Hi nyazip.
Motorboating is is usualy caused by an excess of low frequency response. The first guess you had (coupling caps with non-original values) is the usual culprit. Not having your schematic in front of me I would say that you should try replacing the coupling caps and also the cathode bypass caps (replacing the filter caps was a good bet too). These are low cost items (around US $1 ea.) but there can be a lot of them.
As to your speaker jacks, they are probably conected in paralell, which is to say the hots and the commons are wired directly to each other. Here's how to tell what's going on:
Plug your amp into everything. Play it to make sure the ext spkr jack is acting up.
Put it on standby and switch the cords in the ext spkr and normal spkr jack. Turn it on an play it again.
If the problem changes SPEAKERS the jack or the cord is to blame. Swap the speaker cords to see if it is the cord.
If the problem switches JACKS (the normal spkr jack now has no output) the problem is either your cord or your speaker cabinet (the cabinet also has a jack and some wire in it, e-mail me or post here if you have a cabinet problem and we'll get it fixed). Again switch your speaker cord to make sure this isn't a cableing problem.
If the problem is your ext spkr jack the fastest thing will be to switch the jack. It should be $1-2. If you look at the jack and see cold solders or broken wires than the solution is obvious. In fact fire up your amp with the chasis out of the cabinet, plug in only your cable and drop the end on the floor so that it makes an anoying buzz, and poke the wires going to the ext spkr jack with something NONCONDUCTIVE (try a chopstick or a sharpie). If wiggling the wire results in turning the cabinet on and off it the wire or the solder. Reheat your old solders, if that fails solder a new wire in.
Your amp's ext jack will not work without something pluged into the spkr jack. This is a saftey consideration for your output transformer, and is a good idea, but you will have to use both at once to find and fix ext jack problems.
In case no one told you before, Those big filter caps you replaced store potentialy lethal voltages. If you are to go poking around in your amp you must be sure they are discharged. They can shock or kill you even if the amp is unplugged (I know from getting shocked). If you want to discharge them it is easy. Unplug your amp and short the hot to ground. You would idealy do this through a 100K resistor, but I have seen techs using a screwdriver, a VOM lead, or the VOM itself set on the ohms scale. Whatever you use make sure that it is insulated, or guess who will also become a path to ground.
Last of all I am willing to try to teach you the basics of electronics as they relate to tube guitar amps. I can explain in english, and I do not need to be paid. I am, however, in Portland Oregon. You may e-mail me at
cartoonweirdo@comcast.net if you like.
Good luck with your Traynor
Carl