I was just testing my AC30 before trading it, and have discovered that the Vib/Trem channel launches into howling if I go beyond halfway on the volume. Bugger. It is dependent on having some input signal, so if I hit the strings hard at halfway, it'll go off on one doing a whale-in-hell impression. But equally, I can turn it almost up to full and brush the strings very lightly and it won't happen. This is why I am thinking it is parasitic. Any suggestions? I've resoldered the ground on the volume control which was dry, and which would launch it into feedback when I nudged it with the faithful old chop stick. The volume control is right before the input to the PI, so will it be something around the PI feeding back into an earlier part of the Vib/Trem circuit? Both the other channels work fine, though there is a faint ticking audible when the volume is very high on the Bright or Normal channels and the Vib/Trem is on. I've been going through the circuit all day and have pushed wires about and re-soldered anything that looked dodgy. Oh and I replaced the Vib/Trem bias bypass cap. So I'm a bit stumped. Any help much appreciated.
EDIT: I received the amp non-working in a trade, and so I'm not sure how long this problem has been here.
Comments
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Have you tightened up all the screws in the chassis? You're depending on these for grounding.
Does the theremin sound change with switching the vib trem switch, or the speed knob?
My gut feeling is something in the oscillator circuit is coupling back to the preamp somehow.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Just a point about the loud noise!
If any of you are contemplating work on amps a load box with a highly attenuated output is a virtual necessity.
Does not have to be "clever" or sound "rock n roll" just allow you to listen to faults in comfort and speaker safety.
Note, you MUST have resistive loads but a 100V line speaker transformer makes a super attenuator, a 3watt jobbie is cheap enough as are the loads (or use a fan heater!)
Dave.
A lot of old 2kW-3kW range heating units - heaters, kettles etc - have DC resistances in the 20-30 ohm range, which is ideal as a "16 ohm" dummy load. The exact resistance doesn't matter much, although it's a good idea to check it. Also make sure you disable any off switch, or at least tape it securely - you don't want to accidentally set your dummy load to open circuit.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein