Selecting frequencies and wave forms from Function Generator for my amp. - Page 1
Forum section: electronics / beginners

author: vinlove
author_rank: Frequent Contributor
timestamp: September 05, 2016, 06:38:50 pm
content: Hi AllI am about to trying to fix my guitar amp which developed buzzing and white noise recently.It is a small transistor amp about 15W output.I have an Oscilloscope and Function Generator, which I would like to connect to the amp, and seewhat kind of signal it will sweep out from it.My question is when you connect Function Generator to the Amp's input, how do you select what frequency is the best?My Function Generator has output frequency ranges from 2 Hz - 20 Mhz.My Oscilloscope is IWATSU 20 Mhz one.I would like to know what is the general principle to select which frequency is the best for different appliances to look at.For example, Radios, Amps and Power Supplies ... I mean how do you select what frequencies for all these different devices whenconnecting them to the Function Generator and then output to Oscilloscope?And also what wave forms are used for which devices and what purposes?TIA

author: janoc
author_rank: Super Contributor
timestamp: September 05, 2016, 10:15:34 pm
content: Hi,It is great that you have a scope and a function generator, but put them away for a bit still. You need to start troubleshooting the problem from the basics. Pushing random frequencies through an amplifier will not really give you a clue about what is wrong.Are you sure the noise is actually originating in your amplifier? Disconnect everything from the inputs and short them. Is the noise gone?- YES! Problem isnotin your amplifier. Check the signal source wiring for broken/missing shielding, broken wires, ground loops, etc.- Nope ... OK, the amp is the problem, indeed.Now, what sort of noise are we talking about? White noise is essentially hissing sound, but you mention also buzzing. Is it a low frequency mains hum? Or more a tone? What is its pitch/frequency? High? Low?Hum and such are often coming from a poorly filtered power supply. Check the voltages of the power rails - are they in spec? You can find the nominal voltages in a schematics for your amp. If you don't have it, then they are often printed on the PCB as well. Usually the voltages tend to be around +-12-15V for any opamps (preamplifier, tone control, guitar effects, etc), 3.3-5V if there is any digital logic and then a higher voltage supply for the power amplifier - that can be anywhere between 12V to more than 100V, depending on the amp.Since you have a scope, now it would be a good moment to power it up and check the power rails. Are they clean? You should see only a straight line on your scope, perhaps with very tiny "wiggles" or "fuzz" at worst. If the power rails are showing some sort of signal superimposed, that's not good. Clean power supply is a must, so that would need to be fixed first.If the rails are clean, then it would be good to actually probe the output of the amp with the scope and post a picture of the signal here, so that we can have a clue what the noise actually looks like.It would also help to post the model of your amp, otherwise nobody would be able to give you an advice beyond very generic stuff.Re generator - a function generator is useful as an artificial signal source when repairing an amp and to roughly check the frequency response by sending different frequencies into the amplifier and measuring the output. However, if you want to do some serious measurements then a function generator is not really the best instrument - they usually don't generate low frequencies (audio is between 0-22kHz, you don't need anything above that ...) and the waveforms are not very accurate, producing distortion. A specialized precision low frequency generators and various noise sources are used instead - sine wave and then white and pink noise generators.However, in order to do basic troubleshooting and repair of a malfunctioning amp you don't need any of that - simply connect a CD player or your phone to the input and play some decent quality music. Human ear is fairly good at detecting distortions and noises already. You can even use your sound card to generate a fairly decent test signal, if necessary.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 06, 2016, 11:28:11 am
content: Thanks for your info and advice. My focus here is not fixing the amp. The amp is cheap practice amp costing maybe about 10 usd. It works ok, but at times, it squeals with hiss and other times it gives white noise, when nothing is plugged in.My focus in this thread was using Function Generator and Oscilloscope as tools for investigating and fixing these problems with any electric appliances we use. - How do you select the ideal frequencies for injecting into the devices in need of repair, and what waves forms would you choose, and why?My Function Generator has freq. range of 2 Hz - 2 Mhz, so it can produce low freq. OK. But I would like to know what is the principle of choosing certain freq. and Wave forms generated by FG for certain devices.You are quite right in that low freq settings of my FG seem very tricky to catch the output from my Oscilloscope. The low freq. wave becomes very small on the screen, and it looks like they are pulses rather than waves in all settings of my Oscilloscope's time base and voltages. It could be due to my FG has faults of the Amplitude button is broken off and cannot change the amplitude output. Would you agree?cheers.

