[tubesorbust]

I finished my Cornet2 and was excited when the light changed color and I didn’t have to use my fire extinguisher.  Unfortunately when I plug it in I get a loud hum.  I jumped on the forums and tried the grey wire from the tranny to the groundnut, twisting wires, and checking all my ground wires.  Unfortunately I still get a really loud hum.   It only happens when I plug in inputs.  If no input is plugged in there is no hum.  It also seems as if the hum is only coming out of the right speaker.  On the up side the music does sound spectacular under all that hum.   Any ideas?

[poty]

Are you use PCB-mounted RCAs? Is it possible that the shells of the RCAs is not soldered (connected) to the signal ground? Maybe problem is in the interconnects? Source?

[tubesorbust]

I am using the AES #S-H267W and #S-H267R as per the upgraded parts list.  I've traced the leads and soldered the wire to where the CP-1435-ND would be attached.  The tabs on the side of the part to the ground and the middle of the RCA plugs to the respective L&R of the board.  I've tried different interconnects and sources from an iPod to a turntable with the same result.  As soon as something touches the input it hums loudly.  I've ordered the CP-1435-ND (original called for part) to try it out just in case something isn't right about my RCA install. 

[poty]

Could you be more specific? The hum begins-
- after you connect an interconnect only (no device on the other end)?
- after you connect the IC cable + powered off source device?
- after you connect the IC cable and power on source device?

Is it possible to post some photos of your Cornet 2?

[el34]

If you swap the input RCAs (from the turntable to the Cornet2) does the hum move to the other channel?  That would narrow it down to the turntable and its wiring versus the cornet itself.

Also, if you have spare 12ax7 & 12au7 tubes, I would try substituting them.

[tubesorbust]

Without the interconnects there is a hum that comes from the right speaker.  When I plug in the interconnects (with nothing attached) the hum becomes really loud and comes out of both speakers.  When I’m holding the interconnects and touch the chassis or the volume knob to the pre amp the hum quiets down a little bit, and when I take the tip of the interconnects and touch them to the screws of the Cornet the hum dies completely.

I’ve added some photos.  This is my first build of anything, hence the 6 too many holes.   

[tubesorbust]

If you swap the input RCAs (from the turntable to the Cornet2) does the hum move to the other channel?  That would narrow it down to the turntable and its wiring versus the cornet itself.

Also, if you have spare 12ax7 & 12au7 tubes, I would try substituting them.

I'm pretty sure it's the Cornet at this point because when I plug an IC into the input without anything connected to the other end of the IC the hum still happens.

[el34]

I'm pretty sure it's the Cornet at this point because when I plug an IC into the input without anything connected to the other end of the IC the hum still happens.

Short the input IC center pin to the cornet chassis, that's a better test, with un-connected inputs, most bets are off.

[el34]

Best practice with those input wires in your picture would be to twist them together with the ground wire, and keep them short.  Do you have a meter you can use to check continuity between the jacks and the board?

[el34]

I'm suspicious of that transformer wire attached to the phono ground lug, what's that for?

[tubesorbust]

I checked the continuity of the plugs to the board with the meter and they all are good.  I pulled the grey wire from the transformer as per the forums.  Well it starts out grey but I made it longer with a black wire.  It's usually connected to the case of the transformer as a ground but some people say that it's solved their hum but pulling it through and grounding it to the plug.   

[poty]

What you were describing (and after I looked through your photos) I can guess that there are problems on the input part. What I'd like to check:
- if you have ohmmeter (multimeter) - turn off your device completely and measure the resistance between the central pin and the shell of input RCAs;
- please unsolder everything from all RCAs (input and output) and check with the multimeter if the shells are isolated from chassis. Connect each RCA with its own twisted pair of wires (from the shell and pin) to PCB, do not connect shells directly between them;
- after soldering the wires from the RCAs in place, check the input resistance again, check connectivity of central pins to pin 2 and 7 of the V200 tube, check connectivity of shells to ground lug.

[el34]

Poty's plan looks smart, I 2nd that.

