Classic 30 blowing fuses...

[HojoMojo]

Hi, guess I'm new here... and somewhat needing help

After my power tubes stopped working, I opened up the amp and found a filament power jumper broken. I replaced it, along with some other jumper wires that were looking pretty sad.

Now whenever I turn on the amp it blows the first fuse, the chassis-mounted 3A one, within about half a second (I briefly tried a 3.2A fuse, which didn't blow, but was definitely going to destroy my input transformer. The tubes started to get REALLY bright, when I yanked the plug).

I can't seem to spot any bad solders or broken jumpers on the board, and I'm fairly clueless at the moment... don't know much about amp repair. Any suggestions on what could cause such a large current flow would be greatly welcomed!

thanks!
-Kyle

[lowell]

hi I would double/triple check the wiring that you did on the filaments.... then fire up the amp with no tubes in it and see if it blows the fuse still.

[HojoMojo]

thanks for the suggestion -- I remade my solders and checked them with a multimeter... still blowing the fuse

[lowell]

Did you pull tubes and still blew fuse?

[tubeswell]

Here is a copy of the C30 schematic and layout diagram and parts list (can I say that these are courtesy of Steve Ahola and PV in the same sentence like that?).

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

http://www.blueguitar.org/new/schem/peavey/c30layg1.jpg

http://www.blueguitar.org/new/schem/...parts_list.jpg

If you don't really know what you're doing, take it to a tech, esp if your amp needs repairs

[HojoMojo]

I'm sorry - meant to say that it still blew the fuse without any tubes in it. (which *initially* would suggest to me either a ground short, a bad transformer, or an open circuit resulting in a non-loaded transformer? but why would it still have supplied massive current to the circuit, notably the tube filaments when they glowed white-hot...)

Thanks Tubeswell, that should help... I'm going to take it in to work tomorrow where I've got a signal gen and an oscilloscope and see if I can "follow" a small AC input through the power section...

I'm an engineer, so that means I have a big enough tech ego that I'll take it to someone else only after I make everything worse myself

Thanks again.

[EDIT] As near as I can tell, I must have plugged the input transformer's secondary taps onto the wrong pins once and blown pretty much everything in the bias circuit. My bias rectifier diodes are all measuring 600 ohms one way and 1300 the other, and some of the large filter capacitors have less than 5k ohms resistance. Anyone wanna confirm that plugging the "red" plugs onto the "orange" (as seen in c30schem.gif) would have made everything worse? LOL.

[Enzo]

Revered tranny wires was what I was about to write until you mentioned it. The red wires are the high voltage and the yellow ones are the low. Orange? OK, I always called them yellow.

Look between the little two pin connectors and the fuse clips on the circuit board. See the little perforated patterns that make an "R" and a "Y"? Those are there to indicate Red and Yellow. The yellow/orange wires connect to the pins nearest the board edge, and are served by a 10A fuse. Th red wires feed the pins more towards the center of the board and are served by a 1A fuse. The red wires also wind up at the four little rectifier diodes.

[HojoMojo]

Trying to see if my transformer is buggered, would anyone be able to inform me if this makes sense:

primary winding 4.3 ohms, 20mH
high voltage winding 1.0 ohms, 0.03mH
low voltage winding 1.7 ohms, 0.36mH

I'm using a 3A circuit breaker (originally was a 3A fuse)

with a 60W lightbulb hooked onto the low voltage tap, the lightbulb slowly starts to dimly glow and then the breaker pops (after about 3 seconds) -- does make sense

with the same 60W lightbulb hooked onto the high voltage tap (and a shield in between my face and the lightbulb ), the lightbulb doesn't light up at all, and the fuse blows after about 1 second. -- doesn't make sense??

with no load, the circuit breaker will blow in about 1 second.

Thanks for any wisdom.

[Enzo]

No, what doesn't make sense is connecting a 60 watt bilb to the secondarues of the tranny. The use of a 60 watt bulb would generally be in series with the primary - in place of the main fuse for example - to indicate excess current draw.

Ohm meter readings of transformers are not usually of much help.

The high voltage winding tries to produce several hundred volts at not much current. That 60 watt bulb will try to drag well over an amp from the poor thing.

Disconnect the secondaries - let those red and yellow wires dangle in mid-air. If the main fuse now still pops, then the transformer is bad.

[HojoMojo]

Thanks Enzo. I knew that connecting a bulb to the high voltage side was kinda dumb, but I was skeptical that that tap was putting out any voltage at all (and evidently it wasn't)... I've now hooked the lightbulb in series with the primary, and measured 115V across the bulb, 4.5V across the primary, 1.5 across the low tap and 0.0V across the high tap. I guess that confirms my suspicion.

I'm thinking I overheated the high voltage coil by getting the plug connections backasswards, and maybe now the coil's shorted out, but not open, giving me a short-circuit in the high voltage coil. I'd really like to know what the heck I did, I might rip this transformer open and look at the windings.

Anyone care to suggest replacement transformer ratings (and/or suppliers)?

Thanks for the help!

[edit] Opened it up and the offwhite plastic covers are a lovely golden-brown around the high-voltage coil Live & learn...

[tubeswell]

Best place for PV PT is PV.

[Alex/Tubewonder]

Disconnect all PT secondaries from the board. If it still blows the fuse the PT is done for, if not you got a short circuit on the board.