author: ebclr
author_rank: Super Contributor
timestamp: September 06, 2016, 01:38:40 pm
content: That's is not a easy question. Depends on what you are looking for, you will chose one frequency or another or even a sweep. Same for waveform. But for fixing purposes the starting point is the standard 1Kz sinewave.

author: janoc
author_rank: Super Contributor
timestamp: September 06, 2016, 04:11:59 pm
content: Quote from: vinlove on September 06, 2016, 11:28:11 amOKQuote from: vinlove on September 06, 2016, 11:28:11 amThat's difficult to say, usually when one is troubleshooting an amplifier, the generator is not something that gets used that often. In general, you use it as a signal source to check whether the amp even works at all, check for distortion/clipping (but harmonic distortion below few % is very difficult to see on an oscilloscope) and measure the frequency response. It is a tool used for fairly specific tasks, not really a general purpose troubleshooting instrument in this case.Perhaps if you saywhywould you want to inject those frequencies (to check/measure what?), someone could answer you better.Quote from: vinlove on September 06, 2016, 11:28:11 amThat could have various reasons. The knob not working could well be one. However, that wouldn't explain why are you seeing pulses instead of a continuous signal (you have increased the time base on the scope to match, right?). That sounds more like a cheap generator that is unable to work at such low frequency properly. Post a model of your FG.

author: edavid
author_rank: Super Contributor
timestamp: September 06, 2016, 04:42:22 pm
content: Quote from: vinlove on September 06, 2016, 11:28:11 amWhen you are repairing an audio amp, obviously you use audio frequencies. What is the specified frequency response of your amp?The waveform chosen depends on the specific problem with the amp.QuoteIt sounds like you are using a frequency that is too low.QuoteNo.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 06, 2016, 04:58:27 pm
content: Quote from: janoc on September 06, 2016, 04:11:59 pmI thought Function Generator, Audio Generator, Signal Generator and RF Generator were all same thing under different names. When I connect a loud speaker to the FG, it produces audio which varies as I change the frequency of the FG.I also thought they are all signal source to be injected to audio and radio devices for diagnosing problems, and also to see how well they are processing the signals into the output.Quote from: janoc on September 06, 2016, 04:11:59 pmMy FG is very old but good one, maybe from 1990s, but it is well built, and has ranges of 2 hz - 20 Mhz. The model is Thandar TG2001.My scope is IWATSU 5702.It is just it has broken Amplitude knob, which cannot adjust the output voltage of the FG. So it is always outputing at 4V. When it goes down to the very low frequency, the display of Oscilloscope becomes very small and like dots, even I try to increase the Vertical value, it does not increase. I was thinking maybe I had to increase the output amplitude of the FG to compensate it, but it cannot be done due to broken knob, which is disconnected from the variable resistor in the unit.I get good waves on the scope from the TG2001 say 10 khz - 20 Mhz range. Below 10 Khz, it is not displaying very well at all. Well it displays, but it cannot be adjusted at the scope by the Vertical and Time base and Trigger controls.

