The vertical section is the same for both beams, and seems to be working well.\u00a0 The timebase and triggering are are also in good shape, but the timebase is off by 3% – 8%\u00a0 across almost all of the ranges.\u00a0 Here’s output from my 181 timemark generator at 1ms.<\/p>\n 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.\u00a0 Out of the factory, accuracy was within 1%, and I’d like to see about getting that restored.<\/p>\n The manual doesn’t have a section for calibration, it’s a separate document – both available on the Tek wiki, here<\/a>. 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: <\/strong>What’s written below is mostly incorrect (though I’m leaving it for historical purposes).\u00a0 I was able to get the timebase into calibration by following the instructions.<\/em> Before calibrating the sweep, you’re supposed to calibrate the sweep magnification.\u00a0 This seems completely counter-intuitive, and I never use the mag, so I didn’t bother with this step.\u00a0 This turned out to be a giant mistake.\u00a0 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.\u00a0 That points to\u00a0C180A, perhaps I’ll try to replace it another time.\u00a0 For now, I’m calling this instrument done.<\/p>\n <\/p>\n This pot is already as far as it will go, so no luck there.\u00a0 Turning it the other way increases the error.\u00a0 What’s interesting is this adjustment doesn’t actually alter the rate of the sweep ramp per se, it changes the amplitude.\u00a0 There are trimmer caps for some of the faster sweep rates, but there’s nothing in the ramp generator to alter global timing.<\/p>\n 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. 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.\u00a0 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.\u00a0 For example, I could get a few measurements within spec if I altered the sweep length beyond normal or listed ranges.\u00a0 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.\u00a0 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.<\/p>\n I’ve gone back and forth between suspecting that the issue is in the ramp generator or the amplifier.\u00a0 Like I said, the important thing is the slope<\/em> of the ramp, and that can be adjusted (within reason) in either section.\u00a0 All of the timing resistors & capacitors are within 1%<\/p>\n Here’s a bunch of the measurements.\u00a0 It’s extremely challenging to find a 5% error among readings that are 10% off. This is getting annoying.\u00a0 I may just solder that damn resistor in place and move on with my life.\u00a0 More to come.<\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Relatively speaking, the 551 is one of the less useful scopes in my collection, as specially considering it’s size.\u00a0 With a separate power supply at 42 lbs, and the mainframe 10 lbs over that, it comes in over 100 lbs … Continue reading ![\"IMG_5128.JPG\"](\"https:\/\/paulcarbone.com\/blog\/wp-content\/uploads\/2015\/10\/IMG_5128.jpg\")
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\nSo what’s wrong with mine?\u00a0 10ms worth of 1ms markers are being displayed in less than 10ms of divisions (1ms per division).\u00a0 That means by the time the 10th marker goes off, the beam hasn’t yet gotten to the right spot on the screen.
\nWhat 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:\u00a0 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.\u00a0 Is this cheating?\u00a0 It feels like it.\u00a0 I haven’t soldered it in yet, I’m just using a substitution box.\u00a0 At higher sweep speeds, the capacitance of the leads have a significant impact on the waveform.<\/p>\n![\"Screen](\"https:\/\/paulcarbone.com\/blog\/wp-content\/uploads\/2015\/10\/Screen-Shot-2015-10-23-at-1.14.44-PM.png\")
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