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inverters going along a twisted pair, into an AND gate 8====D e: hello new page
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Jul 16, 2021 16:40
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Dominoes posted:Design in a big label that says "Don't tap the mic" Cojawfee posted:Set up snipers to shoot anyone about to tap the mic. lol I should probably clarify that this isn't actually a karaoke night microphone or something. Its a wearable device that detects audio and does some DSP trickery. Because its wearable, its susceptible to getting knocked/banged/bumped and this causes high amplitude spikes that clip the ADC and ruin the signal.
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Jul 16, 2021 17:00
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Cyril Sneer posted:We're doing some audio stuff at work and one issue we always have to contend with is high-amplitude low-frequency impulses saturating the ADC, which arise from things like physical tapping of the microphone. I don't want to reinvent the wheel here; I feel like this is something that the audio world has surely dealt with. I'm vaguely aware of things like limiting circuits and soft clippers, but looking this stuff up its largely presented in the context of intentional distortion. Any ideas or some keywords here would be most helpful! Question: Does this saturation cause other issues than hard clipping in the ADC? If not then I'd say a software fix might be preferable; it will involve clipping anyway so a DSP limiter is probably desirable anyway. Another question: why is your analog saturation level higher than the ADCs? This potentially means dynamic range is being sacrificed; though I can see a design targeting optimal sensitivity ending up in this situation. The hardware fix will be some variation of non linearity, hard or soft clipping is AFAIK typically just done with diodes; the higher the source impedance the softer they will clip. Fixing it right at the ADC (ideal if there are issues beyond the saturation itself) would probably involve putting current steering diodes to the maximum positive and negative voltage the ADC can handle (and perhaps a slight attenuator before the ADC). Fake e: seems like the following isn't the issue but it might have some educational value Or is the issue that a large e.g. positive transient causes the ADC to saturate for a long time after the transient, i.e. for a positive pulse the ADC saturates high during the pulse then stays saturated at the low for some longer time after? This behaviour is most easily fixed by decreasing the AC coupling caps to increase the minimum input frequency to some higher value. To maintain DC balance the integral of the positive pulse must be the same as the following negative trail; if you increase the cutoff frequency the negative trail will have a higher amplitude but will be quicker. You could possibly fit some clipping diodes to help with this effect, or e.g. add some active clamping circuitry. E: or obviously you could put in a high(ish) speed variable gain/automatic gain amplifier, but since it must respect causality (i.e. it can't see the future) it can only respond after the clipping has already started.
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Jul 16, 2021 17:13
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Cyril Sneer posted:lol Is it possible to use compression? You can do simple audio compression with a simple optical circuit, like a photoresistor picking up an led for use as feedback. https://en.wikipedia.org/wiki/Dynamic_range_compression#Design
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Jul 16, 2021 17:17
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somebody on craigslist is giving away a heathkit oscilloscope in working order and i'm currently shamefully attempting to undercut whoever dibsed it with offers of money b/c oh man, man oh man oh man  i actually need a scope to boot but i never expected to run into a gorgeous old analog model to start off with
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Jul 16, 2021 18:40
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I'm thinking for the mic tapping problem that a highpass filter is in order. Is this intended to normally transmit the human voice? There's not really any useful information below about 120-150 hz or so, you can chop freely below there.
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Jul 16, 2021 19:22
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compressor/limiter is the first keyword that comes to mind. I used to be really partial to an old Symetrix model, the 525.
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Jul 16, 2021 22:14
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PRADA SLUT posted:Let's assume I can hit any trace on the board. How would I apply a correct voltage to the MCU, while still allowing for uploading a program over usb from the computer, without drawing USB power? I can see how I do the former, but not both at the same time. Would I need to shunt USB +V but keep the data pins? IF you can hit any trace on the board, figure out which one is sucking all the power down and lift it off the board, then see if the chip boots. It's probably down to one pin on one port having something wrong inside. This isn't a problem with USB power (probably) or weak voltage regulator; it's probably an issue with the main MCU chip itself having gotten something spicy somewhere and being turned into a big diode with not-big-enough series resistor. If you keep powering the thing in this condition, you'll turn your MCU into a smoke-emitting diode.