author: janoc
author_rank: Super Contributor
timestamp: September 06, 2016, 06:59:18 pm
content: Quote from: vinlove on September 06, 2016, 04:58:27 pmErrr ... nope. All of these share only the principle that they contain some sort of oscillator or synthesizer (DDS, micro + DAC, ...), but that's all they have in common.Function gen - usually from audio frequencies up to 20MHz or so, generates at least sine, triangle and square wave. Some can generate arbitrary waveforms too. These are used for general checks and testing, high accuracy and low distortion are not really the key parameters here.Audio generator - something that works in the audio frequency range (0-22kHz). Typically generates only sine wave, often very low distortion and at precise levels. These are used to measure distortion of amplifiers, for example.RF generator - usually only sine wave and generates high frequencies - typically from ~1MHz up, some cover bands up to multiple GHz, depending on type. These are used to test and measure wireless/radio gear.Signal generator - general name for any of the aboveQuote from: vinlove on September 06, 2016, 04:58:27 pmWell, yes and no. E.g. something like thishttp://www.keysight.com/en/pd-1000002817%3Aepsg%3Apro-pn-E4432B/esg-d-series-digital-rf-signal-generator-3-ghzwould be completely useless to troubleshoot anything audio related (and enormous waste of money), because that generator has lowest possible frequency of 250kHz. That's a typical RF generator.Conversely, for radio gear an audio generator or even function generator (depends on model & frequencies needed) would be useless, because it wouldn't cover the frequencies used (and lack other functionality).Quote from: vinlove on September 06, 2016, 04:58:27 pmThat's certainly not due to a broken knob. If the FG is outputting 4V, that you have to be able to see on the scope. Oscilloscopes can display signals in millivolt ranges. Furthermore, a line input to your amp is about 1.4V RMS (or 1V RMS for pro gear) maximum, if you used 4V as input signal you could blow something. At the very least it would be overdriven and badly clipping.That sounds like your generator is failing at those low frequencies - perhaps problem of the range switches or something more involved.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 06, 2016, 07:49:18 pm
content: Quote from: vinlove on September 06, 2016, 11:28:11 amIt depends entirely on what you're trying to do, and what you suspect a problem to be.I do a fair amount of work on old radios, mostly from the vacuum tube era, with various bits malfunctioning. If, for example, a receiver is not working, I might inject an audio signal at the place where audio is demodulated, to see if it's showing up at the speaker. The audio signal could be virtually anything near the middle of the audio spectrum, but I'd probably choose something around 500Hz or so out of habit. If this works, it tells me the audio amplification is working, so the problem is earlier in the signal chain. If it doesn't work, it tells me the audio amplifier is not doing its job, and I need to figure out why. If the audio amplifier is working, then I might inject a modulated RF signal at the radios IF (intermediate frequency), which will depend on the radio, but might be around 455 kHz. If that works, I might try backing up to an earlier stage. In each case, I'm trying to do a "divide and conquer" test, to determine whether the problem is before or after the point at which I inject the signal. The signal frequency, modulation, and voltage which I inject is determined by what type of signal I would expect to see at that particular stage if the radio were working properly. That's a "back to front" approach, checking the last stages first, and working your way toward the front end antenna input.Another approach which sometimes works well, depending on the radio and the observed problem, is the "front to back" approach. You inject a very weak modulated RF signal at the antenna input, simulating what a transmitting station would produce. Then use the oscilloscope (or perhaps just an RF voltmeter) to observe the signal through the various stages, to see how far it gets before things go wrong.I haven't diagnosed and fixed guitar amplifiers. It sounds like your amp is working to amplify the input signal, but it is also introducing unwanted noise in the process? If the noise is audible with no input, I might just leave the input disconnected, and probe with a scope to see which stage is getting the noise.Perhaps I might use a variation on the "front to back" strategy that I used for radios. For a guitar amp, I'd inject a very low level pure sine wave audio signal (500 Hz or so, but could probably use any pure signal within the range of the amplifier's capability, at the lowest level that the amplifier can amplify to make audible) at the input jack, listen to the output at the speaker, and see if it sounds distorted or noisy. I'd look at the waveform at both the speaker and at the input, and if there's an audible difference, I'd expect it to be visible on the scope. Once I got a picture of what the noise looks like on the scope, I'd go through the various stages of the amplification, to see where the noise was being introduced. I'd also check things like power rails to see if the noise was coming from there.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 06, 2016, 08:40:46 pm
content: Great points and posts. Thanks.I measured output voltage of the FG at different frequency settings (100Khz, 10Khz, 1Kz, 50Hz) on my DMM, and they are all same 4.37 V.But strange, from about 10 Khz downwards, the output waves on the scope decrease drastically. No matter how I try to adjust to bring them up into good sizes using the Vertical and Horiziontal and Sweep controls in the Scope, they cannot past very small ripples. I can see the small sign waves in the ripples.Another thing I note is that last night I made up a BNC to BNC cable to connect the FG and Scope. For that I have used about 50cm length of Satellite Dish cable instead of the 50 ohms coaxial cable. I do not know what the impedance of the Satellite dish cable is, and wonder if it could be possible source of the anomalies?I will try to take some photos of the FG and Scope, or even video and upload to Youtube, when I get chance.cheers.

author: janoc
author_rank: Super Contributor
timestamp: September 06, 2016, 08:57:26 pm
content: Quote from: vinlove on September 06, 2016, 08:40:46 pmThat's definitely not normal. I am not familiar with your FG, but it seems it has some issues there. If the signal "dies out" even if you aren't touching the volume/level knob and only changing frequency, it could be some output buffer which isn't working right. I would open the generator, clean all the pots and switches (there could be a lot of dust and grime accumulated!). Then check things like electrolytic capacitors - they could be dried out, especially on an instrument of this age. Further troubleshooting would depend on the architecture of your generator - try to get a service manual for it.Quote from: vinlove on September 06, 2016, 08:40:46 pmAt the audio frequencies you can use even a piece of speaker wire or a guitar string, if you really need to, to connect the generator. It wouldn't make any difference. The only benefit of the coax would be the shielding, so the scope isn't going to pick up external noise. However, any shielded cable (or an oscilloscope probe) would give you that.Impedance matching starts to get important when you are working with radio frequencies, let's say some 500kHz and up. There the reflections from an incorrectly terminated cable will start to have an impact on what you see on the scope. However, then you would need the scope to be 50 Ohm terminated too - yours likely isn't (most scopes have 1MOhm input impedance by default), unless you use an external terminator.In short, don't worry about the cable and impedance. At audio frequencies it really doesn't matter.Quote from: vinlove on September 06, 2016, 08:40:46 pmThat would be great.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 06, 2016, 10:44:35 pm
content: Quote from: vinlove on September 06, 2016, 08:40:46 pmIf the output when measured by a DMM is constant as you change frequency, but the output looks diminished at low frequency on the scope, that suggests a scope problem. A problem in the FG should show up on the DMM as well as on the scope. Do you have the scope set to DC coupling? (you should) Can you use the scope to probe a AA battery and have your scope more-or-less agree with your DMM regarding the voltage measured?

