[fishydan]

Hello-

I've just completed my Cornet Octal, and it passed the smoke test.  Yippee!!!  It also passes a signal through both channels.  But there is some hum, which sounds like a ground loop.  

Do you have suggestions for troubleshooting?  Most likely candidates?

I am using the upgraded RCA's.  Should I use shielded wire to hook them up?

I also made the octal socket by epoxying an octal pc to a 9 pin pc.  It works great, but I'm worried that the proximity of the leads, which criss-cross at several points, may be inducing hum.  

Thanks in advance for any help.

Dan

[hagtech]

It's not a socket problem.  Are the RCA jacks insulated from chassis?  Best if you can post a good photo of the guts.  Then I can see what we have here.

jh

[fishydan]

Hello Jim-

Here are a couple of photos-

http://home.comcast.net/~fishydan/cornet.jpg

http://home.comcast.net/~fishydan/cornetii.jpg

Thanks,
Dan

[fishydan]

Hello Jim-

I forgot to mention that the RCA's are isolated from the chassis.  

I measured the heater voltage at 6 VDC.

Thanks,
Dan

[hagtech]

Construction looks perfect.  I forgot to mention that the black anodize of the chassis is an insulator.  This can cause problems with the ground lug and connecting the pieces together.  They must make contact.  It is also good if one of the transformer mounting screws connects the bells to the chassis ground.

I usually scrape away anodize under the screw mounts.  Maybe not enough was removed to make good contact.  All chassis pieces must make solid ground connect to each other and to back panel, which must connect to ground lug (connection point to PCB).

jh

[fishydan]

Hello Jim-

Thanks for the info.  Your tips have eliminated most of the hum.  At my normal listening level, it's pretty much gone.  I'm wondering if some of the remaining hum might be tube related.  It's slightly louder than my Curcio PAS 3, which is pretty darn quiet.  

Any other ideas to reduce the hum even more?

Thanks,
Dan

[hagtech]

It's possible some of the hum is coming from tubes, but not likely.  More often it is cabling and system setup.

What residual do you have?  A buzz or hum?

jh

[BobM]

I also have a slight buzz coming from my (non-octal) Coronet. I'll have to try the grounding wire trick from the transformer and see if it goes away.

It's noticeable when putting my ear close to the unit, with a very little bit trickling through the speakers (very little but it's there, and not there when I switch away from the phono source).

Thanks for this thread,
Bob

[fishydan]

Hello Jim-

I'd have to call it a hum, not a buzz.  It increases with the volume, so I'm concerned about what might happen should I use a lower output cartridge.  I am currently using a V15VxMR.

I have hum using the Cornet with two different TT/Pre-amp/Amp combinations- one solid state and one tube.

Thanks,
Dan

[fishydan]

Hello Jim-

Well, the hum is almost completely gone, very quiet indeed!  At the suggestion of another Cornet Octal fan, I used shielded wire at the inputs.  This dropped the hum considerably.  I'll do the outputs later.  Then on to tube rolling...

Thanks,
Dan

[BobM]

I tried the ground wire from the trannie and ... no change. Mine is more of a "buzz" that doesn't change with the volume. I already have shielded wire on the inputs, going all the way to the 1st set of resistors and bypassing that first long board trace. Swapping tubes also doesn't change it.

It's very low in volume, but it does come through the speaker and is definitely not tube-rush. I can hear it very plainly with a stethescope to the case, or the transformer, or very loud to the large varistor under the transformer by the regulator tube.

Any other thoughts?

Thanks,
Bob

[hagtech]

Bob, you hear audible noise from the Cornet itself (eve with speakers disconnected)?  Then it could be transformer issue.  Make sure the bolts on tranny are tight.  You might also have dc on the power line!

Which tranny are you using?  The 270BX or 370BX?  What is your line voltage?

jh

[BobM]

Yes, there's audible buzzing from the unit disconnected from the speakers on my workbench. I tried putting some EAR isodamp under the trannie, just in case it was vibrational. It reduced the level somewhat - it's low in volume now but its still there. Meaning, I can barely hear it standing over the unit, and I need to put my ear close to the unit to hear it clearly, but hear it I do.

I've got the 270BX trannie. Don't know my actual voltage these days (air conditioner's tend to drop it somewhat), but I've measured it at 115-117V in the past.

I have the unit plugged into a Jon Risch choked power conditioner when its in the system (no power conditioner on the workbench). It doesn't change the noise either way.

One other possibility, I didn't have the noise originally. It popped up sometime after I swapped out the orange drop caps for Sonicaps (in most locations in the non-power circuit). Not immediately, but about 2-3 weeks later. I probably should double check my solder connections to see if I've got a cold one. Perhaps?

Bob

[andrew]

Hi Bob,

  What heater voltage do you have?

     cheers,

-- Andrew.

[hagtech]

The 270BX has been known to be a mechanical transducer.  Mostly because it was designed for 115V and doesn't have much headroom (hindsight is great).  This is one reason I switch over to the more expensive 370BX.

You could actually try a 10 ohm 3 to 5 watt resistor in series with the primary.  That would drop about 3 volts.  

Or perhaps maybe just some rubber mounts will loosen it from a resonant chassis?

jh

[BobM]

Thanks Jim. Yes, I think it's definitely a mechanically induced buzz. Kind of confirmed when the EAR Isodamp pad under the transformer reduced the level.

I'll measure my line voltage and try the resistor. Now that you mention it, I believe the buzzing is lower these days than it was during the winter months (no air conditioners running everywhere reducing the line voltage), so that makes sense to me.

Thanks again,
Bob