Microelectronics | Hewlett-Packard 3312A Function Generator


I have decided to repair my old function generator. While it was still usable, a strong cat urine smell came from inside. This could only mean one thing: bad electrolytic capacitors. I have opened it up -- a very simple operation by all means -- and I have quickly filled the room with the bad electrolyte smell. And I mean really bad. An inspection of all capacitors in this unit revealed that there are only two wet-electrolyte capacitors. And both of them are in the power supply section. The rest of them are either silver-mica, film, or tantalum formulations.


The original capacitors were Sprague, rated 500 uF / 40 V. Surprised as I might have been, Mouser still stocks these 500 uF Sprague parts. But I decided to beef up. Thus I have used Vishay 120 ATC series modern parts rated 1000 uF / 63 V. These capacitors are rated for an usable life of 8,000 hours at +125 degrees Celsius. There is nowhere near this temperature in the old generator. It might go up to 40-45 degrees in a prolonged use during a hot summer day. This practically means that they will last for the entire remaining life of this signal generator. By comparison, the former capacitors pretty much lasted for about 45 years. How does it sound to use this generator for the next 45 years?

I carefully de-soldered the old parts and cleaned the tracks and the PCB area beneath the capacitors with isopropyl alcohol to remove any electrolyte residue. Then I installed the new parts. I have used thermo tube on each lead of the two axial capacitors. Less chances of short-circuits when measuring voltages for the alignment procedure.

Speaking of which: after mounting the new capacitors I have adjusted both positive and negative regulated supply rails to an incredibly steady ±17.01 V. Great mains transformer, quality linear regulators, Bourns trimmer resistors, you name it, they did it back in the '70s.


There is nothing much to be said here because it was not a real repair. But the pictures will really do this unit justice. This is how 1970s American engineering looks like. I particularly like the gold plated tracks. This is something we don't really see these days. The component selection is wild as well. All come from reputable manufacturers.

Now this generator still has some problems. The most annoying one is on the frequency adjustment potentiometer. The frequency is not stable between the 1 and 2 gradation. If I jiggle the potentiometer then I can get a stable reading. Otherwise there is absolutely no change. The other problem is the sweep generator command switches. They behave erratically and require a lot of jiggle to even start the sweep. But these are subject for another repair session. For my current needs, the generator is performing great at this moment.

Copyright © 2004- Alexandru Groza
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