BK Precision 1685B Constant Current Faulty

[peterrash]

(Note: mechanical engineer, never repaired a power supply before, please forgive any ignorance  )

Has anyone attempted repair of the BK Precision 1685B Switching Power Supply? Or even better, got access to a schematic or service manual?

I picked one up for cheap on eBay, and its got the following symptoms:

• Current shown on the display is always 180mA above the actual current (even when open circuit)
• Constant current mode doesn't activate even if the current is above the set limit (eg. set at 1A, increase voltage, current goes above 1A)
• Once current gets high enough above the set limit (about 100-200mA or so above the setpoint) the supply kicks into overload protection mode

I initially thought that something in the ammeter circuitry was faulty, but its interesting that the OLP still seems to work properly.

[edit: 180mA, not 18mA  ]

[peterrash]

I've disassembled and inspected the supply, nothing seems amiss. I also performed some more testing, and found that the overload protection always activates at 125mA above the current setpoint, even though the constant-current mode doesn't activate. This seems to indicate that the setpoint adjustment and current sensing are working, but for some reason the supply doesn't try to activate constant current mode and doesn't report the correct current.

I also noticed that if I set the current limit to zero, the constant current mode does come on, but the readout still shows 180mA.

Any ideas greatly appreciated...