Troubleshooting The Deck

I'm definitely getting my feet wet in the realm of instrument testing.

Adjust One has been a much needed challenge for me as things around here were getting boring. There's a decent amount of manual tuning that I have to perform on each unit which means getting a box into the proper power range or frequency level can be a chore. Many of the PC boards are loaded with potentiometers that can take up to 20 turns before a signal peak pops up on my test rack's oscilloscope or spectrum analyzer. Sometimes I see what the technicians refer to as a "false peak" during a tuning adjustment. A false peak looks like a good signal on the display, but if you crank a pot just a few more turns you'll get an even cleaner, stronger signal peak. It's important to find the right one before moving on to the next tuning adjustment. So far the whole process has been straightforward enough. I think I'm doing okay with it.

My real headaches come from that no-good RF Deck assembly. The RF Deck runs down the length of our units on one side and it's loaded with shiny gold plated microcircuits. Each microcircuit is held to a metal back plate by numerous screws and also made more secure by a series of metal rigid cables. I hated building these things, but now I've discovered trying to troubleshoot them is even worse than simply putting them together. During testing sometimes I'll get a real dirty signal up on the display. Instead of a nice clean signal peak that resembles the silhouette of a tall traffic cone I'll end up with a jagged runt of a triangle. Or I'll get what the techs refer to as a "suck out" or a "power hole". What those are is a total loss of radio frequency power like a signal dropped by 10 to 20dB. On an oscilloscope display it looks like a sharp spike pointing at the floor. All of these indicate a real problem somewhere on the RF Deck.

To locate a hidden problem in the RF chain of microcircuits and cables you have to be methodical. The first thing to do is check to make sure all the rigid metal cables are seated properly on microcircuits and torqued down all the way. Sometimes I'll find a loose nut on a cable and the messed up signal I see on my equipment instantly cleans up. Most of the time however I'm not that lucky, I have to begin tearing the whole microdeck section down instead. I have to pull the deck out of the unit and mount it on top of an extender board. The extender board makes a connection between the deck itself and the instrument's motherboard. Then I loosen up microcircuit screws to give them a little play and start removing sections of rigid cable. I hook up a power sensor with a 90 degree angle section of rigid cable so the sensor can be placed very close to microcircuits. There isn't much room to work with around them. Once the sensor is hooked up I take more measurements until I find which microcircuit or section of cable was causing a weak signal. It's a really time consuming process of elimination.

Obviously I don't understand the electronic theory behind how all this stuff works, but I've asked the techs to try and break things down for me in a bonehead sort of way so I at least have a little idea of what each microcircuit is there for in the box and what it is supposed to do. TC gave me a pretty good overview the other day when I asked him about it.

TC said, "Here is what I know about the RF Deck. The YO (YiG Oscillator) is the oscillator that makes an RF signal between 2-7Ghz. The YO feeds into the YTM (YiG Tuned Multiplier) which multiplies the frequency from the YO to 2-26.5Ghz. The YTM feeds into the Dual Mod. This is where the amplitude or frequency modulation is added. It is also where the feedback from the Detector is adjusted. From there the signal is fed into the YTF (Filter) that separates the unwanted frequencies from the frequency that the unit is tuned at. The YTF is fed into the Doubler. The Doubler doubles all the frequencies so that we can get 50Ghz out of a 26.5Ghz unit. At the end of the RF Chain is a Crystal Detector. The Crystal Detector is used as a feedback loop. When the power goes too high, the Detector converts the RF signal to a DC level, and then sends a correction back to the Dual Mod. The Dual Mod adjusts the power so that it remains the level that the instrument is set at. The last component is the Attenuator. The Attenuator acts as a resistor would in a DC circuit, but it does the same thing with power (measured in dBm)."

When TC finished up explaining the RF Deck to me, it kind of reminded me of that old song about skeletons. You know, the one that goes something like the leg bone is connected to the hip bone the hip bone is connected to the whatever. I did come away from the talk feeling like I had a much better working understanding of what all those little gold plated boxes do. So that was good. As far as my training is concerned with TC, Wah, and the Crazy Redhead lurking about, I think I'm in very competent hands here.