power-on-off thump

[nyazzip]

this is actually a SS question, not tube, but....
i just dragged out my old SS Pioneer amp , circa 1981...i hooked it up to my nice(for me)Klipsch speakers, 100 watt/96db....anyway immediately at power on i can see the speakers travel a startling-looking distance, and there is a loud thump. volume is down. input sources are off.
1)should i worry, and 2)what could i do to stop this? i'm thinking a capacitor across the power switch maybe?
....this unit has a 2 prong cord, so i intend to take it apart(maybe tonight) and install a proper one, so as long as i'm in there, maybe i could do something about the thump

[Geek]

Sounds like the power-on mute circuit/relay is shot.

[20to20]

edit:

[coolhandjjl]

Another suggestion to consider (unless you are already doing this) make sure you are turning the preamp and devices on first, and the power amp last. And at shutdown, power amp off first.

[nyazzip]

thanks
makes no diff about the sources; the speakers thump immediately upon pressing the power button. when i have it open to replace the cord i will check for relays, although admittedly i know nothing about them. i did snoop around at an old PC carcass i have, looking for a "555" relay IC that i read about last night but i didn't find one. i found a couple DIY delay circuits online but i was hoping it wouldn't be that complicated. i thought a capacitor from hot to gnd was supposed to ease the big turn-on spike? or an inrush limiter? no...?
my last resort solution is to always remember to switch the speaker selector to "B"(it is unused) before turn on, and before turn off. this solution relies on a good memory/knowledge by unfamiliar operators, though

[battradio]

Just make sure the DC offset is less than .1 volts , also check the cap at the bottom of the feed back loop is good , if its week or bad i can cause a turn on thump .
Make sure that the + & - power supplies come up at the same time and equal , bad power supply caps can also cause the thump .

[burnedfingers]

Just make sure the DC offset is less than .1 volts , also check the cap at the bottom of the feed back loop is good , if its week or bad i can cause a turn on thump .
Make sure that the + & - power supplies come up at the same time and equal , bad power supply caps can also cause the thump .

Won't a bad feedback cap to ground contribute to a high DC offset ? Also, won't that high a DC offset cause additional possible problems at a higher output level? When working on SS amplifiers I won't run them for very long unless I can maintain a DC offset lower than 100mv and try to get this down to less than 30mv or lower before I send them out the door. As a course of normal repair I generally just replace the feedback cap to ground if the amp has any age at all. If DC offset higher than normal results I match up another set of transistors for the diff input and in the case of the Ampzilla/SAE group a matching of the quad diff transistors is in order.

Yes, you are correct in that the power supply caps, the feedback cap, and sub power supplies like + and - 15 volt supply caps for opamp front ends also need replaced.

Another thought.... sometimes resistors need to be added to the power supply caps to aid in both the power up and power down to maintain a nice even charge and discharge of the power supply.