Anritsu MF76A Repair - Part I

I recently acquired a nice Anritsu MF76A 18GHz frequency counter in a non-working state off of eBay. While older, this is still a very usable counter that goes for quite a bit of money when working. This one sold for cheap so I figured why not, I'm sure I'll be able to get it working. I didn't really need it as I have a good EIP 545A 18GHz counter, but this one looked fun to fix. The images of it indicated that it was powering up and had the cpu portions functional as the display went through its power up cycle and displayed all zeros on the display. Typically with frequency counters in this state and not counting, you either have a failed 10MHz reference or a damaged front end from over-driving the input. I was hoping for the former as I already have a stable rubidium reference I use with all of my gear.

Anritsu equipment is notoriously difficult to repair due to the lack of service manual availability. I ran into the same issue with my second MT8801C communication analyzer / spectrum analyzer that I had purchased in a non-booting state. I was ultimately able to acquire a service manual for it but it took some serious searching and asking around. Once I obtained it, it was nothing like the older HP and Tektronix manuals I was used to. This manual went through a basic troubleshooting process to identify the issue to a board level, no further schematics or information was available (in the manual I had anyway).

Upon arriving, I powered this MF76A up on the bench, it was in good overall condition and did power up as it was supposed to. Upon applying a RF source to either the 200MHz or 18GHz inputs resulted in no counting. In a way this was good, it could indicate an issue further down then the first converter stages. The back of the unit has a switch to either output the internal 10MHz reference or accept an external one. Checking this with a counter, the internal reference is working as expected. Damn. Time to open it up.

The back of the unit had the calibration stickers broken which was expected, this came from a calibration house / test equipment reseller and I'm sure someone mucked around inside with some attempt to repair it. With covers off, everything looked at least complete, this unit was constructed very nicely.

The power supply is tucked into the back of the unit, not the easiest to reach to check voltages, but the power comes off of it and is transferred to the backplane which has wider traces for increased current making it easy to identify. There are no voltage markings or test points on the boards but it was pretty easy to determine which were power. Probing voltages, everything looked fine except for one strange voltage reading which was around +6.8 volts. Also noticing that a -12V rail was present but no +12, I was betting that something simple like a bad cap was holding the +12V rail down to +6.8 volts, very similar to my EIP 545A repair.

Pulling all the boards and reinserting them to locate the source of the issue resulted in the +12V voltage drop being caused by the input IF brick at the front of the unit. This was fed directly from the 18GHz input, the 200MHz input bypassed it and went right to one of the digital boards. As a test I left the 18GHz section unplugged and ran a 50MHz source into the other input. At first there was no response but I realized that one of the coax connectors coming off of the 200MHz input was not connected in the correct location. Whomever had been in this unit messing around didn't put it back together correctly. Once it was where it was supposed to be, success. 50MHz displaying correctly.

So the counter itself is working fine, just the 18GHz converter is causing problems. This was always a concern, you see this often from someone providing too much input and damaging the input section. So while 200MHz is great, 18GHz is why I wanted this counter. At this point I pulled the input brick out and started taking the covers off of it. The cables for this are long enough to allow you to work on it outside of the unit, I so love designs that allow for repairs.

Each side of this brick was made up of multiple boards all of which actually had labeled power input pins. This made locating the problem board very easy. Measuring +12V to ground I was reading roughly 1.2K ohms through all boards. Through process of elimination I was able to isolate the boards that were sinking the +12V down to two boards, both exactly the same. Pulling the +12V cable off of each board returned the rail to normal levels. Both boards were labeled '342U7363', each containing a handful of passives, some transistors and MMICs, along with what looked like a large thick-film precision resistor.

By following the traces on the board and divide and conquering by cutting some traces, I discovered the +12V sinking was actually coming from this large mystery device package. Looking it up, it is an NEC MC-5156 broadband amplifier in a sip package and was nearly shorted from vcc to ground.

Removing these two amplifiers from each board returned the +12V rail to normal, so these two parts were at least indicating to be bad. Was anything else bad? Maybe, it is hard to tell for sure and without a full schematic it is really difficult to trace the signal paths between these boards to guess what else may be blown. Luckily these old amplifiers are still available as NOS from some suppliers, so with two ordered we shall see. Right now with these two amplifiers removed, the unit powers up and counts correctly on the 200MHz input. I am also able to see some RF on the input to each now missing amplifier, so RF is at least getting that far. Is this the only problem? Maybe, maybe not as this would be too easy. I'm betting there are more issues than just this but I can't do any more at the moment until the replacement parts arrive.

