Laney VC30 near short to ground, popping fuses, odd behavior!

[OwenM]

Hi all,

I was really confident I could have a go at this one and work it out on my own but I seem to have come against yet another thing I don't get...

All my gear is failing at the moment, again, in new ways I hasten to add, not from my older repairs

My Laney VC30 is popping the mains fuse and lighting a lightbulb limiter brightly when just the power switch is on. I was in the process of working through the things that are powered up with the just the 'power' switch on (heaters, rectifier etc.) when I noticed this really odd behaviour.

When plugged directly into the wall or anywhere else it pops the mains fuse on the PT primary. Switched off and tested no shorts can be found on the primary loop (at least so far, but theres not much to test of course).

When I plug it into my 4 gang switched extension on my bench, with the socket switched OFF I can measure a short to ground from the IEC socket neutral pin. The amp power switch is in the off position, the fuse is out and it only shows shorted on the node from IEC socket neutral to open switch. It's the only thing plugged into the 4 gang plug extension on my bench.

If I unplug the amps IEC power cable from the socket, the short from neutral to ground disappears and goes open.

If I plug in through my lighbulb limiter, from this 4 gang extension (socket on the extension still switched off) I can see the short to neutral to ground.

If I switch off the lightbulb limiters own socket, the short from neutral to ground is still present

If I switch off the main wall socket that powers the switched 4 gang extension (extension socket still switched off) the short again disappears and goes open.

The amp, with fuse, and only power switch on, plugged directly into the wall socket lights the bulb limiter brightly. It does the same on my 4 gang bench extension.

What the heck? So there is something wrong with some sort of short, clearly. And I have a feeling that measured short to ground has something to do with it. What I can't get my head around is why it fails and lights the limiter/blows a fuse when tested in EVERY socket, but ONLY measures a short to ground from the neutral when plugged into my 4 gang switched extension lead, which still requires the main wall socket to be on (even if the extension switch is off) to read the short.

One thing that maybe is of mention is that I haven't measured to see that there is a short to ground on the IEC neutral pin when plugged into the wall AND the wall switch is switched on, just because I'm not gonna be poking around for resistances on the neutral pin while it's in a live socket. It seems so weird though, the short appears when using the extension socket switched off, as long as the extension is plugged into an energised wall socket. I should mention that all the other electrics in my house are working just fine, but this amp will blow a fuse, knocks out the house circuit breaker sometimes and will light a limiter till it hurts to look at it.

Could it be something about my switched extension? I feel like whatever extra circuitry is in my extension lead it shouldn't present a short to ground from neutral under any circumstance, and all other gear plugged into it works fine (I've tried several sockets for the amp and confirmed other gear working in those same sockets of the extension lead), so it MUST be the amp, but why wouldn't it show up when plugged into the wall? And this amp blows in any socket I've tried. But either way, this has flummoxed me.... Any help gratefully received, though I'm willing to just take this one to a shop if the behaviour I'm seeing makes no sense!


WAIT - so when my extension board socket is switched off, it goes open and grounds the sockets neutral pin rather than JUST going open? haha, maybe thats it, and I can go back to checking all the usual stuff!

EDIT 2 - Nope, testing the bare wires on a plug inserted into my extension lead (unplugged from the wall) confirms all three pins isolated from each other when the socket is switched off...

[Helmholtz]

I might have missed something, but did you consider that safety ground and neutral are connected at the building's grounding point?
Be careful with those measurements on live mains sockets!

Please post schematic of the amp.

[OwenM]

No, I didn't, I know very little about mains electricity, and was anticipating I may end up knowing a little more after this thread! My confusion would be that though I can see see why safety ground and neutral could be connected in my home, that doesn't seem to be the case when I plug directly into a wall socket, I see no short. The short only appears when I am plugged into my extension 4 gang socket.