author: vinlove
author_rank: Frequent Contributor
timestamp: September 07, 2016, 10:00:42 am
content: Quote from: AG6QR on September 06, 2016, 10:44:35 pmI am still waiting for the new scope probes for my IWATSU ordered from Chinese seller. I got the IWATSU only a few days ago, and it arrived with no probes.Hence why I made up a BNC to BNC cable using Satellite dish cable cut offs to test the FG and the Scope at the same time.Before the IWATSU, I had again an old KIKUSHI 538A. I still have it on the desk above the IWATSU with the original probes, but it doesnt use BNC socket on the probe. It is just a old speaker terminal type probe end.When I hook up the FG into the KIKUSHI 538A Scope, it does more or less the same as with the IWATSU. It displays the FG freq. output very well above 100 Khz, but below that, the display diminishes into a thin line of ripples with the sine waves still in it.I wonder if this Youtube video has any significant answers to my problems.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 07, 2016, 02:59:18 pm
content: Quote from: vinlove on September 07, 2016, 10:00:42 amThat's a good video on why a function generator's output voltage setting often has a mismatch with the voltage as measured by a scope. The scope probe setting may be mismatched with what the scope is set to expect, and the source impedance of the generator may be mismatched with the load.But neither of those problems would cause the issues you're reporting, where high frequencies can be measured correctly, but low frequencies cannot. If that's what you're seeing, you've got some kind of intentional or unintentional high-pass filter in the system. The one that immediately comes to mind would be the one in the AC coupling circuit on the scope, but it could be something else.Take the function generator out of the equation by just trying to measure the voltage of a battery, as a start.Compare your scope's readings with the readings of your DMM.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 07, 2016, 10:16:27 pm
content: Quote from: AG6QR on September 07, 2016, 02:59:18 pmYou are right. The Iwatsu Oscilloscope has fault in the 2nd channel. The 1st channel seems ok, but the 2nd channel does not read DC Voltage from a battery. Nothing happens on display. I have been using the faulty 2nd channel all the time.It also seems have problems with trigger and time base too. The display of the signals are not consistent on both channels.I will try to sort the scope out (try to return it, and get something else in full working order), and come back to see where I am.Thank you for your info and advice. I have never imagined or doubted the Iwatsu 5702 scope would be faulty.But it is clearly showing the fault. Maybe these old electronic gears are not reliable anymore without proper servicing and repairing them time to time, or even all the time.Time to go and get the new DSOs like RIGOL, Agilent, OWON or even Teks?

