I just received a new piece of hardware from one of my many forays into Ebay. It's a HP4951A protocol analyzer from Hewlett Packard. It appears to be just the piece of hardware I need for all of the serial communications hacking I do with my antique computers.

However, when I plugged it in, it made a few encouraging beeps, followed by nothing. The screen briefly showed a green dot in the lower center of the screen, but then nothing.
My background is digital, but my instinct told me that this was probably a display problem - maybe in the horizontal sweep? I carefully pulled the unit apart. I'm absolutely amazed at how well this thing is put together. The copper traces on the circuit board are gold plated. Almost all the resistors are precision 1% tolerance. Most of the chips, however, are printed with a house part number, so repair of the digital circuitry may be impossible.
I found a company in Canada that appears to rent service manuals for various equipment, so I've put an email in to them asking if they are able to rent me the service manual for this piece of equipment.

I did some searching on the net to see what I could find. I didn;t come across very much at all. One gem I did come across was this usenet posting. There were no responses, but it definitely was the description of my problem. I decided to email the gentleman, and to my surprise I received a response within a couple of hours detailing his problem was in the focus circuit, and identified a 750K resistor that was open. I stripped down my unit, and sure enough my unit had the same problem! unfortunately, it's Saturday afternoon here, and there's nowhere open to find a replacement, so I'm going to have to wait until Monday. The fact that I've found the very same component that failed is very encouraging.

I posted two images in my moblog showing the board and the component. It's difficult to see with my cell cam, but the faulty component is located just to the left of the focus pot (it's removed in this picture). I'll let you know how I got on once I get a replacement part.