How the MIC input Oscilloscope works
Every PC / laptop has a MIC input - and this can be used as a basic 2 channel oscilloscope ! Of course it's band-width limited - the often 'claimed' 20Hz-20kHz will be more like 40Hz to 10kHz, however for tracking faults in audio amps it is all we need !
Most MIC inputs will tollerate 'Line' level voltages (+/-1v), however most are designed for microphones that deliver 10-100mV
'All' you need to do is feed the probe signal to the MIC socket and record the 'sound' using any Open Source application such as Audacity
Input circuit
To diagnose audio-amp faults, you will need to probe voltages up to 50v or even more.
The typical MIC input will operate best at voltages from an actual microphone (approx 10mV) up to a low volumn Line input (about 100mV). Most will limit at +/- 1v or so.
This suggest an 'ideal' target of +/- 25mV max.
Assuming you are using a standard 'x10' probe (which comes with a 9Mohm in-line resistance), a 1M ohom termination will drop 50v to 5v. To get this down to 25mV, you need an over all 2000:1 reduction (50v to 25mV)
The 47k pot. together with the 9M ohm probe sets the 2000:1 division ratio.
The additional 470k has mninimal effect (other than to protect the circuit in the event that a x1 probe is used)
The back-to-back Schottky diodes will provide some additional protection (they should keep the levels below about 330mV)
The 1uF cap. prevents DC levels reaching the MIC input (chances are MIC-in has an input cap anyway)
Most MIC inputs sample at 16bits and at 44.1 kHz. The more 'up market' ones also offer a 48 kHz (or even a 96kHz) sampling frequency.
Note - the 'rule of thumb' 10x signal sampling means the 96kHz audio system = a 10kHz Oscilloscope
It's to be noted than most MIC inputs provide a 5v DC supply (typ. via 10-22k in-line current limit) to power an electret type microphone (whihc is about the cheapest microphone available).
Note also that most 'higher quality' dynamic microphones have no built-in pre-amp and might, or might not, deliver sufficient voltage levels to the MIC in socket.
For software, just Google 'Open Source PC MIC Oscilloscope' (you can even find some tested with Windwos 10 - see, for example, Soundcard Oscilloscope
Parts cost
!! NEVER, EVER, BUY ELECTRONIC COMPONENTS FROM ANY UK HIGH STREET STORE !!
Over the last few years, the rise of 'hands-on technology' teaching in UK schools ('STEM') means that the UK High Steet store (such as Maplin) can now gouge the average doting ignorant parent 10 to 100 times the 'going rate' for individual 'electronic' componets = these soon-to-be-poor ignorant saps have no idea that they can literally purchase 10 times the qty. for 1/10th the price on eBay (or Amazon) FROM CHINA post free (UK sellers will be at least double the cost)
SO :-
Project Box = I used a 0.1L 'Really Useful' box which cost me about 60p purchased from a 'general hardware' store (a 'small project box' from eBay will cost at least £1.50 (and similar from Maplin at least £2.50))
2x 100MHz probes (switchable 10x 1x) = about £3.50 ea. from eBay (China) NB. 10x = 9M ohm, x1 = approx 350 ohm = £7 for 2 (Maplin charges £15 for a single 60MHz probe)
2x BNC co-ax sockets = pk5 for £1.19 eBay (or 10x for £2.02) = 48p for 2 (Maplin, £3 ea.)
2x 470k = taken from 1/4w kit (10x ea. E12) 300pcs kit at £1.04 = 2 for 0.7p (call it 1p) = Maplin wants 10p ea. (a big reduction from the previous 35p ea.)
2x 50k pots. with knobs = pk5 for 99p eBay (China) = 2 for 20p (Maplin, £1 for ea. pot. + 70p for the knob)
4x Schottky diodes = actually, 2x dual Schottky diodes cost 2p each (from CPC 'Bargain bin') = tot. 4p
2x 1uF 6v cap = taken from a uF cap. kit, 12values, 120 caps for £1.33 (eBay, or 25 values, 210pcs for £2.30), so approx 1p ea. = 2p for 2
Stereo 3.5mm flying lead = I chopped the end off a spare, however you can get a 3.5mm-3.5mm stereo cable from the £1 stores, so half of that = 50p
Total 'front end' cost = £ .
Note - I used the BNC sockets and Potentiometers as 'anchor points; to solder everytimg else tyo, so no 'bread-board' needed, however a small piece of 'matrix' (Vero) board costs less than £1.