Tektronix 2246 Repair

Last year I had talked about how I accidentally destroyed my Tektronix 2246 oscilloscope by removing the cover. There was a small dent in the metal bottom panel that while removing it had snagged a heatsink on a Motorola 151-0846-00 labeled TO-39 transistor, specifically Q702 on the A10 main board. This ended up ripping the pins out of the transistor can leaving me with a unusable scope. Unfortunately this was not a common part and trying to locate one is next to impossible. I had attempted to use a 2N3866A with no success, the scope was definitely not happy with that transistor in it, an original replacement part would be necessary.

I did ultimately find a donor board. Late last year I found a complete used replacement board on eBay for around $40 which contained both of the 151-0846-00 labeled transistors. This option was definitely cheaper than a whole parts scope and I would hate to ruin another 2246 which may be repairable just for this one transistor. Once I replaced this transistor in my 2246 it was good as new.

I want to focus the attention now to a second Tek 2246 scope that I own with its own set of troubles. I purchased another 2246 a few years ago for cheap, I think it was only around $100. This scope had a lot more use than my original one, while it was in good cosmetic shape it had some issues mostly related to old capacitors. The display was not stable, the character generated osd and cursors would jump around and upon probing the supply rails you could see there was some noise present. A re-cap would be necessary and possibly some additional caps on the A-10 main pcb if necessary.

The 2246 main power supply is pretty basic, much more simple to work on than the Tektronix 2445B power supply which I have also done. One interesting observation upon accessing it is that all of the caps look to be large axial electrolytics:

Trktronix 2446 Power Supply

The bottom of the board told a different story, and once removing a capacitor and testing it was indeed a standard radial cap with an interesting third lead out the top that wasn't connected to anything. Maybe it was designed for additional stability? I know this scope was a popular portable model so my guess it was just some additional ruggedness built into the design. Interesting regardless, at least I can replace them with standard radial caps which are much easier to source.

Replacement caps were all Panasonic 105 degree C. units which are always my first choice, then using Nichicon capacitors in cases where the Panasonic's were not available. The final rebuilt version looked like this:

Tek 2446 Power Supply Rebuild

Quite a difference in size. For good measure I also replaced all of the high voltage film capacitors on the board, many looked stressed as they did in the 2445B. I left the large primary switching capacitor alone, they are rarely every a source of trouble. Everything else looked okay.

This is where things started to get interesting. After putting everything together and powering the scope back up, I had strange display issues. The entire display was shifted left. I started looking around to see if I had missed a cable or possibly had a connector loose during the reassembly but didn't see anything obvious. This scope was working just fine before the power supply rebuild so this issue was definitely something that I caused. At this time I went to turn the scope over on its side while powered up that the display snapped back into alignment. After some more poking around I realized that if I put pressure on the chassis, twisting it just lightly I could get the display back in alignment. So it must be a bad ground, loose connector, bad solder joint, or some other mechanical failure where putting pressure on the chassis would complete whatever broken connection was occurring. It will just be a matter of tracing down where the issue is at. More to come in part II.