Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: Loquah on November 12, 2012, 06:17:32 PM
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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"?
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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.
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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!
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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! :o
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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)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.flickr.com%2Fphotos%2F86948442%40N06%2F8181238603%2F&hash=2a4f6bdb5d9ea4d66b5f935036c9b7dd0dfe05b2)
http://www.flickr.com/photos/86948442@N06/8181238603/
Is that normal?
P.S. Quadruple checked the switch - it is definitely breaking the circuit.
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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
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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
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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?
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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?
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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?
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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.
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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
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm9.staticflickr.com%2F8339%2F8184563709_a325b93506.jpg&hash=33ff26fdb7470540688676ab7c860aa67c46786a)
Transformer
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm9.staticflickr.com%2F8486%2F8184601208_df57413bfa.jpg&hash=10940f325b44264be08a88a8d7f497348e36553f)
B Terminals
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm9.staticflickr.com%2F8488%2F8184562865_95b65c7a02.jpg&hash=c612c24b3c4f27977e0d1c45bf4c9c40ce8e34d8)
A Terminals
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm9.staticflickr.com%2F8485%2F8184600394_fa6e52a1fe.jpg&hash=785b056dc96b0ca9c5dc3adb432005769aa66022)
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Still can't understand why the unit is permanently on regardless of the switch position.
Have you looked into any of the measurements suggested?
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Can you post a close-up photo of the switch/A/C socket?
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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!!
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I still have full system power regardless of the switch power position - very confusing!!
The switch is shorted shut.
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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.
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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.
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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...
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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.
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[...]
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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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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? ???)
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Have any spare tubes? Bad tube can be a source of hum. It's the easiest thing to check first.
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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.
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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?
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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.
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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).