IFR COM-120A need help

[bh7juo]

hello everyone
I have an IFR COM-120A service monitor,now the"deviation meter"failed in self-test.And I tried to replace most of capacitors on "BASEBAND TRAY"but it still failed.(why most of them but not all?It maybe has some "non-polarized electrolytic capacitor" 10uF,16V,I have no found the replace yet.)
Another problem is it had OPTIONS "OUTPUT AMP"in menu,so in operation it display AUX port can output +13DBm max,but I check if AUX out level less than-13DBm,no problem;but up to -13DBm(such as -12DBm),actual output is -12-26(amp gain)=-38DBm,level increase as set....oh my good....
so does anyone know.....
how to solve this problem?need calibration?I asked no expert or company can do this here.
and what's wrong with AUX output?
My 120A really possible re-assembled by someone before I got it,CPU board may not original.
I'm looking forward to someone can tell me,thanks.

[Wowbagger]

The "Deviation failure" may be no more than needing to be recalibrated after you replaced the capacitors. It may also be a similar issue on the gen or receiver tray - after all, the deviation test is looping the TX back to the RX.

The aux amp issue sounds like somebody switched on the option in software but without that pesky little step of actually adding the hardware.

[bh7juo]

The "Deviation failure" may be no more than needing to be recalibrated after you replaced the capacitors. It may also be a similar issue on the gen or receiver tray - after all, the deviation test is looping the TX back to the RX.

The aux amp issue sounds like somebody switched on the option in software but without that pesky little step of actually adding the hardware.

thank you Wowbagger for long time support
Should I need replace all electrolytic capacitor on BB board?
I should tell you I had repaired GEN tray at first,because first time I got this 120A the GEN had no any output,and base on maintainace manual I found 5 MMIC's in GEN tray gone bad(no amp ability,check by votage and spectrum analyzer),4 are MSA-0486 and 1 is MSA-0286,after replace then the GEN output is OK,but Deviation still failure.
What should I check next?

And another issue is if I put it no work more than one day,turn on it,it comes"DI,DI-----------------,DI"beeps,first comes 1 short beep,no display,follow with a long beep about 5 seconds,and 1 short beep,and screen come up.If run continuous running for some minutes,turn off,and turn again, it comes no long beep,only first "DI" and some seconds after "DIDI".Is the back battery run out?

[Wowbagger]

No, that's not a battery issue - that sounds like the CPU has issues - probably bad caps as well (We got bit by "capacitor plague" pretty hard - damn cheap ripoff artists).

I would suggest replacing all the caps you can.

[bh7juo]

No, that's not a battery issue - that sounds like the CPU has issues - probably bad caps as well (We got bit by "capacitor plague" pretty hard - damn cheap ripoff artists).

I would suggest replacing all the caps you can.

OK,I will try to replace more capacitor......And you mean that power supply have too much ripple?or could you tell me the main power supply output votage in 24-pin plug?I will use oscilloscope and multimeter to check it.
And any outher opinion with deviation meter fail?(see my reply up)

[bh7juo]

OK,I checked the power votage ripple,found +12V line has about 500MV P-P ripple,and VP-P value changing with what show on display........I have to check power module.

[bh7juo]

hello Wowbagger,I had checked my 120A spectrum analyzer is work fine,but the max span is only 1MHz,even if it has option track generator,but 1MHz span is not enough to do lots of test.I checked CPU firmware version is 1.01,I don't know if 120A mode has only so narrow max span,or I can update firmware to get more span select?Thanks

[Wowbagger]

The "A" firmware was very limited. It wasn't until the "B" CPU and firmware that a lot of cool stuff, like wide span analyzer, was added.

Unfortunately, unless you can find a B CPU, you are stuck with what you have.

[bh7juo]

The "A" firmware was very limited. It wasn't until the "B" CPU and firmware that a lot of cool stuff, like wide span analyzer, was added.

Unfortunately, unless you can find a B CPU, you are stuck with what you have.

OK thanks,I have no hope to wide span.and this time most want to confirm the "deviation failed" issue position......
What test can I do to help you analyze?Or check any votage/waveform/signal? I have digital oscilloscope and spectrum analyzer.

[Wowbagger]

The simplest test is to replicate the self test manually:
Duplex TX: 105.005 MHz, T/R port, -40dBm output, GEN1 1kHz tone, triangle, 3kHz peak deviation.
Duplex RX: 105.000 MHz, T/R port, FM, 15kHz IF.
Select distortion meter (discriminator input), deviation meter (range auto), power meter (range auto).

Give me the results from the meters on the receiver screen.

[bh7juo]

The simplest test is to replicate the self test manually:
Duplex TX: 105.005 MHz, T/R port, -40dBm output, GEN1 1kHz tone, triangle, 3kHz peak deviation.
Duplex RX: 105.000 MHz, T/R port, FM, 15kHz IF.
Select distortion meter (discriminator input), deviation meter (range auto), power meter (range auto).

Give me the results from the meters on the receiver screen.

I tried to test as you said,and my operation detail is:
(1)Loop ANT and T/R port
(2)At DPLX screen,set TX and RX as above,both TX and RX modulation is FM.GEN out set to T/R port and REC use ANT port
(3)Move cursor to REC,press ZOOM to adjust more parmeters
(4)Zoom distortion meter,I can't change input way(SOURCE fixed on DEMOD),deviation meter AUTO,can't set to power meter but REC LEVEL is OK.

