Tektronix CFC250 Teardown

Introduction

Time for something different – and perhaps the start of several new articles containing teardowns. In our first instalment we examine the Tektronix CFC250 100 MHz frequency counter circa 1994: Not the most spectacular of designs, but it has worked well right until the present day. The update speed of the display wasn’t lightning fast, however for the time it would have been quite reasonable. Here is a short video I shot last year comparing it against a small frequency counter kit:

However after staring at this thing every day on my desk for a couple of years it has now become impossible to overcome the temptation to have a look inside. Therefore the reason for this article. You can click on the images to see the full-size version. So let’s go back to 1988 and check out the CFC250…

External tour

A quick look around the outside. The casing is reminiscent of the Escort brand of test equipment from the era, and (I suspect that) they OEM’d the CFC250 for Tektronix. (Interestingly enough Agilent bought the assets of Escort in 2008). Moving forward, the external images of the CFC250 starting with the front:

… and the rear. The AC transformer is tapped out to accept four different mains voltages, which you can select with the slide switches:

Opening up the unit involves removing screws from the base. The first ones were only for the feet, so they could stay put:

It was the screw on the right of the foot that was the key to entry. After removing them from each side and the other pair on the rear-bottom, the top casing pulls off easily…

Internal tour

… leaving us with the internals for all to see:

Although the LED display is a fair giveaway to the age of the CFC250,  a quick look around the PCB confirms it… and the display is ultimately controlled by an LSI Systems LS7031 “Six decade MOS up counter” (data sheet.pdf). It is matched to some DS75492N MOS-to-LED hex digit driver ICs (data sheet.pdf) and some other logic ICs. It is interesting to compare the number of parts required to drive the LEDs compared to a contemporary microcontroller and something like the TM1640 used in this module.

Now for the LED display board:

Nothing too out of the ordinary. A closer look at the rear panel shows some very neat AC mains wiring:

Now for some more close-ups. Here we can see the use of the MM5369 17-stage oscillator/divider (data sheet.pdf). I haven’t seen one of these for a while, the last time we used them was for a 60Hz timebase. However in this case it would be used to create an accurate timebase within which the CFC250 would count the number of incoming pulses:

 The removal of two more screws allows removal of the main PCB from the base of the cabinet, which reveals as such:

There is also an opaque plastic sheet cut to fit, helping insulate the PCB from the rest of the world:

 The PCB is single-sided and very easy to follow. I wonder if it was laid out by hand?

It reminds me of some old kits from the past decade.  Moving forward, there is a metal shield around the PCB area of signal input and low-pass filter:

A quick desolder of three points allows removal of the shield, and reveals the following:

At the top-left of the above image reveals a resistor in a somewhat elevated position, as shown below:

If anyone can explain this one, please leave a comment below.

 Conclusion

What impressed me the most during this teardown was the simple way in that the unit was designed – all through-hole parts, mechanical connections either soldered or nuts and bolts, and all components labelled. I can imagine that during the lifespan of the CFC250 it would have been relatively simple to repair. Such is the price of progress. And yes, it worked after putting it all back together again.

In the meanwhile, full-sized original images are available on flickr. I hope you found this article of interest. Coming soon we will have some more older-technology items to examine and some new tutorials as well.

In the meanwhile, have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.
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John Boxall

Founder, owner and managing editor of tronixstuff.com.

9 Responses to “Tektronix CFC250 Teardown”

  1. The posts for the resistor are probably heat sinks.

  2. jme says:

    About the resistor on the poles: from the silkscreen it appears to be a spark gap, hence the jaggered marking, probably a lot like a jacobs ladder. And since the resistor is in place across the gap, it is probably there to make the spark gap smaller. This would help reduce noise and zapping.

    • asf says:

      Spark gap? I don’t think so. The little ‘jaggered marking’ is the standard symbol for a resisor. The resistor is rasied off the board becuase it might get hoter than others. Heat will most likely change the resistor’s value slightly, and as this is a piece of measureing equipment, those varriances need to be minimized for it to be acurate.

  3. David Thomas says:

    without seeing the manual/circuit diagram, I would suggest that it is a terminating resistor for the nominally 50 ohm impedance input (being 47 ohm 5% CF) and it is on pillars for either “select-on-test” or being replaced when excessive volts are applied to the counter input..

  4. marcwolf1960 says:

    As it is on a sgnal imput I believe it would act as an impedance/fuse. Easily sacrificed and repaired should an overload situation occurs.

  5. Gavin says:

    Based on the age of the unit, resistors mounted like this on posts were usually done this way in order to calibrate a particular part of the circuit.

  6. alex says:

    i believe it was referred to as vector pin mounting brought that the depths of my memory maybe someone wants to try and establish why, for me life’s tom short.

  7. alex says:

    apologies the word mounting should be after vector pin

  8. Renato Silva says:

    Looking at the photo we can see that the resistor is in series with a capacitor. If this is near the signal input, there are big chances that the resistor is part of a compensation circuit to correct the waveform, like that present in the oscilloscope probes. This is a 100MHz circuit and after all put together may be necessary to fine adjust the complex impedance interactions between the various components present at input. Anyway this type of mouting is always to adjust something in the final test/calibration phase.

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