HP6125 DC power supply

[kj4rbd]

I purchased a HP6125 DC variable bench power supply circa 1970?? The output is a 0.6 Khz sine wave at around 1 Volt p-p. This is the output even when the switch is in the off position. When voltage is increased, the peak of the output waveform becomes more angular and the troth becomes flat. Is there a fix to this problem which would yield a DC output for experimentation.


Thanks

Bob Anderson

 

[(*steve*)]

I would think it's unlikely to be anything this simple, but I would first check all the electrolytic capacitors and make sure that none of them need replacement.

It sounds like the power supply is oscillating, and it's not at a frequency I'd expect if it were a switchmode power supply (unless something has made it run *very* slowly).

The fact that you're getting output when it's turned off is a worry.

What happens if you place a small load on the power supply (say a 1k resistor)?

 

[kj4rbd]

Thanks Steve

I placed a resistor between the negative terminal and the ground terminal which apparently solved the problem. I now have a steady DC output where expected. This device is over 30 years old; therefore, I think it predated the development of the switching power supply.

Bob Anderson

 

[(*steve*)]

OK, that's not exactly what I meant, but it clearly did the trick.

If your power supply has both a +ve and -ve output as well as a separate ground, it it typical to connect one of these (usually -ve) to ground.

However that shouldn't be required.

Were you measuring the voltage between -ve and +ve or between ground and +ve? The latter would be incorrect.

 

[kj4rbd]

I was working with an op-amp and used both v+ and v- for Vcc and Vee. I tested the v+ and v- out against the device ground with no change in output. I then used the ground from my signal generator and received the anticipated response. I guess somehow the ground is the issue, which may explain my trouble with the integrator circuit.

 

[shrtrnd]

I'm not familiar with the 6125, but I'm thinking about this so I thought I'd mention it.
A lot of high-end power supplies have a voltage out, WITH current sense outputs.
If you try to read voltage without considering the current sense outputs, you read
no output.
You might check to see if your model needs the current sense inclusion in your
test set-up.

 

[kj4rbd]

Thanks; however, this is not a high end product. It's a basic regulated DC regulated power supply I purchased from E-bay. I think it's around 30 years old

 

[shrtrnd]

My friend, anything made by HP is high-end, even if it's 30 years old.
Did you check the output terminals to see if they have the current sense terminals?
Up until they got revamped as Agilent, HP was the Rolls Royce of test and measurement
equipment.

 

[john monks]

I'm not familiar with the 6125 but I would think that it has a lot of dried out electrolytic capacitors. Troubleshooting old power supplies is very hit and miss. I would try paralleling the electrolytic capacitors and seeing if that makes any difference.

 

[Wabajig]

The + and - are your supply terminals, only referenced to each other. The ground connection was used to tie your equipment outside metal to earth ground to protect you from a poke. You could use it as a +supply with - and ground together, or -supply with + and ground tied together. If you leave the earth ground off, then it becomes a floating ground in which you have more flexiblity hooking up to other equipment without some feeback through the different grounds of each equipment. Hope that helps.