Crack switch conundrum

[Paul Birkeland]

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

The switch is shorted shut.

[Loquah]

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.

[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  ).   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.

[Loquah]

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.

[Loquah]

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.

[Loquah]

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.

[Loquah]

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.