VOX AD100 motorboating / machine gun sound out of nowhere...

[Tilman]

Hi,

I got an AD100 SS VOX modeling amp here. After warm up, it starts to "motorboat". It happens with no guitar plugged in and ALL pots down. It stops when I plug in head phones but I can still hear some motor boating through the headphones.
This is not my video but the machine gun sound on mine is the same:

http://www.guitarsolos.com/videos-vo...lHtY9q0%5D.cfm

They mention bellow the video that some B+ decoupling capacitors might be damaged or old and cause the problem. Hard to believe since I guess (no schematic) that the Master Volume is behind the preamp and ahead of the power amp, but the motor boating happens with the Master pot closed.
I had this one on my bench 2 years ago but could not find the reason. I took every thing apart, cleaned it and re soldered many components. It did not cure the problem.
At that time the motor boating happened only once a day or even only once a week. Now it happens every 10 minutes or so. The odds are better now.

Maybe some of you guy have had a similar problems with one of these fine amps and can give me a hint ?

Thanks!
Tilman

[frankeg]

B+ decoupling capacitors might be damaged or old and cause the problem. Did you check your power caps? I have seen this before and it was a bad cap.

[Tilman]

Hi,

I think I have fixed the ugly thing! On the main board there are +-15V voltage regulators 7815 and 7915 (IC5+IC6). Both are getting very hot - no heatsink on them. The one close to the power amp IC caused the trouble once he was getting too hot to touch.
I have replaced both, installed a small heat sinks and it seems to work just fine.
Tilman

[cdubya]

Hi,

I think I have fixed the ugly thing! On the main board there are +-15V voltage regulators 7815 and 7915 (IC5+IC6). Both are getting very hot - no heatsink on them. The one close to the power amp IC caused the trouble once he was getting too hot to touch.
I have replaced both, installed a small heat sinks and it seems to work just fine.
Tilman

Thanks for posting this information years ago! My AD100VT was acting up the same way so I changed those two regulators and it seems to have fixed the issue. In case it helps anyone else, the 7815 equivalent is NTE968 and the 7915 equivalent is NTE969. I replaced both just as is, no heat sinks... let's hope it lasts!