DC Negative fuse will blow when I connect amplifier to mains outlet.

[Makila]

Hi,

I have a 13.8 Volt DC  linear power supply that I use to provide power to my ham radios.

I also have a valve linear amplifier for 144 Mhz that has it's chassis connected to mains ground.

When I connect the two together, the 30 Amp fuse in the negative line to the radios will blow.

So... I investigated to find out how this is happening and discovered that the DC voltage on the Negative terminal of the power supply has 24.6 volts above ground potential, and being a 40 Amp supply, the negative fuse blows due to the amplifier, connected to the radio through various cables being at mains ground potential.

So the power supply really needs to be at ground potential, the negative line is not connected to ground. There is no circuit diagram for this power supply, a QJE QJ-1840SC, and I was thinking - can I tie the negative to mains ground as it is currently isolated?

Any thoughts welcome.

Many thanks

Steve 

[aqibi2000]

Use a lead acid battery with a fuse in series to power it up  and report back the outcome:

[fcb]

Sounds like your PSU is not floating.

I would expect that even if it is not floating, at least the 0V output would be tied to ground, especially as a google search shows the PSU has a cigarette lighter output on it as well. Probably worth taking the lid of the PSU.

[TheMG]

Sounds like the unregulated positive rail of the power supply is somehow shorted to chassis ground or something.

This could be due to a broken insulator on one of the pass transistors on the heatsink or the rectifier diodes, or something as simple as a connection/wire touching chassis somewhere.

With the power supply off and capacitors discharged, measure resistance between positive of main filter capacitors and the chassis ground. It should be an open or high resistance.

[Makila]

Measuring the resistance between the ground and positive rail is about 9 Mohms so is quite high. The DC voltage is rather odd when measured against the ground, which is 25 VDC on the negative connector and 11.5 VDC on the Positive connector. So why would the positive have a closer potential to ground than negative?
Is there a link anywhere to get a circuit diagram of this device, QJE QJ-1840SC? I have found nothing on it apart from pictures.

Thanks Stev