author: janoc
author_rank: Super Contributor
timestamp: September 07, 2016, 11:13:53 pm
content: Quote from: vinlove on September 07, 2016, 10:16:27 pmAre you sure you don't have the channel set to AC coupling? There is a switch on your scope for that - a three position vertical switch for each channel, labeled DC-GND-AC. Make sure the lever is in the DC position (down). Also make sure the V-Mode switch is in the CH2 or DUAL position if you want to see the channel 2. Setting it to CH1 will turn the channel 2 off and you will see at best some cross-talk noise, while CH1 seems to work fine.However, even on AC coupling the scope should be able to display a signal of few kHz unless it is defective.Quote from: vinlove on September 07, 2016, 10:16:27 pmThat scope is 30 years old at least. If nobody has ever cleaned it, there could be poor contact in the switches and pots from the accumulated grime and dust. The capacitors could be dead, connectors could be broken, etc. That's par for the course for such old gear. It needs maintenance as anything else.Quote from: vinlove on September 07, 2016, 10:16:27 pmIf you have the money, sure. Teks are expensive junk today, they don't have a competitive model for some years now. Agilent is expensive ($1000-$2000 at least for an entry level scope but good), Rigol is the king of the low end, entry level stuff today, with the $400-$500 getting you a great four channel 100MHz scope. Owon is cheap and buggy junk, not really worth the money, IMO. If you want to buy a scope, there are other threads on the forum about the pros and cons of the individual models.However, I think your problem is more that you are missing some theoretical and practical basics about how to troubleshoot things - buying a new scope will not help you there, really.For audio gear your old Iwatsu is plenty enough. In fact, the simplicity of the instrument is an advantage for you while you are learning, because it doesn't hide anything from you, unlike some modern digital instruments - e.g. no chance for a signal to alias or averaging "cleaning up" a noisy signal, making you think everything is fine. The modern digital scopes are insanely powerful tools, but the learning curve is quite a bit higher than with an old style "button & knobs" analog scope.I am pretty sure the bad channel on your Iwatsu can be easily fixed. Most of the scope is working since you have a one good channel - so the screen, power supply, time base, triggering are all in working order. The problem is most likely in the input for the channel 2 - which you could actually troubleshoot using the working channel of the scope! Just be careful if you decide to poke around it - as with any CRT equipment, there are high voltages inside. My tip is that simply some switches for the channel 2 are stuck/not making contact, not letting the signal through correctly.BTW, since your other scope doesn't display the low frequency signal from the FG neither, I think that the generator has a problem as well. It is fairly unlikely that two scopes would be having the same sort of fault. But make sure you can get your scope show a flat trace matching a battery voltage first, as suggested by AG6QR above, before attempting to troubleshoot the FG. If you need a test signal, your Iwatsu has the probe calibration output. That should display a square wave signal of 0.3V at about 1kHz. It is labeled Cal Out and is next to the external trigger connector on the right side.Don't worry about the lack of proper probes - at audio frequencies or DC it really does not matter.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 08, 2016, 05:51:50 am
content: Quote from: janoc on September 07, 2016, 11:13:53 pmI agree with absolutely everything janoc has said in the above post, but I have one point to note about this sentence. In an earlier post, vinlove says:QuoteSo we have two instruments -- the FG and the multimeter, agreeing that the FG's voltage is unchanged at low frequencies. And we have two other instruments: the two scopes, saying that the voltage is lower at low frequencies. I'm not sure what to believe, but if the FG were actually reducing its voltage, it seems very unlikely that the DMM would fail to pick that up. It also seems unlikely that both scopes would be wrong in the same way. I'd want to double check all the measurements, and also verify that the scopes are capable of measuring DC, before I'd be confident on which instrument(s) are giving misleading readings.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 08, 2016, 09:10:10 am
content: I have been working only with my DMMs before, because that's all I needed for what I do. But recently I decided to learn Oscilloscope and Function Generators, so thats why I got them. So yes, I am a truly newbie. Yes, it is set to GND to start, but flicked to DC to read the DC voltages.It seems clear that the IWATSU scope Channel 2 cannot read DC voltages. I just made a little video and am uploading to Youtube. Will post the link later.For learners like me, I would have thought cheap old reliable scope like Iwatsu 5702 would do. It was cheap and old, but reliable? Not sure. No I am not thinking of opening it up to fix it unless I definitely decided to keep it. It can just be returned for full refund.I think scopes like the Rigol 1102E for about £200 would be ample more than ideal for me. Anything higher spec. than that would be redundant.For FG issues, I am thinking of getting another FG maybe ranges between 1 - 150 KHz, and see how it comes up on the scopes here. I got another old FG the other day to test out the scopes and also compare with TG2001, but that one was dead on arrival. No output at all.

author: alanb
author_rank: Frequent Contributor
timestamp: September 08, 2016, 10:16:36 am
content: Quote from: vinlove on September 06, 2016, 08:40:46 pmAre you sure that you are not measuring the DC offset of the FG? The TTI Function generators have a variable DC offset that is added to the AC signal. If your DVM is on DC volts it would show this regardless of the amplitude of the AC signal.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 08, 2016, 10:27:47 am
content: Quote from: alanb on September 08, 2016, 10:16:36 amNo. DC offset on the FG is not even switched on. Now I am using DC volts output from a regulated Power Supply to check the ch2 and ch1 of the scope, and the result is same.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 08, 2016, 10:30:15 am
content: Just uploaded a quick video to show that the IWATSU SS-5702 ch2 is faulty.

