My HP 6632 Power Supply has got crazy (SOLVED)

My 6632A s/n 3145A-06821

 

Symptoms:

• power on is ok; no self test errors on the display;
• VSETing from the front panel to values greater then 5/6 Volts triggers OVP.

Checks:

• with these symptoms always double check the sensing terminals (S+ and S-): they must not be left floating. Be sure they are directly connected to output terminals on the rear terminal strip or to a remote load. For testing purposes mine were connected to the terminal terminals on the rear output strip;
• checked OVP circuit as per SM: voltages at U110 are ok; both R158 and R155 checked ok.

Further investigation:
    • 0.1 Volts < VSETs < 4/5 Volts work but the readback (front panel) is out of, say, 15% and slightly unstable; the actual output is about 4x VSET and highly unstable: it jumps randomly up and down few volts.

NOTE: from now on I  connected my Arduino GPIB controller to the unit so that I can quickly issue commands to it without pushing the control panel buttons.
      
This is the result of “VSET 2” GPIB command:

The first spike is the direct consequence of VSET 2; the second is just pure random noise. Please note the rms voltage is more then 4 Volts instead of 2. The peak reached by the first spike was 20.4V, barely not enough to trigger OVP.

VSETting to > 7/8Volts produces an output spike. Fortunately the working OVP circuit triggers @ 21 Volts:

 

 

this is the result of VSET 2 while the output was @ was 1 Volt:

A 12.8V spike, .. and then down to about 4 volt, then instability and random noise. WHAT A MESS!

 

Where this kind of behavior can come out from?
OVP is not the cause: it works as expected triggering as soon as the voltage overcomes the limit.
So, what else?
I suspected the "voltage monitor subsystem" (sometime called the “error amplifier”).
Please note that these power supplies have sensor terminals useful to monitor the load voltage directly at the load terminals.

 

The S+ and S- terminals are available for this purpose and they are high impedance inputs (very sensitive to noise). Instabilities on the monitoring circuit behind the S+ terminal have great effects on the output voltage stability.
So I started looking at those terminals on the main board and ...BINGO!
Some of the PCB traces and solder joints in that area shown signs of corrosion.

 

An electrolytic filter cap (C110 - 470uF 35V) is present in the affected area. It appears it could have leaked, but it is in very good physical shape (no bulgin, no fresh leak around it).
The cap, measured off circuit with my LCR meter, was found to be in excellent electrical conditions: 472uF and ESR=0,1ohm.
In any case, while working on the board, I replaced it with a new ELNA one.
I also cleaned the offended PCB area with IPA (isopropyl alcohol).

 

THESE OPERATIONS SOLVED THE PROBLEM.

 

Here is my 6632A following a bunch of VSETs without any problem.

Aftermath note:
an incorrect voltage calibration can trigger OVP at turn on. This is because the rig resets to "VSET 0" during the turn on routine, but if the zero setting is well below zero, it fails and goes up to MAX V, triggering OVP.

Hope this helps!

Cheers.