Need help with my KGST

[meeskees]

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.

[Pars]

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.

[meeskees]

They are indeed Phoenix connectors.  I'll start working on it tomorrow.  Thank you for this practical tip.  The problem cannot be very serious, because the amplifier also works well from time to time.

[meeskees]

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

[mwl168]

Maybe try to swap the left and right channel boards and see if the hum follows the PCB board? 

 

[meeskees]

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

[Pars]

When you were checking the connectors yesterday, did you loosen the screws and reseat each wire? I presume in order to swap channels, you had to disconnect/reconnect them all? I'll be interested to see if it now stays quiet.

[meeskees]

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

[MLA]

Next step is usually swapping the tubes between boards to see if it's the boards or the tubes. In the process, cleaning the tube pins might be a good idea; if some have oxidized and have poor contact, hum may occur.  

[Pars]

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.

[meeskees]

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. 

[chinsettawong]

The hum on my amp associates with the attenuator.  On some positions, it’s quiet and in other positions the hum is quite disturbing.  I’m using the Alps RK27.  Could that be something similar to you?

[sorenb]

On 6/11/2024 at 2:56 PM, meeskees said:

After adjusting the offset and balance, everything worked properly again

What makes you do that over and over again? do both phases require adjustment?

it might be a clue to look at the CCS's, and whether those are properly connected to the heatsinks ... 
You might look at the amount of current being supplied by each CCS with and without hum
 

[meeskees]

My amplifier also contains an Alps R27.  The position of this potentiometer has no influence on the hum.  The current state of affairs is that, after switching on, I have 3 minutes of hum in the right channel (after exchanging the boards and then the amplifier is silent and everything functions as it should.  The tubes are not the cause of the hum, because the phenomenon also occurs with other pairs of tubes.  Both with GE and RCA tubes.  So apparently there is an imbalance that is resolved after three minutes.  Fortunately I can just listen to music, but if anyone has a good tip?

[kevin gilmore]

suggest you solder all connections associated with the filaments. and then solder them again.

[spritzer]

Could be an internal short in the transformer too on the heater windings. 

[meeskees]

Par already pointed it out to me and now Kevin Gilmore has done so too, so who am I to doubt these authorities.    This afternoon I removed the board again and re-soldered the connector for the filaments.  To be on the safe side, I also provided the tube socket pins with fresh solder.  Once everything was put back together, I turned on the amp.  And.......no hum this time.  Unfortunately we cheered too early.    After dinner I wanted to listen to music and the hum was present again, but fortunately it disappeared again after 3 minutes.  

A short circuit doesn't seem likely to me, because when I swapped the board, the hum moved with the board, while I had connected the other winding for the filament voltage

[jamesmking]

12 hours ago, meeskees said:

Par already pointed it out to me and now Kevin Gilmore has done so too, so who am I to doubt these authorities.    This afternoon I removed the board again and re-soldered the connector for the filaments.  To be on the safe side, I also provided the tube socket pins with fresh solder.  Once everything was put back together, I turned on the amp.  And.......no hum this time.  Unfortunately we cheered too early.    After dinner I wanted to listen to music and the hum was present again, but fortunately it disappeared again after 3 minutes.  

A short circuit doesn't seem likely to me, because when I swapped the board, the hum moved with the board, while I had connected the other winding for the filament voltage

Here are a few suggestions. My apologies if you have tried any of these already.

If the hum is changing after a few minutes then this implies to me that thermal expansion could be the cause. Are you sure all the pins of any socketed components are gripped well by their sockets? I believe there have been cases where over time some of the teflon tube sockets pins have started to lose their grip..

I would also very careful with the amp off and all capacitors in the psu drained and using a high resolution multimeter check the resistance between the psu connectors and the amp boards to make sure there is not a poor wiring connection due to oxidized wires etc this test will also check for any potential problems with an umbilical if you have the psu in a seperate box from the amp with seperate wires for the left and right channel.

I think at this stage you need to start being very systematic and seeing what is going on.. so do you have access to an oscilloscope?

I don't have a KGST but I assume it has separate and identical + and - amplification stages for each channel with a phase splitter at the input stage to allow single ended or balanced input... 

With a scope you can CAREFULLY (10x probe and dont connect the probe ground lead to anything that is live) probe the output pins at the stax socket one by one AVOIDING the bias pin and see if you have hum on both the + and the - outputs of a channel or if it is only the - or + half. This could potentially eliminate half the components of a channel from being faulty. If both - and + of a channel has hum then suspect something common to both which is usually the psu or input phase splitter. If you have seperate psus for each channel then you need to look at the psu rails to see if there is excessive ripple on one of the psus. If each channel shares the psu and only one channel has an issue then its very unlikely to be the psu but it could be the wiring from the psu to the effected channel.

If all else fails a scope is an essential debugging tool and you are going to have to compare the good and bad channels and see what is different.

good luck

regards

James

 

[meeskees]

Thanks for your detailed response, James.  So far I have only resoldered the tube sockets and filament connections.  I will re-solder the other soldered connections in a few weeks.  Unfortunately I don't own an oscilloscope and I wouldn't know how to use such a device. I'm not that technical.  For now I'll leave it at that, because the amplifier will eventually function normally after three minutes and my health won't leave much room for any effort in the coming weeks.