17-11-2017, 12:05 PM
(16-11-2017, 06:02 PM)Bonky Wrote: The inbuilt sq wave generator for calibrating the probe didn't work (and still doesn't). The output is meant to be 0.2V but was only 0.1. I've adjusted the trimmer on the pcb to its max (but only 0.125V). Has anyone any advice?
So it is working, but not giving the expected output voltage? There's a world of difference between the two...
I'm sorry to say this, but by twiddling that trimmer, you have made a fundamental mistake here. You are new to this, so this is forgiveable, but it's really important that you take this on board and learn from it.
Before twiddling that trimmer, you made a huge assumption. You assumed that the vertical sensitivity of the 'scope was correct. That was an invalid assumption. You know nothing about the pedigree of that 'scope, so you have no reason to believe that any part of it is correctly calibrated. You placed your trust in something that wasn't deserving of it.
This is a big deal. You need to trust your instruments, and that trust must be earned. In other words, assume nothing!
If that seems like I'm telling you off, I'm not really - well, perhaps only enough to get your attention! But please, take the lesson from this - no mistake is wasted if you learn from it. Don't become the "Phantom Twiddler". Adjusting pre-sets is the last thing to do, not the first. Especially in a bit of test gear.
And of course, once you've found out what the problem is, you'll have to go back and readjust the calibrator preset. But how will you do this with the required 1% accuracy? Think about that one...
You haven't described why you believe the calibrator is low. Perhaps you do have good reason to suspect it, but you haven't explained that - therefore I'm assuming nothing.
So is it the chicken or the egg? Or both?
An important check - is the error the same on both channels?
If so, then it might be the calibrator that is genuinely low, as the two channels are fairly separate internally, and it's unlikely that both channels will have the incorrect gain by the same amount. However, that's not conclusive, because actually, the two channels do share a common amplifier after the beam switching. But nonetheless, it's a check that should be done.
You have a multimeter that you trust? And you can lay your hands on a battery? If so, you have the means to check if the problem is with the generator or the input channel of the 'scope. Remember, 'scopes measure DC as well as AC
Ref's suggestion of checking the variable gain controls is a good one - a common mistake that we all make from time to time. Some 'scopes light a red LED when the control is not in its calibrated position, but this one does not.
Let's think more deeply about this...
The calibrator circuit is a CMOS IC running from 5V. The 5V rail is likely to be OK because if not, there would be many other problems with the 'scope. However, it's easy to check, and would be the first thing I'd do if I knew the calibrator was the culprit. Next, the 4011 CMOS IC making the signal is generally reliable in service, but when chips fail, they tend to fail completely. If you were unlucky, it's possible that a failure of 1 of the 4 NAND gates (either a or b) could cause the output signal to half in this particular circuit. It's easy to check - just make sure that you have a 5V square wave on pin 3 and pin 4 of IC2001. For anyone not looking at the service manual, each of these two square waves goes through a 15k resistor to a summing point which has 270 ohms and the 100 ohm preset going to ground, and this is the output...
So with this particular circuit, there is a possible failure mode that could result in the signal being half the size you expect. That's relatively unusual. Whereas when it comes to a gain error in a vertical channel, that's ordinarily much more likely. There is a lot more circuitry, switch contacts and many more pre-sets to play with (DON'T!), so on purely statistical grounds, that's where you tend to point the finger of suspicion in the first instance. As I say, a battery and a multimeter is all you need - literally a 30 second job.
Final point. Despite the name, the calibrator isn't there to calibrate the 'scope. The voltage accuracy of the calibrator might be quite good (1% in this case), but often they aren't as good as that, and they are intended to provide a quick check but nothing more rigorous - calibrating the vertical channels of a 'scope requires something with more precision (the spec is 3%, so as a rule of thumb, a calibrator ought to be accurate to around 0.3%). The primary purpose of it is to check the HF compensation of the probe. That much you might already have gathered, but it's worth writing down for anyone else in a similar position.
I hope all this helps. It will need a couple of re-reads, but if nothing else, try to take the general guidance - try to understand the thought processes that I'm explaining. Yes, specifics are important, but one of the most important things about becoming an engineer is to develop the right mindset - that's what enables us to take on new challenges and problems.
