Tektronix 2213 Restoration - with IR Thermal Pics!

[toxuin]

Hello all.

At the local Makerspace, I've volunteered to fix an old oscilloscope, namely Tektronix 2213, a 60MHz CRO. Problem is that it just does not turn on. There is a green LED, subtle transformer hum and that's it.
Usual routine: I open scope up, check voltages: EVERYTHING is WAY off. 8.6V is at 1.1V, 5.2V is at 0.(something). Visually there are no blown components, nor caps have popped.
Being quite clueless on the course of action, I take out my thermal camera and whoa, an inductor near the transformer is glowing >120C!

I then googled for service manuals and found two scans, made by apparently different people, both of which are missing schematic #9, "Power Supply and CRT", where this inductor is depicted. It is labelled L971 on the board layout but that's pretty much info on it in those manuals.
But, knowing Tektronix has scopes that internally have similarities, namely 2215A and 2235, I dug up the service manuals for those. No wonder, both those scopes have inductor at the same spot, near 4 parallel placed diodes. In 2235 this inductor is labelled L960 and it is doing something on this schematic:


So question is: why does this inductor gets so hot?.. Is this a failure of one or multiple diodes? Is this a transformer winding short?.. Please advise.

Also, does anyone have Schematic 9 labelled "Power Supply and CRT" for Tektronix 2213? This would be handy for future repairs (I kinda do enjoy this lol).

I've attached the original FLIR JPEG if you guys want to measure different spots.

Thanks for the help!

[TiN]

Welcome to the forums

Something down on that +8.6V rail is shorted to ground, causing excessive current to heat up poor inductor. Look down what is connected and powered from that rail and try to find a bad part. Could be protection zener of transistor down in the path.

[tautech]

Welcome to the forum.

L1971 is part of a Pi filter that smooths the rectified voltage after rectification.
https://en.wikipedia.org/wiki/Capacitor-input_filter

The likely reason it's hot and the rail is low (V) is too higher current draw in the circuitry it supplies.
Failure points to check are any Tantalum caps and electrolytics.

Your thermal camera will help. 

[singapol]

2213 full schematics service manual:
http://w140.com/svc2213.pdf

The schematic of 2235 ( crt ) is slightly different...in 2213 tektronix save 2 rectifier diodes by tapping from +8.6V even though it has that extra winding for 5V regulator (78M05). If the related components linked to your hot inductor is ok then know that +8.6V goes to a number of places like schematic 1 ( ch1 & 2 attentuators) ,schematic 5 ( sweep generator & logic) , schematic 9 (probe adj, crt  ,to trace rotation adj.) and schematic 7 ( x/y ,horizontal output).

My guess is horizontal circuit.  


PS..there is another hot spot behind inductor, looks like C951 10uf. related to circuit before + 8.6V ?

More likely power circuit..related to inductor. See schematic 9.

[toxuin]

Thank you guys!

In the finally found schematic 9, I've noticed the +8.6V is quite different from 2235 and 2215A: it uses a voltage regulator and that inductor basically serves power to 5V as well. Well, there were bridges marked on the schematic too, so I thought I lift them one by one and see where the short is.
Surprisingly enough, when I lifted the bridges, voltage was still around 1V and inductor still heated up. That would mean only couple of other devices might be shorted: low-pass cap right after the inductor, diode on the output of the voltage regulator and the 5V low-pass filter capacitor.
I initially thought that regulator might be dead and not regulating, but then there is a diode that should protect the out path from shorting to ground, and seeing that diode being quite big & beefy that's less likely to be the case.
Then I've noticed a red drop of capacitor blood on the 5V low-pass filter cap! It was there all the time and should've caught my attention on the visual inspection stage! I even have it visible on the thermal pic! No wonder when I replaced it the voltages went back to their values and I got a "line" on the screen. Whoo-hoo!

Here, blue is the inductor that gave a clue, red is the capacitor I've replaced.



I've replaced the 840 microfarads 12V with a 820 microfarads 16V nichicon one. I have no clue where would you get a 840 microfarads cap these days as they do not appear to be a common value at all. Should I put in series 20 microfarads + 820 microfarads with higher voltage rating for ESR compensation?..

After doing these manipulations, I can see a square-ish wave on the screen with no probes attached. On my other scope this pattern is a line. I was a bit short on time that day and couldn't investigate further in terms of set settings and\or probe compensation business. Can this be a consequence of the new cap being not exact matched value?..

Thanks again!!!

[tautech]

I've replaced the 840 microfarads 12V with a 820 microfarads 16V nichicon one. I have no clue where would you get a 840 microfarads cap these days as they do not appear to be a common value at all. Should I put in series 20 microfarads + 820 microfarads with higher voltage rating for ESR compensation?..

After doing these manipulations, I can see a square-ish wave on the screen with no probes attached. On my other scope this pattern is a line. I was a bit short on time that day and couldn't investigate further in terms of set settings and\or probe compensation business. Can this be a consequence of the new cap being not exact matched value?..

Thanks again!!!

Very unlikely.
As that one had failed there'll be others most likely sick too but as you have another scope it's time to check all the other rails for ripple, the specs for which should be in the service manual.

[David Hess]

There is nothing critical about the value of the capacitor and I am surprised they used such a large one at the output of the linear regulator.  An 820uF or 1000uF capacitor will be fine.