Need help with Allen and Heath system 8 mk I

[Timjimbaker]

I just bought an Allen and Heath system 8 mk I 16-8. It was a good deal but was as is. The channel strips seem to work fine but the main left and right vu's get pinned as soon as the power gets turned on. I checked the power supply, the voltages are supposed to be +15, -15 and 48 for the phantom. The positive voltage is about twice what it should be. The other two are within 10%. Could this be the problem. What else Could I be dealing with. Some insight would be greatly appreciated. Thanks.

[djmukilteo]

Bad power supply

[AnalogBrain]

Probably the power supply.

You should replace the power supply for that desk anyway, whether it is working or not. The stock PS is very weak. There is a thread on here somewhere where Jim Williams recommends I believe a power-one power supply. Search for the thread.

Hope you get it figured out!

-Neil

[Timjimbaker]

Thanks for your reply guys... Turns out, the power supply has been rebuilt. I fiddled with it a bit and I am now getting some proper voltages out of it. The master VU's still max out as soon as I plug in the console. I was wondering if maybe the plug has been wired wrong. There is a diagram on the back of the console that reads from left to right 1. -15v 2. & 3. ground 4. +15v 5. +48 pp. Now, the plug should be a mirror image of this. correct? I have attached some photos. The first of the diagram of the pinout and the second, a hand drawing of the layout of the plug and what the actual voltages are. Seems right to me but is it? If this is all right the problem must be inside the console. Where do I turn next. Thanks again for your previous help. tim.

[djmukilteo]

Looks like the +15V and +48V phantom power are in the wrong places from your hand drawing.
The plug should have pin numbers on it embossed on the inside shell...

Sorry maybe not
have you check those with a meter..?

[Timjimbaker]

The phantom power and the +15 are in the right place. The numbers on the drawing are taken from a multimeter. The phantom power is a bit high but the other two are pretty much right on. Thanks.

[djmukilteo]

Originally Posted by Timjimbaker

The phantom power and the +15 are in the right place. The numbers on the drawing are taken from a multimeter. The phantom power is a bit high but the other two are pretty much right on. Thanks.

Well I guess the next step is to get a schematic.....did you have a schematic?
Open it up and look over every board for blown or burnt caps or resistors etc. anything really hot or anything that smells like burnt resistors...
You don't need to supply the board +48 to test and check.
You could disconnect that at the plug temporarily for now. It's only for phantom power at the mic inputs after the switch and usually doesn't have anything to do with the board itself...Just the +/-15V.
Start checking test points.
I would find all of the test voltages listed on the schematic and start by measuring those.
If you find a voltage that doesn't match the schematic values then focus on that, check each part in that portion of the circuit.
After that you'd need a function (sine wave) generator or test tone and an oscilloscope would be real handy...hook up to the input channels and start checking signal paths....

[Timjimbaker]

Hey thanks man. I didnt think about disconnecting the phantom. I'll get my hands on the schematic. Thanks for the great advice. tim.