Yes, I am playing it very carefully with the measurements, I have so far measured nothing when the amp was actually powered (any socket the amp has been connected too whilst measuring has been turned off). With the amp ON but in an OFF socket I get a short from neutral to ground, but ONLY if I am in my extension lead, not into the wall. But both on the extension lead or the wall the lightbulb limiter is on as bright as can be when I try and power it though that.

Maybe it would be helpful for me to record a video? I could do that tomorrow, it really is very very strange behaviour and I can't get my head around how to explain why its happening..

[Helmholtz]

Remember schematic..

[OwenM]

Remember schematic..

Apologies

I don't have the exact revision, but mostly it just seems to be pad numbers on the PCB that are different. So far all I have been testing is only in the PT primary loop, thats where I read the short, though I may as you say only be grappling with how my house wiring measures...

Laney VC30.pdf

[Enzo]

When you are turning off the socket, you really have no idea just what the switch is disconnecting. I agree, most of your problem can probably be "blamed" on the connection between ground and neutral in your service panel.

meanwhile your problem was that when plugged in, it blew its main fuse when power switch went to ON. That doesn't imply a primary side issue, it suggests a secondary side problem.

[OwenM]

Yeah, cool, thanks guys, I was in the process of investigating the active stuff on the secondary when I discovered this anomaly and tripped up, I'll go back to investigating the amp post secondary.

[Helmholtz]

When plugged directly into the wall or anywhere else it pops the mains fuse on the PT primary.

The amp, with fuse, and only power switch on, plugged directly into the wall socket lights the bulb limiter brightly. It does the same on my 4 gang bench extension.

This shows that your problem is not related to the switched extension.

Apart from that, I can't see why a connection between neutral and ground in the extension would cause the fuse to blow, as neutral and ground always are connected somewhere. I'm not sure, though, if safety regulations allow such connection within equipment. Wouldn't be allowed in my country.

As your problem shows with standby switch open and there is a heater fuse, it could be a PT problem. But we can only be sure, if the problem persists with secondary windings disconnected.

[OwenM]

I keep thinking I find something worthwhile and then quickly realise I've missed something obvious! Would I have to disconnect all three of both the heater and 17v sources secondary wires to adequately test?

[Helmholtz]

Would I have to disconnect all three of both the heater and 17v sources secondary wires to adequately test?

Disconnecting 2 wires of heater and 2x17V windings each should suffice.

[Dave H]

When you are turning off the socket, you really have no idea just what the switch is disconnecting..

The switch on UK mains power outlets is a single pole switch disconnecting the live (hot) conductor. It would be unusual to find a double pole switch there.

[Dave H]

I keep thinking I find something worthwhile and then quickly realise I've missed something obvious! Would I have to disconnect all three of both the heater and 17v sources secondary wires to adequately test?

The fault may be a shorted diode in the B+ supply. You could test for that first by seeing if the limiter bulb glows brightly with the standby switch in the "standby" position i.e with the switch open (or remove the 630mA fuse)

[OwenM]

The fault may be a shorted diode in the B+ supply. You could test for that first by seeing if the limiter bulb glows brightly with the standby switch in the "standby" position i.e with the switch open (or remove the 630mA fuse)

Thanks, I'm not sure I follow though! The power switch on, but the standby switch open, lights the limiter brightly. Removing the 630mA fuse opens the primary and thus nothing lights up, I've checked the B+ rectifier diodes and they check out ok in circuit FWIW but there are a few of the smaller ones I'm yet to find, but they're not on the B+...

[Helmholtz]

I understand that all the mains fuse blowing happens with standby switch open (?).

[OwenM]

Disconnecting 2 wires of heater and 2x17V windings each should suffice.

Thanks, the two grey wires from the transformer just run along the heaters until the last 12ax7. As far as I can see if I removed all the valves that would disconnect the heater 3v ac, is that a viable alternative to desoldering the connections? I can easily desolder but its a bit tight and thought that would be a way of achieving the same thing. I know you don't want to run an amp with no valves, and it will fully charge up the big caps too, but with standby 'open' and through a lightbulb limiter for a couple of seconds, is that ok?