Refurbishment of Tektronix 2465

Note: Click on (most) pictures to embiggen.

I picked up an Tektronix 2465 scope at an estate auction. It "works", except that the fan was dead. The fan on the 2465 is notoriously complex and unavailable, so the best solution seemed to be to replace the original fan with a common "computer" fan, which is what Tektronix did with the 2465A and -B scopes.


Front view of the instrument showing temperature reading of 36.9 C.


Rear view of scope showing added computer fan with cobbled up fan regulator.

The fan I chose is close in diameter to the existing fan port on the back of the scope. It runs off of 12v (nominal); the scope supplies 15v (unregulated) to the (stock) fan. My fan regulator is just a 7805 regulator with resistors and a small trim pot so the output is adjustable from 8v to 12v. I built it on a small scrap of perf board.

The fan voltage is adjustable because I had this idea that I could run the fan slower to make it less noisy. However, the temp rose high enough with the fan at full speed that I don't think I'll experiment with this. Besides, the fan noise is "not that bad"; I could easily live with it.

This is not necessarily my "final" repair, but it gets the scope working right away. Since I don't plan to carry it about, the frailty of the stuff on the back panel isn't an issue; this will live on my bench. Besides, my handle is missing (this scope had the rack mount option).

To mount the fan, I drilled and tapped two 6-32 holes in the back panel (careful to not drill into anything expensive), and also a larger hole (grommeted) through which I run the 15v supply leads to my fan, and the temperature probe (an LM35 analog temp sensor).

I've left the plastic back cover off; it doesn't seem to solve any problems and I'd want to hack holes in it to increase air flow, so I'll just leave it off.

Initial Testing

The first test run with the new fan saw the internal temperature rise and plateau at 36.9 C over several hours. I think this temperature is OK.

Schematic

Perhaps I will supply a schematic for the fan regulator eventually, but really, just hook up an 7812 regulator to feed 12v to the fan and be done with it. My solution is needlessly complex.

"Rifa Madness"

(with apologies to EEVblog)

It's well known that the "Rifa" brand capacitors have a high failure rate. One of the Rifa caps in my 2465 had failed; the body was cracked and split open, and some of the "magic juice" had run out and spread out on the PCB. (Fortunately, this stuff cleans off easily with rubbing alcohol.)


Failed Rifa cap. You can see the brown stain from the leaking cap spreading out to the left of the cap.

Another view of the Failed Rifa cap on the PCB


Failed Rifa cap removed from the board. The body is cracked all the way through, such that it almost crumbled to pieces in my hand.

This cap is a .068µF 250V "X" capacitor. The X means it's a "safety" cap, designed to not fail with a short circuit when placed across the AC line. Therefore, one should replace it with the same kind of safety cap.

There are two other Rifa caps in the power supply; though they hadn't failed, I've replaced them out of "an abundance of caution".

FAQ

Q. Will you fix mine?

A. No, I'm retired and so not interested in a job.

Disclaimer/Warning

This is just my documentation for my repair. I don't claim that doing this is safe or recommended.

Soldering irons are dangerous, be careful. Oh, and don't eat the solder.

There are live mains voltages, plus other high voltages, in this unit. Be careful when poking around in the power supply area.

William Dudley
June 29, 2022 (initial discussion of fan failure)
July 09, 2022 (added "Rifa madness")

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