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Jul 17, 2021 07:10
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This article discusses building active clippers to avoid going out of an ADC's range: https://www.analog.com/en/technical-articles/op-amp-precision-positive-negative-clipper-using-lt6015-lt6016-lt6017.html The key thing is that you have to use op amps that can live with the positive and negative input voltages being different by a large amount. A lot of precision op amps don't allow that.
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Jul 17, 2021 13:57
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longview posted:Question: Does this saturation cause other issues than hard clipping in the ADC? If not then I'd say a software fix might be preferable; it will involve clipping anyway so a DSP limiter is probably desirable anyway. Yes, at its core its a dynamic range issue - we're going for sensitivity. Our signal of interest is quite weak, so our gain stage is designed to bring it into (more or less) the full range of the ADC. So, when large amplitude events happen, they saturate the ADC input. I've seen this diode approach mentioned elsewhere but I'm not sure it'll really work. It seems you're just trading off ADC clipping for diode clipping. The issue with the clipping isn't component damage, its that under an FFT, it causes all sorts of artifacts -- presumably the diode clipping would do the same. Cyril Sneer fucked around with this message at 20:20 on Jul 17, 2021 |
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Jul 17, 2021 20:14
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Jonny 290 posted:I'm thinking for the mic tapping problem that a highpass filter is in order. Is this intended to normally transmit the human voice? There's not really any useful information below about 120-150 hz or so, you can chop freely below there. We have one around 150 Hz, but a real filter doesn't provide perfect attenuation, so big stuff can still get through. Additionally, the frequency content of these impulse events can be fairly broad.
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Jul 17, 2021 20:18
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petit choux posted:compressor/limiter is the first keyword that comes to mind. I used to be really partial to an old Symetrix model, the 525. Yeah, I've been looking into compressor/limiters. This is for an IoT device so I can't stick a 525 inside it 
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Jul 17, 2021 20:20
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The optical compressor seems like the simplest one but I've never implemented them per se. Seems like it could be simple, but the problem is the characteristics depend on the specific parts you use. Also it uses a photoresistor, I mistakenly said phototransistor before. From: https://www.izotope.com/en/learn/4-types-of-analog-compression-and-why-they-matter-in-a-digital-world.html quote:Optical Compression
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Jul 17, 2021 20:41
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I actually just stumbled upon these soft-limiter circuits which use parallel diodes across the feedback resistor to kill the gain when activated. Seems promising? https://sites.google.com/a/davidmorrin.com/www/home/trouble/troubleeffects/distortion-overdrive/op-amp-distortion-and-overdrive
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Jul 17, 2021 20:46
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I just happened to skim that section of my old text book the other day and I think you're right about the diode being more for protection and will resemble the kind of clipping your ADC would otherwise do (but without actually causing the ADC to limit out which I suspect could have a recovery time associated?)
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Jul 17, 2021 21:03
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Diode pairs like that are very ubiquitous in radio land because their characteristics are just about right for clamping RF energy that receivers are Just Not Meant For and radios are built to be able to sanely take signals varying over about a 100-120 dB dynamic range. The signal gets very ugli-fied, but it does save the "can hear a mouse fart in a hurricane" front end transistor in a receiver.
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Jul 17, 2021 21:09
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Cyril Sneer posted:I've seen this diode approach mentioned elsewhere but I'm not sure it'll really work. It seems you're just trading off ADC clipping for diode clipping. The issue with the clipping isn't component damage, its that under an FFT, it causes all sorts of artifacts -- presumably the diode clipping would do the same. Cyril Sneer posted:We have one around 150 Hz, but a real filter doesn't provide perfect attenuation, so big stuff can still get through. Additionally, the frequency content of these impulse events can be fairly broad. If you're doing spectral processing and the noise signal is an impulse you're kind of screwed regardless aren't you? Clipping will add more harmonic components sure, but if the impulses are broad band even before clipping there's no analog filter that can help with that. And this clipping could also be occurring in the microphone itself, and in that case you've lost before you started. Can you do the real obvious thing and just detect clipping and ignore those samples +/- some time around the event? In radio communications this is done using a noise blanker circuit that just detects the envelope of the total signal energy coming in (before narrow filtering) and disabling the receiver during those impulses. The super nice ones use a hardware delay line to make sure they can in fact ignore causality and respond "before" the impulse. In this case it should be fairly trivial to do in a DSP. Another alternative I see is adding an automatic gain control circuit that can reduce the gain; this will need to either be slow to detect the event (i.e. it will "leak" some impulse noise through) or it can be made sensitive (using e.g. the derivative of the signal), but then it will also be sensitive to e.g. high frequency signals. If you really need that signal with no lost samples then I'd say the most practical way of doing it is to add another ADC channel set to a lower gain (or even better, get a higher dynamic range ADC, and/or rethink the noise analysis of the circuit to see if you can't sacrifice some gain). You could run the same processing on the other channel or detect when the high gain channel is saturated and switch to the low gain one during the event.
