Vox vt100 volume drop and buzzing - VOX Amplification VOX VT100 Valvetronix 100 Watt Tube Amp

Is this it?

https://www.google.com/search?q=VOX+VT15+guitar+amplifier+need+a+input+PCB+BOARD

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You can use it, just keep the volume low. The amp doesn't care what yu play through it, but the speaker isn't designed for loud bottom end.

If your amp isn't producing signal, your best bet is to take it into a local Vox autorized repair shop. There are leathel voltages involved and if you're not an experienced tech, you can get hurt, not to mention screwing to amp up worse. Here's a link to the Vox contact page http://www.voxamps.com/contactus
And one to find the list of Vox repair shops.
Korg USA Musical Instruments

that is because it is two different output channels. eventually you will blow the speaker or your amp. It wasn't designed to handle the impedance of an external speaker through the stereo
headphone jack. Are you sure that your speaker is hooked up or plugged in?

There is NO "reset" button.

You LIKELY have a bad filter capacitor. The additional load of the Triode load is probably dragging the supply causing flattopping and the power supply ripple frequency to come through. Have the unit serviced.

Look for bad soldering around the volume control pot leads. I think you need a new amp tech that can troubleshoot a problem.

Yes you could blow the speaker. There is either a problem in the power supply or possibly the power amp chip has shorted to one of the power rails. Some of the vox stuff uses th LM3886 chip as the power amp. While these are supposedly protected, they can go bad. Download A schematic that MIGHT be close to your unit... it is for the VT30 but probably is similar:

http://elektrotanya.com/vox_ad15-30vt_sch.pdf/download.html

Scroll down to "Get Manual" and click on it to download the schematic. Do this test: Adjust the power level control. IF it doesn't vary the level significantly, then check the voltage across the speaker... IF it isn't less than 1 volt DC (bias) replace the LM3886 chip. Use thermal grease and any insulators they are using. Chip about $8 from Digikey.com. Note these come with two different pre-bent leads... make sure to get right ones. IF the output bias is with 1 volt DC of ground, check for bad filter caps and rectifiers.

I suspect you may have either a circuit board crack or the volume pot is bad or soldering of it is bad. Vibration will make things come and go. Look for broken solder around the pot leads first. Then look for a cracked circuit trace on the board near the ground end of the volume pot. If you put cleaner in the pots NEVER use Deoxit or Blue Shower. Use CRC226 which is available at Home Depot in the electrical dept.