HP 6115A going CC over 72V

[LazyJack]

Hi,

I just got a very nice HP6115A power supply for cheap, as it was blowing fuses. That was quickly traced back to a shorted rectifier and sorted out.
However, an other problem persists, the supply works nice up to 72V, over that it goes to CC mode, independent of the setting of the current limit.

I'm sure it's going to be possible to fix it, once I dig deeper into the unit (which is a paint to remove the main board), but maybe someone has already experienced similar problems and  could give some pointers.

Thanks.

[BravoV]

Just fyi, I made a "stiched & colored"  schematic in single page for this PS, you can get it from -> HERE , beware, its huge image.

Hope this helps in troubleshooting.

[LazyJack]

Wow, thanks! This helps a lot. This is a great piece of work.

[alm]

First place I'd suspect would be the overvoltage crowbar circuit. Either it is adjusted to 72V, or due to a defect it does not go beyond 72V.

[LazyJack]

That is unlikely, the crowbar once triggered stays on until power is removed and the output is clamped to 0. Here, the output stays at 72V, but changes to CC mode. Once the voltage is lowered, normal operation is restored.
I'll dig into deeper today.

[LazyJack]

All right. So fixed it.
I did have a suspicion that the gross current limit circuit may be faulty, which later turned out to be true.
However, i first went ahead to localize what is going on, why why it goes into CC mode. Turned out that it was not the constant current circuit, but the voltage regulator, that went out of regulation, which also results in lighting the CC LED.
The reason it went out of regulation, because its input voltage wouldn't go above 70 something volts. The way this power supply works is it has 2 sets of winding, rectifier and capacitor, both giving around 75 volts. They are in series, but if the load is high, the gross current circuit disconnects one, throttling back the voltage and the dissipation.
Now, this always happened, only one was used, therefore the regulator never had input over 75 volts. This was not immediately evident, as simply measuring the voltage at the filter capacitors showed the correct value.
Once I found this, it was easy to locate the problem, a shorted VR2, a 6.24.2V Zener. Replaced and it works very well, voltages are spot on. Not bad, that I only paid 30 bucks for the "for parts" unit.