author: janoc
author_rank: Super Contributor
timestamp: September 08, 2016, 01:30:22 pm
content: Quote from: vinlove on September 08, 2016, 10:27:47 amNo, you have misunderstood. If yourmeteris set to a DC range, it will measure only a DC offset and not the actual AC amplitude. Your FG could well be faulty (it is certainly suspect) and be adding a 4V offset regardless of the position of the controls. In the worst case it could be outputting 4V DC and no AC signal at all (probably not your case, though - you have reported seeing some signals before on the scope) and your meter would be happily reading 4V. So make sure the meter is set to AC voltage in order to eliminate this possibility.Quote from: vinlove on September 08, 2016, 10:27:47 amThat scope is fairly old already, rather add a bit of money and go for the newer 1054Z series. It is a much better scope. I had the older Rigol DS1052e (50MHz version of the 1102E) and now have the DS1054 and there is no comparison - and the price is only about 100 bucks more. It really doesn't make much sense to buy the old series.

author: janoc
author_rank: Super Contributor
timestamp: September 08, 2016, 01:46:05 pm
content: Quote from: vinlove on September 08, 2016, 10:30:15 amOK, that scope really does have an issue with the CH2. Seems to be dead like a dodo. A wonderful troubleshooting opportunity, though! Those scopes are fairly well documented and simple, so could be fun to try to fix it. Especially since you are quite well equipped for basic signal tracing.That switch on the right side you were confused about is the trigger source - which channel should the scope trigger from or the external trigger input (the BNC on the right side) (that's the EXT position).The switch above it is the trigger coupling - whether the trigger should be DC coupled (most commonly used), AC (ignores any DC offset) or TV - likely can trigger on analog TV sync signals too. It certainly will not output to a TVYou have the controls described here:http://www.ece.rice.edu/~jdw/data_sheets/ss5702.pdfAnd here is the service manual:http://www.radiopistoia.com/pdf/iwatsu.pdf

author: vinlove
author_rank: Frequent Contributor
timestamp: September 08, 2016, 06:39:12 pm
content: Thanks for your info about he Xmode - I now know how to use it and what it is for.I have uploaded 2nd video for testing the TG2001 with the SS-5702.I was using a cheap Chinese made action cam on my baseball cap, and the frame is jumpy and audio quality is terrible. Also the video has stopped abruptly due to wrong cyclic settings in the cam. Sorry about that.Please point out what I am doing wrong, or other things to try out in order to trouble shoot these devices. Thanks.I am looking to get another FG for more proper comparative testing, and also a more modern scope in the near future.

author: janoc
author_rank: Super Contributor
timestamp: September 08, 2016, 11:29:56 pm
content: Quote from: vinlove on September 08, 2016, 06:39:12 pmWelcome.Quote from: vinlove on September 08, 2016, 06:39:12 pmA much better way is to simply stick the camera on a tripod. That will at least not make your viewers motion sickQuote from: vinlove on September 08, 2016, 06:39:12 pmWell, first thing - why are you running the channels on AC coupling? You should be measuring on DC, even when measuring a sine wave signal!The AC setting basically connects a capacitor in series with your signal and that will act as a high pass filter, attenuating your low frequency signal a lot. It is useful to eliminate DC offset, but normally you measure on the DC setting (the AC-GND-DC switch in the down position!). The documentation says that the -3dB point with AC coupling is at 4Hz, so my guess is that the AC position isn't working completely OK - maybe dirty switch contacts or a bad capacitor with much lower capacitance than it should have, thus attentuating even a 100kHz signal already. That would also explain why your square wave looked only like spikes up and down - that is exactly what you would get if you high pass filtered it.I think you may have got confused because of the previous discussion about the multimeter. On a meter the AC/DC setting determines which part of the signal is getting measured - only AC or only DC (unless you have a dual display meter, like some higher end Brymens).However! On a scope the AC/DC label on the vertical axes is actually a misnomer. DC means the signal is connected directly to the input amplifier and the scope will showbothAC and DC components of the signal, as they are. The AC setting connects that capacitor and thus makes the scope show only the AC component of the signal, blocking any DC (or low frequency) content. That's why one uses the DC setting most of the time, even for AC signals - you want to see the signal as it is, without artificial filtering or deformation by the instrument.There is no point in turning the time base knob - the scope isn't triggering because the level is too low. You must first adjust the level knob (on the left), so that you have a signal, then adjust the trigger level to get the scope to trigger on it (you get a stable waveform). THEN you can play with the time base knob to "stretch" or "compress" the waveform in the horizontal axis.The correct settings for conducting measurements are actually described in the manual as well.I recommend that you watch this video by Alan Wolke W2AEW (he is on this forum as well):It is long, but excellent introduction into how scopes work and how to use them. He also explains some capabilities of digital scopes vs. analog ones.BTW, you have three scopes there - can't you compare the signal between them? Should give you a clue which instrument is fibbing.

