Very low frequency oscillation in amp

[nee]

Hi all

I picked up an old solid state amp (unknown brand) at a yard sale, replaced the four fried rectifier diodes and - hey presto! - it works.

When I hook my JBL 4311Bs up and turn the amp on, after a few seconds one woofer in particular develops a large oscillation at about 8 - 10 cycles per second. Not enough to rip the woofer apart, but worrying all the same. If I plug the port of the JBL the movement stops, which implies to my tiny brain that there must very little power driving the oscillation if restricting the airflow makes it stop.

I know I'm asking people to look for a needle in a haystack here - unknown amp and no schematic - but does anyone have any ideas as to where I might start looking for a solution?



Many thanks!

IanG

[SY]

Power supply and decoupling, especially the electrolytics. Look up "motorboating."

[sakis]

make a few pictures and RTFM : vintage amplifier repair/upgrade manual

happy repairing !!!!

[Dr.EM]

Yeah, sounds like "motorboating" for sure.

If plugging the port stops the movement all together I'd rather think that doing so is changing the electrical parameters of the woofer voice coil just enough to pull the amplifier back into stability. If it just reduces it then this is the normal effect of a sealed cabinet compared to an unloaded ported cabinet.