Monday, January 6, 2014

How I Accidentally Destroyed my Tektronix 2246

I have talked many times about my Tektronix 2246 oscilloscope in the past as it is by far my favorite analog scope. Simple, reliable, and inexpensive and was the first 'real' scope I had purchased used about 10 years ago. So to destroy it, all I had to do was take the enclosure off.

All Tektronix scopes of this era usually have handles attached to them for portability which is a nice feature. My problem was that I never take it anywhere, it stays firmly situated in the center on my bench as it is my go-to scope for most basic troubleshooting. Having the handle attached resulted in one problem, it prevents you from easily stacking anything else on top of it. With an ever growing bench of test equipment I really needed the real estate on top of it, so the handle had to go.

Removing the handle requires unfortunately more work than necessary. There is no external bolts to remove to take it off, instead the bolts attaching it are on the inside requiring the entire enclosure to be removed. While somewhat annoying, the handle has a well designed strong attachment to the scope. If I were to transport my scope around I would have no concerns of the handles integrity.

Tektronix 2246 Handle Mount Inside the Enclosure

Normally the covers of all scopes in this era come off easily, typically there are 4 screws on the back of the scope and maybe one or two on the sides. Once removed the back then falls off and the entire cover slides off the back. Normally I place the scope standing vertically and lift it straight up via the faceplate while the cover then remains on the bench. My Tektronix 2246, 2445B, and TDS-420 all have this very similar design (and have removed the later two without consequence).

So what went wrong? There was unfortunately a few dents on the bottom of the units case that I did not consider to be an issue. When I began sliding the cover off, it slid about 6" or so and had some resistance. Not seeing anything to be a problem I pulled a little harder (not excessively) and the cover did continue to come off as normal. Once removed I saw a small round heatsink laying on the bench. That's odd I thought, as I don't recall hearing anything rattling around inside previously. Once I looked to see where it came from I realized what had happened.

The dents on the cover that were pushed in had snagged a transistors heatsink on the bottom pcb. This had then put force on a TO-39 transistor ripping one of the leads off of it.

The transistor itself was not salvageable as the missing lead had been physically torn out of the can. Looking into the part itself it looks to not be an easy replacement. The markings on it reference it to a custom Motorola / Tektronix part number of 151-0846-00.

Searching for this part I discovered two things. 1. I'm not the first person who this has happened to. 2. There are no easy to find replacements for this part. The part is critical to the scopes operation as it will not power up without it in place.

The part cross references to a SRF5286 or 2N3866A. The 2N3866A is possible to find and purchase inexpensively, but reports from others say that since it is not an exact replacement for the original Motorola part, it will not work. I'll probably give it a try anyway and also be looking for a 2246 parts unit in the meantime. 

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