View Full Version : N* Horizon PS voltage very high? Yikes!

March 22nd, 2011, 07:39 AM

I plugged in a N* Horizon that I plan on restoring. I measured the voltage from the wires that connect from the PS to the MB. The voltage for the +16V line was +20V. The voltage for the +8V line was +10V. Yikes! Given that, I suspect the PS section has some sort of voltage problem.

I see three big caps in there and a transformer. I looked at the schematic at:
and couldn't understand exactly what those components are on Page 9, because they are not labeled.

Does anyone have any advice on bringing the voltage back to correct levels and/or diagnosing this problem?


March 22nd, 2011, 08:07 AM
Were these voltages measured with the circiut under load? If you had the motherboard disconnected and checked the voltages, I'd expect them to be higher than advertised.


March 22nd, 2011, 08:34 AM
I measured with it disconnected and connected to the MB. Same voltage. I did not plug in any cards because I didn't want to damage them.

March 22nd, 2011, 09:03 AM
I'll try to check one of mine when I get home tonight.

March 22nd, 2011, 09:19 AM
I measured with it disconnected and connected to the MB. Same voltage. I did not plug in any cards because I didn't want to damage them.Sounds OK to me. Measuring voltages on a linear supply like this without any load can be misleading, but there's really no way for them to become higher than the design; on the other hand voltages that are too low under load can sometimes be a problem. Every card has its own regulators; consider that every card and/or drive that you add will pull the input voltage down a little, but the 5V supply for example has to remain higher than its minimum of 7.5 V. 8 and 16V are really ideal values assuming a full set of cards and drives, and the fewer cards & drives the higher the voltages will be. BTW, the maximum voltage is around 35V.

If you're really concerned, connect an old scrap hard disk or two and see what that does to the voltages.

Which other components are you asking about? They all seem to be labelled, nothing unusual, just rectifier diodes, voltage regulators and a few caps.

March 22nd, 2011, 09:35 AM
The 78xx regulators used in circuits of this type are usually rated for at least 30V input. The indication should be the heat dissipated by them when operating.

Unregulated DC supplies driven with sine-wave input will naturally tend to charge their filter capacitors to the AC peak voltage value. In other words, if you have a transformer whose secondary is rated at 8VAC RMS, it will tend to charge the filter capacitor to about 1.414 times this value (less rectifier drop), or about 11.3VDC.

It's the load that will bring things down. This is all as it should be.

March 22nd, 2011, 11:03 AM
Thanks folks! I'll try adding in some boards and take another measurement

March 22nd, 2011, 03:11 PM
Here is one of my system's readings:
+8 = +10.3
+16 = +21.7
-16 = -21.8

So, sounds like you are good. This system has been in use off and on for the last 9 months.

March 22nd, 2011, 11:28 PM
Thanks Kipp!

March 24th, 2011, 06:40 AM
Basically what MikeS/Kipp said; you're ok. Before you plug in any boards though, the next thing you need to check are the three voltage regulators on the back left of the mboard. are those regulating the power properly? First you can check the voltage going to the disk drives by checking the voltages coming to the drives through the two drive power connectors. Compare and contrast with similar connectors from another computer that you know works. If they're OK, then trace the wires back to the regulator, and test the remaining one(s).

I have worked on three n* boards and I have had to replace at least one regulator for each, but now they're all OK. For some reason, the N* is one of those systems that today will usually require some work to get running. Age is not always kind to these.


March 24th, 2011, 11:10 PM
Thanks Bill. Someone else also advised me that the VRMs sometimes need replacement in older boards.