through the copper affecting the internal ceramic structure? 

Principle used (might be useful for others who want to perform this test):
Equipment:
- Low cost Chinese RTL-SDRv3 dongle with Satsagen software to make the SDR dongle work as a spectrum analyzer
- Low cost RF Active Probe 0.1 - 1500 MHz from Ebay (similar to this: https://www.ebay.com/itm/332253695460?hash=item4d5bdd0de4:g:CzsAAOSwWlNZzA4E)

Setup
- generated a 1500Mhz 0dBm signal via RF3, looped back to RF4 in (1500MHz to be in the band used by the 843MHz IF)
- CMU200 was put in narrow band power mode, with a center frequency of 1500Mhz (to ensure a continuous IF-signal)
- RxTx card installed in CMU without its shielding/heatsink so probing is possible (after the test I checked with an IR camera, and the max temperature I could see was 65°C)

Thanks again for pointing this out 

[Michael-VK2BEA]

I tried purchasing filters here but with no luck...  after 3 weeks the order is still 'pending' and they do not answer my email. I'm not sure if its a scam but it doesn't look good 8-)

The filters ar Murata monoblock (Gigafil) devices DFC3R836P025HHD (836MHz) & DFC31R84P075HHA (1842.5 MHz)..  (see datasheet)
The filter design is described in the attached IEEE MTT paper. It is effectively a combline filter using a ceramic dielectric.

From what I can tell, the corrosion is just on the top surface (and top edge). If push comes to shove, it may be possible to electroplate copper back on to that surface. I bought an RXTX board on eBay and used the filter from the TX side (The same filters were corroded on that board but there is an identical filter on the TX side which is not in contact with the RF absorber ... and thus in good shape).

Michael

[_Wim_]

From what I can tell, the corrosion is just on the top surface (and top edge). If push comes to shove, it may be possible to electroplate copper back on to that surface.

I did a "quick & dirty" test with some copper tape as a proof of concept. I think the results speak for themselves. This is still with the shield/heatsink of the board removed. Some more careful tape-cutting and maybe soldering it in place (to prevent the glue from coming loose in the hot environment with the shield installed) is definitely a worthwhile improvement! And I would add a layer of capton tape on top to prevent future corrosion for the RF absorber foam.

[_Wim_]

For those who are wondering about the pattern in the tape, my role of copper tape is similar to this: https://www.newark.com/3m/1245-tape-1-2/tape-copper-foil-54ft-x-0-5/dp/66F1197?st=3m%201245

(part number mentioned on mine is 3M 1739-7)

[Sas]

Not all CMU200 units have this problem. Some units, probably newer ones from the end of production, have rx / tx modules which do not have this problem. The originally fitted RF absorber is much thinner and does not touch the filters. The modules can be recognized without opening, because they have the serial number printed on the pcb, not glued on the sticker. I don't know if this is the rule. For about 20 CMU200s, only 3 had such a newer module with a similar serial number without the problem of corrosion of the filters. They passed the test 1-> 4 / 3-> 2 RF Loop Path 1. I attach a significant fragment of the module photo without this problem.
The copper foil should be smooth, thin, without glue, adhere as well as possible to the ceramics and not cover the larger BPF holes. For me, using the foil, it was not possible to get the correct test 1-> 4 / 3-> 2 RF Loop Path 1, i.e. +/- 2 dBm in the entire range. Later I will attach photos of modules and filters with problems and I will try to explain more. The original 836.5 MHz murata filter is probably impossible to get, I have been looking for it for 2 years. It can be desoldered from a damaged FSH3 board.
I am looking for a method of home copper coating of ceramics without the use of electricity and an initial layer of carbon or palladium. Service companies dealing with catalytic copper plating, after learning about the size of the object and the problem of covering the holes, just laughed at me :-)

[Michael-VK2BEA]

Interesting..  The number to the right of the S/N is probably the date code. Yours shows 17/18, mine shows 14/09 (maybe 2014 week 9).
Its likely that they began seeing the problem after several years and fixed the design.

The 1842 MHz filters are available from one source I found in Italy. The 836 MHz filters are unobtanium in the original pinout but can be found in a slightly different package (BHD).

