Correct horizontal frequency??

[ohohyodafarted]

I will be the first to admit that I am not the sharpest tool in the shed when it comes to television theory. So I need an explanation of the following:

I am working on a 15" Westinghouse for a gentleman who has asked me to restore the set electronically for him.

I replaced a defective trimmer cap in the horizontal oscillator ckt and needed to re-adjust the circuit to the correct frequency. So far no problem.

I go to the Sams and it says 5250 is the frequency. I am reading about of 17Khz with my frequency counter, no where near 5250.

So I take a look at the original Westinghouse factory prints and that same waveform is indicated to be 15750 cps (exactly 3 times what Sams says)

Obviously the correct frequency is 15750, So why does sams say 5250??

In addition, Sams says vertical frequencies are 30 and the Westy schematic says 60 cps.

So can someone please enlighten this old fart as to why the numbers are different in the Sams from what I believe are the correct numbers on the original Westy schematic. I am sure there must be some reason the Sams indicates 5250 but when you actualy measure the frequency it is actually 15750.

In any event I have the Westy horizontal frequency adjusted to 15735 and the horizontal hold locks up perfectly. (it seems the set doesn't want to settle at 15750) and I discovered this is because color sets have 286 lines/field. There was no way you could possibly adjust it to 5250.

I would like to understand this so the next time I run into this I will understand what I am looking at on the Sams. If it wasn't of the fact that I had the original Westy print with the actual frequency, I would still be scratching my head in bewilderment.
Thanks for educating this old fart.

[etype2]

Just went through my Westy restoration notes from Mike. Not much help to offer except to say that Mike found several discrepancies between Sams and factory manuals from RCA and Westinghouse.

[jr_tech]

It appears that the frequency showing in the Sams schematic is the sweep frequency setting for the oscilloscope. (Remember when scopes used frequency rather than a calibrated time base for setting horizontal sweep). Note that 3 cycles of the waveform are shown.

jr

[old_tv_nut]

5250 and 30 seem like typos.

NTSC black and white TV scanning frequencies were 60.0 Hz vertical (matched to the power line, has an advantage that any hum in the scan circuits causes a stationary distortion) and 525/2 x 60 = 15750 horizontal. (So, 525/2 = 262.5 lines per field.)

Color kept the same number of lines as black and white, but there was a problem with the color subcarrier - it could be interleaved with the monochrome video or the 4.5 MHz audio, but not both - it just wouldn't work mathematically. The 2nd NTSC decided that moving the 4.5 MHz audio frequency slightly might cause some trouble with existing monochrome receivers, so they moved the scanning frequencies instead, by the exact ratio of 1000/1001. Thus, the vertical became 59.94 and the horizontal became 15734 (15734.2657343... to be more precise).

Since the field rate and frame rate were no longer exact integers, this required the invention of SMPTE "drop-frame" time code. Actually, no frames are dropped, but the count is adjusted periodically so program time in seconds and minutes keeps pace with wall clock time.

https://en.wikipedia.org/wiki/SMPTE_timecode

By the way, since the exact new frequencies including the chroma subcarrier are derived by integer ratios from the old integer values, the ultimate precise chroma frequency is 3579545 + 5/11 Hz, but with a tolerance of +/- 10 Hz. The 5/11 Hz is never called out in standards or FCC regulations, but it comes out that way, as practically all modern broadcast plants are tied to a 10.00000... MHz GPS reference. Before GPS, the major networks for a while used their own atomic clocks, and before that, broadcast gear used oven-controlled crystal oscillators to guarantee the +/- 10 Hz chroma tolerance.

In analog broadcast and professional gear, the chroma frequency and scanning rates are locked to each other. The horizontal scan is the chroma frequency x 2/455; the vertical is the horizontal x 2/525 (i.e., 262.5 lines per field). Digital sources for analog sets (such as DVDs) generally provide these exact numbers, but home VCRs do not lock scan rates to the chroma. In this case, the scan rates are determined by the rotating head speed, while the chroma frequency comes from an independent crystal oscillator. The sound frequency also comes from an independent oscillator. Old video games for analog sets generally use progressive scan with 262 lines per field.

[old_tv_nut]

Originally Posted by jr_tech

It appears that the frequency showing in the Sams schematic is the sweep frequency setting for the oscilloscope. (Remember when scopes used frequency rather than a calibrated time base for setting horizontal sweep). Note that 3 cycles of the waveform are shown.

jr

Duh - of course!

[old_tv_nut]

BTW, I did a bunch of edits on my long post to correct typos and try to make some things clearer, so refresh before rereading.

[Electronic M]

Originally Posted by jr_tech

It appears that the frequency showing in the Sams schematic is the sweep frequency setting for the oscilloscope. (Remember when scopes used frequency rather than a calibrated time base for setting horizontal sweep). Note that 3 cycles of the waveform are shown.

jr

+1

My favorite way to dial in the horizontal frequency is to take a NTSC video source (usually a cable box or DVD player) , feed it into a scope and use the distance between H sync peaks to calibrate the distance between osc peaks in the osc. It's fairly easy to do, and all you need besides TV and signal source is a decent scope.

[ohohyodafarted]

Thanks guys. Never having had an old style scope, I guess a 1954 Sams would relate to test equipment of that era.

If I was 20 years younger, I would buy one of those newer scopes that gives you a digital frequency read out. But instead I have a cheep Chinese freq counter that I use along with my TEK 475A. Good enough for as often as I use it.

[Popester]

Originally Posted by jr_tech

It appears that the frequency showing in the Sams schematic is the sweep frequency setting for the oscilloscope. (Remember when scopes used frequency rather than a calibrated time base for setting horizontal sweep). Note that 3 cycles of the waveform are shown.

jr

+2 Yes, that referred to what you set the scope sweep at. That I do remember from working at the tv shop.