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HP Recommended
Z800 Workstation
Microsoft Windows 7 (64-bit)

I've replaced everything in this Z800 except the fans. Used, from another working Z800 PSU, memory, graphics card, CPU's and motherboard. I found it 'dead' after an electrical storm, although it was on the surge protector. Router and cable modem was not affected. I don't think anything came through the network.

All soon as I plug in the AC, all the fans start running on low. After 30 secs, the CPU fans go to high speed. This happens with both the old and new motherboards and the old and new CPU's.

I disconnected everything, including the bay and back chassis fans and still the Z800 starts itself when powered.

No video comes out of any graphic card I install, so no bios to see. Odd?

I get 3 beeps when I press the start button, one time but no red light flash, just one green power on flash.

Could this chassis be possessed?

So First, what would make a Z800 start without pushing the start button?

SIP
5 REPLIES 5
HP Recommended

the z800 does have a bios option to auto start after power fail, set to disabled to stop this

 

or just let it power on, startup then do a normal shutdown from the OS and see if that resolves the auto power on

 

the fan issue is most likely due to a failed "ambient tempture sensor" that is embedded in the power harness

 

you can either replace the fan, or use this sites search function for z800/z820 ambient temp sensor to locate previous posts that describe how to replace the NPN 2n2222a plastic transistor that has most likely failed

HP Recommended

I can't believe this NPN 2n2222a plastic transistor does anything worth while. Both the plastic case and the heat shrink insulates it from everything. Even a thermistor needs free air to work properly. Maybe I'll try putting a 2.2K ohm across the emitter-collector and see if the fans stop. The NPN is probably open. Considering this unit is built so well, why anyone  would use a transistor for a heat sensor is beyond belief.   

SIP
HP Recommended

HP uses the NPN 2n2222a  in the circuit because it works, and is quite sensitive in detecting tempture changes. and no,.. placing a resistor across the two transistor leads will not do anything towards reducing the fan noise. the fact that it's in a plastic covering has no effect on it's ability to detect the ambient tempture changes

 

 the transistor is used (instead of a thermistor) BECAUSE it has a known accurate/reproducable/reliable curve in translating even small temp changes to voltage very quickly when the transistor is used as a diode (that tracks voltage changes not resistance) , and is unlike a thermistor which uses resistance changes and is considerably less sensitive

 

 

i recomend you spend some time reading about the diffrences between using a thermistor vs a diode in measuring/tracking tempture changes you will find that both circuits are very diffrent on how they measure temperature and how  sensitive the diode circuit can be compared to a thermistor based circuit

 

HP Recommended

I did check out using a reverse-biased diode or NPN transistor as a thermal sensor connected to a comparator. A transistor with a metal can TO-3 case would make more sense as a conductor especially for sampling convection air flow.
Hiding it in a wiring harness, away from the chassis vent is a odd placement. When I was looking for temperature sensors on this Z800, I couldn't find any that were not on the motherboard. If HP would have placed it conspicuously in the air flow vent, it could have saved me hours of troubleshooting.

Now I've got to dig through my NPN supply and hope I can find one that works. Even if I need to add a trim pot to adjust the bias, it will be faster than waiting for USPS to deliver this 5 cent part.

SIP
HP Recommended

I can't believe it. I opened the box and the 2n2222 were right on top. See photo. Good DSC11091801.jpgKarma...

SIP
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