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Jul 17, 2021 22:21
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Never mind 
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Jul 18, 2021 03:04
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longview posted:True, this is only necessary if the ADC were misbehaving, but clipping is correct behaviour when the input signals are too strong so adding additional limiting isn't going to help. Soft clippers may change the spectrum you get but it's still a non linear process that will always add distortion. Yes, understood. It might be good enough though. You can always tap harder, but if we can get rid of / reduce the typical tapping, this would still be a nice improvement. longview posted:If you're doing spectral processing and the noise signal is an impulse you're kind of screwed regardless aren't you? Clipping will add more harmonic components sure, but if the impulses are broad band even before clipping there's no analog filter that can help with that. Absolutely, which is why I'm exploring non-filter based strategies. longview posted:Can you do the real obvious thing and just detect clipping and ignore those samples +/- some time around the event? This might work but doesn't really solve the problem. From a DSP perspective, blanked-out samples is the same as artifacted samples (i.e,. we can detect the artifacts, and hence clipping easily). The hope is to try to eliminate or at least reduce these events from occurring to begin with. longview posted:Another alternative I see is adding an automatic gain control circuit that can reduce the gain; this will need to either be slow to detect the event (i.e. it will "leak" some impulse noise through) or it can be made sensitive (using e.g. the derivative of the signal), but then it will also be sensitive to e.g. high frequency signals. The problem with AGC is that our algorithms are dependent on absolute noise and signal levels so this would screw things up. longview posted:If you really need that signal with no lost samples then I'd say the most practical way of doing it is to add another ADC channel set to a lower gain (or even better, get a higher dynamic range ADC, and/or rethink the noise analysis of the circuit to see if you can't sacrifice some gain). Now this is interesting, and might be doable. The IC we're using does actually have unused ADC inputs, hmmm.
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Jul 18, 2021 05:06
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longview posted:If you really need that signal with no lost samples then I'd say the most practical way of doing it is to add another ADC channel set to a lower gain (or even better, get a higher dynamic range ADC, and/or rethink the noise analysis of the circuit to see if you can't sacrifice some gain). Isn't this how "HDR" mode works on your phone's camera? It just takes a low-exposure, normal-exposure, and high-exposure and composites them together with magic? It's a very clever solution for audio.
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Jul 18, 2021 05:57
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Hey! I'm going to record a little bit of audio from this f'ed up drum machine I got from Goodwill, and ask if you cats and kitties can guess as to the diagnosis from the audio. I bet you can tell me something. http://homeofthegnome.net/07162021/shmurdge-1.wav
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Jul 18, 2021 18:16
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Ambrose Burnside posted:somebody on craigslist is giving away a heathkit oscilloscope in working order and i'm currently shamefully attempting to undercut whoever dibsed it with offers of money b/c oh man, man oh man oh man turns out this merely “turns on”, the seller hasn’t actually tried to use it for scopey things. hmmm gonna snag it regardless because i can get it for “nearly free”, worst case scenario i resell it as a decor item for a profit- that said, i actually want/need a scope, so my intent is to refurbish n recalibrate it if possible on that note, what should i be looking for to assess its condition? the dude already powered it on so any damage from that is done; aside from capacitors going kaput, what other failure modes or issues are likely for something ~75 years old like this? Any particular problems bad enough to make it not worth fixing? I’d guess the display tube being hosed could be a serious issue if i can’t source a replacement, anything else stand out?
