what cheap digital oscilloscope for analog audio? - Page 1
Forum section: electronics / beginners

author: sheiku
author_rank: Newbie
timestamp: December 11, 2019, 10:48:28 pm
content: Hello to all. Sorry for my poor english.I am happy that i find your forum. I read from it for few days all i can about oscilloscopes..Here is my problems. I have about 10 old amplifiers and receivers (SS, not tube, around 1975 year) I am a hobbist.This week 3 of them started to make very loud pops, hiss, or distortions..I have nowhere to repair them. And i try to do by myself..In my youth i dealt with some basic electronics, but never with oscilloscopes.So i understand that i need to have an basic osciloscope for best results on repair and diagnose. ( hiss, pops and distorsions)I intend to buy only digital oscillo.I see that almost every recomandation is for old cheap and good analog OLD oscillo. I canot find at all in my country and i do not want to deal with old oscilloscopes.My budget is maximum 200 usd.What should i search as specs for analog audio?i understand that 20Mhz is minimum. Also 1 channel is enough.Can i do some acceptable work with something like : DANIU ADS5012H ? I can afford it as price..Or of course other recommandations for minimum specsThank you

author: tggzzz
author_rank: Super Contributor
timestamp: December 11, 2019, 11:12:13 pm
content: Check the simple things first.Look slowly and carefully at each component, checking for any signs of discolouration due to heating, or leaking/bulging electrolytic capacitors, or poor solder joints/wires, or poor switch/potentiometer contacts.Then measure voltages with a meter. Start with the power supply DC voltage and AC ripple. Then, with no input, look at various points in the signal path, starting at the output and moving towards the input. Observe voltage readings when you hear a crackle/pop (analogue needle meters help here!), and draw conclusions as to whether the source is earlier/later in the signal path.There's a good chance the problem is with electrolytic capacitors, potentiometers, or switches.

author: andy3055
author_rank: Super Contributor
timestamp: December 11, 2019, 11:24:45 pm
content: Quote from: sheiku on December 11, 2019, 10:48:28 pmOld audio gear can be easily repaired just using a multi meter to troubleshoot. I never had an oscilloscope until recently but that never stopped me from fixing the older amps. You may need a scope if you are fixing newer amps as most of them have digital control circuitry that were never there in old gear. In fact, you won't need even a digital multi meter as long as you have a decent analog multi meter in my opinion.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 12, 2019, 06:44:28 am
content: You might start with a PC sound card oscilloscope. The sound card works as a signal generator too.All you need is an analog front end with a couple of opamps and input dividers for large signals.Just search the net and you will find plenty of information and free programs to display and generate audio signals.

author: tggzzz
author_rank: Super Contributor
timestamp: December 12, 2019, 08:18:27 am
content: Quote from: not1xor1 on December 12, 2019, 06:44:28 amThe OP is a beginner and likely to make mistakes. A mistake might fry his computer, which would have been more valuable than the amplifiers!With thought, care and understanding, you can do a lot with a voltmeter, a few switches, a few LEDs and resistors, and a potentiometer. That's all I had when I designed and built my first computer from individual components such as a 6800 CPU, 128bytesRAM, and TTL (not even LSTTL) chips.

author: sheiku
author_rank: Newbie
timestamp: December 12, 2019, 08:39:50 am
content: Thank you all, really many answer, i did not expected to so many answers.Yes i am beginner. I only build some amplifiers from 0 to functional in my youth but i have no experience in trouble shooting and repairing stuff. It was easier to build as new with schematic in font than to search defective components.So..for now i have an ( quite good) digital multimetre, and an cheap chinese ESR metre.I will try to diagnose the problem only with them, but i am afraid that i have to desolder the component to test from pcb. From what i read i see that excepting the capacitors no measurement is trusty on the pcb or ..in circuit. I am afraid to damage something at de soldering at soldering again.I have no good soldering gun ( but i can buy one)For that reason i readed that using an oscilloscope will be easier to isolate the part of the circuit with problem..Even so , even a cheap oscilloscope as the mentioned (DANIU ADS5012H) will provide some benefits in diagnosing my damaged amplifiers over a simple multimetre?If no, i will forget it and try to repair only with classical methods.Thank you again for all.PS:the story is:my main and most expensive amplifier ( a sansui au 9900) i bought about 2 weeks ago with 1000 euro:-at the beginning it has almost no bass and had strong hiss on right channel-I tested with input from other source as pre amp and found that it sounded excellent in separated position with external preamp ( so i supposed that the internal sansui pre amp was quite defective)-after that i deoxided it very good. After that he recovered and had a very strong bass, but hiss still present strong hiss in right channel.( the hiss it is audible in all the room, and decrease as the amplifier is warming up)-after 3-4 days of intensive use the bass and sound stage became more and more good. It seems that capacitors suffered a sort of burning as much as i use the amplifier. So excepting the strong hiss all was great with it and he have a beautiful sound.but suddenly 2 days ago in a morning it started to pop and crack very very loud from ( i suppose) both channels. So loud as my children started to cry..and i thought i damaged my speakers..I never used from that moment.(Similar symptoms i have with another sansui au 505 but the pops are not so loud. The pops are much much softer as volume.)Thank you

author: andy3055
author_rank: Super Contributor
timestamp: December 12, 2019, 08:51:04 am
content: You have said " One amplifier cost me about 1000 euro .. " Can you tell us more about this amplifier? Make, model etc. Or, did you spend that much building it yourself? That sounds very expensive for an old amp. If you did it yourself, can you post the schematic and tell us more about the problem you have?

