grounding on mpc 3000

Hello everybody, I have a heat problem on my mpc 3000, the bridge diode rectifier ( the chip with 4 legs with a little heatsink mounted on it next to the fuses) on the power supply is getting mad hot. I tried another bridge with the same specs, it is doing the same. It is doing this only when the cpu board is hooked to it so I think it may be a short somewhere or there is too much current drawn by the circuit for the 5V line. The rectifier for the +12v and -12V lines has no heat problem. The grounding on the mpc 3000 is a little bit confusing. There is FG (frame ground?), AG if I look in the power supply schematics is the ground for the 12V and -12V. There is another ground, just marked as ground (the ground for the rectifier and regulator 5V).

Do someone had a problem with the rectifier like me and know what it might be? I'm a little bit lost right now and I would really like to fix this issue. Also if someone could check the voltages on a non-heating unit it would be great. And if someone know a little bit more about the grounding and could clarify, it would be appreciated thanks

Try cleaning out the inside of your 3000 and adding a small amount of ceramic or carbon thermal paste/grease between the bridge rectifier and the heat sink. Don't add a whole bunch but just a small amount.

I was under the impression that the MPC60/3000 power supplies were supposed to get pretty hot. They are designed that way.
I dont think you have anything wrong and you wont find someone with a unit that doesn't heat up unless they have completely replaced the power supply with a different design.

linn4life wrote:It is doing this only when the cpu board is hooked to it so I think it may be a short somewhere or there is too much current drawn by the circuit for the 5V line.

How much current are you measuring on the 5V line? That small piece of info will tell you enough to let you know if something is shorting out.

It may be possible that there is a leaky capacitor somewhere in the chain but I wouldn't go changing parts blindly unless there is a problem. If there isn't a problem and you want to lower the temperature, you'll need to better transfer the heat to a heatsink or find a component with a lower forward voltage drop.

linn4life wrote: And if someone know a little bit more about the grounding and could clarify, it would be appreciated thanks

FG is Frame Ground (chassis ground) AG is Analog Ground (audio) DG is Digital Ground (CPU, digital).

These are often kept separate to prevent unwanted noise from bleeding through. You don't want ground hum in your audio paths. You will see multiple grounds distributed through a design to prevent ground loops.

Thanks for all your answers, I already putted thermal grease between rectifier and heatsink, I know the mpc power supply have the reputation to heat a lot but when the mp is on for ten minutes, i can barely put my finger on the rectifier since it is too hot, I think it is not too good for the rectifier and that there is a problem somewhere. I will try to measure the current on the 5V line when I will have some time. Also I found out that the ag and dg pins on the main board are connected together. (there is an inductance in between so they are not connected from an ac point of view but from dc, they are.) how much your 5U (supposed to be 5V unrectified I guess cuz its at the output of the rectifier just before the regulator). Is it supposed to give 5V ? I have more than that