OCP5502 Optical Power Meter Temperature control fault

Dave Wright

Checking out a recently arrived OCP5502.

Fails on 'ZERO' operation.

TEMP indicator illuminated continuously.

Hot smell, faint smoke from region of optical receiver module after a minute or two...

Does indicate optical power, though off by ~1-2 db

From all of these symptoms, and a read of the manual, I suspect the detector temperature control loop is failed, and the heater is running full power. Have not left it on for more than a few minutes at a time so far...

Service manual..? Schematics..? Wisdom from those who have gone before..?

My hope is the problem is external to the detector module, and any hints as to where the thermistor(?) signal might be probed, and / or the pass transistor for the heater is located would be appreciated..!


_Dave KC6UPS

Dave Wright

Some new information...

The failure code upon a ZERO operation is 2434, which the manual describes as "O/E closed loop zero high (possible defective measurement board)"

The response to the GPIB TEMP? command is -1, which means cold.
From the manual: "The TEMP? command returns the status of the temperature controller on the OCP photodiode. If the controller is functioning properly, a 0 is returned. If the photodiode is too hot, a 1 is returned and a 6020 event is generated. If the diode is too cold, a -1 is returned and a 6021 event generated."

So, this leads me to believe that the temperature sensor, or the conditioning electronics are at fault, and the control loop is asking for heat. The heater and its drive are certainly working, evidenced by the smell and smoke...

I suspect the sensor, thermistor or whatever, is inside the optical module in thermal contact with the photo-diode. If that has failed, I am probably out of luck...

However, if the feedback amplifier is where the problem is, there may be hope as there is an area of the PCB labelled HEATER CONTROL, a likely spot for more investigation...

_Dave KC6UPS