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sx-990 thumping at 240 hz

K

kep1113

Member
It's been a while since I fired this one up, but I figured I would let my teenage son put it in his room. A few years ago I did a total recap and I thought it was working OK, but maybe not. Anyway, when I turned it on yesterday I get a rhythmic thumping (not really thumping but I can't think of another term), about 4 beats per second. Doesn't matter what setting it is on, Speaker A, Speaker B, FM, AM, etc. It seem to start slow when first turned on, then quickly speeds up to the 4 bpm thing. Does anyone have a general idea of what this might be caused by? I know it could probably be multiple things, so even just a general direction to look into would be helpful!

Thanks!
 
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does it have a pre/ out main switch or jumpers on the back? if it does pull it or flip it, as the case may be, to isolate the main amp from the everything upwind
 
Agree with basic troubleshooting... preamp/power amp? Both channels? Headphones?

Sounds like a capacitor discharging, probably a small one because you haven't stressed too much.

I'd have a close look at all the solder joints under a magnifying lamp. Also recheck soldering of recap work.

You might be able to pick this up with a multimeter as a spike in DC volts, try on the speaker posts
then work back(or whatever) following the schematic.
 
Sorry, life got in the way for a while. It does have a set of RCA plug jumpers for Pre-Out/Main-In on the back. I suppose I would need to rig up some RCA plugs for the speakers to see if the thumping is coming from the Pre-Outs? I'll take the advice above and give it a closer look. Will report back on what I find. Thanks for the input!
 
The pre out/main in connections are for connecting a separate pre or power amp. I would not attempt to connect speakers to these under
any circumstances. Assume you don't have another amp with pre out/main in RCA's (Pre from one amp to main of the other...)
Some people have connected their ipod, ixxxx via suitable cable to the main in, the ipod acts as the preamp stage, however this should
be done at minimum volume, start at zero vol and increase a little.

Is thumping on both channels?
If yes then maybe a power supply issue
If not on both channels then you could remove the pre out/main in jumpers and replace them with an RCA cable but criss/cross them.
That is Left pre out connects to Right main in. The other RCA cable from right pre out to left main in. If thumping stays on the same channel
the power amp issue. If it changes channel then preamp issue.


You could also try capturing this at the speaker teminals with a volt/multimeter, dc voltage.
Also, remove pre out/main in links and try to capture at pre out links measuring inside the rca with the red probe.
In both cases black probe connected to chassis, phono ground or black speaker post.

Just downloaded service manual from hifiengine. The preamp is populated with 2SC458LG's. These have a strong
history of going noisy. It's generally accepted that these should/must be replaced, kind of a shoot on site. KSC1815
or KSC1845 are suitable replacements. Do you have the skills/tools... to replace transistors?
 
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