HP HSTNS-PL18 server PSU strange noise

[OpaHoppenstedt]

Hello everyone,

I tried to increase the output voltage of an old HP server power supply to 13.8V.
The model number is HSTNS-PL18 rated at 750W and I found two ways to do that
on the web.

The first involves bridging a resistor next to the voltage setting pot. That worked but
when I hook up my little ICOM IC-7300 transceiver and slowly increase HF power the
PS starts to make a strange noise at about 12 amps and higher. The higher the
current the higher the pitch and louder the noise. To me it sounds a bit like arcing
and this makes me quite nervous.

I thought that maybe the PSU picks up HF from the transmitter. So I got me an electronic
load and the result is exactly the same.

So maybe the PSU is defective. I got me another PL18, did the same mod but exactly the
same noise starts, although this time at 25 amps.

Removing the modification back to the original 12.3V I can draw 400W, the max my EL
can take, and both PSUs remain completely silent.


At this point I am looking for advise.

- where does this noise come from?

- will the PSU go up in flames if I ignore it? would my transceiver be at risk?

- are there better ways for modification or some of the other pots to change?

Thanks for reading.

73, Wilhelm

[eblc1388]

It is possible that 13.8V, your are operating the power supply very close to the output overvoltage shutdown voltage level.

Try reducing the 13.8V output to 13.4V or lower and check if the strange noise go away. This slight voltage reduction should not have much impact to the IC-7300 transceiver RF power output.

[OpaHoppenstedt]

Many thanks for your kind reply.

Overvoltage protection does not seem to be a problem here,
at least not as long as there is no load. I turned the voltage
up and could easily get to 14.5V. I stopped there not to
overdo it, but it could go higher.

You are right, the noise went away when I reduced the setting
to 13.4V. With a load of 400W the voltage then drops to 13.2V.

While the IC-7300 works fine at this level I have some other
mobile transceivers for 144/430MHz and those really seem to
be designed for the higher voltage in a car. Therefore I had
hoped to reach the nominal 13.8V.

I tried the PL18 because it was available at a reasonable price
but I am open to alternatives. The PSU should be compact, quiet
and be able to deliver 50 amps or more for some hours.

So if you are aware of some other brand/model that can do this
I would appreciate to hear about it.

Best regards,
Wilhelm