Is scope and freq generator needed for repair vtg solid state audio gear?

[RiRaRi]

Hello. I do basic repairs of vintage solid state radios, boombox, walkman etc. I change everything needed in order to work correctly and test all bigger caps so i know that nothing will die in near future. But as i heard no serious repair job can be made without scope and function generator. Is that true? Also, i would like to ask if someone is willing to write me what is procedure of faulth finding with scope and function gen? Because i can not find good info online except how to use these devices.
As far as i have seen, i have to feed circuit board with function and start to probe from the point of feeding to other end and look where the wave is getting unusual noise. Please correct me if i am wrong. I am new to frequency field and would really like if someone can explain how to do it. Thanks in advance.

[Shock]

Without a scope you will be limited to repairs you can diagnose with a multimeter within it's bandwidth capability. It rules out any complex signals as most of the time you are measuring the amount of signal rather than showing it's contents or in the case of distortion it's unwanted contents.

The basic rule is probing with a scope or connecting any ground referenced test equipment without experience, is it can cause damage to your gear and device. The most common way is by shorting a circuit signal/voltage with your probes ground wire. So watch the youtube video, how not to blow up your scope.

We get around these problems by not connecting grounds without careful consideration. By using an isolation transformer for the device under test, so we can introduce a ground point in a controlled way. By using an isolating scope probe (active differential probe) or other safe probing technique that limits the necessity for a ground wire.

Signals can be injected anywhere it makes sense, for AF/RF along the signal path is a good idea, likewise for checking a signal with a scope. You're only limited by your understanding of the circuit. Connecting your signal/function gen to a power supply or speaker output of an amp could be a bad move. So the next rule is understand what you are working on and observe the input/output range and limitations of your test equipment.

To get yourself started a simple battery operated low power amp circuit can be probed all over as long as you don't introduce two grounds. So a battery powered mp3 player or old phone running just on it's battery is an ideal audio source to inject a sine wave into the amps input. Or you can buy or make a basic battery powered signal generator. Not exceeding the amps input voltage with the signal gen? Not exceeding the scopes input voltage on the output of the amp? Only using one ground connection on your mains powered scope? No mistakes like plugging the phones charger in (can add a ground). Good to probe.

I suggest browsing through videos on youtube for repairs where scopes are used. Other test equipment like spectrum/distortion analyzers, component testers may also be necessary to make accurate measurements or uncover a problem.

[u666sa]

You better off having scope and signal gen. Simple real life example: yesterday I was working on a router, and all voltages checked out. I pulled out a scope, measured clock crystal, no clock. I now know where to dig further. It lets you see signals, so it is another set of eyes.

Ideal scope for your application is Owon HDS2202S. It is portable, so you can't mess it up in power supplies. It has signal gen. The only down side to it, are its probes, so you will be buying new probe.

You also need LCR meter. It would be like THE money maker in your line of work. I recommend Hantek 1833C.

Both these tools are cheap on Aliexpress and a must have on your bench.


Plus you need multimeter, obviously. If you have a cheap one, I suggest another cheap one. UNI-T UT61E, not the newer one E+, the old one. It has more favorable diode mode voltage, so you will be able to test in circuit mosfets better.

[armandine2]

https://www.eevblog.com/forum/oshw/paul-carlsons-super-probe/

you might be interested in this device too

[rstofer]

I don't work on audio and have less than zero interest but...

The Digilent Analog Discovery 3 has two floating scope channels and they float as long as you don't use the BNC adapter. This can be very helpful.  If I were interested in distortion I would want a way to get a FFT and the AD3 does a great job of this. Not only do you get scope functionality, there are other displays that are quite handy.

It also has 2 channels of Arbitrary Waveform generation.  There outputs are not floating AFAIK.

All the necessary tools in a small box.  It's $379 so it's pretty expensive but definitely a useful tool.  There's a 15% Off Sale at the moment.

You can download the free Waveforms software and play with the Demo device.

https://digilent.com/shop/analog-discovery-3/
https://digilent.com/shop/software/

[themadhippy]

How did  we manage in days of old with not much  more than a crystal earpiece,signal source was your finger on the input

[Shock]

Here is an example of a very simple (for illustration purposes) audio amp diagnosis solely performed with a scope. It adds more information than just a measurement with a multimeter, it's observing the absence of other weird faults that would otherwise lead you around in circles.

How did  we manage in days of old with not much  more than a crystal earpiece,signal source was your finger on the input

Is that a calibrated finger?

[themadhippy]

Is that a calibrated finger?

only at 50Hz