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Need help with my KGST


meeskees

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After almost ten years of loyal service, my KGST is starting to show signs of malfunction.  Since last week I have a loud hum in the left amplifier channel.  I replaced the tubes with another pair and readjusted the balance and offset.  No hum could be heard and I could enjoy the music undisturbed again.  The next day the hum unfortunately returned.  Readjust everything and the hum was gone.  After switching the amplifier off and on again, the hum returned.  I have swapped the filaments, but that didn't solve the problem. I also left the gyro turned off, but that doesn't make any difference either.  Hopefully someone has a good tip to try.

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Someone with more experience in the stat amps should come along with some suggestions. From a general troubleshooting perspective, I presume you have checked the rail voltages and verified that they are correct? I would think they would be as biasing and the balance and offset adjustments would have been difficult or impossible if not.

Given its age, I would check connections to ensure they are tight. Don't ignore the IEC ground to chassis connection. I would presume this uses those god awful* phoenix screw down connectors? I would loosen and reconnect all of those as they can loosen over time.

 

* In my opinion. I greatly prefer pluggable connectors such as Molex, Amp, etc.

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Today I removed the left amplifier board from the housing.  There was nothing wrong with the Phoenix connectors.  They were still perfectly connected to the PCB and the connection to the cables was as it should be.  I cleaned the bottom of the print with alcohol to remove any flux residue.  After adjusting the offset and balance, I turned off the amplifier and activated the servo.  After switching on I heard the familiar hum again, only softer this time.  After a few minutes the hum disappeared and I can enjoy the music as usual.  I don't feel like I've really solved the problem yet.  So if anyone has any suggestions, please let me know

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This morning the hum was prominent immediately after switching on. Today I had some free time and I swapped the amp boards to see, as mwl168 suggested, if the hum moved with the board.  I have no idea yet, because after switching the amplifier on and off 3 times, no hum has been heard and I am now listening to music.  I don't know yet whether this has solved the problem.  In any case, thanks for the suggestions made

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Last night I wanted to listen to music.  Unfortunately the hum was back, but this time, as expected, in the right channel.  After adjusting the offset and balance, everything worked properly again.  I have no idea why I have to readjust everything after every listening session.  All suggestions are welcome

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Hmm. So it seems pretty clear that it is something on that board. I don't recall where the tubes mount on the KGST? If they are on sockets on each board and mover with the board, then MLA's post above would be something to try as well.

I would take the board out and examine all the solder connections, probably reflowing them or at least ones that look suspect. I'm assuming you have a soldering iron, etc. as from a search it looks like you built the amp? Since you are in the EU, I presume this used lead-free (ROHS) solder?

Another thing you could do is take a non-conductive wooden or plastic rod or stick and poke around the bad circuit board and see if you find anywhere that effects the hum level. If so, that should narrow down an area where there may be a bad connection.

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In Europe, at least in the Netherlands, lead-containing solder is still widely available.  When I cleaned my boards of flux residue, I checked all solder joints with a loupe for cold welds or bad connections.  I couldn't find any errors and thought I had actually soldered it very neatly ;) Coincidentally I discovered today that the hum disappeared after just over three minutes.  Until now I switched off the amplifier as soon as a hum was heard.To be on the safe side, I replaced the tubes of the right channel with another pair, but that makes no difference.  The hum also lasts about 3 minutes before it disappears.I am going nuts of those tubes. 

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