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Jul 18, 2021 18:56
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babyeatingpsychopath posted:Isn't this how "HDR" mode works on your phone's camera? It just takes a low-exposure, normal-exposure, and high-exposure and composites them together with magic? Basically, the issue is as always going to be merging the signals together smoothly, but at least in the audio case the two samples can probably be made coherent, whereas camera HDR has to compensate for movement in the pictures. petit choux posted:Hey! I'm going to record a little bit of audio from this f'ed up drum machine I got from Goodwill, and ask if you cats and kitties can guess as to the diagnosis from the audio. I bet you can tell me something. Yeah it sounds terrible. There's a positive square-shaped pulse that repeats regardless of the signal level, the period of that might give a clue as to what's going on. Shot in the dark: DAC reference smoothing capacitor is bad. Pretty much impossible to diagnose without knowing how the thing works, at least what chips are involved.
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Jul 18, 2021 18:58
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Ambrose Burnside posted:turns out this merely “turns on”, the seller hasn’t actually tried to use it for scopey things. hmmm It's a heathkit, so there are probably instructions online for it with all the values needed for various components. As long as the CRT still works, you can probably start replacing things until it works. It's a kit after all.
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Jul 18, 2021 19:06
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Ambrose Burnside posted:turns out this merely “turns on”, the seller hasn’t actually tried to use it for scopey things. hmmm What are you planning to use it for? A scope that old is neat to mess around with, but not super useful for modern electronics work. It is about as simple as you can get: one input channel, your basic scale and timebase controls, and a simple rising/falling trigger. It is really meant for things like repairing TVs and radios in the 1960s -- you'd use it to look at the frequency of oscillators and tune them and so on. You won't be able to do much with digital electronics. However, I remember that you mostly do those old-fashioned decoherers and things so this might work fine for those purposes. Basically, you need to be working with regular repeating signals for an analog scope to be useful, and with only one channel you'll be limited to studying simple devices. Calibrating a scope requires another working calibrated oscilloscope. If the display is not working, you definitely won't be able to source a replacement display tube other than buying another working model and swapping it out. Sagebrush fucked around with this message at 19:10 on Jul 18, 2021 |
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Jul 18, 2021 19:07
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I speculate Ambrose is looking for it as a collectable/antique, vice for use towards a practical goal.
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Jul 18, 2021 19:18
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Assuming that's a tube scope (e: other than the CRT, like the amps/trigger/sweep gen are all tubes too and all running at supplies that'd turn anything modern and its owner to charcoal), I'd hit the input with a multimeter in volt mode first just to make sure nothing is shorted to some hundreds-of-volts supply. After that, hit it with some square waves from any audio source of known frequency you can find. This can be a phone playing "10 minutes of 1kHz square wave" from Youtube or some function generator app or whatever. If it triggers on that, rising and falling edge, and displays it as approximately square with approximately the right width, and it stops displaying it beyond a certain trigger setting, you're probably good. You can also hit it with some known voltages but really, if it triggers and the time base is kind of accurate, you're 90% of the way there. If it doesn't then you have a problem you'd need a second scope to debug. Stack Machine fucked around with this message at 19:47 on Jul 18, 2021 |
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Jul 18, 2021 19:43
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Stack Machine posted:After that, hit it with some square waves from any audio source of known frequency you can find. This can be a phone playing "10 minutes of 1kHz square wave" from Youtube or some function generator app or whatever. I use this site with my phone all the time as an ersatz signal generator: https://www.szynalski.com/tone-generator/ Works pretty well actually
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Jul 18, 2021 20:33
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longview posted:Basically, the issue is as always going to be merging the signals together smoothly, but at least in the audio case the two samples can probably be made coherent, whereas camera HDR has to compensate for movement in the pictures. Well I'm pretty much bound to open it up now because I always wanted one of these. Kind of suspecting this isn't going to be something visible. When I do I'll take a snapshot.