[_Wim_]

The copper foil should be smooth, thin, without glue, adhere as well as possible to the ceramics and not cover the larger BPF holes. For me, using the foil, it was not possible to get the correct test 1-> 4 / 3-> 2 RF Loop Path 1, i.e. +/- 2 dBm in the entire range.

Would conductive paint be an option? Something like this: https://www.ebay.com/itm/194247218349

I only have copper tape with structure currently available. Another option might be to this the surface with a layer of solder (for less badly corroded parts at least, so the solder has something to adhere to)

[Sas]

This module 1718 is from CMU200 calibrated in 2018. Based on the dates on the power supply (January 2005), hard drive (October 2004), fan (week 33 2004), R&S Electrical Safety Inspected sticker (March 2006), I assume the unit was manufactured in 2005 and sold or safety tested in 2006 year. There are numbers 0301 on the power supply (production January 2005, i.e. 0301 is not a date). Other numbers on the modules are 0309 0608 4000 0208 1104 0400 0300 1500. If it were weeks and dates, the device would be rebuilt many times. In this device I am almost sure that if something was replaced it was only the txrx module, although it may be the original module.
I have another CMU200 calibrated in 2015 and it has a sticker 1607 on the rxtx module. The rxtx module is almost certainly not from this unit. I bought this unit incomplete, without the dsp module and repaired it. Then, in 2020, a buyer cheat opened it, replaced the parts (keyboard, ccfl lcd, rxtx, dsp) and returned the damaged one. Now he is trying to fix it again. Based on the dates on the power supply (September 2004), hard drive (July 2004), fan (week 12 of 2003), R&S Electrical Safety Inspected sticker (March 2006), I assume the unit was manufactured in 2004 and sold or safety tested in 2006 year. There are numbers 0301 on the power supply (production September 2004). Other numbers on the modules are 1300 0400 0200 3800 0203 0208.
Another CRTU calibrated in November 2006 by R&S has a sticker of 1026 on the rxtx module. I think it is the second calibration after a year from sale, traces of removing the first glue on the housing. R&S Electrical Safety Inspected sticker for testing performed in November 2006. Based on the dates on the power supply (August 2001, no additional sticker), fan (week 38 of 2000) and hard drive (August 2005, numbers 0100), calibration and stickers, I assume that the unit was produced at the end of 2005. Other numbers are 2700 (cpu) 0300 (main board) 0409 0500 0100 (hdd) and 1026 (rxtx). By suggesting that this date, it would be difficult to manufacture a device in January 2000 from parts from future years.
EDIT: The numbers 1026 on the module rxtx are: Part Number, Serial Number and Modification Index (e.g. the number 10.26 can be read after pressing the INFO key on the CRTU). It has nothing to do with the production date.

I have 1842 MHz filters. I ordered 836 MHz filters in the BHD version a month ago, when the topic of cmu / crtu repair came back.

Painting cavities with the Chemtronics CW2000 marker pen does not give positive results, it additionally damages the copper layer. Painting with a conductive fabric based on graphite does nothing. Painting with copper-based paint does not do anything, the layer is not electrically conductive. Painting with Armack Kupfer Spray advertised by the manufacturer as powdered copper in an aerosol (does not contain paint) gives little, the layer is not electrically conductive. I added various copper strips, it is better but it is not good, fails the 1-> 4 / 3-> 2 RF Loop Path 1 test. I have not tried to solder the surface, it will probably damage the filter irreversibly. Each piece of the filter is individually tuned on a ceramic core. The core has defects on the ceramic surface prior to the copper plating process. These surface defects are filled with a copper layer and affect the filter tuning. Perhaps later the filters are measured, grouped and marked. I will add photos from the microscope soon.

Edit: Here are two photos of a working filter. I am posting two more extremely corroded filters after being returned from a buyer cheat. He will post a few more photos soon.

[Michael-VK2BEA]

Would conductive paint be an option? Something like this: https://www.ebay.com/itm/194247218349

I tried silver paint but it only partially restored the loss. I assumed because of the higher resistivity (compared to the original electroplated copper).

[Sas]

Photos of the 836 MHz filter before and after cleaning. The enlarged view shows the ceramic clearances without the copper layer. The ceramics are dark gray (inverted orange).

[Sas]

Photos of the 1842 MHz filter before and after cleaning. The enlarged view shows the ceramic clearances without the copper layer. The ceramics are dark gray (inverted orange).

