HP 8082A Pulse Generator

The HP 8082A is from 1974, but you would not know it from its specs. It offers pulses down to 2 ns with 1 ns rise time up to 250 MHz repetition rate. More over, both the leading and trailing edge rise times are independently adjustable. You also get complementary outputs, with optional polarity inversion and adjustable DC offset. It also supports ECL levels.

The very unique leading and trailing edge capabilities, together with the speed, make them ideal when working with core memory, and that is indeed how we made the core memory video.

And this is all achieved by superb analog design. The secret of the exceptional performance comes from HP proprietary ECL ICs, developed specifically for this instrument. Which is great when they work, and terrible when they don't, as in my instrument, because you cannot find replacements. It also seems that the HP 5081-3011 that failed in my instrument is prone to failure and is bad in mnay other instruments. But I made a replacement circuit using modern surface mount components to replace the failed gate, and in the end it worked very well, yiedling the original performance.

The HP 8082A was developed in Boeblingen, Germany. The HP Memory site and the HP Journals give a nice account of its development history:

HP 8082A Repair Videos

HP 8082A Documentation

HP 8082A ECL Inverter Gate Replacement

In order to repair my HP 8082A, I had to make a replacement for the failed gate of the U2 IC on board A3, part number HP 5081-3011. This is an HP proprietary ECL IC, but fortunately there was enough information in the original service documentation to show us the layout of the original IC and make a reproduction possible. Mattehw D'Asaro led the way with his resurrection of an earlier version of this chip for the previous generation of this pulser (the HP 8007):

• Matthew D'Asaro HP 8007A Pulse Generator Repair

I inspired myself from D'Asaro's work, but went with a slightly different solution, choosing to only reproduce the gate that had failed in a separate, RF connected PCB, which is inserted in the instrument circuit by simply moving some RF cables around.

HP 8082 ECL Inverter Gate (HP 5081-3011 replacement) Project Files

Epilogue: Repaired HP 8082A

After I finished the video, I tried the circuit with the faster transistors and it performed even better, giving shorter pulses below the required 2 ns. But the non-delayed pulse was still about 1 ns longer than the delayed pulse, itself now a bit too short. Turns out, there is a factory selected resistor (A42 on board A4) that needs to be adjusted to balance the pulses. After I did my own "factory" resistor selection, then adjusted the minimum width back to 2 ns using the R240 pot, the instrument exhibited ideal characteristics at the fastest pulse speeds.