Anritsu MS2651A repair (need service manual)

[Pacotech]

Hi there, just to explain whats inside the 1st mixer..
once desoldered all the wires, below the closed area there were a central pin to upper side ( 2 double diodes and probably 4 Resistors) IN MY CASE THE ATTENUATOR z1 is opened in input, so need to change them with a 4 / 5 dB ATTENUATOR.
tHE INPUT 10-20-30-40- dB ATTENUATOR IT HAS BEEN REPAIRED WITH SMD RESISTORS  0,1%.
Now waiting the new smd to change the Z1.
For all you guys , with the doubth .. what's inside... here the pics..

[Cwvto]

Hi all,

Because the cov19, I finally have time for playing with my MS2665C SA.
After looking around for the service manual, I finally come to this post here.

My unit look pretty much the same symptoms like your guys said and the RF board look very similar to jimon posted.
So may be I can share what I had found, may be someone someday need these information.


Lets share my story.

I got my unit from the china online second hand shop.
The display level is over 0 dbm over the whole frequency range. The level cal cant help, but the displayed level will change after each level cal.
The interesting thing is that if I connect a external signal, say 5GHz 0dbm to the input while doing level cal, the level seems more reasonably normal, at least I can see the spectrum peak response on the correct frequency. I suspecting the cal signal cant switch in to the RF path, but confirm I am wrong after a simple check.

I study the block diagram, and check if the signal can reach before the IF output, when operating at zero span. Luckily, IF level follows the SG level change.

I check all LO that I can reach on the RF assembly, they are

 a. 3.5-7.5GHz 1st L.O,
 b. 100MHz Ref,
 c. 94-106MHz for SAMPLER.

They all seems fine and in lock. When I look closer to the spectrum line, the peak seems swinging, and a pulse like signal shown when the C.F below 3GHZ.
I wondering may be the level cal process referenced to a wrong level because some LO drifted at the sampling instant?

One LO I can not check at the beginning is the 2nd LO, which operating at 4GHz inside the 3GHz converter. I don't want to touch every single screw I see, coz experience tell me this ack may end up to a tragedy.
 
The missing 4GHz was found when I figuring out which screws holding the 3GHz assembly, it is moving like sweeping. This signal should be a CW signal at exactly 4GHz for 2nd mixing, moving around definitely abnormal.

The 4GHz can be check on the SMA connector shown in Signal Flow.png.

Since no schematic is available, the only way to fix is trial and error. After probing around the pcb trace, I confirm the 4GHz VCO is controlled by "N5" - SOP8 IC marked as 812. The -11V is controlling the VCO freq. on N5's pin 4. May be adjust the R22 and R40 can make it lock on again.
So I started my iteration, I adjust R22 a little bit and install it back and check the 2nd LO. I am so lucky that the signal now not swinging any more. Then I adjust more to find if there is another sweet spot and check the test point LND15 (-11V), which is accessible when the module installed back, with oscilloscope A.C couple mode to find a spot the amplitude of the "noise like" signal, which is the error voltage of VCO, is minimized.

Finally, the problem is fixed, and the cal status are all zero which means every thing is good. The minor issue is when using a very small span, 20kHz, a FM like signal is observed but actually a CW signal is feeding. May be this is the phase noise of 4GHz or other L.O, but its good enough for me and I don't want to create new problem while hunting for the minor problem.    


Forgive my long article, I just finish the repair and so happy to share the information with all you guys.

[JohnnyS]

Hi all,

Because the cov19, I finally have time for playing with my MS2665C SA.
After looking around for the service manual, I finally come to this post here.

...

Hi all,
I got from my company junk an Anritsu MS2665C spectrum analyzer, it was supposed to have problem with the input attenuator and 3GHz up-converter, but when I try to power on I discovered that the power supply is not working.
The power supply unit is the Anritsu 34Z112975 fitted in the SA series MS266x, the schematic is not available, it blows the fuse at power on, the input stage is ok, no burned component, so it is very difficult for me to repair. I'm thinking to replace with external unit, but there aren't voltage labels on the connector, so I'm trying to find the pinout and the voltage levels.

This is what I find:

         A   B
V1     1   1   V1
V1     2   2   GND
GND  3   3   GND
V5     4   4   V5
V5     5   5   GND
GND  6   6   GND
V3    7   7   V3
V3    8   8   V3
V3    9   9   GND
GND 10 10 GND
GND 11 11 GND
V2   12 12  V2
V2   13 13  GND
GND 14 14 GND
?     15 15  ?
?     16 16  GND

For sure V3 is +12V and V5 is negative.

Any help will be appreciated.

[JohnnyS]

After some reverse engineering of the PSU schematic, I was able to find a faulty MOSFET of the PFC circuit. I removed it, since it's not strictly necessary and the PSU restart to work. Now a stand-by voltage is present on a pin of the interface connector.

I will check the rest of the PSU and finally connect back into the instrument, going on with the rest of the instrument.

I will try to put in clear the schematic of the power supply, for every one that may need it.

[dannyx5321]

Hello,

I've been reading for a while through this post since a won an Ebay auction for the same Anritsu analyzer and in hindsight I think I'm going to regret it. For a couple of hundred dollars more I could have got a working HP. The one I bought has power supply issues and so far no schematic is to be found. I'm hoping that you'll post that diagram when you have a chance,

Thanks, Daniel

[Halcyon]

Not sure what it is about this thread, but why are we getting pointless reports to moderators? Spam bots perhaps?

[hansgt21]

Hi JohnnyS, All,

I have MS2661C with a defective power supply.
I have replaced the AN8032 controller chip and a blown 10 ohm resistor.
All other parts seem to check out ok so far.
However, when powering up the supply with my current limiting bulb it blinks periodically.
Currently waiting for a high power differential probe for some safe measurements with my scope.
Have you had a chance to publish the re-engineered schematic of the power supply ?
Any idea whether there is a chance to identify the main controller chip ( black square plastic package ) ?

Thanks for any help !