Hi, I have a crate vtx65 on the bench at present. First fault was relatively simple being a couple of pots with broken tracks which after a good deal of research manages to track down replacements. After doing the bench test with everything appearing to be functional - utilising the good old 100w lightbulb short circuit tester and a bench speaker, I re-assembled the unit to try it properly. Still plugged to the short circuit tester (thankfully) switched on to a loud hum and a bright bulb! Ok I had swapped out 3 op-amp chips to removed some distortion (I do this by de-soldering and fitting a socket to ease future changing). Replaced these chips again just in case. No difference.
I connected an external speaker through the socket on bottom of amp and bulb went dim and carried out a quick test with a mic. All working A1. Disconnected ext speaker, switched on again - Bright bulb, loud hum !!!. Disconnected internal speaker and switched on - no problem, reconnected int speaker - all works A1 so eliminate speaker fault.
Having powered up yet again with int speaker connected, again bright light and loud hum - however plugging a bare jack plug to ext speaker socket then removing it again allows the amp to work A1.
I am missing something really simple here.
Has anyone come across such a fault or hazard a bit of knowledge - I have attempted to Email Crate but their  website takes me to another which is closed.
I do not have a schematic for this amp either which would help.
Please can anyone help me out
Thanks
From Scotland UK