Tektronix 485 troubleshoot and repair

This video shows the troubleshooting and repair of a Tektronix 485 dual channel oscilloscope. It has a bandwidth of 350 Mhz , 50 ohm input. And 250 Mhz, 1 Megohm input. It has a 1 nano second sweep rate. This particular unit has a date sticker of 1971.

[@t1d100]

Great find + Great Repair = Great Unit! Kudos! My guess is that the two resistors that were bodged in are not OEM... Because, I don't think that I saw tube isolation pipe/heat shrink on their pins. If that isolation is not in place, I suggest adding it... But, you know that... Cheers!

[@richardkimble8775]

Question. Why did you use a curve tracer, and why is that better than the small transistor tester. Isn't it more accurate than the older equipment.

[@toddanonymous5295]

Hi Richard. The curve tracer is able to test devices to their rated limits under different operating conditions. The small testers run on a nine volt battery so miss things like breakdown voltage and leakage. I do use the small testers and like them. Once you get a curve tracer you will wonder how you ever got along without one. They are pricey but the units I have did not cost much. Of course, they did not work when I got them which is why I was able to get them in the first place. Most of my equipment started out as for parts or repair. You can get some great lab grade equipment if you are willing to repair it.

[@aa7jc]

I have a 485 that I bought rebuilt. All new Tantalums etc.. It worked perfectly for a while. One day I turned it on and nothing at all lit up. Fuse is good. It is sitting on my todo project shelf now. My guess is that it is something simple. Any idea what the #1 suspect might be?

[@williamburleson2976]

If it does not stay powered up I would look for a shorted tantalum cap.

[@toddanonymous5295]

Hi aa7jc, Being completely dead , I would focus on the power supply. Try tracing from the power input through the power inverter board. If the defect is beyond the power supply, it should still attempt to start and then shut down, cycling every few seconds. Use care on the non isolated side of the power supply. Good luck with the repair.