Hi, just a story about HP/Agilent 5060-9444 5 VDC Rotary Pulse Generator RPG +, replace LAMP by LED.

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I had the opportunity to get an 8350B with an 83595A (0.01-26.5GHz). This device had to be stored for a few years. In short, I had to partially disassemble it to remove dust and some traces of corrosion. I changed the battery of course and fixed several problems like the front panel which was partially working. I had to do a complete alignment, now everything is OK.

The 83595A seemed to be in better shape, but the RPG was not working. I removed it from the module and opened it up...after examining the circuit, it was easy to see that the lamp was burned out. I studied the module and how it worked, so I tried a repair by replacing the lamp with an LED.

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I found that this little module was quite ingenious. All the parts are on the top of the PCB and underneath 2 photo detectors in a dark room, where there are two plastic sheets in the shape of a circle, a hole followed by another one on top translucent. A small metal piece bent at 45 degrees, serves as a support for the lamp but also as a reflector to direct the light through two tunnels to the photo detectors. The two tunnels have screws allowing them to partially obstruct the light, allowing the two circuits to be balanced. If the light intensity is sufficient, this will trigger the gate 7413 (Schmitt Trigger). When the light levels are balanced by the screws, the duration of the pulses, for a constant speed, should be approximately the same.

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Using an LED recovered from a defective LED bulb, which was the perfect size to fit and fill the rectangular area where the bulb was located, I was able to determine the LED current for minimum operation of the RPG. The currents were 7mA for one side, and 11 mA for the other. I tested up to 30 mA for proper operation. I reduced it to 20mA, found the resistance needed for this current in the LED at 5V. The LED dropped 2.721V. I balanced the circuit with the screws and closed everything. Now the RPG of my 83595A works correctly. I will just have to make the necessary alignments for optimal operation.

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-Yves