Presonus Monitor Station Schematic

[Martin Pedder]

It looks like Presonus are very private about their circuitry. There seem to be hardly any schematics for Presonus products in the wild. I have a Monitor Station on the bench which won't switch on. I have started drawing out the power supply board, which doesn't seem to be as simple a circuit as I was expecting. If anyone has it, you could save me some work finishing it. Otherwise, I will finish it off and will be happy to post what I draw for other's benefit.

If you can help, would you be kind enough to post here or email me at mpedder@lineone.net

Thanks, Martin

[Martin Pedder]

I have drawn up the PSU board and attached it here for anyone interested. This is my first attempt with the RS DesignSpark software - Free circuit diagram and multilayer pcb layout software. I have a lot to learn, but I am pretty impressed with it. DesignSpark PCB | DesignSpark

Monitor Station PSU - Schematic.pdf

Now I have a circuit, it should be straightforward to work out why it isn't powering the Monitor Station.

Martin

[Martin Pedder]

Indeed it was straightforward. Right from the start I had suspected a couple of bulging caps. Unusual in something less than 3 years old.

I took the board home to draw up the circuit of the PSU as I was far from clear what it was doing. Having seen it, I must confess I am still somewhat in the dark.

It is a peculiar circuit, with half of the power supply being on even when it is switched off on the back panel. Even the relay remains energised, or at least it does now that I have fixed the board.

Couple of three points; It is 16V AC not 18V AC as labelled. I have no idea how to get rif of those little "x" on the RS DesignSpark software yet, but it was my first attempt. Sorry I didn't bother with mist of the component values. Many are SMD. I just wanted to see what was going on.

C2 and C5 were bulging. They are only rated at 25V. I assume there are two 1000uF in parallel just for space reasons. I also assume that the reason the positive rails uses 2x1000uF and the negative rail (C10, C11) uses 2x 470uF is because the positive rail is also used to derive the 5V rail and so needs more smoothing.

When the unit is running, these caps have about 23V across them. However, if the customer switches the power off on the back of the Monitor Station, the first half of the PSU is still running, including the relay. Worse still, as there is minimal load, the voltage on the caps rises to around 25.2V. This for my money is designed to fail. How odd though that it only fails when it is switched off!

I replaced the 2x1000uF/25V with a single 2200uF/35V. Problem solved.

If anyone can shed any light on why Presonus have designed the power supply this way, I would love to understand it.

The TIP31C take their juice from the already regulated LM3x7T anyway so they aren't beefing up the current. If the pos rail fails, it will pull down the neg rail, but not the other way round so it doesn't appear to be for protection reasons. They could have sensed both rails and used the relay to kill both rails in a fault situation. I assume the relay is just there for a delayed switch on/off.

Why haven't they just switched the incoming power?
Why having decided to put in a relay did they not switch that?
Why is the OP-Amp, TIP31C part there at all?

The item is on its way back to the owner, bit I like to understand these things.

Martin

[Martin Pedder]

Schematic re-uploaded 12 July 2013

Due to the file loss problem, here it is again (rotated 90 degrees so it is a different file size).
Martin

[apollomusicservice]

I have problem with Presonus Monitor Station, so I need service manual/schematic. Thanks.

[jmo1112]

I'm aware this is quite an old thread. Did you conclude that the reason the Monitor Station wouldn't turn on was because of the two faulty caps? I have a Monitor Station, as well, that will not turn on. Visually inspecting the caps is not really showing me anything. Hoping you can provide some clarity as to what I should look for and test here.

Thanks.

[Jazz P Bass]

Just so you are aware.
That power supply is a switch mode type.

Very dangerous if you don't know what you are doing.

[jmo1112]

Dangerous as in I could fry the unit, or fry myself?

[Enzo]

Fry yourself.

A switch mode power supply - also called SMPS - works by DIRECTLY rectifying and filtering the AC mains coming into the amp. This results in about 340vDC with all the current the mains can supply, just waiting to kill you. Also, the circuit is not referenced to earth. For example the control circuits of many if not most of them is referenced to -170vDC. It may LOOK like ground on the schematic, but if you clip your meter or scope to that "ground" you will have huge DC voltage all over it.

You need an isolation transformer to safely work on the SMPS.