I had a gearbox jam a few years ago preceded by an brief odd mechanical noise. When I stripped the box I found the bushes were worn elliptical by heavy wear so allowing a gear to tilt and jam against another gear (it had had a lot of hammer for years but I think the primary cause was the motor was over-adjusted and bearing too hard into the gearbox). The fix was simple, just a new set of bushes.
By the way, no problem about linking to other sites - no such rule on this forum. 
I would suggest having a word with the previous owner to find out if it has metal bushings fitted.
If it hasn't, and has had a lot of action, it could be excessive wear as CW suggests.
Another option is that the power cut out at a point where the switch won't make contact. This kind of lock-up is usually fixable by running the gun in full auto for a few shots. Have you tried that? Semi won't do it, as that's the cause of this particular lock-up.
If that doesn't fix it, dismantling will be required. I can't remember for sure, but I don't think the anti-reversal latch can be reached without getting the GB out of the lower receiver, so it's not an easy job if that's all the problem is.
It is possible to just connect power to the motor by removing the grip, but I'd not recommend doing that if you don't know what your doing with electricity - it could very easily make things worse. Shocks, flames, bad smells, etc. 
If none of that helps, and if you can't find anyone more local (I'm in South Essex), PM me and we'll see if I can look at it.
I've used http://www.fire-support.co.uk/for this sort of thing and they did a good job. I'm sure plenty of other retailers do the same.
The clicking is the motor trying to turn but not being able to, this can be due to motor not being alligned properly with the gearbox or more seriously somethings f**** inside the gearbox stopping the gears from turning.
German
Russian
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Almost anything you do to see what is wrong will require the gearbox to be extracted from the receiver because of the way the thompson is put together.
So it's either time to take the plunge and be a lot more knowledgeable of the workings of an AEG or send it somewhere to be fixed.
If you burn out the motor it can be replaced.
Basically take the grip off and ensure the motor doesn't get too hot.
Oh... the symptoms you mention could be as simple as knackered battery! Try other batteries and try that battery in other guns.
Oh... the symptoms you mention could be as simple as knackered battery! Try other batteries and try that battery in other guns.
Yes, although the blown fuse points to a seizure I'd definitely start at the simple end of the spectrum - the battery. You could also take the pistol grip off (single screw) and slacken the motor right off with an allen key. Squeeze the trigger and see if the motor whizzes. If so start winding the motor back towards the gearbox, bit by bit, testing with the trigger as you go. If the motor changes from whizzing to total stop then it is a gearbox issue. But you might be very lucky and find it is just an overtightened motor bearing too hard into the gearbox, which can lock up the gearbox.
Like others have said, Have you tried the batterys you have in any other guns to see if they work ok? It could be that the battery is crap! (especially a china one) No point in opening the gearbox if there is no need too.
In the likelyhood of your gun still not working properly it could be a few things.
Wires have started to wear. Make sure that the wires push right into the connectors both at the battery and the connector.
Shimmed incorrectly
Piston tooth has snapped and "jammed gears"
Failure of a gear or other gearbox part.
Pinion Head from motor not been adjusted correctly (causing a jam)
My TM FAMAS blew 2 fuses in a day and that was my alarm bells ringing. On opening the plastic bushing had failed and the gears had started to eat away at the case. This though was expected as i had the gun for about 6 years and had pumped about 175k - 200k rounds through it.
Good luck on the fix, the version 6 gearbox isnt too hard to work on. (alot easier than the version 2 "M4" gearbox) i did an upgrade guide on mine which might come in use which is posted on the fourms. Or just search on you tube for Thompson Airsoft takedown's.
had a similar problem, worked for a while just decided to play dead. so ran round with a rental m4 for the rest of the day (how fun that was, id forgotten how badly i hate the high cap rattle)
Anywho long story whort, a wire had worked its way loose and was a thirty second job once it had been field stripped back at home. how silly i felt then as i'd taken it to our teckies with a sad face.
Hope you get it fixed soon mate
had a similar problem with one of mine, it was a bad wire to the trigger contact plate, just de-soldered them, drilled a small hole in each to stop the solder coming off then soldered them back on wrapped through the holes, problem solved
If something is blocking the bb's from entering the hop unit that would mean that either the Air Nozzle is broken or more than likely the tappet plate which holds the airnozzle in place is broken. I would at a guess say that the tappet plate is broken as on the version 6 gearbox these are one of the weakest area's. But marui did there homework and at least chosen a very inexpesive part to break in this gearbox.
A link to what tappet plates look like and a rough idea on price although i didnt see a thompson one.
My Thompson upgrade guide
http://www.ww2airsoft.org.uk/php/phpBB2/viewtopic.php?f=13&t=12014
A picture of the tappet plate (it leads from the top left gear to the front of the red cylinder head.)
I had a similar problem with an M16.
Have you tried stopping the motor when it's spinning? A good motor should be nigh on unstoppable when spinning but if the brushes are shot then it'll stop under pressure. Maybe this is the problem and the motor isn't cycling the gears at all.
Try a spare motor if you have one.
Also check whether the trigger contacts are ok. The short that blew the fuse could have caused a carbon build up over time.
To me it sounds like something has jammed in the gearbox. This could be almost anything and you won't really know for sure until you open it up.
