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Tek 520A NTSC vectorscope disassembly


Greg Muir
 

I had an occasion to recently part out an old Tek 520A NTSC vectorscope that was one of two lying around in storage given to me by a past client. To say the least I was impressed about the packaging design of this unit given its age.

The unit is obviously a more complex instrument since it was assigned the task of precisely measuring both amplitude and phase components of the analog video signal. All of the several printed circuit boards were highly populated with discrete components.

Mechanically it appeared that the Tek packaging designers were given wide latitude to utilize as many unique approaches to their input as needed. The sheet metal in itself was a significant design effort to get everything to align properly. The printed circuit boards were retained not by normal fattener means but, instead utilizing small plastic catches that secured the edges of the boards. Simply flexing them away from the board allowed it to be released.

All except a couple of instances all of the high number of connections to the boards were made using the small pop-type wire connectors allowing each board to be removed without having to resort to desoldering wires. I can see where this would be rather handy for board repairs or replacement (and probably factory calibration during assembly) but it also could possibly introduce a certain factor of intermittence over time (I did find a couple of connectors that were rather loose when parting the unit out).

A large number of the semiconductors were also socketed on the boards. As to why this approach was made is not certain except from the possible standpoint of serviceability.

Of particular mechanical note was the increased complexity of the front panel and mounting of several of the controls. Without going into detail I will say that some of the designs were not of the normal variety. And included in this was the design of the phase adjustment controls where a mechanical approach was made to achieve adjustment rather than an electronics means. But it must be remembered that this was an early foray for Tek to provide quality test equipment for the broadcast color television industry.

The CRT itself was of the obvious Tek custom design. It utilized a dual graticule arrangement to achieve two different measurement display modes. There was a dual illumination scheme to achieve this approach. For phase measurements one set of lamps illuminated a custom glass faceplate overlay permanently bonded to the CRT that contained legends indicating the proper phase relationships for the I, Q & Y signals. Then when simple video amplitude and timing were to be shown a second set of lamps illuminated the “normal” plastic graticule that mounted in front of the CRT. And to assure accurate calibration of the displays a dual focus coil arrangement surrounded the CRT.

Compared to the typical Tek oscilloscope product I have to say that the engineers really knocked themselves out to develop a dedicated product that served the television industry for many years. If something of this nature were to be designed in the present era it would have been considerably less complex given the availability of ASICs and processing power available.

In all it was a great learning experience to better understand the earlier years of Tek engineering.

Greg


Greg Muir
 

As usual my fingers and brain got ahead of me when writing my post. A couple of issues:

“The printed circuit boards were retained not by normal fattener means....” should read : “The printed circuit boards were retained not by normal fastener means”

And…

“And to assure accurate calibration of the displays a dual focus coil arrangement surrounded the CRT.” should read: “And to assure accurate calibration of the displays a dual alignment coil arrangement surrounded the CRT.”

Sorry.

Greg


ppppenguin
 

I've worked on the PAL version (521 and 521A) which is mechanically near enough identical. As usual it's a superbly engineered instrument. There was actually an earlier Tek vectorscope, the 526, (NTSC version may have had a different number) which wasn't the most wonderful of instruments but at the time it was all there was. The phase shift arrangments were a row of pushbuttons for coarse adjustment and a hefty multi-turn dial for fine.

The socketed semiconductors were commonplace in other Tek instruments of that era. In my opinion this made reliability worse for no great benefit.

The word "vectorscope" is actually a Tek trademark that fell into common use. The generic term is "vector monitor".

Except for a few legacy composite analogue video applications, vectorscopes are totally obsolete. Some modern video testgear will still give a vector style display even though it has little relevance in the digital age.

Jeffrey Borinsky www.becg.tv


Greg Muir
 

Thanks for your input Jeff,

Yes, socketed devices can lead to a myriad of issues. I guess I had never noticed before Tek’s use of sockets in past instruments. But the sockets in use in the 520A were quite impressive in regards to their contact tension/grip strength.

I have a few other vector scopes of various manufacture that I am weeding out of storage. It seems like I either became the recipient of them from clients or simply felt bad seeing them heading for the dumpster when there was the possibility of scavenging parts for hobby purposes (as if I need more “stuff” hanging around).

I won’t get on my soapbox to comment about the DTV conversion. Analog was suitable enough for general viewing and fraught with far less problems in regards to reception. The human mind was efficient in filtering noise and other artifacts out of the viewed video and didn’t have to deal with the “cliff effect.”

It’s sad to see technology move more into the “neat and nifty” arena rather than “simply practical.” I personally don’t have a television that covers one entire wall of my house since I don’t watch it that much.

Greg