The disk emulator is now spinning up virtually when an image is downloaded to its RAM and the command is issued to turn on the drive. It goes ready and should be producing sector marks, rotating sectors at the proper rate and otherwise acting right. I hooked up the scope and did some initial testing.
I pulled a wire to cause the Strobe signal to go active, but with a cylinder request of 255 which is invalid. As expected the logic responded with a Logical Address Interlock condition since only values between 0 and 201 are valid for seeks.
I will set up the logic analyzer allowing me to check the timing of all the signals involved in a seek. Too, I have to see that all the lines respond as they should.
TEKTRONIX 7854 SCOPE REPAIR
I popped open the scope and discovered that I had a wire loose. Soon fixed and my horizontal scan is back in operation. Unfortunately, still have the interference between the digital and the analog sections, causing dashes across signal traces and smearing sideways of digital characters.
Hmmm, guess there is something else to find and repair but I was close. The chip I replaced is indeed the switcher that determines whether the CRT horizontal plate amplifiers are driven by the digital or the analog side. I am seeing mixing of both - digital signals on screen at the time of the analog sweeps and analog signals interfering with digital signals.
Since my switching chip was blown out, but now works properly, I have to assume that the event that caused its failure also took out one or more other components in attached circuits. Time to study the schematics and look for candidates to investigate.