Overall the build looks neat and clean, but I'd make the following changes:

Safety ground from the power inlet should attach to its own dedicated chassis connection, not the turntable ground post. You have an anodized chassis, so you will need to scrape away the surface to get shiny metal for a good contact, then connect the safety ground close to the power inlet with a screw, toothed lock washer, and nut.

What is that wire from the transformer to the turntable ground post?  That still doesn't look right to me.

[tubesorbust]

Poty: I'll do all that tonight once I get home.

EL34:  I'll move my safety ground to the chassis tonight as well.  Would it be okay to move the ground wire from the transformer to the same post as the safety ground? That's the wire currently on the turntable ground post.

[el34]

Poty: I'll do all that tonight once I get home.

EL34:  I'll move my safety ground to the chassis tonight as well.  Would it be okay to move the ground wire from the transformer to the same post as the safety ground? That's the wire currently on the turntable ground post.

Everything I read about the safety ground says that it should be its own dedicated connection, with a lock tooth washer, close to the power inlet.  Don't forget to scrape some of the anodizing off at the spot where the lock washer attaches (that should be on the inside of the chassis.)

So that extra wire is the center shield on the transformer?  I would run that to the board power supply ground to keep it away from the signal ground, but it may or may not matter in reality.  I would run the signal grounds (RCA jack outer connections) and the turntable ground all to the same spot (the ground solder point for the RCA jack) with twisted pairs (ground/signal) for the signal wires.  The main Idea is to keep the signal grounds separate from the power supply grounds (until they rejoin through the power supply)

Soldering on those RCA jacks is not easy, it helps to crimp the wires on tight first, make sure both wire and jack heat up good then solder. (not so much as to melt the plastic insulation, though)

[tubesorbust]

I would run the signal grounds (RCA jack outer connections) and the turntable ground all to the same spot (the ground solder point for the RCA jack) with twisted pairs (ground/signal) for the signal wires.

Since the turntable ground post is touching the chassis doesn't it ground there?  Are you saying to run a wire from the turntable post to the RCA center pin ground on the PCB? 

Also here is a pic of so far what I think I'll be doing tonight as well as making measurements and connectivity tests. 

[poty]

tubesorbust and el34:
according safety ground and signal ground: there may be several approaches. In this device the signal ground is on PCB, the ground from turntable is not related to the signal ground, so it directly goes to the "ground point". To the same poit there are connections from safety ground and signal ground. I'd save it as is.

[el34]

Since the turntable ground post is touching the chassis doesn't it ground there?  Are you saying to run a wire from the turntable post to the RCA center pin ground on the PCB? 

Yes, something needs to connect the chassis to ground on the PCB.

Also here is a pic of so far what I think I'll be doing tonight as well as making measurements and connectivity tests. 

It looks like you're on the right track.  Do the wire twist on the input wires too... it's more important there because the signal level is only a few millivolts, see how the board has ground traces surrounding the signal traces on the input side?  the concept is the same for the twisted pair wires.  It would help to have thinner wire to get a tighter twist, and avoid a big loop, ideally a short straight connection.

[el34]

tubesorbust and el34:
according safety ground and signal ground: there may be several approaches. In this device the signal ground is on PCB, the ground from turntable is not related to the signal ground, so it directly goes to the "ground point". To the same poit there are connections from safety ground and signal ground. I'd save it as is.

Yeah, it might work as is, but the standard advice everywhere is safety ground should be it's own connection, be locked down tight, and go direct to chassis.  I agree the turntable ground would probably be fine going to a point electrically near the power supply, but I suggested the other point because I know it works fine based on my cornet (original board). The connection from the transformer is the one that bugs me in terms of noise pick-up, I think that should be kept away from the signal ground to avoid polluting it.

[tubesorbust]

Yes, something needs to connect the chassis to ground on the PCB.

In that case, the wire from the turn table ground to the PCB should suffice.  I can't trace a line so I drew arrows to show the line from the turntable plug from the chassis that goes into the PCB.  It's hard to see it because it's black.  I didn't have any green wire at the time.  The Saftey ground is also soldered onto that connection as well.  So as it stands now there are 3 things connected to that turnable ground: the PCB, the saftey ground, and the transformer ground.