Counterpoint SA-1000 Hum in left channel Pinging Anatech! - diyAudio
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Old 14th April 2013, 03:16 AM   #1
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Join Date: Jan 2013
Location: San Diego
Default Counterpoint SA-1000 Hum in left channel Pinging Anatech!

I have a hum in the left channel of my Counteroint SA-1000 preamp. now before I start I have tried all the tricks to narrow it down with no great success. I fully upgraded this preamp a few years ago and was in heaven with the sound. Until....it slowly developed a hum in the left channel after 8 months of no hum issues and which I thought was a tube going bad. so I changed it, same hum.

now the question is is it a 60hz or 120hz hum? I would love to tell you but my scope is down and the 2 dmm i have one doesn't have a freq measurement and the other that does for some reason is not picking up the hum.

I did the chop stick trick and resolder all solder joint again but not all at the same time.

i have replaced and upgraded all components in the filament, B+ supply active and passive components plus check all the grounds. even replace all the components in the relay circuit. All the B+ caps have been replaced and upgraded to Mundorf Tube cap and Mundorf HV MLAL caps. replaced all active comeponents, Mosfets, Zener diode, regulators twice just to make sure.

All film cap were upgraded to Auricaps (and replaced coupling caps in the linestage again with Dynamic caps), all resistors around the tube have been upgraded to Mills or Caddock MK-132. all tube sockect were up graded to ceramic gold plated. Volume and balane controls were upgraded to TKD, all wire and connectors were upgraded to vampire. upgraded tranny to the plitron for this preamp from altavista audio.

So here's what I did and what the results were. I shorted the signal to ground before the volume control...no change in hum. Shorted the signal after the output coupling cap hum was gone. so it seem like the hum is definitely between those 2 stages of the tube and mosfet. I did do the same in the phono section but no change in hum. this hum originally doesn't change in volume when you turn volume up. Always the same. Now it sounds to me like a 60hz hum cause it seems at the same freq, as the fridge whe it running which give off a 60hz hum noise.

I did find a 200k resistor on the left channel that measured 210k. replaced both 200k resistors in both channels no change in hum.

all the wires from the tranny are twisted now and no change in hum. I'm going to hit all the solder joint one last time before i through this preamp out the window but take out all the parts before I do that! and i will also solder the grounds from the RCA screw ground point to the ground wire connected to the ground spacer rings that are screwed down for ground on the connectors just to make sure.

Any help would be greatly appreciated.

Thanks
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Old 14th April 2013, 05:33 PM   #2
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Join Date: Jan 2013
Location: San Diego
Default Counterpoint SA-1000 hum UPDATE!

Hello all,
I finally got my O-scope up and running with that probe that I have.
Ok, here's what I fond out today and have been suspecting all along. The HUM is 60hz. But its only coming out of the left channel and doesn't not change in loudness when when Volume is turned up. So I'm suspecting is a bad solder joint somewhere in this preamp.

I have retighten all the RCA connectors and replaced the 200k resistor that I found out of spec that was measuring 210k on the Source lead of the Mosfet and the the other lead of the 200k resistor is attached to ground. I have even again replaced the input and output coupling caps in the linestage just to be sure and replaced all the caps and regulator again in the Filament supply just incase. Still the hum is there!

The only thing I did do from the original design was put a bigger output cap from 1uf/400 to 2.2uf/450 auricap then I replaced the output caps with Dynamic caps 2uf/450. This shouldn't have any affect due to the 100 ohm resister between the output cap and the Mosfet that is used as a buffer between the Tube and output cap.

Like I said in the beginning there was NO hum after I did all the upgrades and it slowly came over 8 months. This Counterpoint preamp was really sounding just wonderul until that Hum and to be honest still even with hum sounds really nice.

I know there will be a very small hum in both channel once I get rid of this Hum in the left channel, but original littel hum was so low you would have to get up to speaker to hear so that is just the nature of the beast with this system so that 60Hz hum in the left channel is audible from 8 feet away so I know this is not normal.

Any help now that I have identified the frequency of the hum would greatly appreciated. I will keep at it but any advice is really appreciated it.

Thanks everyone for all the help and suggestions! Please keep them coming!
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Old 14th April 2013, 09:16 PM   #3
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Join Date: Jan 2013
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Default I think I may found the problem....

I started doing resistance checks again and notices that the area where that 200k [COLOR=#009600 !important][COLOR=#009600 !important]resistor[/COLOR][/COLOR] (the one that was mearsuring 210k)and 100 [COLOR=#009600 !important][COLOR=#009600 !important]ohm[/COLOR][/COLOR] resistor attached to the Source lead of the Mosfet in the bad left channel was measuring 2.1 meg to [COLOR=#009600 !important][COLOR=#009600 !important]ground[/COLOR][/COLOR]. The good channel was measuring around 200k to ground. snipped the Source lead of the Mosfet in the bad channel and measurement immediately went to 200k, did the other mosfet too which was measuring close to 200k and now has the same measurement of 200k. I must have damaged the Mosfet in the left channel when I was either changing it or was damaged due to the original 200k resistor being out of tolerance. Hopefully this will be the cause of that hum.

I will post back the results in a few days.
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Old 17th April 2013, 03:01 AM   #4
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Join Date: Jan 2013
Location: San Diego
Default I Give up! it wins!

I have wasted to much money and time on this preamp. I will just move on to my Aikido project.
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