author: w2aew
author_rank: Super Contributor
timestamp: September 09, 2016, 02:16:56 am
content: Here is a more recent "how to use an oscilloscope" video, that is geared towards looking at signals inside a working AM radio. Please pardon the music/radio in the background.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 09, 2016, 07:57:19 am
content: Very good explanation Janoc. Thank you.I have tried other old scopes in there, but result is more or less the same. As you said, maybe it is because I have been using the AC settings.I will try again with DC settings, and see how it will show.I tried with the DC before, but many times, the signals in the display of the scope kept disappearing, and I had to pull it down or up using vertical control.And on some freq. settings, nothing was showing in the display, so I switched it back to AC. In AC settings, at least signals are present most of the time.I will also watch the Youtube video, and try to learn more about the scopes. Thanks.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 09, 2016, 07:59:09 am
content: Quote from: w2aew on September 09, 2016, 02:16:56 amThank you. It seems a great video for Scope Fundamentals.

author: janoc
author_rank: Super Contributor
timestamp: September 09, 2016, 06:34:40 pm
content: Quote from: vinlove on September 09, 2016, 07:57:19 amThat is actually completely normal - if the signal has a DC voltage superimposed on it (= DC offset), the trace will move up and down to match if you are in DC mode (same thing as when you have tested the scope with your power supply). Make sure the DC offset knob on your FG is set to zero if you don't want to have it there.In AC mode the DC offset is blocked - at the expense of some potential distortion/attenuation and you not being aware that the signal actually has a DC offset. Which could be in many cases important - e.g. 50V DC superimposed on your signal due to some fault could blow out an input of the following amplifier. Also DC offset on the amplifier output is really bad news for loudspeakers because the speaker coils will burn out. So it is typically something you want to know about.Quote from: w2aew on September 09, 2016, 02:16:56 amAh, mention the devil ...I liked the older video better, because there has been an explanation of some things I didn't know neither when I have got my scope - like the probe compensation or how to use XY mode to measure phase. But both are good material, your channel is a real treasure trove, Alan.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 10, 2016, 03:40:57 pm
content: I also have the Thandar TG2001 function generator and although waveforms start to get somewhat distorted particularly above 10MHz, it should be producing very clean triangle and reasonably nice square waves on the 200KHz setting. You were getting something resembling a square wave on the triangle setting and the waveform produced on the square wave setting was unrecognizable. I have attached some photos to illustrate what the output should look like on the scope for comparison. These were taken with the function generator set to 200KHz and the scope input at 1v/div and timebase at 5uS/div. The waveform display you would get on the 2KHz setting with the timebase set to 0.5ms/div should be identical. I had the amplitude set to 4.06v which is quite close to your 4.37v. Although I was using a 50ohm RG58 co-ax cable rather than a 75ohm satellite co-ax cable to directly connect the two instruments as per your video, I don't think this kind of difference can be accounted for simply by impedance mis-match or abscence of terminators. The wave forms (other than sine) shown in the video were simply not correct and would unfortunately seem to indicate that your function generator is not only malfunctioning when trying to display wave forms at 2KHz and below, but is also not displaying square and triangle wave forms correctly. Sorry to be the bearer of bad news, but it looks it will definitely require repair.PS, while what has been said about DC offset is essentially correct, if you have not depressed the Offset button and set an offset using the DC offset control, then the DC offset should be zero. The trace should not move from the center position on the oscilloscope screen regardless of whether DC or AC coupling is being used. If it does, then this means that there is a DC element present on the output when there is not supposed to be which might also suggest a problem with the generator. The DC offset can be displayed on the LCD by switching the display mode switch to the Offset position and depressing the offset button. It should change as you rotate the DC offset control. On the oscilloscope, the trace will move up or and down vertically as the DC offset adjusted. As soon as you turn off the Offset button, the DC Offset should change back to zero and the trace should return to the center position.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 10, 2016, 08:32:18 pm
content: Thank you WaveyDipole for your detailed explanations and also the photos of the devices. It is very comprehensive, and clear proof that my FG2001 is faulty needing repair. It seems OK up to about 200Khz down to 150Khz, but below that the signals just diminish into thin lines. It is happening with when connected to other scopes too.My IWATSU SS-5702 is also faulty on Channel 2, not reading any DC Voltages at all, and responding erratically which made the whole thing very complicated for me to figure out.But with all the advice and info on this forum, I am able to know now these device I have needs repair before put to use.Thanks again & kind regards.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 11, 2016, 06:15:44 pm
