26 February 2012

Vintage scopes are better part 4

Vintage scopes are better (also see part one, part two, and part three).

Reason number 4: Repairabililty and inspiration.

It is nearly impossible to get a useful service manual for a modern oscilloscope, and even if you could, the parts to repair it are generally unobtainable custom ICs. However, many vintage scopes are repairable, and complete service manuals are readily available for many models. While some vintage parts are getting hard to find (cough, tunnel diodes, cough), many repairs are possible with easily obtained parts. In his second book, Jim explains,
Older equipment offers another subtle economic advantage. It is far easier to repair than modern instruments. Discrete circuitry and standard-product ICs ease servicing and parts replacement problems. Contemporary processor-driven instruments are difficult to fix because their software control is "invisible", often convoluted, and almost impervious to standard troubleshooting techniques. Accurate diagnosis based on symptoms is extremely difficult... Additionally, the widespread usage of custom ICs presents a formidable barrier to home repair. [1]
In addition, vintage scopes are extremely well-designed and well-constructed. Much can be learned from examining and exploring the internals of classic oscilloscope.
The inside of a broken, but well-designed piece of test equipment is an extraordinarily effective classroom... The clever, elegant, and often interdisciplinary approaches found in many instruments are eye-opening, and frequently directly applicable to your own design work. More importantly, they force self-examination, hopefully preventing rote approaches to problem solving... The specific circuit tricks you see are certainly adaptable and useful, but not nearly as valuable as studying the thought process that produced them. [2]
His love of vintage scopes was a continual source of inspiration. He studied, referred to, and borrowed extensively from classic instruments and their service manuals. For example,
  • The high-speed ECL logic in the "King Kong V/F" in App Note 14 was inspired by the trigger circuitry of the Tektronix 2235.
  • The adaptive threshold trigger circuit in Figure 130 of App Note 47 was also inspired by the Tektronix 2235.
  • In his discussion of CCFL power supplies in [3], Jim explored the high-voltage resonant power supplies in the Tektronix 547 and the Tektronix 453.
  • In his discussion of low-noise power conversion (App Note 70, Appendix A), he discusses circuits from the Tektronix 454 and 7904 (Figures A1 and A2).
  • Figure 12 in App Note 98 was derived from the calibrator circuit in a Tektronix 485.


[1] Jim Williams, "There's no place like home," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 17, pp. 269–277.

[2] Jim Williams, "The importance of fixing," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 1, pp. 3–7.

[3] Jim Williams, "Tripping the light fantastic," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 11, pp. 139–193.

1 comment:

Todd Bailey said...

I'd suggest two more advantages as well:

For us younger types, it's amazing to see that engineers wanted to, and DID, solve the same problems we often want to solve 40 years later. It was as useful to record a waveform in 1968 as it is now, and engineers solved the problem then with special phosphors and flood guns. So, old scopes keep you humble.

The second, again perhaps more useful to young types than a guru like Jim, is that old scopes teach you not to be afraid of discrete design. Old scopes were often economical in design, because of the kinds of active devices available. Scope designers did not hesitate to use a cathode follower when a buffer was needed (one active device) rather than an op-amp (many transistors). It's easy to get intimidated by trying to design systems with discrete parts today, since it is so easy to use ICs -- but it is instructive to see that with a few active components with not-amazing specs, and creative design, you can make a device which is accurate and durable for generations. So, old scopes make you less afraid to think discrete.