New Spectrum Analyzer: Anritsu MT8801C

The Spectrum Analyzer is by far the most important piece of gear for any RF design work, unfortunately spectrum analyzers are also one of the most costly pieces of gear you can buy (A VNA is right up there too, but that's a different post). I have had access to spectrum analyzers at a few previous jobs which is great whenever you need to test your latest RF design. The problem is when you are at home working at your own bench at 2am,  it's annoying to not be able to have access to this gear all of the time. Buying an analyzer is ideal, but costly. Anything new is pretty much out of the question, so the usual source of eBay is the place to go. Both Tektronix and HP/Agilent have some amazing pieces of gear for an 'affordable' amount (the Tek 49N series and HP 85NN series come to mind), the problem is any of these models can easily cost over $1000 in good working condition. The other issue with this equipment (like all older gear) is their age. Since most were used in a production or lab environment, they have been powered on for 8 hours a day for years. This can result in some crt burn in, the devices being way out of calibration, instabilities and other problems as most of the equipment in this class is 15+ years old (note that makes it affordable). I have had a good run with all my HP / Tek gear as this equipment is really built extremely well. As an example, my HP8614A signal generator was built in the 1960s and still works perfect today. So what happens if you want a spectrum analyzer but don't want to spend $1000+? As I found, there are a few options:

1. Buy a really old analyzer. Some of the older HP models will go for under $500. Keep in mind that these models usually have a max frequency range of no more than a few hundred Mhz.

2. Buy a lesser known brand. There are a handful of analyzers by Chinese companies that go for cheap. They may be perfectly fine, I just prefer to go with a good established brand if I'm going to invest in one.

3. Watch local auctions. There are a ton of company liquidation auction houses such as Dovebid that sell off large companies test equipment assets. These are great places to pick up gear. The issue with this is that there is no guarantee that the gear works (no one tests it) and if it is a valuable piece it will most likely get bid up pretty high. Packing and shipping can cost hundreds of dollars as well if you are not able to pick up the gear locally.

4. My favorite option. Buy gear whose primary purpose is not a spectrum analyzer, but has an analyzer hiding inside it. A lot of communication analyzers and cell phone test sets have an available spectrum analyzer option. I will look up unusual gear on eBay that seems to be selling for cheap and read the product literature on them. You will be surprised on what you will find. I have purchased both of my spectrum analyzers this way.

My first spectrum analyzer that I bought a few years ago is an HP8922H GSM test set. It is designed to replicate a GSM cell station to test GSM cell phones. It also has a bunch of options included one of which is option 006, a 10Mhz to 1Ghz spectrum Analyzer. You can find these for around $500 or less. Now 1Ghz is great, but you eventually reach the limits of what you can do with it. One of my current projects is building a hydrogen line radio telescope which at 1420Mhz is outside of my analyzers reach. I needed something to at least 2Ghz at this point to test my down converter so I began my search again.

While recently looking at more gear that was available I came across an Anritsu MT8801C radio communication analyzer. Not being familiar with Anritsu as most of my gear is HP/ Agilent and Tektronix, I did a bit of research into this particular model and discovered that not only was it an amazing piece of gear, but much like my HP8922H, it has an option for a 300Khz to 3Ghz spectrum analyzer (Option 07):

Being a communications analyzer it has a bunch of other nice features such as a 300Khz to 3Ghz RF frequency generator, and an RF power meter:

 

 

Here is a full span of 300Khz to 3Ghz to my outside wideband antenna:

A couple nice things about this analyzer is that it has a large LCD screen, is capable of displaying a full frequency span, it has a resolution of 1Hz, and a very fast interface. I checked its calibrated accuracy with my HP 8656B RF generator and it was spot on which made me very happy as well:

The additional 3dBm loss above is from the mini-circuits splitter I was using between the generator and analyzer.

Included with this unit was a nice shielded RF test chamber for no extra cost. I can only guess what this had cost new:

 

It will be a great tool for testing devices within a completely shielded environment from external interference.

 

So all said and done this complete setup cost less than $600. Quite a steal considering new this was a $30K+ piece of gear. With that being said I'm finally excited to test my down converter further and also get my RF front end for my radio telescope underway.