content: With regards to CH2 on your oscilloscope, having read the thread, I'm not sure whether you are not getting a reading on DC only, or whether its dead on both AC and DC signals? Your video makes clear that you are not getting a DC reading, but your comment that CH2 is "responding erratically" suggests that you are getting a response to a signal sometimes?If you are getting a response for AC signals but not for DC signals, this may be down to the channel switch. Looking at the circuit diagram, the AC signal passes through a single set of contacts, but a DC signal must pass through two sets of contacts in the switch before it reaches the input attenuator and amp. If it is behaving erratically in the sense that sometimes it picks something up and other times it doesn't then this also might suggest switch contacts. A squirt of Servisol (don't be tempted to use WD40 - it is NOT a switch cleaning lubricant nor the right stuff for the job) or similar switch cleaning lubricant into the switch contacts and then working it a few times might help.Of course if you are consistently not getting any input on any volts/div setting on AC or DC, then the problem probably lies elsewhere in the CH2 input amp, perhaps a blown input FET or something. The good news is that it should be a relatively minor repair since the scope is otherwise working.Although it seems that there is more going on with the FG, I did wonder whether dirty switches might be at least partially responsible for the problems you are getting with that device as well. It does appear to work for some frequencies - at least with sine waves. It might be worth trying a squirt of switch cleaning lubricant into one of the switches for a frequency range that does not work properly and working it a few times to see what happens. From the circuit diagram it looks like the 20M switch (SW9-A) in particular looks like it might cause just about every frequency range to fail if it were not making contact properly, except for perhaps the 200KHz range... The type of switches used in this instrument allow their operation to be easily checked using a multimeter via the pins protruding from the top.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 11, 2016, 08:48:41 pm
content: Great points WD.The Ch2 of the Scope is not reading DC at all, but it displays sine waves from the FG when set to AC but only above 100 Khz.As you say, I am not sure if it is dirty contact on the Voltage selection switch, or if it is more serious such as blown FET, diodes or Transistors, or even burnt out resistors or dried out Capacitors.I would think it would be straight forward process of checking every suspect components one by one on CH2 PCB, and compare them with the ones on the CH1 PCB, and if there are noticeable differences in values, or bad components, then I would know what to replace. But it would be much time and effort consuming process I would reckon.Another point is, although the scope seems operating OK on AC, I am not sure if it is working 100% accurately either. I could compare it with other scopes on this issue, but they are all very old vintage scopes which dont give much trust. For that, I would need a FG and scope which work perfect, and then compare properly all the aspects of the operation of the devices?Yes, I will open up the FG2001 and try to apply the Maplin switch cleaner in the pots of the freq. selection buttons first, and see if it will improve things.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: September 12, 2016, 09:57:12 am
content: Well, we know that your FG is malfunctioning and only produces signals on the 200KHz range only, a behavior which you have duplicated on another scope. However, if you are getting a sine wave on CH2 when switched to AC mode then it would seem that the input amp is actually working, in which case this does likely point to a dirty or possibly worn switch as the source of the problem. The switch that I am talking about is not the volts/div switch, but the AC/GND/DC switch. It should be straightforward enough to squirt a bit of switch cleaning lubricant into where the contacts are and flick it up and down a few times so see if that helps. The decoupling capacitor is switched in for AC mode and would therefore seem to be OK.Does the sine wave match what you are getting on CH1 with the same volts/div and time/div settings?I would not worry too much about calibration or 100% accuracy just yet - that can come later. You are correct not to expect vintage scopes that may have been in storage or of unknown provenance to be accurate and a comparison with another scope that is in calibration and showing the same signals will give you an idea of how accurate it is. Quite often they are adequate enough and even if not working perfectly, will still be a useful tool that will help you visualize what is going on in a circuit even if you can't measure it accurately. Your basic DC test seemed to be quite accurate though. I have a vintage Hameg and Tektronix scope. neither is 100% accurate, but both are fairly close to the mark and more than adequate for what I require.With the FG, start with the 20M switch. That may be the source of the problem with the other frequency ranges. Please don't be tempted to twiddle with any of the presets! If you would like the circuit diagram for the FG then please PM me your e-mail address.

author: vinlove
author_rank: Frequent Contributor
timestamp: September 15, 2016, 08:52:42 am
content: I had to do some other jobs around the house, so left tinkering with electronics gear for a while. I will get around, and trying to fix the devices when time is available soon. But yes, your instructions sounds practical and very clear.If I need the service document and circuit diagrams for the FG, then I will be in touch. Thank you very much indeed.