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The RU800 - A Drop-In Replacement for The Tektronix U800

The RU800 - A Drop-In Replacement for The Tektronix U800

The RU800 is a drop-in replacement for the U800 found in Tektronix 2400 series oscilloscopes. However it has some inherent limitations compared to the original U800 which I describe below.
Changes for Ver 2.1
a)  Calibration of the XY system and of the cursors previously required 0805 resistors to be manually replaced. This is no longer necessary. The RU800 Ver 2.0 can now be calibrated the same way as the original U800, i.e., the X1 system, X-Y system and the cursor system are all aligned. This is the single biggest improvement.
b) Readout jitter has been reduced in both X1 and X10 modes. The readout remains readable in all ranges although the x10 mode in the very slow ranges (10mS/div and below in X1) still exhibits a little jitter but the readout remains completely readable. (This is only a problem in the x10 mode.)
c) X10 traces are sharper right through to the 500ps/div range
d) The switching supply that provides 5V has been replaced with a linear regulator. This runs hotter but offers improved noise that reflects in clearer and more stable traces
e) The overall bandwidth has been improved. The Transient Response control in the original U800 is no longer needed to tune the 500ps/div range. However expect about a 5-7% loss of accuracy in the 500ps/div range.
f) Tactile switches have been replaced with micro-jumpers for easier HF rolloff tuning to allow you to tune it very precisely if things drift with age.
Original Description for Ver 1.0
Presented here is a drop-in replacement for the Tektronix 155-0241-XX, also called the U800 in Tek schematics, used in the 2400 series oscilloscopes. These [in]famous chips suffered from field failures, and now being unobtanium, command premium prices on this site. Would it not be good to have a replacement that could do a decent job 90% of the time and preserve those real U800s for that "special occasion"?
1. The RU800 is a "drop-in replacement" in that nothing needs to be removed or replaced on the main PCB of the oscilloscope except of course the original U800. All features available with the original U800 are also available with the RU800. In particular, you get the X1 Mode, X10 Mode, B Sweep, and X-Y Mode. Furthermore the existing adjustment controls and trimmers on the 24XX board work as they should with the RU800- these include Horizontal Center, X1 Gain, X10 Gain, Transient Response and Beam Find. However, if you wish to calibrate either the cursors or the XY mode in your specific 'scope using the RU800, it may require you to replace 1-2 0805 resistors as explained below.  
2. Before you buy, you need to know what kind of performance [not] to expect; so please do see the video showing a prototype of the board in action at:
and decide for yourself if it's worth it. Questions are most welcome.
3. Mechanical and electrical compatibility. I have only tested the board using a 2465A. I have not tested this with other 'scopes. Please make sure the board will be compatible with your model of oscilloscope - in particular, you need to check with me to make sure it will fit in the available space the way it does in the 2465A. A helpful buyer has confirmed it fits in a 2467.
4. Though the RU800 is an all-SMD design, it uses larger-footprint SMDs for the most part. The board uses only standard parts - no custom ICs or firmware! This will allow you to do some troubleshooting and repair yourself at a later date, and I will be available to guide you though the process. All you need is the ability to solder 0805s and SOICs for the most part, a skill which can easily be mastered, if you already know soldering, with a weekend's worth of practice. It also has a few test points to allow you to verify correct performance at key points on the underside of the board which most people can't reach when it's plugged in.

5. Additionally, if you feel adventurous, a pair of micro-tactile switches is piggybacked onto two resistors which will allow you to make individual adjustments to the HF rolloff behavior of the positive and negative outputs of the differential output, using the two trimmers you see. This will allow you to fine-tune the accuracy of the fastest 3 ranges, viz., 2nS/div. 1nS/div and 500ps/div.
6. All parts are brand-name parts. In particular, passives on the board are sourced from Digikey, Mouser and Farnell/element14 for peace of mind.
7. Known limitations:
The RU800 not being a microcircuit, is more exposed to noise and more susceptible to disturbances - (the amplifiers on  the board operate with bandwidths exceeding 300MHz). Furthermore, you are likely to operate this with a socket so that you can swap it for the original U800 on occasion when you need the original "Tektronix Performance." Therefore you should be aware of the following:
7A. Runtime limitations
- Readouts and traces are clear and usable but may not be as sharp as with the original U800
- B-sweeps may exhibit slight distortion on the A-sweep, when viewed with the A-sweep.
- Range-to-range accuracy, especially at the highest sweep speeds of 2ns/div and above, may vary in sweep accuracy by about 2% (1 minor division).
- A small amount of noise is also present in the X-Y mode.
7B. Calibration limitations
Additionally, because the design of the RU800 was based on some guesses about the innards of the U800, the following limitations have been encountered in calibration. In each case, if you encounter the need to calibrate at any time, I will guide you through this process and provide specific instructions based on the value of the inaccuracy you report.
- Calibration of the gain of the X-Y mode requires manually replacing one 0805 resistor on the board.
- Calibration of the cursor accuracy requires manually replacing a single 0805 resistor on the board.
- If you want to perform a complete horizontal cal of your 'scope, this is best done with the original U800.
- Some parts have been soldered by [my] hand. Expect them to look like they have.
Whether you view it as reincarnation of, or surgery to, your Tek oscilloscope, don't expect it to regain its youthful vitality. Nevertheless, I would be very happy to get feedback about its performance so I can try to refine the design!
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