Crack switch conundrum

[Mosez]

[...]

There is no continuity from the L or N to the chassis.

I had missed that you had already answered this one. And PB is right (as always I quickly learn). Guess I better stick with plumbing.

 

[Paul Joppa]

If there is 50 ohms (that's about the primary DC resistance for a 240v transformer) from line to neutral no matter what the switch position, then line and neutral are connected to the transformer primary. Either there is a wiring error, or the switch is shorted. If the switch works, then that leaves a wiring error which bypasses the switch.

This is simple logic. Simple logic means I have assumed that all the information needed has been posted already, and that I have missed nothing. Naturally, neither one is a true certainty (!) but that's my best shot right now.

 

[Laudanum]

Im with Paul J here (why wouldnt I be  ;D).  Logic tells me that if the switch measures open in "off" position and their is continuity in "on" position then the switch is working, therefore there has to be a wiring error ... somewhere.

 

[Doc B]

Simply disconnect the switch at one end - the one that connects to the IEC socket - and see if the amp still runs when plugged in. That will tell you whether it is the switch or some other issue.

 

[PassionForSound]

Thanks everyone. Doc B, I must have picked up on your suggestion in my sleep because that thought came to me this morning so I disconnected one lead to the switch and voila! No more power to the amp. I'll go to the local electronics shop today for a new switch.

 

[PassionForSound]

Replaced the switch and it's working a treat now - I can actually switch it off!!

While I was soldering I also added jumpers across the unused TRS terminals as per the post on this board. I'm noticing now some background hum that I didn't notice during initial testing. Could the TRS jumper job cause this?

 

[Paul Birkeland]

Could the TRS jumper job cause this?

There's a very, very slight chance that one of those grounding wires might be poking the chassis plate by the headphone jack.  I'd get a small flat head screwdriver and pry any dangling wire ends up and away from the plate if applicable.

 

[PassionForSound]

No sign of any wires anywhere touching the chassis plate. Anything else I should check?

I might try disconnecting the jumper wire on the TRS and see if that fixes it.

 

[PassionForSound]

Just disconnected the jumper wire and no effect - the hum must have been there all along.

It becomes audible around 50% volume. Is that just a case of "normal" noise (i.e. within design tolerance) or should I be hunting for ground loops? (If so, how do I do that?  ???)

 

[Laudanum]

Have any spare tubes?  Bad tube can be a source of hum.  It's the easiest thing to check first.

 

[Paul Birkeland]

It becomes audible around 50% volume. Is that just a case of "normal" noise (i.e. within design tolerance) or should I be hunting for ground loops? (If so, how do I do that?  ???)

Depending a bit on the headphones, 50% of the travel on the volume control may yield an unbearably loud amount of output with headphones connected, which would indicate that you are just hearing the nosie floor.

 

[PassionForSound]

I'm using HD650s. At about 50-55% seems to be my sweet spot and I'm not a loud listener. My source was an LOD from an iPod Video. Does this sound like I'm not getting full output or is that normal listening level for HD650s?

 

[Paul Joppa]

If it goes up with the volume control, then it must be coming in before the volume control. Ground current noise adds to the signal and does not change with the volume setting, so it must be on the input signal line. It could of course be the source, or (more likely I would guess) it's picked up by the circuitry or interconnects.

To check, first use shorting plugs on the input and see if that kills it. Try at all volume settings; if it is gone at zero and at full volume but loudest around 80% of the volume control then it is picked up at the volume control output.

If it is gone with shorting plugs, try again with nothing plugged into the inputs. If it is quiet at zero volume and maximum at max volume, then it it picked up by the wiring between the input jacks and the volume control.

 

[PassionForSound]

Thanks PJ.

I tried disconnecting my braided wiring from the RCAs to the pot and the sound went.

I've replaced the braid with a high quality shielded interconnect cable and the hum is completely gone. I also made a direct connection from the RCA grounding pins to the main earth on the chassis just to be sure.

There does still seem to be the slightest hint of hum. It's not altered by the volume control and seems to just be system hum. Any thoughts on removing this or is a tiny bit of hum (i.e. only just within audible limits) normal? I can only really tell it's there by switching the amp off and back on again (i.e. I notice it by subtraction more so than by addition).