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Hi Michael.
I changed the subject since it has been about the 500 series SCOPES and not CURVE TRACERS until now.
In the next few days there will probably be many replies to this question. I request that everyone keep the discussion dignified.

For 99% of our members the most important consideration will be which one you can get at an affordable price. They are both wonderful instruments especially if you have nothing else.

Since you are in the 1% category that owns both a 576 and a 577 (and so do I) 1 I will comment on which one meets my needs and why.

The first thing I think of, that I believe Deane also thought was important, was the storage capability of the 577-D1. It has a drawback as well - it can be annoying when it is not adjusted properly. I have followed the internal adjustment procedure carefully for adjusting the storage voltages with little success so I live with the occasional storage artifacts. Another reason I like the 577 is because I am also interested in testing the additional things my 178 "front porch" can test (OpAmps, Voltage Regulators, SCRs, etc).

I personally think the 576 is the most beautiful instrument with a CRT that Tek ever made (In that respect I'm a romantic). Many 576 owners love the readout which helps to avoid mistakes when measuring parameters. This is not something I need. Like my annoying storage, the fiber optic readouts have their own issues. Most 576s have some dark fibers by now and there are no replacement readouts that I know of. I don't think there is a practical way to repair individual fibers either. The 576 has its own specialized "front porch", the 176, for testing power transistors which is important for some users. The 576 was incredibly expensive to build and Tek did not make much, if any, profit on each they sold.

The 577 was designed to fix that problem. Tek engineers borrowed heavily where they could to keep costs down. For example, the entire display section was borrowed from the 5000 series of scopes. My principal complaint about the 577 is due directly to this lineage: there is no Graticule Illumination. That is problematic when I want to take a photo of the curves on the screen and the graticule lines do not show up in the photo so it is almost impossible to know what the V & I of each curve is. Instead I discovered an alternative way to add a graticule overlay after I take the photo of the semiconductor characteristic curves. But this is still an annoying oversight from Tek. One final thing in the 577's favor: it was released several years after the 576 and in the intervening years several new types of semiconductor devices were developed and the design team made sure the 577 could test those devices.

You are probably wondering why I find the storage of the 577-D1 to be essential. Here are four examples:
1) Junction (bipolar) FETs are notoriously temperature sensitive in a curious way. Below a certain bias point the temperature coefficient is negative, above this bias point the temp coefficient is positive. So display a FET on a non-storage 577-D2, pick some typical gate bias voltage steps, and look at the curves you get. You can't tell where the gate bias point is exactly at the midpoint between the positive and negative temp coefficients. If you do the same thing with the 577-D1 and turn the storage on all you have to do is change the temperature of the FET by applying a little heat to the FET and the curves above the critical bias point will move up and curves below this point will move down. The storage displays the bias point where nothing moved and that is the critical zero Temp coefficient bias point you should design for.

2) With storage turned on put a transistor in the left socket and another one in the right socket and toggle left to right and you can immediately tell if they match and by how much. The curves won't overlap unless they are matched.

3) This is a little trickier but I have used storage to find two power transistors (one NPN and one PNP) that had Beta close enough to make a near perfect class B output stage. I had the NPN in the left socket and I displayed the curves for it in the upper right quadrant of the 577-D1. Zero collector voltage and current was located at the exact center of the screen. Then I stored the top half of the screen. Next I did a similar thing with the PNP which was in the right socket. Again zero voltage and current were set to the center of the screen and its curves were positioned into the lower left quadrant. Then I stored the bottom half of the screen. There was a perfectly linear load line from the negative PNP collector voltages and currents on the bottom left of the screen to the positive NPN collector voltages and currents in the upper right part of the screen.

4) At extremely low collector currents Miller capacitance, socket capacitance, transistor noise, and AC (hum) pickup distort each collector V vs. I curve into a noisy loop. As you switch to more sensitive ranges the loops occupy a larger portion of each step until the steps themselves are totally obscured by the capacitance, transistor noise, and AC pickup. The solution is to switch to manual +DC or -DC and use the display filter to reduce the noise. With storage you can vary the collector voltage manually and trace out the entire set of characteristic curves just fine on the CRT no matter how sensitive the range you select. With a 576 or a 577-D2 you would have to use a grease pencil to mark your progress on the CRT to see the curves.

Dennis Tillman W7PF