I'm curious to see what the inside of the rotary encoder looks like; I've revived a few of those by just cleaning and relubricating them, if the contacts haven't completely worn away.
They can get dust or lint in them, making skips (if the dust blocks parts of the rotor) or erratic misbehaviour. But unless you fix that in some destructive way there is no real wear.
Many "broken" electronics have fixes as simple as this. There are a lot of cheap kits on Ali Express to practice solder if its something you'd find interesting. One of the benefits of YouTube is the depth of videos featuring diagnosis and fixing of obscure issues on rare hardware.
I have seen claims that as many as 80% of "dead" flat screen TVs are just a dead component in the AC-to-DC power supply. Assuming it's a problem where it doesn't power on at all, not some weird visual artifacting or physical damage to the screen. Sometimes as simple as just desoldering and replacing some capacitors...
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.
An AC power supply is one of the most dangerous places for a beginner to try electronics repair - the capacitors can easily be holding a lethal charge even though the device is unplugged.
It's good that you mention it, but it shouldn't deter anyone really interested. The power quickly dissipates (over the course of minutes), but you should short it with a resistor before working on it, of course.
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.
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.
> the capacitors can easily be holding a lethal charge even though the device is unplugged
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.
Not to mention that many power supplies include circuitry to discharge the capacitors whenever they are not in use (hence why many old devices with hard power switches would still consume electricity when plugged in). Regardless, the myth will almost certainly persist so long as making a low effort comment about safety is behavior people reward with internet virtue points.
You will note that my calculations assume that the caps are charged to the high-side voltages ...
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.
>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.
The PE connection is exclusively for grounding exposed metal parts and is not allowed to carry any current during normal operation. Using it to discarge capacitors would probably trip the RCD circuit in a modern installation. What you really need is a bleeder resistor between the terminals of high-voltage capacitors.
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 :-)
When I was kid my father used to recycle a lot of broken electronics. Most devices could be easily fixed (back then) assuming they haven't been damaged physically and assuming nobody else tried to fix them.
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....
Depending on how obscure the hardware is, there often is a video on youtube that helps. At least for mobile phone and laptops I've always found something.
I definitely recognise this phenomenon. It's amazing how easy it is to apply too much force or apply it in the wrong direction and suddenly you find some tiny plastic clip attempting to break the sound barrier.
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.
The HP/Agilent/Keysight power supplies have service manuals and, more importantly, are designed to be serviced. Most consumer electronics do not and are not.
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.
Can confirm, the knobs on those things are turds. Kinda embarrassing for such an otherwise-nice piece of equipment, too. There clearly are better ways to build rotary encoders, and for a unit with a four-figure MSRP, they could've specified a better encoder!
Like the ones Rohde & Schwarz use. I think the detents are magnetic or something; they feel magical.
I use exclusively Grayhill encoders and pots on my projects.
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.
I guess you might be right. It would be silly to risk entire plant down due to a small PCB broken. Same goes for important machinery that can't be now controlled because an encoder failed.
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.
I love these power supplies! An awesome feature is the GPIB port on the back (it's basically a parallel port with a specific command set).
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 have a ton of old HP/Agilent equipment for my current project and most of it has GPIB connectivity. It seems like a neat protocol and I also finally got an HP-85 with the serial/GPIB modules which have been on my wishlist for a while. Might try to turn it into a controller for some of the equipment.
The protocol isn't too bad but that humongous connector... hoo boy. I think everyone nowadays just hooks it up with those USB-to-GPIB connectors, but the drivers under Linux are often iffy: if you get the wrong one, you need to supply a blob to it, in order to fully boot.
I had an old home cinema amp with a finicky encoder. The only thing I did was to remove the knob, douse the encoder in contact cleaner and put the knob back. I didn't even open the case. It took a few times to get it back to fully working but in the end, it worked surprisingly well, and it seems to last.
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.
Sadly not all Agilent/HP equipment uses rotary encoders like this. One of their function generators was brought to me with a clear rotary encoder fault. I opened it up expecting to find a rotary encoder like this. But instead found they had implemented it using circular tracks on the PCB and the knob turned a metal wiper which shorted out the tracks as it turned. After a couple of years of use the tracks start to develop grooves and wear out. There isn't much you can do without replacing the entire front panel which wasn't economic to do. I think within 5 years we binned 15+ of those units.
Old HP/Agilent equipment was beautifully made but in the few years before the sale to Keysight quality took a real dive.
>Old HP/Agilent equipment was beautifully made but in the few years before the sale to Keysight quality took a real dive.
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.
"If your PSU is still under warranty, expect it to be void after doing stuff like this!"
Ugh, I wish people would stop believing and propagating this falsehood (at least in the U.S.). Your right to warranty service is protected by federal law even if you modify a device, by the Magnuson-Moss Warranty Act.
Not only the right voltage, but a bench supply will have current limiting as well. (It will be constant voltage up to a settable current limit and then will let the voltage drop off to maintain that max current set. You can short the output together and set the max current.)
> You can short the output together and set the max current.
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.
Another really useful thing to have is a programmable load[1]. It's sort of the inverse of a power supply... it's a power sink that you can control. It's really handy to dial up a constant current setting and plug in a power supply that is under test, and observe how it behaves. They're good for testing batteries, solar panels, and most power supply related things.
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.
58 comments
[ 3.3 ms ] story [ 116 ms ] threadWhen 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?
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.
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.
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.
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.
Ugh, I wish people would stop believing and propagating this falsehood (at least in the U.S.). Your right to warranty service is protected by federal law even if you modify a device, by the Magnuson-Moss Warranty Act.
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/