Fixing Obsolete Rotary Encoders for EG&G / PAR Model 5113 Pre-Amp

[RaymondMack]

I managed to snag an EG&G Model 5113 preamp for a good deal. Unfortunately, the unit arived damaged and I ended up getting it basically for free. A small consolation I suppose, though fixing it was a bit challenging.

When I first attempted to turn the 5113 on, I found that the two internal SLA batteries were dead. The preamp didn't come with the original power cable (and I didn't have a spare 5-pin male 180° DIN connector to make one yet) so I set my HP 6236B PSU to +-12V and connected it to the battery terminals. After flipping the power switch, the preamp looked to be okay. But after fiddling with it, I noticed that two of the rotary encoders weren't working right: The push buttons seemed to work okay, but rotating the encoders in one direction either made the menu jump back and forth or do nothing at all. At this point, I figured the encoder switch wafers were toast. I've had a couple of Bourns encoders die this way (cracked ceramic switch wafer). With this in mind I decided to see if I could find suitable replacements.

Code: [Select]

Honeywell/Clarostat
388EN-4P/DJ
2-bit quadrature
4 PPR, 16 detent, (mechanical commutation) with push button (momentary)
1/8in shaft, 1/4in bushing
3/4in from bushing to end of shaft
Sadly, this part has long since been obsoleted; and from what I can tell, there are no drop-in commercial replacements available. And I'd rather not "mod" the unit to accept a different part if possible...

After an exhaustive search, I did manage to find someone on eBay selling a similar encoder without the push button option:

https://www.ebay.com/itm/381485120016?ssPageName=STRK%3AMEBIDX%3AIT

Since the push buttons on my faulty encoders *seemed* to work okay, and since these units use modular construction, I figured I could Frankenstein them together to get a working part. But before I committed to buy anything, I wanted to pull one of the bad encoders apart to make sure my plan would work.






After unsoldering the encoders from the preamp, it turns out that the pins on the faulty encoders had simply sheared off! The push button option allows the shaft to move inward without harming the switch wafer. So my original assumption was wrong: no cracked wafers (which was confirmed later with my DMM on Ohms mode while rotating each switch). Furthermore, one of the push button pins had broken off too...



Using a hand drill and a cylindrical "diamond" grinding bit, I cut grooves into the plastic housing of the faulty sub-assemblies and soldered in new pins (bent beforehand, so they line up with the original pin layout).




That part, while fiddly, wasn't too difficult. Reassembly of the repaired encoder was made surprisingly difficult since I couldn't find any suitable screws to replace the rivets I had to drill out to disassemble the encoders. I thought about making new rivets using some thicker gage copper wire, but the solid core wire I had was either too small or too large to do the job. On a whim, I attempted to wet the stainless steel back plate with some silver bearing solder and, surprisingly, it worked. I had to use a 250W soldering iron and copious amounts of flux, but eventually I managed to get enough solder to stick for reattaching the old rivets.

You might have noticed that I accidentally put the back plate on upside down and the locking tab isn't positioned right... So I had to disassemble that one (again!) and realign the locking tab. In my defense, I was distracted and wasn't really paying enough attention during reassembly. Clearly, I am not a multitasker.




To my surprise, after all the abuse I put the reassembled encoders through, they work!

[RaymondMack]

I reached the attachement limit in the first post.

Here's a few more pictures of the new power cable I made and the replacement batteries.




I've also attached a dump of my unit's firmware (v1.4) for anyone who wants it.

[SoundTech-LG]

Tremendous effort pays off!!!

GREAT JOB!