La-IV PSU issues, So hot toroidal

[dirtyhanfri]

Hi

I´m building a LA-IV Clone, and I got this power transformer:
http://es.farnell.com/jsp/search/productdetail.jsp?SKU=9531688

I´ve built everything ok..., some issues, just replacing a regulator, ok.

But when I connect the compressor circuit to the power supply the toroid gets really hot, I can feel the heat just placing my hand near it (4-6 cm). Also I can clearly hear a Hum coming from it (audio input and output are disconnected).

I have some isssues with the power supply, but the toroid looks ok, I´ve measured his outputs today and it seems to be good, the issue is with the 7918 regulator, wich gives me 21V instead 18V, the voltage in the input looks ok. But I dropped it to 17.8 with a pair or 1M 1/2 W resistors in series. I don´t know if it´s crazy idea...

I´m worried about the toroidal, I think can´t be good it works so hot, or is it usual?

Thanks for your time

 

[ilfungo]

hello
I had a similar problem, and I realized that I had wrong connection on the secondary of the power transformer.
rechecks
ciao

 

[dirtyhanfri]

Bad Wiring? I´m almost sure it´s ok, but I can´t check it right now...

I was thinking in a bad connection in a molex or a solder point touching something wrong...

In one of the 7918 replacements I´ve blow a solder pad, so I wired it point to point and screwed the metal plate to a standoff, touching the chassis. is this ok?

 

[Rybow]

Woah! You have to electrically isolate the metal plate of the regulator. You are shorting voltage to the chassis, which is most likely causing your overheating toroid. Do you have a fuse in there? It should have popped.

 

[dirtyhanfri]

Woah! You have top electrically isolate the metal plate of the regulator. You are shorting voltage to the chassis, which is most likely causing your overheating toroid. Do you have a fuse in there? It should have popped.

Yes, I have a 1A Fuse and it doesn´t popped.

Well, that was a smart, but wrong solution to my damaged pcb, I´ll look for another place for the regulator, isolated from chassis, right?, maybe a plastic standoff and no contact with the pcb?

Could the tansformer be damaged by this? It was plugged it for less than a minute, no smoke, no external damages, I think they´re solid, but not sure...

Thanks

 

[Rybow]

1A is way too big. You'll want 250mA @ 110V mains or 125mA @ 220V mains per channel. Both being slo blo type. With a 1A fuse, the fuse won't go until well after your toroid has more than it's rated current running through it.

Plastic standoff will work. Did you only lift one solder pad? You could just solder the 7918 to the remaining pads, and then run the needed connections to the lifted pad with some component lead or wire.

Transformer should be OK considering it wasn't on for very long.

 

[dirtyhanfri]

Hi

Thanks, You were Right, I isolated the 7918 and everything looks ok now (I keep with 21V and the resistors trick)

The toroidal is cold now and with no hum.

Issue solved then, Thanks.