[Sas]

Photos of the 1842 MHz filter from CMU in very bad condition, corroded, scratched by the buyer (from my previously edited post). The filter was soaked for one day in isopropanol and washed in an ultrasonic cleaner. As you can see, there is not much left of the copper layer. The core is tuned by cutting the sleeve from the inside.
Photo of another 1842 MHz filter, which shows well the damage to the ceramic core, which has been copper-plated. Each ceramic core has more or less damage before copper plating. The damage to the ceramic core is mainly at the edges. The tuning takes into account the copper-bonded faults of the ceramic core.
Re-alignment of the filter may be possible after supplementing the copper layer, taking into account the imperfections of the ceramic core.

[_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]

Re-alignment of the filter may be possible after supplementing the copper layer, taking into account the imperfections of the ceramic core.

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]

I find it hard to believe that they were individually tuned; more likely to be mass produced from calibrated tools (and testing samples).

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]

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).

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!

[Sas]

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

The difference of 0.1 dB is acceptable in the two new filters. I was unable to decode the table structure. I stopped while trying to run VERM and CMU on a virtual machine. There are traces in the configuration files that the software can run under dos itself, without the entire device (for programmers). I think the calibration software is already installed on the device. I tried to figure it out two years ago, now I don't remember much. Re-calibration after the factory is probably done by connecting the standards and sending a few commands via GPIB. I think there is one general table for tx and one for rx that can be calibrated outside of the R&S factory e.g. in LGE Korea. It seems to me that the correction of other tables is only possible at the R&S factory (e.g. temperature corrections).

Where did you buy DFC3R836P025HHD ?

[Sas]

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...

Soldering surfaces is a risky idea. The filter is not soldered from the bottom to the PCB. PCB for RF doesn't like high temperatures. After all, the result of -3.5dB is quite a good result. It is not known how linear the generator is, in my CMU200 it has a deviation of about +/- 0.5 dB for -30dBm in the entire frequency range.
Yesterday I checked the first channel of my CRTU. The worst loss on an internal loop 1->1 is -1.5dB and 2->2 is -2dB. External loop 1->4 is -2.1dB and 3->2 is -2.4dB. The loss of -2.1dB is repeated for -35dBm in loops 1->4 and 3->2, which excludes problems with RF inputs. Probably the 836 MHz filter has too much loss, there is some fault or the module requires calibration. I don't have new 836MHz filters to verify this.

I add photos of filters before and after cleaning from the first channel of my CRTU. I cleaned the upper surface of the filters with a cotton swab and isopropylene alcohol, then a mixture of methyl alcohol, water and detergent (dishwashing concentrate), neutralized with isopropylene alcohol, then cleaned with ketchup (it cleans copper perfectly) and neutralized again with isopropylene alcohol. I isolated the filters from the RF absorber with laminating pouches (0.15mm). I did not want to stick anything to the surface of the filters so as not to damage the copper layer.
I will check the results of the loop measurements after the initial repair in the CMU after the return from the buyer (he replaced the rx / tx board and more ).

[Sas]

FreRes CMU loop 3-> 2 after replacement of BPF 1842 MHz with a new one and BPF 836 MHz tightly covered with copper tape. The tape is soldered on the corners of the pcb filter. The 836 MHz filter repaired in this way has too much loss of about -4.4dB. If the filter factory has a loss of -2.6dB, it is about -7dB after repair. I note that this filter was in a very bad condition (photo 31_CMU_B_836_return_buyer _cheat.jpg).
Internal loop 1-> 1 and 2-> 2 test in the range of 10-1200MHz are from + 0.3dB to + 3.1dB and in the range of 2205-2700MHz they are from -1.4dB to + 1dB. I guess it should be like that, in the internal loop of the same input there is too little isolation in the signal paths. Someone has already written about it.

Is there any way (apart from printing a screenshot onto a CF card) of logging these results (any text format)?
Do you switch the RF4 IN input correctly in FreRes (the analyzer remains on the RF2 input instead of switching to RF4)?

[_Wim_]

Is there any way (apart from printing a screenshot onto a CF card) of logging these results (any text format)?
Do you switch the RF4 IN input correctly in FreRes (the analyzer remains on the RF2 input instead of switching to RF4)?

