Tektronix 2336 YA Oscilloscope repair

Recently a friend asked me to take a look at his oscilloscope. I could write a whole story here, but basically this: it took me a while to find the service manual for this scope (so it’s now on the internet archive here) but I didn’t need it. The symptoms were a burning smell and no trace on the screen except a blip when it is turned on after recently being turned off.

this is the blip

A forum post that helped someone else also seems to have the single issue that may befall this model of scope. I found it here and will quote it in case that link goes down:

I had a similar problem and suggest checking on the A15 board, C108 (.005uf) for shorted, and R113 (1.6K) for burnt changed value.

As info, if the 102 voltage is ok, it suggests that the high voltage circuitry is working ok.

Looking at the parts in question I did find a shorted cap and burned resistor. Tektronix lists the cap as “CAP.,FXD,PLSTC:0.005UF,0.1%,200V” which is… just ludicrous. Most caps are 20% tolerance, even today. 10% or 5% are available in most values, but 1% is just strange. 0.1% is just out of this world nuts.

The shorted cap in question

The actual cap seems to be 1%, which is less stupid of a component choice but still… Also a more modern value would be 0.0047uF because that value at a standard tolerance includes the possibility of being 0.005uF. 200v in the application seems needed, so we need to replace it with a larger value. 450v seems to be common on digikey, but in our parts bin we find…..

1200v! pretty damn god enough

here we see the replaced cap and resistor. I used a 1.8k resistor because it’s what I had. It’s good enough.

There, working, done.

This was an easy-ish fix because someone else had already figured out the issue. I’m here to assure you that component values don’t matter that much and tektronix is nuts for putting a 0.1% 200v capacitor in the 102v power supply circuit in their scope. Good luck repairing things in the upcoming appocalypse!

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