HP/Agilent E3631A Rotary Knob Repair
tomverbeure.github.io
tomverbeure.github.io
For anyone that's ever repaired a more complex power supply, the AC to DC units in a typical flat screen TV are usually really simple and easy to understand.
As a 10 year old kid, I had an old (tube) TV that had been standing on a table for months. I wanted to unsolder some component out of it, and thought it should be safe by now. But as an extra safety measure, I decided to short the big capacitors with my metal screwdriver. Most were as dead as can be. One however, decided to surprise me with a big arc/flash that scared me stiff.
Since them I have a lot of respect to electricity.
None of that applies to LCDs, which are phenomenally safe to work on.
The real issue is that SMPS are very complex and you are more likely to just cause more boom.
Even probing the SMPS requires a lot of knowledge and you can easily stop vital signal that is critical for controlling the circuit and then stuff is flying around at high speeds.
That used to be true when every power supply had a couple electrolytics as big as your fist. Only audio and guitar amplifiers are designed like this nowadays.
At this point, I don't think I've seen a power supply in years that could really deliver even a slight jolt after turnoff--max is about 20uF of electrolytic cap nowadays. For a 20uF cap at 100V, it can deliver 4mA for only about half a second--that is probably just barely capable of doing something to your heart if applied directly.
The bigger danger is you accidentally working on something still energized and you don't realize it.
If in doubt, stay away.
SMPS topologies can use capacitors on high voltage side rather than on low voltage side (like old linear supplies did).
If you don't know what is the topology and how it failed you should not assume it is safe to put your greasy noob fingers in it.
And I'm really, really struggling to come up with a switch-mode power supply topology design that has more than 100uF anywhere.
I stand by my statement. The biggest problem when dealing with power supplies nowadays is getting the shit shocked out of you when they're plugged in--especially if you don't realize it.
That earth pin just zapped me only once in 15 years, I don't remember what brand it was, however it was a beefy (750W+) server PSU.
Any acceptable quality PSU has internal bleed resistors to prevent shocks AFAICS.
Do modern TVs need high voltage circuits unless they're plasmas?
Also, are there any standards which force internal bleeding if there's no earth connection?
>Do modern TVs need high voltage circuits unless they're plasmas?
The way a lot of power supplies work, is that they rectify the 110V/230V to a reasonably high DC value, and change that to a high frequency AC signal so it can be transformed to a lower voltage with a smaller transformer.
This means there's typically some capacitors charged with sqrt(2) * line voltage during normal operation.
Thanks. Didn't know that.
Electronics repair on ordinary equipment is less dangerous than crossing the street. Folks just need to educate themselves on the basics of electrical safety, preferably with an experienced mentor, and then just get started with a healthy level of caution.
Energy discharge from capacitors found in things like appliances and electronic instruments is very unlikely to be fatal. The way people usually get hurt from caps is from an involuntary jerking response to the jolt and the subsequent scraping of skin on sharp metal edges.
5 years ago my big old Plasma TV stopped displaying a picture. Took the back off, reseated all the connectors, stared at the multitude of SMD boards, put the back on, and it's been fine ever since!
The daily driver, a 50" plasma, just celebrated its 12th birthday a few weeks ago :-)
It's also true that you can't usually put the case back together after you've gotten the damn thing apart and found the problem. At least, not unless you already had a case to practice on....
The next time you open the same thing it seems completely trivial.
Usually, finding and fixing a problem in a PSU like that is much tougher. These are very well designed (I know because I own one) and they are usually well protected from simple faults.
On the other hand they are built to be serviced. You can easily disassemble and reach any part in it. The biggest issue you will face is finding parts.
My success rate on random consumer electronics is pretty good, firmware problems notwithstanding, but then it turns out something was glued or even ultrasonically welded or requires an alignment jig or was just designed by a madman and is impossible to reassemble.
I've even been in the situation where something that I was on the design team for is busted and I know exactly what the fix is because I know that product but the assembly is one-way and so there's nothing I can do. (In this case the product required an IP rating and low cost, so ultrasonic welding it was.) It's very frustrating.
In college I bought a USB to GPIB adapter on ebay for like $100 and a few cables (you can daisy chain them together) and wrote some VBA macros to automatically sweep the voltage output over a range, and also connected to a frequency generator & oscilloscope to capture the data. My EE labs were done in a fraction of the time compared to my classmates!
I already had the expensive ($100+) USB to GPIB interface anyway. Earlier, I wrote a blog post about controlling my TDS 420 oscilloscope through GPIB: https://tomverbeure.github.io/2020/06/27/Tektronix-TDS420A-R....
Replacing the part is obviously better, but contact cleaner can do wonders if you want to try a cheap and dirty solution.
Note: I used the 3-in-1 (WD-40) brand because it is what they have in my local hardware store. I suppose that DeOxit and other brands work just as well if not better.
Like the ones Rohde & Schwarz use. I think the detents are magnetic or something; they feel magical.
Which is to say I don't mind paying extra for a knob that I will be regularly using but probably would be revisiting the choice if I had to make 100 thousand of these.
I mean I know why I buy them. Because I just like the feel of it and am not restricted by cost. My time is more valuable than dealing with cheap stuff.
Military? Aerospace?
There is just no connection between the cost of components and the value they are generating and possible costs of downtime.
On the other hand you have probably low production volumes because there is no mass market for these parts. And that then causes some stupid stuff to costs hundreds or thousands of dollars.
Keysight was actually formed by spinning-off Agilent's electronics test-and-measurement businesses (leaving behind really I think just the life-sciences stuff?) so there was no sale per se.
When they're worn, they're worn.
On the other hand, I can't remember encountering a worn-out optical rotary encoder - can they even wear out?
https://twitter.com/tom_verbeure/status/1384010826177122306?...
Wonder what sort of grease do these usually use?
I've been meaning to buy one for years, and only bought one recently at an estate sale for 30 buck.
It's just so convienant having the right voltage for testing, and repair.
Please, read user manual first.
Not all bench supplies are safe to be shorted.
Also there might be a small current peak that might be damaging your device over time without you even noticing it.
For example, my PSU has a special button to press and you dial in current limit while the button is pressed. You can do it without interrupting supply to the load but more importantly it does not stress components.
One example that I'm familiar with (it's what I use on my bench at home) is the Rigol DL3021A[2]. There are a lot of choices out there though, spanning a wide range of price / capability / power handling points.
[1]: https://en.wikipedia.org/wiki/Programmable_load
[2]: https://www.rigolna.com/products/dc-power-loads/dl3000/