The failed results are stored in a file cst.err when running in continuous loop test mode. The good results unfortunately are not. A workaround is installing an attenuator in the loop path so all results are bad...

To get the cst.err file, you have to have a keyboard attached to the CMU. When all the tests have run (in continious test mode), you have to hit alt+F4 to exit in dos modus. The cst.err file is found in the firmware folder (if I remember correctly). This file has to be renamed/copied in order to prevent it from being deleted automatically.

After renaming, you can reboot the CMU and transfer the file to your hard drive using the "remote service tool" software.

There might be an easier way, but that is the only way I know of.

[Sas]

At CMU (buyer's cheat back), after several hours of warm-up, the RF3 OUT output died    . Yes, I thought the screw heads were damaged for a reason. He also replaced the RF IN / OUT module  .
And I wanted to try the log method with the cst.err file. The generator works on RF1 and RF2 (loop 1 i 2 is ok). It may RF3 OUT work when it cools down, but finding such thermal faults is not easy. We'll see tomorrow. I'm fed up, I'm going to sleep .

Edit:
The MMIC amplifier at the RF3 OUT output is damaged. The cool one worked as normal, but as it warmed up it limited the gain to the band above 2.2 GHz, and then the full band. Cool it worked, warm no, and over and over again. After disassembling the in / out module and heating the amplifier with hot air, it got a short circuit on the output (and power supply) of 60 ohms. It is now completely dead.

[Michael-VK2BEA]

Where did you buy DFC3R836P025HHD ?

The replacement 836 filter I canablalised from another board bought on eBay (from the TX side .. it also has corroded RX side filters)


I placed an order for the BHD version on the dubious site I mentioned earlier (ecTransistors.com) but have not heard anything from them despite several emails. The 'order' still shows pending and the PayPal payment is the 'approved' state. I'm not optimistic though.

After seeing Wim's message the other day, I did another search and found some (BHD) on eBay which I ordered. (I had looked on eBay previously but not found this)
The really curious thing is that the price on the dodgy site and the eBay seller are identical (except they added a bunch of fees and taxes). Maybe its someone scraping eBay and then charging high shipping and fees.

BTW .. If you need just a couple, send me a message.

Michael

[Sas]

After seeing Wim's message the other day, I did another search and found some (BHD) on eBay which I ordered. (I had looked on eBay previously but not found this)

I ordered BHD filters on 23 Dec 2021 from the same ebay source. Theoretically, the package is flying to me. I wonder who will get first
 (USA vs Europe send 29 Dec 2021). If I did not get it within a month then I will send a message.
According to the IEEE MTT (thanks a lot) that you posted, BHD filters can be older type filters, with an electric wall. I still have to wait for the MMIC amplifiers to the RF3 output, I can't do anything else until the parts are delivered.

[Michael-VK2BEA]

According to the IEEE MTT (thanks a lot) that you posted, BHD filters can be older type filters, with an electric wall. I still have to wait for the MMIC amplifiers to the RF3 output, I can't do anything else until the parts are delivered.

Unfortunately it's going to be difficult to get it to fit. I'm thinking that it might need an intermediate PWB (with castelated edge 1/2 holes) to mount the BHD and lift the end of the (bigger) filter over some other smd components. 

Michael

[_Wim_]

I can happily report that the BHD type filter can be bodged on with good results 

Probably not ideal as not all the filter castelations are grounded, but seems to work ok. I first covered the small components above the filter with capton tape to prevent shorts, then moved the small cap at the input of the filter a little to the side (but still on its original pad), aligned the filter such that the cap fitted just inside the castelation input of the filter, and the solder the ground and the output of the filter.

Covered the new filters with capton tape to prevent future corrosion from the RF absorbers and reassembled. All tests past, even the FM Calibration that failed before!

Remark: removing the 836MHz original filter was not easy. I do not know if this was related to my soldering abuse from before, but I had a hard time getting the filter of the board. Luckily nothing got damaged in the process.

[Michael-VK2BEA]

I didn't notice before but your layout is different than my RXTX board. In mine the 936 MHz filter is at the edge of the board.

[_Wim_]

I didn't notice before but your layout is different than my RXTX board. In mine the 936 MHz filter is at the edge of the board.

