Electronics > Repair
Anritsu MS2651A repair (need service manual)
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jimon:
Hi, got a broken unit from ebay, hoped it will just work, but not so fast ... so my first repair :) let's see what we can do.

Symptoms so far :
- no/very small signal (was unable to track anything reliably from rigol dg4102, don't have any other reliable signal sources atm, anything I can buy cheaply for it?)
- huge (like 50db or more) amplitude level difference between 0-3GHz full span and 0-100MHz span, when spectrum analyzer in full spane mode picture looks like normal (with noise level on -70db or so), in 0-100MHz mode noise is somewhere around 0db, no idea whats going on there, is there even a different path for different frequencies?
- no other visible issues
- cpu board shows self check is ok.

Looks like there is no internal test signal to check if this thing works, 'cause I've tried to disconnect whole RF stage and there were no complains on a screen.
Checked input DC block and attenuator - they seem to be fine. Checked almost all test voltage points - they look fine.

Does anyone have an idea where to look for service manual ? Tried to google it, but was not so lucky.
And any tips what to check next ? and maybe how IF should look like on oscilloscope ?
nidlaX:

--- Quote from: jimon on March 30, 2016, 09:07:37 pm ---Hi, got a broken unit from ebay, hoped it will just work, but not so fast ... so my first repair :) let's see what we can do.

Symptoms so far :
- no/very small signal (was unable to track anything reliably from rigol dg4102, don't have any other reliable signal sources atm, anything I can buy cheaply for it?)
- huge (like 50db or more) amplitude level difference between 0-3GHz full span and 0-100MHz span, when spectrum analyzer in full spane mode picture looks like normal (with noise level on -70db or so), in 0-100MHz mode noise is somewhere around 0db, no idea whats going on there, is there even a different path for different frequencies?
- no other visible issues
- cpu board shows self check is ok.

Looks like there is no internal test signal to check if this thing works, 'cause I've tried to disconnect whole RF stage and there were no complains on a screen.
Checked input DC block and attenuator - they seem to be fine. Checked almost all test voltage points - they look fine.

Does anyone have an idea where to look for service manual ? Tried to google it, but was not so lucky.
And any tips what to check next ? and maybe how IF should look like on oscilloscope ?

--- End quote ---
If you haven't already, I recommend watching some of Shahriar's spectrum analyzer repair videos to get an idea of how the input stage typically works and where you might have a problem. If you have a scope, have you tried following the signal from your Rigol through the input stage?
jimon:
Not really sure what's wrong with that unit, I've tried to bypass input attenuator and put 1Mhz -40 dBm into input of RF front end ... and I found nothing at with oscilloscope all around first mixer. Also nothing shows-up on output of LO ... it might be the same as here http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=274605

Will I even be able to see LO output on 100 MHz oscilloscope ? Or should I look for whatever driving LO ? (there is probably some sort of PLL for it)
sherlock:
I don't have that analyzer, however I had bought an Advantest SA a few years back that didn't display any signals on the screen.  In my case the mixer diodes were shot, along with a couple of burned resistors in the attenuator.  I too had questions about the LO circuit working.  You're correct in that a 100 MHz scope won't show anything.  Since I was mostly interested to see if the LO was working or not, and not on frequency at this time, I used an old radar detector for the car that I had lying around.  I powered it from a small 12 volt supply and used the detector as a probe.  In my case I was able to pick up the microwave LO signal so I knew it at least worked.  You might have to remove some shielding to pick up anything.  You can also check if you have the large "spike" at the left side of the screen (0 MHz).  Or set the center frequency for zero Mhx, full frequency span, and see if you get a zero indication in the center of the screen.  If you get that, then the LO and mixer is probably working.  Then you can troubleshoot where the input signal is getting lost.  Good luck with your purchase.
jkf1000:
Hi Jimon, did you manage to get any further with your analyzer? I have just purchased the "B" model of the same analyzer as a spares or repair unit. The fault with my unit is a display issue, the LCD displays nothing but vertical lines, I have scoped the ACRTC chip and have found signals are missing for the Vsync and Hsync and have ordered a new PLCC chip for it. It appears as if it is functional as far as the RF circuits are concerned as I can get the unit to run its own self cal. routine by pressing the shift-cal-F1 while monitoring the IF out on the scope.

Did you manage to get hold of any service data for your unit as I have found nothing online and Anritsu UK could only supply the user manuals!!

Given that we are looking at 2 completely different faults we may be able to help each other out given the lack of supporting manuals. Karl
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