Tektronix 2215 Oscilloscope Repair
Tek 2215 Repair
2020
This was another repair item I got on Craigslist for relatively cheap. It was listed as broken, having had smoke come from the power supply area and then stop working. I expected this to be a simple blown regulator or maybe a shorted capacitor or similar, but it turned out to be slightly more in depth than that. In the end I repaired it and sold it to someone in need for a very reasonable price. This again would have been an item possibly going in the trash, so I was very glad to get it working again.
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I should note that in this repair I worked around many High Voltage areas, make sure you are careful and know what you are doing if you attempt something like this. I ensured everything was discharged and safe prior to moving on to the next step every time.
All I had to work with to repair this was that the power supply presumably let out the magic smoke as well as a very bad smell. This seemed like a pretty obvious part overheating, and the smell coming from a burning semiconductor possibly. I did not power this up prior to fixing it as I did not want to risk anything else blowing up.
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After taking the cover apart I firstly inspected everything on the digital logic boards and the analog boards for any small damages. The PSU was under a metal shield which I had to unscrew. After doing so the smell became much stronger which made me even more sure the PSU was to blame.
My eyes went directly to the large heatsink in the PSU, as that is where something was supposed to be getting hot and most likely would be faulting and burning in this case. The transistors Q940 and Q942 were the two devices bolted to this heatsink. After analyzing the schematic, this were easily the highest suspects as they seemed to be the main switching transistors for the switch mode power supply. This adds even more to this suspect as these are high power transistors on the primary side of the main transformer, which would explain the burning if something went wrong elsewhere.
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I de-soldered said transistors and looked on amazon for some replacements. I had to look for transistors with similar beta values and power ratings. After replacing the transistors it worked again! I put the heatsink back on and measured and logged the temperature of the devices over a few hours to make sure it was all ok. The temperature started to rise surprisingly quickly, so I added a silent fan into the scope. It even had a cutout and vent hole for the fan so it fit perfectly in place and worked well.
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The last problem was that the scope was emitting a decently loud humming noise. I suspected this was from the main power transformer and that something was drawing an excess of current. Transformers tend to hum like this when there is a lot of current being drawn from it, this causes the core to vibrate due to the magnetic effects of the AC current flowing through it (also known as the Magnetostriction Effect(thanks Google)).
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This problem ended up being caused by a blown capacitor and resistor I didn't see initially. After replacing these the scope worked perfectly well!