Electronics > Beginners

what cheap digital oscilloscope for analog audio?

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tunk:
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.

Gyro:
Just to note that the DSO150, while being a handy little toy, starts to alias above 100kHz sinewave. Its 200kHz spec is rather optimistic.

magic:

--- Quote from: sheiku on December 12, 2019, 08:39:50 am ---strong hiss  in right channel.( the hiss it is audible  in all the room, and decrease as the amplifier is warming up)
--- End quote ---
Not sure what that might be.


--- Quote from: sheiku on December 12, 2019, 08:39:50 am ---suddenly 2 days ago in a morning it started to pop and crack very very loud from ( i suppose) both channels.
--- End quote ---
A 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.

andy3055:
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.

magic:
Perhaps you could insert some 5W 1kΩ resistors in series with the speakers to reduce risk of damage to the speakers or your ears.

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