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I took a few photos to update the tek wiki page, and figured I’d add them here as well. In all it’s calibrated glory, sitting atop my 519 Left side Right Side Bottom Closeup of the lower beam … Continue reading
Relatively speaking, the 551 is one of the less useful scopes in my collection, as specially considering it’s size. With a separate power supply at 42 lbs, and the mainframe 10 lbs over that, it comes in over 100 lbs once you include plug-ins. It is the least capable of dual beam scopes in the 500 series, having only a single time-base connected to a single set of horizontal deflection plates, and a lone intensity control shared across both beams.
Nevertheless, I’m in the mood to work on a 500 series, and this was the only ‘normal’ one that’s currently the apartment; frankly I’m still too intimidated to work on the 519 or the 661.
Here it is on workbench B (aka the dining room table). On top of the power supply is my TU-75 variac which I was using to check the power supply on the 181 time mark generator (more on that later). Pardon the underwhelming photo quality, proper glamor shots are on the winter todo list.
The vertical section is the same for both beams, and seems to be working well. The timebase and triggering are are also in good shape, but the timebase is off by 3% – 8% across almost all of the ranges. Here’s output from my 181 timemark generator at 1ms.
Parallax induced by the fact that the graticule is forward of the CRT surface makes it difficult to take an accurate picture, but the in the photo above, the left marker is aligned with the left graticule, and the right most marker is at the 2nd to last small tick; making this about 4% off. Out of the factory, accuracy was within 1%, and I’d like to see about getting that restored.
The manual doesn’t have a section for calibration, it’s a separate document – both available on the Tek wiki, here. To calibrate the sweep, there’s a trim-pot on the back of the ‘Horizontal Display’ control, visible in the photo below, just behind the red test-lead.
Update: What’s written below is mostly incorrect (though I’m leaving it for historical purposes). I was able to get the timebase into calibration by following the instructions. Before calibrating the sweep, you’re supposed to calibrate the sweep magnification. This seems completely counter-intuitive, and I never use the mag, so I didn’t bother with this step. This turned out to be a giant mistake. Once I did that, I was able to get the sweep calibrated quite easily, with the exception of .1/.2/.5ms, which are running a little slow. That points to C180A, perhaps I’ll try to replace it another time. For now, I’m calling this instrument done.
This pot is already as far as it will go, so no luck there. Turning it the other way increases the error. What’s interesting is this adjustment doesn’t actually alter the rate of the sweep ramp per se, it changes the amplitude. There are trimmer caps for some of the faster sweep rates, but there’s nothing in the ramp generator to alter global timing.
The sweep signal is generated in the miller run-up circuit, given a variable DC offset to adjust the horizontal placement of the trace on the display, and then is turned into a differential signal and amplified before being sent to the horizontal deflection plates.
So what’s wrong with mine? 10ms worth of 1ms markers are being displayed in less than 10ms of divisions (1ms per division). That means by the time the 10th marker goes off, the beam hasn’t yet gotten to the right spot on the screen.
What do we need to do to correct this? The slope of the ramp is determined by both the RC constant of the ramp generator, and the subsequent amplification stage, so it’s possible (within reason) to adjust for a slow ramp in the amplification stage. In the photo above, that’s what I’ve done: By putting a 500k resistor in parallel with the 100k resistor that’s in series with the trim-pot, I was able to get the sweep into cal. Is this cheating? It feels like it. I haven’t soldered it in yet, I’m just using a substitution box. At higher sweep speeds, the capacitance of the leads have a significant impact on the waveform.
It’s easy to make erroneous conclusions if you don’t understand the effect the controls have on the measurements, or don’t heed & grok all of the notes on the schematics. While it’s possible to get the voltages listed on the schematics, if you have to deviate too far from the stipulated settings, or adjust other controls to their extremes, there’s probably something else wrong. For example, I could get a few measurements within spec if I altered the sweep length beyond normal or listed ranges. In another case, I measured the length of the sweep ramp at the cathode of V173, and found it to always be about 5-10% too long. At first I thought this was indicative of the ramp running too slow and I’d found the issue, but then I realized that the sweep length control effects this, and it was only a coincidence.
I’ve gone back and forth between suspecting that the issue is in the ramp generator or the amplifier. Like I said, the important thing is the slope of the ramp, and that can be adjusted (within reason) in either section. All of the timing resistors & capacitors are within 1%
This is getting annoying. I may just solder that damn resistor in place and move on with my life. More to come.