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

[Helmholtz]

Sure.

I'd say, if the problem stops with all tubes pulled, we know that one of the tubes was the culprit - if not, well...

[OwenM]

Yup, and thats what I'm hoping for, getting pricey for a new transformer for this amp on a quick search!! I'll pull the tubes, do a quick test and if its still the same remove the PCB and desolder the final PT secondary leads.

[OwenM]

Pulled all the tubes and same result.

Before I got the front plate off and PCB out to desolder the 17v secondary I had a thought, I can just remove the 500mA fuses, top right of the schematic. I don't think the 17v goes anywhere else as far as I can see, it just feeds the low level DC voltage circuit.

Unfortunately, the limiter is still lighting as bright as can be...

EDIT, well, as far as I can see the transformer is isolated and I'm getting 2 Ohms between the heater and 17v secondaries... Limiter still bright, I assume because the grounds of the heater and 17v secondaries are now acting as their own secondary to ground?

[doombass]

If you have two 500mA fuses instead of the two 10R R55, R56 you can pull those. Also you can remove the T2A heater fuse to prevent grounds to interfere with secondary winding measurments.

[OwenM]

If you have two 500mA fuses instead of the two 10R R55, R56 you can pull those. Also you can remove the T2A heater fuse to prevent grounds to interfere with secondary winding measurments.

Crap, I've just realised I'm referencing a schematic that has R55 and R56 replaced with the two 500mA fuses, they were the ones I mentioned pulling above! The schematic thats closer to my amp I only have in a printed version at the mo. I'll try find the closer one and replace my upload schematic. I can't actually see the heater fuse on my amp, making me thing perhaps mine doesn't have it? Well, I'll have a look anyway and try isolate at least one of the grounds.

I assume, if my idea is correct that the heater and 17v secondaries are shorted then if I lift the ground on one of those secondaries the bulb limiter will turn off as the primary seems to have no short?

Edit - updated schematic

[g1]

If there is a fault in the transformer then you will get the bright bulb no matter what secondaries you disconnect.
If you have that 2A fuse in the heater winding, remove it and recheck that resistance from heater to 17V windings.

[OwenM]

If there is a fault in the transformer then you will get the bright bulb no matter what secondaries you disconnect.
If you have that 2A fuse in the heater winding, remove it and recheck that resistance from heater to 17V windings.

Thanks, excuse my ignorance but if the secondary was completely disconnected from circuit and the primary was intact and correctly isolated from the secondary why would the bulb limiter illuminate? In my current understanding it can't induce any current flow in the secondary, broken or otherwise, as all the leads are disconnected and we would just be passing mains through a big inductor (the primary). That would be, in my current understanding, the same as if we were passing mains through the primary of a known-good transformer with its secondaries disconnected. I can totally see why, if the primary was shorted in some way, that we'd get the bulb limiter to light up. I measured open from primary to secondary at this point in time/testing

Not disputing in any way, I'm just missing some understanding and as always, learning TONNES

I'll look for the 2a heater fuse, and if I cant find it I'll go in fully and desolder everything from the transformer secondary!

[Helmholtz]

Thanks, excuse my ignorance but if the secondary was completely disconnected from circuit and the primary was intact and correctly isolated from the secondary why would the bulb limiter illuminate?

The most probable PT defect with the symptoms observed would some shorted turns of either primary or a secondary winding. This behaves like a shorted secondary and reflects to the primary as an AC short. Often impossible to detect by DCR measurements.

[OwenM]

The most probable PT defect with the symptoms observed would some shorted turns of either primary or a secondary winding. This behaves like a shorted secondary and reflects to the primary as an AC short. Often impossible to detect by DCR measurements.

Thanks, gotcha

[The Dude]

......if the secondary was completely disconnected from circuit and the primary was intact and correctly isolated from the secondary why would the bulb limiter illuminate......

By unhooking the secondary from the circuit, you are NOT isolating the transformer primary from the secondary. The primary and secondary are still AC coupled within the transformer itself. If there is a short in any winding, the bulb limiter will light. It's NOT simply two separate chokes without any EMF relationship.

[Dave H]

Removing the 630mA fuse opens the primary and thus nothing lights up,

Sorry, I should have have said the 630mA fuse on the high voltage secondary. It was there on the first schematic you posted but not on the latest schematic so I guess you don't have it in your amp.

[OwenM]

By unhooking the secondary from the circuit, you are NOT isolating the transformer primary from the secondary. The primary and secondary are still AC coupled within the transformer itself. If there is a short in any winding, the bulb limiter will light. It's NOT simply two separate chokes without any EMF relationship.

Thanks, I need to get my ass back to transformer school... By the primary isolated from the secondary I meant not shorted 'electrically', but now I think of it, secondary wires disconnected or not I can see how the EMF field on either side would still be generated and the picture would be more complex than my simple choke idea.. Good to know I can get the transformer completely isolated and the limiter will clearly tell me if its at fault! I'll do that tonight..

Sorry, I should have have said the 630mA fuse on the high voltage secondary

Ah I see, sorry that was my bad! I didn't realise the PDF I posted had a few more differences to my printed schematic than I had noticed..

[OwenM]

From the heater and 17v secondaries I have 2 wires each as the center tap. The labeled Black/Yellow on the 17v, and what I think is two Black wires from the heater. I can't see a 2A heater fuse anywhere, oddly enough. Does it matter what I do with the pairs of center tap wires? Ie the 17v has two center wires going to ground, should I leave them disconnected (taped off individually) or maybe splice them together or perhaps it doesn't matter? My hunch is it doesn't matter but this is getting to the edge of my comfort zone and I'm taking the time to make sure I've asked every obvious question along the way (not to mention what I have assumed obvious has proved wrong many a time )!

[Helmholtz]

Best remove all secondary wires from circuit and tape them individually. Maybe take a picture before you start.

[OwenM]

So center taps of the 17v disconnected, the 2 fuses on the 17v ac removed, heater center tap disconnected and all valves removed so the heater 3v either side is disconnected and standby switch open and the result is... light bulb limiter still glows very bright.

I no longer read the small resistance heaters to 17v but of course they were connected because both the center taps were going to ground, so that was one of my classic excited fails.

But that seems to suggest the transformer is bad? I guess I'll decide wether to buy a new one straight away or wait a bit. What sort of things should I start looking for to see why it may have failed? Or is it a case of installing it and checking voltages carefully etc? The amp was running absolutely fine up to the moment before death so I guess it could be a sudden component failure or gradual strain on the transformer?