Ground not at 0V

[Navyblue]

This should be a simple mistake, but for the life of me I can't figure out where the problem lies.

I am putting together a mic preamp kit from ebay. I don't have its schematic, so please bear with me.

Transformer secondary = 12 VAC
Rectifier, V across negative pin and positive pin = ~15V
LM7809 regulator, V across ground pin to input pin = ~15V
LM7809 regulator, V across ground pin to output pin = ~9V

Seems good, but:

V across chassis ground to rectifier positive pin = ~5V
V across chassis ground to rectifier negative pin = ~-5V

V across rectifier is supposed to be ~15V. What happened to the other 5V?

And:

V across chassis ground to LM7809 output pin = ~0V

If I connect this to an amp it would trigger the DC protection relay. The output is capacitor coupled, but of course that doesn't stop the DC because the problem is with the ground.

Connecting the rectifier negative pin to the chassis ground doesn't solve the problem. What could have caused this?

Thanks for reading.

 

[JonSnell Electronic]

As I read your question. the negative of the bridge is 0volt and circuit ground. The positive goes to the smoothing capacitor +ve and feeds the regulator. The centre pin of the 9v regulator goes to 0v which is connected to ground.

 

[Navyblue]

Yes, they are.

Edit: Btw, this is a PCB kit. So unless I stuff the components the wrong way, or there is some kind of short circuit, wrong connection is very unlikely.

 

[sofaspud]

You really should draw up a schematic.
We don't know what "chassis ground" is or is supposed to be...

V across chassis ground to rectifier negative pin = ~-5V

and

Connecting the rectifier negative pin to the chassis ground doesn't solve the problem.

You seem to be saying this results in no change. That's difficult to understand.

 

[Navyblue]

Sorry, by "chassis ground", I meant the earth connection from mains.

By connecting the rectifier negative pin to that would of course yield a voltage of 0V, but the output would still trip the DC protection relay of the amp, which is what I meant by doesn't solve the problem.

 

[JonSnell Electronic]

Have a look at this simple circuit. It should do all you need.
9V Power Supply Using LM7809 | CircuitDiagram.Org

 

[Navyblue]

Yes, I know how to make a regulator circuit with LM78xx chips. And as far as I can tell, the connection on the PCB are liking the right pin to the right parts. But that is unfortunately not all I needed.

The major mystery is what happened to that missing 5V across the rectifier. I suspect this is where the problem lies. I can not imagine such a scenario where this would be possible, so I would like to borrow your imagination.

 

[sofaspud]

Chassis ground and earth (mains) ground are not necessarily the same thing. Nor is circuit ground the same as those two. If the circuit ground and earth ground are connected via a chassis ground, try separating them - chassis to earth and circuit with its own ground.

The major mystery is what happened to that missing 5V across the rectifier. I suspect this is where the problem lies. I can not imagine such a scenario where this would be possible, so I would like to borrow your imagination.

If chassis ground and circuit ground are electrically the same point, there should be 15V and 0V chassis to rectifier, not a negative voltage or missing voltage.

 

[Enzo]

Ignore what you know and ignore how you think the circuit should be. Take your pcb and draw a schematic of what is actually there. Do not assume any connections, verify all of them.

I suspect your missing 5v comes from assuming a circuit path that is not present.

 

[AndrewT]

The retailer of the "kit" should supply all the customer support required to build their "kit".
They have the schematic. They have the same components to hand. They should have all the expertise required to support the "kit" builders.

If they don't, or won't, or can't, support the buyers of their "kit" then you simply send back the non working "kit" and demand your money back.

 

[DF96]

How, if at all, is the 'chassis ground' connected to the circuit signal ground (which may or may not be the same as PSU negative)? Your measurements may be consistent with such a connection being missing, which may or may not be correct. If it is missing, then some of your measurements are actually meaningless so need not be a cause for concern.

 

[Navyblue]

I found the the smoking gun, or rather, the smoking rectifier. It turns out it was something else.

I think it has to do with 2 separate circuits that has grounds that are not at the same potential, which formed a short circuit, which need to be isolated from each other. The rectifier actually got fried once when I first put the circuit together, then I thought it was just a bad rectifier and I replaced it.

Thanks guys.