Tektronix 2246 Repair Part II

In the process of debugging the strange intermittent display issue on my Tektronix 2246 after re-capping the power supply, things are not looking so good. Things have actually gone from bad to worse. First of all, the short (or open connection) somewhere in this unit which I can return the unit to normal operation by twisting the chassis has been more difficult to find that planned. I pulled and went through the entire power supply again and did not find any issues. Same for all of the connectors, all are seated correctly without issue. Every screw was in place and I checked to make sure there were no poor grounding issues throughout the scope. What happened next is the result of my own stupid mistake (again) which is just making things worse.

In the process of removing the power supply, you have to remove the high voltage anode cable to the crt which is generated in the main power supply. This cable runs out of the supply towards the front of the unit and has an insulated high voltage connector that is clipped on the the metal chassis between the power supply and front panel board. When I was pulling the power supply out for the third time, I wasn't careful and unplugged this connector only a minute or two after powering the scope down. At this point I let the cable go and it fell towards the A16 main processor board letting a nice spark jump from the high voltage cable to the main board most likely due to stored capacitance that had not been bled off yet. I didn't see exactly where it hit, but it didn't matter. The stored voltage had hit the board and I knew damage had been done. Upon reassembly and power up it was confirmed.

The display was in bad shape at this point, everything to the left say 2/5ths of the display was not showing up and squashed into a vertical line. Definitely not good. So now my priorities have changed, now I have to debug this issue to make sure I haven't burnt out something serious like the main display DACs or readout processors as these would most likely be difficult parts to locate and replace.

I started right with the schematic, which was my next stop anyway in locating the intermittent display issue. The full service manual including schematics is easily available for this scope which is another reason why I love the older Tektronix gear. I focused right away at the character generator and display readout circuitry.

After some probing around, I noticed that output from the A16 processor pcb was odd, the horizontal output drive for the display was not looking right, the bottom half of the waveform was clipped which would explain the left half of the display being smashed into a vertical line. Further up the signal path, the signal was looking correct from the DAC and the multiplexer. So the final op amp stage was looking to maybe be the culprit which makes sense as these op amps definitely wouldn't survive a direct high voltage hit.

Output of mux on left, output after op amp on right

So only the final output stage from the A16 pcb was looking bad, this is a good sign as the actual logic does not look to be damaged. Only the final outputs through an op amp look to have been hit. The op amp in question is a TI TL074. There still may be more damage but I'm going to focus on this op amp for now. I didn't have any in my parts bins, so I placed an order for a handful with Digikey which I should have in a few days. Stand by for part III.

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.

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.