TDS 500 600 Series (and TDS 820) Monochrome CRT Horizontal Linearity

I have a batch of TDS 620A which everything looks shiny new, but the display will stretch vertically and eventually gone unstable after turning it on continuously for a long time.

Turns out there’s a specific batch of flyback transformer (120-1841-00) on the monochrome CRT driver board (640-0071-06) that has quite a bit of infant mortality. The newer the unit looks (of the CRT tube looks shiny new without burn-in), the more likely it’s a victim of the bad batch.

The transformer is almost impossible to source (other than getting another CRT driver board), but I was able to find a Chinese supplier who makes it. It was usable, but it’s a nightmare to get it on because it’s really done with the stereotypical Chinese (PRC) manufacturing caliber.  When I received the unit, it’s a WTF moment! Leave me a comment if you want me to write about it.

The replacement transformer is only pin and functionality compatible, but it’s not a drop in replacement (not even geometrically). The characteristics are different and I had to adjust the trimmers all over the place.

I was able to get the screen width I want by adjusting the variable inductor (L105, HOR SIZE) by nearly pulling ferrite rod out, but the horizontal linearity was way off (the left side is very squeezed):

I looked into the TDS520B Component Service Manual (Same CRT board circuit diagram) and found this:

But L100 looks like this, which doesn’t seems trimmable:

The right hand side is the same L100 choke I extracted from a CRT driver board (same model) that I’ve disposed of. I saw two suspicious pieces of metal-like objects strapped on the choke on the left (installed) which I haven’t seen in other identical boards. 

Thinking that by changing the magnetic property of the core, I can adjust the inductance of an otherwise non-adjustable inductor. I took a few bits of magnets sitting on my bench and swing it around the L100 choke, the horizontal display widens/narrows depending on which pole of the magnet is facing the L100 choke.

After a few trial and error, I picked the right amount of magnet discs to correct the horizontal linearity so the squares have roughly the same width:

I guess I cracked the code! Here’s the result of correction by magnet:


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RTC / NVRAM in Test Equipment

HP Infiniium Series Atlas III Motherboard (Oldest generation Infinium oscilloscopes still branded as HP):

  • [RTC. There’s no exposed batteries] bq3287AMT = DS1288

Tektronix TDS Series (500, 600, 700, 800)

  • Older TDS (without alphabet suffix): Dallas DS1245Y
  • Model number that ends with ‘A’:  Dallas DS1650Y

The NVRAM in TDS series only contains the options data. Since I can reprogram the options, I can start with a blank NVRAM without reprogramming it). 

Will keep updating this page when I came across more test equipment.

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C Traps and Pitfalls

Here’s a concise paper describing common C programming pitfalls by Andrew Koening ( corresponding to be book with the same title.

As a reminder to myself, I’ll spend this page summarizing common mistakes and my history with it.

Here are the mistakes that I don’t make because of certain programming habits:

  • Operator precedence: I use enough parenthesis to not rely on operator precedence
  • Pointer dereferencing: I always do *(p++) instead of *p++ unless it’s idiomatic.
  • for() or if() statements executing only first line: I always surround the block with {} even if it’s just one line. Too often we need to inject an extra line and without {} it becomes a trap.
  • Undefined side effect order: I never do something like y[i++]=x[i]
  • char* p, q: very tempting since C++ style emphasize on pointer as a type over whether the variable is a pointer. I almost never declare multiple variables in one line.
  • Macro repeating side effects: use inline functions instead whenever possible. Use templates in C++.
  • Unexpected macro associations: guard expressions with (). Use typedef.

Did these once before, adjusted my programming habits to avoid it:

  • counting down in for() loop with unsigned running variable: I stick with signed running variables in general. If I’m forced to use unsigned, I’ll remind myself that I can only stop AFTER hitting 1, but not 0 (i.e. i=0 never got executed).

Haven’t got a chance to run into these, but I’ll program defensively:

  • Integer overflow: do a<b instead of (a-b)<0. Calculate mean by adding halfway length to the smaller number (i.e. (a+b)/2 == a + (b-a)/2 given a<b). Shows up in binary search.
  • Number of bits to shift is always unsigned (i.e. -1 is a big number!)

What I learned from the paper:

  • stdio buffer on stack (registered with setbuf()) freed before I/O flushed: use static buffer (or just make sure the buffer lives outside the function call).
  • char type might be signed (128 to 255 are -128 to -1) so it sign extends during upcast. Use unsigned char go guarantee zero extend for upcasting.
  • toupper()/tolower() might be implemented as a simple macro (no checks, incorrect /w side effects)
  • Can index into a string literal: "abcdefg"[3] gives 'd'

Mistakes that I usually make when I switch back from full-time MATLAB programming:

  • Logical negation using ~ operator instead of ! operator.

Common mistakes I rarely make because of certain understanding:

  • Forgetting to break at every case in switch block. It’s hard to forget once you’re aware of the Duff’s device.
  • sizeof(a[])/sizeof(a[0]) after passing into a function does not give array length: hard to get it wrong once you understand that array (declared on stack) has meta-info that cannot be accessed beyond the stack level it’s initialized.

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