Thursday, January 28, 2016

Racal-Dana 1992 repair

Racal-Dana 1992 in my lab on top of an old signal generator.

I recently got an old Racal-Dana 1992 frequency counter from the dumpster. The unit looked very good and did power up and in fact did measure frequencies. However, the user interface was completely non-responsive. It was stuck in Freq A mode. After googling around, I found out that a typical failure mode of these instruments is that the push buttons themselves die. The rubber springs have hardened and deformed after all these years (the quality control stickers and IC date codes seem to indicate early 1989). I looked through Mouser, but these switches or anything that would directly accept the switch hat are no longer made.

Front panel removed.
Front panel PCB with the broken switches. The hats are held on the switches just by friction.

Inspired by a post in the EEVblog forums about replacing the buttons, I ordered a bunch of 6x6 mm tact switches to mount diagonally with two of the legs cut off. The original poster used tact switches with a long shaft, which were then shaped to accept the hat. I wanted to go a different route, as I considered shaping the shafts too tedious. I decided to have the hats mounted on the original plastic pieces of the old switches and then have these pieces attached to the tact switches. For this I needed to drill a 3.5mm hole in the bottom of each hat holder piece, to fit the shaft of the tact switch, and then fix the piece to the tact switch shaft with a drop of super glue.

6x6 mm tact switch with two of the legs cut off. This fits perfectly in place of the original switches.
Halfway there. I forgot to take pictures of the desoldering process.

There are 33 switches, so the process of desoldering the old ones and replacing them with the new ones took one whole evening. After this I could finally test the instrument, and find that this repair had indeed fixed the problem. A quick look into the operation of the instrument shows that repairing this unit seems to be well worth the time and effort I've put into it. The drilling of the hat holders and gluing them on the tact switches then took one more evening.

All switches replaced and board cleaned from flux residue.
Testing the instrument with the newly replaced switches. Everything's good!
Front panel PCB back in the front panel casing. Switches seem to line up. Next up is gluing the hat holders on the tact switches.
Hats in place and glue curing.
Front panel in place and everything working correctly. Win.

Overall, I'm very happy with how everything worked out. It is a beautiful instrument with very interesting features. Next up is to check the calibration somehow. Then perhaps I can finally think of getting rid of my old oscilloscope, as I wouldn't need it even for its frequency counter any longer.

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