Buying Used Products
Buying used music equipment can be problematic. Will it work? Will it last? Can I get it fixed?
I'll break this topic up into a few categories.
Unless a guitar is physically damaged, odds are it will last for many years. Physical damage includes not only visible breakage, but also the climate conditions where the item was store. Outdoor garage storage can rust the metal parts, and warp the wood parts causing a permanent failure which may be unrepairable. Since no one in their right mind would sell such a damaged guitar for anything other than parts, I'll move onto the electronics.
Nearly every item in the guitar internals can go bad. Wires can become 'unsoldered', the cable jack can get worn or dirty, capacitors on the tone control can change value or outright fail, the volume and tone controls ( potentiometers or 'pots' ) can become dirty or fully broken and even the pickups could possibly either short or open causing no or weakened sound.
Luckily, guitar electronics are (usually) simple to repair. If the volume/tone pots make crackly sounds when turned they can simply be cleaned using something like WD40 ( yep, I said that ) or just replaced. They're usually under $10 - you don't need to get the 'original' control - just match the ohm rating and approximate size and you're good to go. On the other hand, you might be that guy that simply won't settle for anything but OEM ( original equipment manufacturer ) parts. This will most often be the most expensive.
Likewise, the jack and wiring is easily replaceable. Shielded cable ( enough to wire 10 guitars ) can be had at just about any yard sale using some RCA, or other, type audio cables you'll find in that box in the back of the garage.
The coils ( pickups ) are the
only possibly expensive part IF they go bad. I've never actually seen a
bad pickup in my life, but I'm sure they go bad. It highly unlikely one
will fail without some forceful provocation - like Pete Townshend
slamming his guitar into his HiWatt amps.
The bottom line: Don't worry about buying a used guitar although personally I like to pick one up and play it before buying.
Rack Mounted FX:
In this area there are a few things to be concerned about. While a used RMF may work today, it may not tomorrow. Certain components can fail, usually capacitors, that can cause a unit to become intermittent, noisy, lose some functions, or totally die.
Another area of concern in the battery. Older units used batteries that, if not replaced regularly, can leak acid onto the circuit board. While this can certainly be repaired, the cost can be prohibitive in your have a shop do the job for you. It's also possible that collateral damage can exist that doesn't show up until some time later, like IC legs being eaten through, or shorts underneath internal multi pin connectors.
While not always provided, a photo of the unit with the cover off so you can see the circuit board is a big help. Look for bulging capacitors, discoloration around the battery, or signs of insects living inside. Here in Florida I've come across many unit with roaches inside ( yech ) and it has a distinct pungent smell even without the case opened. Just stay away from those - the little vermin can damage every part of the circuit besides just the plain gross out effect.
Certain models, like the old Digitech
GSP-21 Legend, mounted the pots on the circuit board and the metal
frame. This made a lot of these units physically breaks the pots off at
the legs. It's an easy fix but you have to be able to disassemble,
remove, and install new pots. I do sell them ( link needed ). If you
come across one of these that has 'no sound output' open it up and look
at the pot - see if they're separated. Less than $10 to fix yourself.
Because technology has come so far so fast, and can emulate all the old effects ( well, maybe not the Morley EVO-1 oil can echo ) I'd stick with new models unless you REALLY want a specific piece of vintage gear for some reason.
Capacitor failure is inevitable in ANY electronic device - some just
fail faster than others. The older the unit, the more likely some caps
are going, or already gone.
There's a whole lot that can go wrong, and a whole lot that can go right, with keyboards and synthesizers. Compound the electronics issues of rack mount FX mentioned earlier, compounded with the mechanical issues with the keys themselves AND the multiple connectors usually found on most keyboards, these units will take the longest to check out.
In a velocity controlled keyboard ( the harder you hit the key the louder it gets ) the system that triggers the sound is similar to that found on any television remote control - a rubber pad with a conducting material over a circuit board only there are TWO of these for each key. The timing difference is how the keyboard knows how LOUD to play a note. It usually results in digital 0-255, which is midi code for softest to loudest. If one is not working, the key may or may not produced a sound.
If you're lucky you can gently clean the contact by removing the rubber pad. There are some products on the market which *may* repair a pad that's lost it's conductivity. If that doesn't work you'll need to replace the pad which may be difficult to find - this is where a parts machine will come in handy.
Anything that has USB connectors have a high likelihood of failing. The AKAI Mini MPK is notorious for this. Most manufacturers mount the USB to the circuit board with no other support. Pushing and pulling the USB cable eventually rips the connector off the PCB. This can result in PCB damage ( torn traces, cracks ). The repair is to repair and pcb damage and replace the USB connector - but this will fail again. I'd use the 5 pin MIDI connector instead of USB - it's been around since the '80s and they rarely break.
Here's an alternative way to fix the USB connector that will last much longer, although it's not pretty - info-akai-mini-mpk-usb-port-repair-v2.html
While much smaller than rack mount units, stomp boxes have the same types of issues - noisy pots, bad caps, dirty connectors, broken switches... These are more like "they either work or they don't" but "working" is a subjective term for some people. I used to buy broken TV sets - people would call and say things like: "It works, it just doesn't turn on" or "It's probably just a fuse". If it did turn on, they'd neglect to tell me it only got channels 2-6, or the sound would cut off, or the controls were sticky or dirty. So... subjective is the term. You should ask: "Do the controls all work?" "Are the pots noisy?" "Do all of the functions work?" "Does it run on both batteries and AC?"
(work in progress)