Ach indeed, you board does look quite different. Something to watch out for when attempting this repair. It was already very tight on this layout, but luckaly it just fitted.

[_Wim_]

Now that I look at the pictures again, I did also install the filter in reverse (output used as input). I presume these filters are fairly symmetrical, but should maybe check if no additional spurs are seen by doing it this way....

[Michael-VK2BEA]

Now that I look at the pictures again, I did also install the filter in reverse (output used as input). I presume these filters are fairly symmetrical, but should maybe check if no additional spurs are seen by doing it this way....

I think you are right ... they should be palindromic (50 ohms in and out, passive and linear). I'll see if I can confirm on the VNA. I think there should be a way to fab an intermediate board that gives a better ground (and lifting above the other components).

BTW It seems R&S used these filters in other devices. I spotted them the other day in Shahriar's (Signal Path) repair video of the FSL spectrum analyzer. (see them here in the bottom left  https://youtu.be/60hasxvl5KU?t=1044)

[_Wim_]

I think you are right ... they should be palindromic (50 ohms in and out, passive and linear). I'll see if I can confirm on the VNA.

 

BTW It seems R&S used these filters in other devices. I spotted them the other day in Shahriar's (Signal Path) repair video of the FSL spectrum analyzer. (see them here in the bottom left  https://youtu.be/60hasxvl5KU?t=1044)

Good find. Probably not a "cheap" donor though... 

[Sas]

All tests past, even the FM Calibration that failed before!

Congratulations on the repair . Can you do the 1-> 4 and 3-> 2 loop test with a positive result?

BTW It seems R&S used these filters in other devices. I spotted them the other day in Shahriar's (Signal Path) repair video of the FSL spectrum analyzer. (see them here in the bottom left  https://youtu.be/60hasxvl5KU?t=1044)

SA R&S FSH3 also has the same filters.

I don't have MMIC amplifiers yet and work stopped.
I got the same filters from the same seller. The interesting thing is, each of the filters is different, one of the filters has large offsets between the sections. The entire 3 pole BPF consists of three separate filters.

[_Wim_]

Congratulations on the repair . Can you do the 1-> 4 and 3-> 2 loop test with a positive result?

A single frequency fails on the TX1 => RX4. But this is related to CH1 of the frontend, which was a separate issue as reported in the first post (and as far as I remember, CH1 has always been a less good performer since I have the CMU). I typically do not use CH1, so I never looked into this.

There does seem to be a small jump (+-0.6dbm) when switching IF-frequencies, but will try to fix that with the frequency dependent attenuation settings

[_Wim_]

There does seem to be a small jump (+-0.6dbm) when switching IF-frequencies, but will try to fix that with the frequency dependent attenuation settings

A little better

[_Wim_]

Meaured spectrum of a BG7TBL RF noise source (https://www.epanorama.net/blog/2017/08/29/bg7tbl-rf-noise-source/)

No strange jumps or peaks, so I expect most is working fine, certainly sufficient for my modest needs in RF (my low cost China RF cables and connectors are probably creating at least as much deviations...)

[Sas]

I fixed the CMU.. The CMU passes all tests without errors, including the 1-> 4 / 3-> 2 loop test. Adds the RF3 OUT -> RF2 loop test image from FreRes after repairing the RF3 output and replacing the 836 MHz BHD filter 

[_Wim_]

I fixed the CMU.. The CMU passes all tests without errors, including the 1-> 4 / 3-> 2 loop test. Adds the RF3 OUT -> RF2 loop test image from FreRes after repairing the RF3 output and replacing the 836 MHz BHD filter 

Congratulations. Looks very good! Can you post a picture how you installed the BHD filter?

[Sas]

Congratulations. Looks very good! Can you post a picture how you installed the BHD filter?

I soldered the same, isolated with Kapton tape. I removed the solder mask and added a GND solder pad (PB tin) under the filter on pcb (i marked the size and location in the photo). I soldered the filter with hot air, then soldered the top of the filter to ground and the in / out connection with a soldering iron (new toy on board JBC CDE-2BQA  ).

