Crack switch conundrum

[PassionForSound]

My Crack is built and sounding sweet, but when I switched it off (using the onboard switch only) the tubes stayed hot and glowing (specifically the 6080, not sure about the 12AU).

When I test the switch at the terminals using my voltmeter, it appears that the switch is breaking the circuit between the 2 terminals on the back of the switch (i.e. infinite resistance in one switch position and zero resistance in the other). I can't understand why the tubes are staying hot and glowing - am I missing something? Could the switch be faulty and yet still show infinite resistance when "off"?

 

[Paul Birkeland]

Do the tubes glow perpetually after the switch is switched off?  It will take a few seconds for the heaters in the tubes to stop glowing, and maybe 5-15 minutes for the tubes to cool off.

 

[STURMJ]

it could be the caps are powering them, but that should olny last a few seconds.  If thats not the the case you need to troubleshoot, for safety reasons and to save your tubes!

 

[PassionForSound]

They stay hot indefinitely. I haven't tried the sound output after the switch is "off", but I just confirmed the issue by plugging in power after confirming infinite resistance across the switch terminals. Sure enough, the tubes went from cold to hot just like the amp was switched "on".

I wonder if the wiring is meant to somehow be different for 240V transformers? I have checked and re-checked my wiring, but it's interesting that the "L" label on the power socket is on the bottom right corner (when looking at the back/terminal side of the socket. There is no tab where the "L" is. The only tab other than the earth and neutral tabs is on the top left corner (as pictured in the manual), but it's not labelled and there's no external sign of it being connected to the "L" circuit.

I'm SO confused!

 

[PassionForSound]

Had a brainwave and tested the power socket terminals. There is no resistance between the neutral and live tabs. (I.e. top left and top right when looking at the back of the socket - see image)

http://www.flickr.com/photos/86948442@N06/8181238603/

Is that normal?

P.S. Quadruple checked the switch - it is definitely breaking the circuit.

 

[Doc B]

Plug the amp in and measure all of the test voltages with the power switch on, and again with the power switch off. The power switch switches only the primary side of the power transformer, so the secondaries will either all be on or all be off, i.e., either you will see no voltage with the power switch off, or you will see voltage on all the test points because the switch is shorted and needs to be replaced. Let Eileen know if the switch got shorted in assembly and she will send a replacement. queen at bottlehead dot com

 

[Paul Birkeland]

Yes, if the switch is off and the amp is unplugged, there will be infinite resistance.

With the switch closed (on), you will get the impedance of the primary winding of the transformer, which is relatively low.

It is possible that the switch arced over and is shorted open, the easiest way to know would be to listen to the amp with the power switch off...

The tubes will not glow with the amp not operating (unless you undertook some unusually creative wiring).

-PB

 

[PassionForSound]

If the switch is faulty shouldn't it show zero resistance in both positions?

I think the power socket is more likely at fault based on my previous post above. Thoughts?

 

[Doc B]

You sent me a photo of the socket, which looks correctly wired.
I supposed one could get the socket hot enough that it might short between pins, but I suspect it would look pretty distorted and yours looks OK. And I think that shorted condition would blow your circuit breaker whether the amp is switched on or not, because there would be a dead short when the power cord was plugged into the wall. Also seems like the amp would not work at all if the L and N pins were shorted together. Can you post a photo of the whole back end so we can see the wiring from the socket and switch over to the power transformer terminals? And is the resistance reading across L and N very close to 0 ohms or more like 20 or 30 ohms?

 

[Mosez]

PassionForSound - you mentioned you're in Australia. In europe, or at least in the Netherlands, power plugs are not polarized - meaning that the hot wire could end up on either prong. That's why switches are always double pole double throw. Could that be the case here?

 

[PassionForSound]

Thanks for all your responses.

The resistance across the L & N tabs is 0 ohms.

I'm away from the unit now, but will post a pic tonight. The power terminal soldering went without any hitch so I don't expect that it over-heated.

Would polarisation make a difference? I would have thought that it should still be a broken/open circuit when the switch is off.

 

[PassionForSound]

Here are some images of my wiring job. I should add that all of the voltages and impedances in the manual checked out ok. Still can't understand why the unit is permanently on regardless of the switch position.

Full board

Transformer

B Terminals

A Terminals

 

[Paul Birkeland]

Still can't understand why the unit is permanently on regardless of the switch position.

Have you looked into any of the measurements suggested?

 

[jimiclow]

Can you post a close-up photo of the switch/A/C socket?

 

[PassionForSound]

OK, I've had the chance to do more testing. Here are the results:

There IS resistance between the L & N tabs on the power socket (I made a newbie mistake and measured with the ohm-meter set for high impedance readings so it appeared to be zero). Actual value is just under 50 ohms across the L & N tabs.

I've measured voltages (as per the manual) with the switch on and off. There is no significant difference in the voltages recorded (i.e. 1 volt here or there, but I think that's standard margins of error)

I measured the impedance on the switch again. With the dot on the switch in the up position, there is no continuity. With the dot down there is a circuit with 0.04 impedance.

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

I still have full system power regardless of the switch power position - very confusing!!

 

[Paul Birkeland]

I still have full system power regardless of the switch power position - very confusing!!

The switch is shorted shut.

 

[PassionForSound]

Caucasoan Blackplate, that was my initial thought, but wouldn't that make the impedance on the switch solder tabs stay the same no matter what position the switch was in? As it is, altering the switch position appears to break the circuit from an impedance perspective, but is not preventing power flowing to the circuit.

It does sound like a switch issue, doesn't it? The measurements are confusing me and I don't know enough to troubleshoot in further depth.

Perhaps it's just time for a new switch as the next step.

 

[Mosez]

Do either hot or neutral have continuity to chassis ground?

I'm with Tim on this one - still think it's a power cable issue. Did you check whether L or N have continuity to ground? Alternate test would be to pull the fuse (from the power inlet) and see if it indeed cuts the power.

 

[Paul Birkeland]

I'm with Tim on this one - still think it's a power cable issue. Did you check whether L or N have continuity to ground? Alternate test would be to pull the fuse (from the power inlet) and see if it indeed cuts the power.

A power cable with hot to ground will blow a breaker...

 

[Paul Birkeland]

It does sound like a switch issue, doesn't it? The measurements are confusing me and I don't know enough to troubleshoot in further depth.

If voltage measurements don't change with switch position, the switch is stuck closed, period, end of story.

I'm not sure where this "hot to chassis" or "neutral to chassis" business is coming from, that's not going to magically feed the primary of the power transformer.