Anritsu MS610A repair (faulty 6dB pad)

This repair is my first foray into the almost-microwave realm.

So, what was wrong with this SA? I bought it unresponsive to the front keyboard commands.

After cleaning the contacts the unit started to be responsive, but the pushbuttons were rather stubborn against my attempts to clean them.

However, there was no response from any input signal if I hooked the cal signal from the back to the input. As I could see the LO feedthrough I was confident that the YTO was working as well as the IF section.

What appeared to be faulty was the input attenuator. Time to disassemble the RF section which wasn’t as simple as I would have liked: rigid coax and SMA connectors had to be dealt with as well as a lot of screws (and I’ve also lost a couple of them… I’ll buy them next time I do an order)

Inside the RF section after a 2GHz filter there is a 6db attenuatorĀ  which was obviously toasted: two resistor were burnt, and a capacitor followed them.

I am still not sure if the input attenator was faulty from the start or if I fried it testing the equipment.

Anyway, back to the repair. The service manual I found is not for the MS610A but for the MS610B and of course the schematic for the 6db attenuator is different from what was inside the RF section of my SA. However after a quick simulation I was convinced that some resistor and capacitor I had in my junkbox would be enought to test if the unit was working or not.

They did and I finally got a response from the SA when I hooked its cal signal. However the unit was not as sensible as it should have been and I couldn’t get the gain calibration right.

As the 6dB pad was fried I suspected that something happened to the first mixer too. There is no schematic for the first mixer, but looking at the PCB it looked like a double balanced mixer.

Unfortunately I wasn’t able to find any information about the 4 discrete diodes that made up the mixer: there was nothing about them in the parts list, no schematic, no marking that helped me identify them. It wouldn’t have been easy to find 4 matched diodes anyway.

I decided to try to substitute them with HSMS-282R, a SOT-363 package with 4 Schottky diodes inside in a quad ring configuration, exactly what I needed.

Desoldering the old diodes was easy, soldering the SOT-363 a little bit less easy. However, after reassembling the RF section, I got the spectrum I expected so at least replacing the 1st mixer was a success.

I cannot consider this repair complete because I am still not able to calibrate the SA with its own cal signal. Moreover, I should find the proper components for the 6db attenuator before going on with the repair.

A.Cicuta

P.S.

I have misplaced four key caps and finally found them inside a toolbox. What was I thinking when I put them there?

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