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Jul 18, 2021 20:45
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Sagebrush posted:What are you planning to use it for? A scope that old is neat to mess around with, but not super useful for modern electronics work. It is about as simple as you can get: one input channel, your basic scale and timebase controls, and a simple rising/falling trigger. It is really meant for things like repairing TVs and radios in the 1960s -- you'd use it to look at the frequency of oscillators and tune them and so on. You won't be able to do much with digital electronics. However, I remember that you mostly do those old-fashioned decoherers and things so this might work fine for those purposes. Basically, you need to be working with regular repeating signals for an analog scope to be useful, and with only one channel you'll be limited to studying simple devices. Ideally I would like to actually use it as a scope, but yeah, every time I've actually had need of a scope an analog would have sufficed b/c of the retro bent to my hobby tinkering. if that's not viable or the thing is broken I'm not too put out b/c it's a lovely shop decor item, or else i can resell it to someone else and make a bit of money Ambrose Burnside fucked around with this message at 23:09 on Jul 18, 2021 |
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Jul 18, 2021 23:02
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In terms of scope usefulness, it's literally worse in every way to an Arduino with that serial scope sketch on it Turn it into a clock or something
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Jul 18, 2021 23:06
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ante posted:In terms of scope usefulness, it's literally worse in every way to an Arduino with that serial scope sketch on it welp, good to know. fortunately thats the other thing im interested in, doing something cool with the display tube. don't wanna get my hopes up until i see what condition it's in tho
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Jul 18, 2021 23:10
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If the crt still works, you can maybe hack it into one of those things that displays images with music.
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Jul 18, 2021 23:42
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ante posted:In terms of scope usefulness, it's literally worse in every way to an Arduino with that serial scope sketch on it Analog scopes are awesome for rad x-y scope demos https://www.youtube.com/watch?v=s1eNjUgaB-g
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Jul 18, 2021 23:47
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Ambrose Burnside posted:welp, good to know. fortunately thats the other thing im interested in, doing something cool with the display tube. don't wanna get my hopes up until i see what condition it's in tho Oh wow, I remembered this video on an old scope and was going to dig it up because it should be similar for any of them, but this might actually be the same model that he's restoring here: https://www.youtube.com/watch?v=RF7CRqb54XM
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Jul 18, 2021 23:49
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It has a 5MHz bandwidth from what I could find online at least? I'm less pessimistic that you could find some sort of use for it given your interests, but the only circuits in modern electronics that live in that below-RF, above-audio range that this could serve and some microcontroller sampling at 10kHz couldn't (that used to be occupied by things like AM radio and analog video) are switched-mode power supplies. Edit: I've been looking at the schematic because I was curious, and the biggest limitation (other than that it's a single-channel analog scope) seems to be that the X, Y, and Z (intensity) axes are all AC-coupled only and there is no trigger in the modern sense, just a sync to zero (really avg value, since there's no DC component) crossings. Compare to something like the Tek 535 to see just how wide a gap there was between low-end and high-end designs of the time. 
Stack Machine fucked around with this message at 00:46 on Jul 19, 2021 |
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Jul 19, 2021 00:01
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Ambrose Burnside posted:somebody on craigslist is giving away a heathkit oscilloscope in working order and i'm currently shamefully attempting to undercut whoever dibsed it with offers of money b/c oh man, man oh man oh man Ehhh, the old tube scopes aren't that useful anymore. They typically only go up to 5 MHz or so. Heathkit wasn't that good of a brand anyway. See that "kit" at the end of the name? Heath sold kits to end users and you'd have to build it yourself. In other words, Heathkit was entry level. That being said, so many end users had the documentation included with their kits that Heathkit is one of the more well documented brands still today. Has it been recapped? Replace all of the old paper wax capacitors with films and the old electrolytics for new ones. A scope that old will just use twisted pair for probing. If you want to use modern probes on it, use a BNC to dual banana adapter. Alternately, remove the banana jacks and put BNCs on. If you want an old tube scope, attend a local hamfest. You can get one for $5 unrestored, or free if you wait at the dumpster when it's over. Honestly, I only ever use my old tube scope when I need XY mode for restoring old test gear, and that's only because I still haven't figured out how to use my modern Rigol scope's XY mode to look like an old tube scope. Protip: you can make a new graticule from a printable overhead projector transparency film, then drawing the hash marks/grid in Excel. Excel will even let you specify cell widths to the inch.
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Jul 19, 2021 02:13
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Is there a place to get extra-wide breadboards? I have a dev board that's too fat for a regular board (I can only hit one set of pins when I place it over the divider). Or, is there a way to hook this thing up that isn't pulling the power rails off two boards and sticking them together (or grabbing the pins directly)
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Jul 19, 2021 02:16
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Place it across two breadboards I guess?
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Jul 19, 2021 02:29
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Ribbon cables, maybe? Like if each row of pins is 20 pins or fewer you can use up to half the conductors in an old IDE cable for each and stick some male header pins into your breadboard.
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Jul 19, 2021 02:31
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