Read out is:distortion meter 11.9~12.4%,DEVIATION 2.51KHZ,REC LEVEL is +++DBm(like out of range),spectrum snalyzer peak readout is -40DBm OK.





I'm not sure you can see photos.....Link from local.

[bh7juo]

If picture is not visable,try to put their links in new browser window.

[Wowbagger]

Actually, you don't need to loop the antenna to the TR - set the receiver to TR as well, and the loopback is done internally.

You need to find a better place to host those pictures, or just attach them to your post here.

[bh7juo]

Actually, you don't need to loop the antenna to the TR - set the receiver to TR as well, and the loopback is done internally.

You need to find a better place to host those pictures, or just attach them to your post here.

I have no found any way to put picture here,so use word.....
Set both TX and RX to T/R port in duplex screen and zoom the RX screen,read out are:
distortion meter: 11.4DB
deviation meter: 2.61KHZ
power meter:0.0mW

And I used 120A to measure my radio power output,both in 29.6MHz and 435MHZ,if radio output is 5W,120A readout is about 3.3W,if input 130W,readout is 70W.

[Wowbagger]

OK, then you do have an issue with the calibration of either the modulator or of the deviation meter. The best way to check that would be to set up as follows:

Duplex mode, RF in = TR, RF out = TR @ -40dBm, Demod FM, 15 kHz RX IF BW, Gen1 set to Sine wave (all other modulators off), FM, 12.5kHz deviation, 3287.8 Hz rate. Go to the Duplex Receive screen, and select the Spectrum analyzer display, 2kHz/div span.

Refer to this web page:
http://www.fmsystems-inc.com/manuals/besselart.pdf

You are going to dial in a deviation level and modulation rate of 3.2878 kHz on Gen1 to get a Bessel null as per the table. You can adjust the modulation rate from the Audio generator pane of the Duplex receive screen (select the AF Gen out field, hit the Zoom softkey). If the modulator is accurate, you should see the first "hump" nulled out. You can tweak the rate up or down to get an exact null. This will give you a mathematically precise check of the deviation of the generator. If it is "close" (say, within a few percent), then the problem is the deviation meter calibration.

As to measuring the power of your radio - I trust the power output of a radio about as far as I can throw the manual: just because the radio's manual states it puts out 100W at full power doesn't mean you can trust that number. Unless you have a trusted way to measure the power (e.g. an RF power meter that is calibrated), don't assume your measurements mean anything. You can do a simple check of the power meter by *carefully*!!!!! feeding a known DC value into the TR port, and taking P=V^2/(50 ohms) (the wide band power meter just looks at the RMS voltage across the 50 ohm power termination resistor, DC is the same as RF as far as that goes). Obviously, use a LOW, known voltage, like 5V.

[bh7juo]

OK, then you do have an issue with the calibration of either the modulator or of the deviation meter. The best way to check that would be to set up as follows:

Duplex mode, RF in = TR, RF out = TR @ -40dBm, Demod FM, 15 kHz RX IF BW, Gen1 set to Sine wave (all other modulators off), FM, 12.5kHz deviation, 3287.8 Hz rate. Go to the Duplex Receive screen, and select the Spectrum analyzer display, 2kHz/div span.

Refer to this web page:
http://www.fmsystems-inc.com/manuals/besselart.pdf

You are going to dial in a deviation level and modulation rate of 3.2878 kHz on Gen1 to get a Bessel null as per the table. You can adjust the modulation rate from the Audio generator pane of the Duplex receive screen (select the AF Gen out field, hit the Zoom softkey). If the modulator is accurate, you should see the first "hump" nulled out. You can tweak the rate up or down to get an exact null. This will give you a mathematically precise check of the deviation of the generator. If it is "close" (say, within a few percent), then the problem is the deviation meter calibration.

As to measuring the power of your radio - I trust the power output of a radio about as far as I can throw the manual: just because the radio's manual states it puts out 100W at full power doesn't mean you can trust that number. Unless you have a trusted way to measure the power (e.g. an RF power meter that is calibrated), don't assume your measurements mean anything. You can do a simple check of the power meter by *carefully*!!!!! feeding a known DC value into the TR port, and taking P=V^2/(50 ohms) (the wide band power meter just looks at the RMS voltage across the 50 ohm power termination resistor, DC is the same as RF as far as that goes). Obviously, use a LOW, known voltage, like 5V.

Thank you,I read that document and tried as your said,I'm sure to see first null(center spectrum peak dispeared) when I turned up deviation from 0Hz.But I really don't know how to understand the bessel mathematics table,SO if audio freq=3.2878KHZ,what deviation level will null center peak or second peak?

[Wowbagger]

The center peak (the carrier) will not go away. It's the next peak that should null out when you get the deviation to be 3.2878kHz - so adjust your deviation until the second peak goes away, and see how close what you've entered is to the actual value.

[bh7juo]

The center peak (the carrier) will not go away. It's the next peak that should null out when you get the deviation to be 3.2878kHz - so adjust your deviation until the second peak goes away, and see how close what you've entered is to the actual value.

Thank you for your help all the time,I have passed all self-test after sent it to my friends to made calibration.Now it's running well and have good precision for test.I consider the failed of self-test was due to bad calibration.