crate glx 65 half the volume, then slow fade to silence

[Roly]

Just my opinion, but I don;t think digital scopes are for beginners, and frankly maybe not the best choice for audio anyway.  We associate "digital" with new and up to date and high tech, but digital scopes have their own characteristics, and if you do not understand those, the results may be confusing or less than helpful.

Well I kinda skirted around saying that, but that is very much my opinion too. 

A basic digital CRO can be built around something like an Arduino very cheaply.  The problem is that the performance is normally so compromised (like in the one above) that it is virtually reduced to a practically useless toy.

By the time you get to a digital CRO that can equal a CRT CRO you are generally starting to pay some serious money.

When looking at any CRO one of the basic telling features of sufficient quality is that you can adjust the trace for a fine line right across the screen.  The correct setting for the brightness/intensity is just bright enough to clearly see, not "blooming" (smudging).  The focus control is obvious, but a control marked "Astig" is to balance the focus between the ends of the trace and the middle.  A fine sharp trace means that the EHT supplies are healthy.


This is actually set a bit too bright.  Note that the traces don't quite align with the graticule, which is what the "trace rotation" control is for (if it has one).

CRO's may have an external graticule in front of the tube, or in better CRO's it is etched into the tube face.

It is possible that an old CRO may have suffered a screen "burn" due to excessive intensity and/or a stationary trace or dot for a long time.  This shows as a darker shadow of the old trace crossing whatever you are displaying, but is generally only a problem where a CRO has been used as something like a modulation monitor of years, and a bit of burn-in isn't really a problem.

Most CRO's these days will have pretty full-on timebase and sync/trigger controls, but really old ones had timebase course and fine, and a sync control.  One of these is still very useful for audio work but the timebase isn't really calibrated; so look for one with a timebase control that is actually marked in "time/div" like 50mS/cm or such.  Similarly really old CRO's just had course and fine vertical attenuator, and the same goes here, useful, but one calibrated in "volts/div" is moreso.

An important feature that is sometimes missing on really old CRO's (and a few later real cheapies, and some Heathkits) is the ability to display DC signals, so look for an AC/DC coupling switch.  More modern CRO's will almost certainly be able to display DC signals.

As Enzo said, just being able to see the signal puts you streets ahead, and all the extra nick-knackery really only serves to make things more comfortable.  So as long as you can see a trace on the screen and make it jump up and down and show hum by putting your finger on the probe, it is fairly hard to go wrong with a first CRO.  I actually built my first CRO, and believe me it was really basic, but even so got used a lot.

Switches and pots in CRO's are like switches and pots everywhere, they get dirty and noisy and cause the trace to jump up and down or the timebase to freeze until the switch is worked a bit, but just like an amp a good clean and squirt of metho or contact cleaner can work wonders.

Some examples;


This is the one that I currently have on my bench.  It is very full-on, 65MHz, and just about all the CRO one could ever want, and certainly for audio work.  I picked this one up for $185AU and I notice they are currently offering on eBay at that price.  This is to give you an idea of how much CRO you can get for under $200.


Another up-market CRO.


Very basic homebrew, uncalibrated and very basic, but still useful.


Typical cheap Chinese "student" CRO, semi-calibrated, ~$100AU new.


Typical basic service workshop CRO (note terminals rather than BNC connectors)


Another fairly basic workshop CRO, a bit more up-market (BNC connectors).

http://hacknmod.com/hack/how-to-build-your-own-oscilloscope/
You could build your own, but by the time you pay for a tube and properly mount it this isn't the cheapest way to go.