author: tggzzz
author_rank: Super Contributor
timestamp: December 12, 2019, 09:00:28 am
content: Quote from: sheiku on December 12, 2019, 08:39:50 amDon't forget that an oscilloscope is only a fast voltmeter showing a graph of voltage vs time. That graph may or may not help you isolate a faulty component.One simple test: if you short-circuit the input, are the pops hisses and distortion still there?Another simple test: use one amplifier to probe the signal path of another. You can make inferences about the source depending on whether a click is heard simultaneously on both amplifiers' outputs.Good luck, and let us know what you find.

author: sheiku
author_rank: Newbie
timestamp: December 12, 2019, 09:01:17 am
content: Hi Andy,i edited the previous post with details about my main amplifier: Sansui au 9900and the problems.Thank youHope i made my self clear. I do not speak english well.my main and most expensive amplifier ( a sansui au-9900 , made in 1975) i bought about 2 weeks ago with 1000 euro: ( i know the prices for sansuis in US are lower than Europe.This is our situation)-at the beginning it has almost no bass and had strong hiss on right channel-I tested with input from other source as pre amp and found that it sounded excellent in separated position with external preamp ( so i supposed that the internal sansui pre amp was quite defective)-after that i deoxided it very good. After that he recovered and had a very strong bass, but hiss still present strong hiss in right channel.( the hiss it is audible in all the room, and decrease as the amplifier is warming up)-after 3-4 days of intensive use the bass and sound stage became more and more good. It seems that capacitors suffered a sort of burning as much as i use the amplifier. So excepting the strong hiss all was great with it and he have a beautiful sound.but suddenly 2 days ago in a morning it started to pop and crack very very loud from ( i suppose) both channels. So loud as my children started to cry..and i thought i damaged my speakers..I never used from that moment.(Similar symptoms i have with another sansui au 505 but the pops are not so loud. The pops are much much softer as volume.)Thank youhttp://akdatabase.org/AKview/albums/userpics/10004/Sansui%20AU-9900%2C%20AU-11000%20Service.pdf

author: sheiku
author_rank: Newbie
timestamp: December 12, 2019, 04:05:14 pm
content: Thank you to all who responded to my problem.I did not expected to receive a such amount of advices.I will try to understand each advice i received.Little harder for me because i need to understand the english language , after that the technical aspect beyond them and ..TO DO that.I will came in few days with news.regards

author: tunk
author_rank: Super Contributor
timestamp: December 12, 2019, 04:58:44 pm
content: The DSO150 can be had for 20-25 us$ and has a 100-200kHz bandwidth.A signal generator may also be handy, you can get one for 2-3 us$ on ebay etc.

author: Gyro
author_rank: Super Contributor
timestamp: December 12, 2019, 05:17:10 pm
content: Just to note that the DSO150, while being a handy little toy, starts to alias above 100kHz sinewave. Its 200kHz spec is rather optimistic.

author: magic
author_rank: Super Contributor
timestamp: December 12, 2019, 05:59:41 pm
content: Quote from: sheiku on December 12, 2019, 08:39:50 amNot sure what that might be.Quote from: sheiku on December 12, 2019, 08:39:50 amA broken connection somewhere? Check resistance of all potentiometers, look for bad solder joints, test connection continuity along the signal path. You don't need a scope for any of that, just a DMM, plus a service manual could be helpful.

author: andy3055
author_rank: Super Contributor
timestamp: December 12, 2019, 06:24:44 pm
content: OK, first things first. Those Sansui amps in that era (and even other makes) have the nasty reputation of noisy transistors and dry solder connections. The issue develops after a few years of using due to heat/in-adequate cooling. Noisy transistors and diodes could have been due to the technology at the time as well.Power down and do the following:Take your soldering iron and heat up all the legs of each and every transistor and diode. Do not be scared. The silicon devices can withstand a lot of heat. If you have isolated the issue to the pre-amp area, half the battle is won. Can you control the noise by turning down the volume control? If so, check the components before the volume control. If not, check what is after it but before the main amp input.Turn the selector to Aux input and see if that has any effect. If there is no noise, then follow backwards from that point towards the inputs. If not, go the other direction towards the power amp area.Once you suspect a component, try interchanging it between the channels and if the noise moves to the "good" channel, you know which part is faulty.If you have noise in both channels, check the components in the power supply.It won't take much time to troubleshoot them if you go methodically and isolate.

author: magic
author_rank: Super Contributor
timestamp: December 12, 2019, 07:35:47 pm
content: Perhaps you could insert some 5W 1kΩ resistors in series with the speakers to reduce risk of damage to the speakers or your ears.

author: gcewing
author_rank: Regular Contributor
timestamp: December 13, 2019, 01:58:21 am
content: Another good inexpensive piece of audio test gear is a crystal earphone.

author: radiolistener
author_rank: Super Contributor
timestamp: December 14, 2019, 05:32:02 am
content: it depends on your needs. If you're needs to analyze signals with high dynamic range, low distortion and low noise, you're needs oscilloscope with high resolution ADC (at least 14-16 bits).For example, you can use something like Picoscope 4262:https://www.picotech.com/oscilloscope/4262/picoscope-4262-overviewFor beginning, you can use just some sound card to digitize signal and analyze it with software. With 24-bits sound card you can get very good dynamic range, but the bandwidth will be limited by half of the sample rate, usually 96 kHz