Author Topic: HP6024A PSU restoration project part II (load switch)  (Read 902 times)

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HP6024A PSU restoration project part II (load switch)
« on: November 25, 2013, 10:27:28 pm »
A while ago I bought a pair of 6024A PSUs:

I ended with stating I wanted to replace the 0.75 turn pot in one and add a load on/off switch. Meanwhile I got the parts and started working. The pot on the left had to go:

The previous owner use a bit of steel or brass wire to hold the replacement pot:

I bought a 10-turn pot from Ebay. It says Bourns on the rear but I'm pretty sure its a low end knock off. The minimum resistance is 8 Ohms so the PSU is likely to have some offset  :'( Then again I had to use a file to make the round pot square-ish to make it fit. There was a risk of damaging the pot. I used some sealant to glue it in place:

Another problem was that the original pot was 2.5k which is very hard to obtain in a 10 turn variant. I used a 5k pot instead with 5k parallel to the wiper and one of the lugs. This method gives a more linear response than connecting a resistor parallel to the lugs.

The next order of business is the load on/off switch. This switch is connected in parallel with the voltage setting potmeter. When the potmeter is shorted the output voltage is forced to 0V. After long consideration I decided the best place for the switch was next to the current potmeter. I didn't wan't to put the switch too much on the side of the front because it may get bumped and break. I would have liked to align the switch with the voltage and current control knobs but that wasn't possible either. It had to sit in the green rectangle as shown in the picture:

Marking the location on the back of the front:

After drilling:

For these kind of jobs I use packaging tape to cover front plates so they won't get damaged during drilling.
The end result is this:

The load on/off switch also provides a nice opportunity to mease the switch-on and switch-off curves. I set the PSU to 24V and got these curves:

The switch off-curve shows the down programmer (current sink) is actually a proper current sink. I guess this power supply isn't very suitable for fast modulation.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.

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