Tektronix 2465 300Mhz 4-channel oscilloscope repair

On a recent trip to Portland, I stopped by Surplus Gizmos, and found a used, as-is, Tektronix 2465 scope that had a calibration sticker on it saying “unit smoked while calibrating”. The scope was listed as $100, but I was able to get it for $60. I know the unit isn’t worth a ton, but it is a 300Mhz scope, and is seemingly considered one of the best Tektronix scopes ever (it’s also considered the last repairable Tek scope, as after this, most scope manufacturers stopped including schematics and other information with the scope). Also, the unit showed to have been manufactured around 1984, which is when my grandfather still worked at Tektronix writing manuals for scopes. So he may have very well worked on the manual for this scope.

Opening the scope up, I figured the issue was going to be the power supply, as that seems to be the common failure in this scope, and really any device of this age. The capacitors dry out, change ESR, overheat, blow, etc. Getting the fan off was really the hardest part of getting the power supply out, but once I figured that out (and broke the mandril holding the fan. =/) I got the PSU out, and quickly found the failure points ((Note, to remove the fan, you loosen the nut, then holding the fan “cage”, PUSH on the nut-screw toward the scope. That will push out the mandril and release it from the shaft. If it won’t release, please be careful. I got lucky and my mandril broke in a repairable way. I’ve seen pictures where they completely shatter, and there don’t seem to be any replacements available)). One of the caps on the low voltage side had ruptured, and also R1016 and C1016 on the mains input were both blown. There are plenty of online guides about re-capping this scope, but the only one I found with actual part numbers and quantities was here. His list is for the 2465B, but the 2465 is very similar as far as the PSU, the main difference is the lack of the SMD caps that everyone says to replace. Instead, there are three through-hole caps on the Digital board that you can replace.

Tek 2465 Digi-Key Parts List, from Digi-key, that I ordered (I make a point to order extras so I have stuff in stock), and which parts they replace.

Recapping the board is relatively easy, but takes time. Thankfully it’s all through-hole, and I have my Hakko FR-300 desoldering gun. Most of the caps I ordered didn’t have the proper leg spacing, so I used needle-nose pliers to bend out the legs then back in. The added benefit being that they’re up off the board now, so it’ll be easier to see if any pop in the future. I may want to glue the caps to the board at some point since they are kind of “up there”. The 3W resistors I bought to replace 1/2W ones also were off on leg spacing, but this is a positive as it forces them to be up off the board, and therefore have better cooling.

Total time to recap everything was probably a good 3 hours, with a chunk of that being replacing the two primary filter caps. Since I purchased radial 330uF 250V caps, I had to fold the negative lead back, and extend the lead with wire. Not a huge deal, but always a bit of a pain.

Once done, I put the PSU back together, and put it back in the chassis. I initially tried to test it with nothing plugged in, but sadly the SMPS just made noises and didn’t actually power up. =/ So, having 99% confidence in what I did (and knowing nothing popped when I did my test power on), I plugged in the connections, crossed my fingers, and hit the power button. The fan didn’t spin, but I was greeted by lots of relay clicking, lights flashing on the board, and the graticule lighting up. No trace. Huh. Since I’d never used this, or any 2465, I started randomly pushing the buttons on the front, and found that the traces were all just turned off, and not triggering. Hitting the trace buttons got them all turned on, and looked good. =) The interface is a bit odd, but it seems to work great, and all the traces and text look very sharp.

Still no fan, however. Sadly, this meant I had to repair the motor, as it seems to have seized. Using the directions here, I was able to unsolder the fan motor, and open it up. Once I had it opened, and saw nothing really out of the ordinary, I added a drop of 3-in-one oil to the rear and the front of the motor shaft. Then I put the motor back together, and using some pliers I manually rotated the shaft a few times to work in the oil. I soldered it back onto the board, and reinstalled the fan. Powering on the scope, the fan just barely moved, so I spun it a few times by hand, then hit it VERY briefly with the air compressor to get it spun up to speed. It then kept spinning, and since has spun perfectly on every startup. I’m relatively certain it was seized due to me removing the fan incorrectly when I first opened the unit. What a weird design. =/

Good luck on this! I now need to confirm calibration, and pick up some probes (or make some) that have the read-out pin, so the scope knows they’re 10X rather than 1X. =/

Here are some links that I found helpful:

Comments

Hilary says

2018/12/31 at 20:50

Thanks for the great detail and also the family heritage with Tek. Another fan of even older Tube Tek scopes is Paul Carlson of Mr. Carlson’s {metrology!} lab.
My prized 2445 just tonight decided to smoke a lot inside, this Jan 31 New Years eve!!!I was tracking some power line noise on HF hwew at W4HDL, and looked over and while everything was running, major smoke was streaming out of the vent holes. I had to immediately unplug it and put it on the covered porch, and was ready to hit it with a hose. I quickly opened it up, and hopefully the damage is all in the AC PSM module. Not sure yet how to get it and the aluminum cover removed.
Definitely glad there is extensive info here on this issue, and MANUALS are available. Telecom power/cooling design is my work so we will be doing all the recapping ect as recommended.