The waveforms look similar, my deviation from +0.7 dB to -1.2 dB. My gain drop is more linear. If you have a problem, it may be possible to correct it with an attenuator (I think 6 dB, I marked in the photo).
It will not change the fact that you may have too low gain (from -0.1 dB to -1.4 dB) in the whole spectrum analyzer operating range or too low signal generator level. I don't know why. Spectrum analyzer gain too low or generator RF level too low. Missing +0.8 dB gain (or generator RF level) causes errors from loop test 1-> 4 / 3-> 2. May require calibration or some repair.

[_Wim_]

I soldered the same, isolated with Kapton tape. I removed the solder mask and added a GND solder pad (PB tin) under the filter on pcb (i marked the size and location in the photo). I soldered the filter with hot air, then soldered the top of the filter to ground and the in / out connection with a soldering iron (new toy on board JBC CDE-2BQA  ).

The waveforms look similar, my deviation from +0.7 dB to -1.2 dB. My gain drop is more linear. If you have a problem, it may be possible to correct it with an attenuator (I think 6 dB, I marked in the photo).
It will not change the fact that you may have too low gain (from -0.1 dB to -1.4 dB) in the whole spectrum analyzer operating range or too low signal generator level. I don't know why. Spectrum analyzer gain too low or generator RF level too low. Missing +0.8 dB gain (or generator RF level) causes errors from loop test 1-> 4 / 3-> 2. May require calibration or some repair.

Thanks for the additional info and pictures. Maybe I will experiment a bit more with the filter placement in the future, but for now I am quite satisfied how it works. Nice JBC you have there, makes my JBC BT-2BWA look "old fashioned" 

[Bravo]

I found some filters on Arrow electronics website. They are smaller than the originals but have the correct spec.
So some kapton tape & thin wire will be required.
They are discontinued & very cheap. They have large stocks ............until throw them out.
The 836.5 Mhz one has a  part No B39841B4147U410  @ $0.45 for one.
The 1842.5 Mhz one has a part No SF1192B  @ $0.73 for one.

[Bravo]

I ordered some filters from 
https://www.ectransistors.com/10-PCS-INDUCTOR-TRANSFORMER-MMCL-DFC3R836P025BHD-3R836P025-DFC3R836P025BH.html

After paying for them & courier shipping, they told me that the price had gone up 500%, so I payed the extra. They then shipped the filters by economy post. Assuming the packet is not empty, they are still cheaper than the ones on Ebay, but you have to buy 10.   

[Bravo]

I fitted the small 836Mhz filters from Arrow,  part No B39841B4147U410. They work fine.

[_Wim_]

I fitted the small 836Mhz filters from Arrow,  part No B39841B4147U410. They work fine.

That good to hear. Hopefully they will keep this stock a little longer.

[450bush]

Dears,

I replaced the 836MHz filter with the B39841B4147U410 and it worked well! My CMU200 now pass the Internal Loopback test!

However, I am still getting some issues with the External loopback 1-4/3-2 that fail some frequencies on 1-4 due to a higher attenuation (just a little...) and also on 3-2 showing less attenuation.

I am working on a small PCB to hold the small filters and be a drop in replacement.

Kind regards,
Pedro

[DavidAzulay5000]

Hey friends
i need help
I need to replace on the RXTX board
the Filter SAW FILTER 836.5MHZ
which was destroyed
At first there was corrosion
I made a temporary solution of coating with copper foil
That temporarily solved the problem
Eventually the filter cracked
I removed the filter using my hot air station
of
hakko fr801
I am now looking for candles to purchase a replacement filter
I found it on eBay
Could this be a decent replacement?

B39841B4122U410 836MHz SAW Filter SMD

https://www.ebay.com/itm/293876605853

Greetings David

[_Wim_]

It will not be easy to fit that without an adaptor board, as that is a TINY component. However, with my limited RF-experience it seems to me that this would at least perform as good as B4147 part from Arrow posted above (and implemented successfully), so worth a try.

Arrow has still almost 18.000 pieces of the B4147 in stock (for only 0.45€ a piece), so that might also be an option.

