Chassis ground blows fuse

[krzanik]

I recently completed an amplifier which has the common ground (a solid copper wire) connected to the IEC connector and then to all the grounds from the curcuit. The amplifier works fine but has a little noise and I realized that I forgot to actually connect this ground wire to the chassis. So I solder a wire to the copper ground wire and then connect it to the chassis with a nut and bolt (after clearing away the paint under the nut). When I turn it on I blow the 1 amp fuse. I put in another fuse and blow it. So I disconect the ground from the chassis and it works fine. What am I doing wrong here? I've seen pictures of amplifiers with a copper wire which is used as the common ground and is then connected to the chassis. It is my understanding that grounding the chassis not only cuts down on noise but is also necessary for the amplifier to be safe. Anyone have this problem and can help me out?

[SY]

You may have a have a bad (leaky) power transformer or a short somewhere in the primary circuit. This could be deadly dangerous, so go sort that out before using that amp again!

[krzanik]

Any recommendations on how to sort it out?

[SY]

Start by checking continuity. Measure each leg of the power cord to the chassis and see which side might be leaky or shorted, then go from there.

If the transformer primary shows any measurable resistance to ground, you've isolated the problem to the transformer. Same with primary to shield (if any)- these should all be open circuits.

If all that checks out OK, you're not home free yet. Disconnect the transformer primary from its wiring and see if fuses still blow. If they don't, then reconnect the primary (power off!) and make sure the fuses start blowing again. Assuming that happens, replace the power transformer.

edit: One simple thing to try first: see if the fuse blowing happens with all the rest of the system disconnected from the amp.

[kevinkr]

In addition to all the other obvious things check the power switch - I've had several vintage (read worn out) U.S made, and several far eastern toggle switches where the bats had actually shorted to the case and hence to the chassis. Quite exciting in a most unfortunate way..

I now check all toggle switches routinely (whether new or surplus) for this fault prior to use.

[krzanik]

I took some measurements from the amplifier as recomended by Sy and have some questions.

Start by checking continuity. Measure each leg of the power cord to the chassis and see which side might be leaky or shorted, then go from there.

I put one end of my volt meter (set to measure resistance) to each leg (I simply clipped one end to each prong in the IEC socket and the other end to the chassis) of the power cord and there is no continuity between them.

If the transformer primary shows any measurable resistance to ground, you've isolated the problem to the transformer. Same with primary to shield (if any)- these should all be open circuits.

I assume that I was checking the transformer primary when I measured each leg of the power cord to the chassis. Why should I see continuity? I don't understand why either leg of the transformer primary/each leg of the power cord would be connected to the chassis. I don't see anything on the schematic which indicates a connection from the power going in to the chassis. This makes me think I'm understanding you correctly.

I don't anticipate any problem with the power transformer as it is a new Hammond bought just recently and only used in this application. Let me know what you think. Thanks.

[SY]

Just because a transformer is new doesn't mean it's good!

OK, you don't have a massive short from primary to ground (good news). Second step: disconnect the transformer primary from the rest of the wiring and see if you still get fuse blowing when power is connected.

[krzanik]

OK, you don't have a massive short from primary to ground

Ok, that sounds good.

Second step: disconnect the transformer primary from the rest of the wiring and see if you still get fuse blowing when power is connected

If I disconnect the xformer primary, from the power supply coming in (it's the only thing it's connected to), how could I blow a fuse? I would basically be plugging a disconected plug into the outlet wouldn't I? I think I'm missing something, which I'm used to. Do you mean the xformer secondary?

[SY]

No, the primary. You've got wiring to the IEC connector, the fuse and the switch that needs a careful look (see Kevin's post for an example).

[krzanik]

.

No, the primary. You've got wiring to the IEC connector, the fuse and the switch that needs a careful look (see Kevin's post for an example).

So I'll have power going to the fuse then to the on-off switch and then it's disconnected from the transformer. I plug it in and see if the fuse blows? Does this sound right?