[DavidAzulay5000]

I know they tiny
But I ran out of options.
It's the only one I found and what's in ARROW
Exactly the same size 3 mm x 3 mm
I have a lot of adapters
I use them for V REG
of linear technology
They are really tiny LT3042
So at most I soldered the filter on the adapter PCB

An example of adapters
This is a multimeter for resistors with values of 5 milli OHM to 10 ohm
that I designed
And built a long time ago

[DavidAzulay5000]

The mission fails
I changed the filter twice
B39841B4122U410 836MHz SAW Filter SMD

In addition, 2 SMD resistors accidentally flew that I de soldered using a hot air station
I looked at the pictures I took before
I'm really mad at myself for not taking a clean picture of this area
From what I've seen resistors are
100 OHM
1 KOHM
The filter I ordered does not pass the internal test
I will try to reverse the polarities
I have no idea what input is
and output
of the circuit
I'm really starting to hate R&S
There is no schematic 

[DavidAzulay5000]

The mission fails
I changed the filter twice
B39841B4122U410 836MHz SAW Filter SMD

In addition, 2 SMD resistors accidentally flew that I de soldered using a hot air station
I looked at the pictures I took before
I'm really mad at myself for not taking a clean picture of this area
From what I've seen resistors are
100 OHM
1 KOHM
The filter I ordered does not pass the internal test
I will try to reverse the polarities
I have no idea what input is
and output
of the circuit
I'm really starting to hate R&S
There is no schematic 

[DavidAzulay5000]

 

[_Wim_]

Hi, where did the resistor come from in the attached picture?

[_Wim_]

And maybe a few other questions: do you measure any resistance between pin 2 and pin 5 of the filter? How does that resistance compare to a brand new filter?

[Ringmodulator]

Hi,

my unit seems to have the same poblems.

Can someone point me to a source for these filters, which delivers to Germany?

Chris

[_Wim_]

Hi,

my unit seems to have the same poblems.

Can someone point me to a source for these filters, which delivers to Germany?

Chris

For the 836MHz part, Arrow electronics as listed above: https://www.arrow.de/products/search?q=B39841B4147U410++&r=true

[DavidAzulay5000]

progress report
The filter 1842.5MHZ
Works perfectly
B39182-B4139-U410 1842.5MHz SAW Filter 50 Ohm SMD

In addition there was a problem with the 9DB attenuation filter
So I made a filter from three size 0402 resistors
t filter
2 24 ohms
1 47ohm
In addition the RF SPDT SWITCH
UPG2179TB
was broken
So I looked for a replacement in other devices
I found the component on the WIFI board
of XBOX 360 E
I removed it and replaced it
The problem remained in the 836.5MHZ filter from the Data Sheet

I think the problem is that the attenuation is too high
at frequencies from 1200 MHz to 2000 MHz
The TYP attenuation
It is over 50DB
And that's why there is a failed internal loop test
Frequency 1200 to 2200
All other frequencies with an accuracy of 0.3 dB
the 1842.5mhz filter of epcos is perfect
B39182-B4139-U410 1842.5MHz SAW Filter 50 Ohm SMD

I do not recommend purchasing the EPCOS 836.5MHZ filter
model B39841-B4122-U410 836MHz SAW Filter SMD
I tried to replace about 4 units with the input and output polarity changed
Exactly the same results in all the tested filters regardless of the polarity of the filter
I thought the minimum attenuation . Maybe you will fall on one of the filters being tested
But they were all measured the same.

[_Wim_]

The problem remained in the 836.5MHZ filter from the Data Sheet

I think the problem is that the attenuation is too high
at frequencies from 1200 MHz to 2000 MHz
The TYP attenuation
It is over 50DB
And that's why there is a failed internal loop test
Frequency 1200 to 2200

Not sure if I misunderstood you, but the goal of this filter is to ONLY allow 836.5MHz to get through (the fixed frequency filter in the attached block diagram), all other frequencies must be attenuated as much as possible. So attenuation can only be too high at 836.5MHz, at other frequencies, it must be as high as possible.

(block diagram copied from here: https://electricalworkbook.com/spectrum-analyzer/)

[_Wim_]

progress report

Do you have an SDR dongle (or other spectrum analyzer) to check if something arrives at the filter (like I have done here: https://www.eevblog.com/forum/repair/cmu200-rx-level-issue-failed-loop-test/msg3917207/#msg3917207)?

My best guess is some RF-amplifier or RF-switch if broken upstream, certainly as you already had to replace another switch in the unit.

[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 

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_]

I don't have another spectrum analyzer
for this moment i give up 

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