Microwave oven repair

So, another round of posts to relieve the backlog of ones that I have pictures for.  We had a wonderful microwave at the house I lived at while in college.  It had all the features I could ask for: a temperature probe.  I know some people want power levels, digital interface, or even a rotating tray, but I really really liked the idea of a temperature probe.  I wonder why modern microwaves dropped this fantastic feature.  My instinct says that morons think “I’m not supposed to put metal in the microwave, therefore I can’t use this feature supplied by the manufacturer”.  The ability to put metal in the microwave has been proved as a fantastic way to heat a solder pot or melt pewter for molds (and it is not bad for the microwave).

Now, I mentioned this microwave didn’t have a rotating tray.  That’s not true, it came with an aftermarket one, but no rotation mechanism.  My theory is that the mechanism had an electric motor on it and it turned quite well until someone lost it before we bought it.  I also mentioned it didn’t have a power level selector.  That’s also not true, it had one, past tense.  This time it had one when we bought it and it doesn’t now.  I’m the one who removed it and I don’t feel bad about it.  Originally the switch was sparking and eventually died as seen here:

The fix I originally had for this was a set of switches that replaced this one (I’m NOT trying to source an equivalent to that).  That worked for a while, but eventually we had a problem of complete failure of the power system.  That is shown here:

The method this microwave uses to change power levels is literally by dissipating it through a huge power resistor and switching parts of it in and out of the circuit.  This method is less efficient than modern methods of sending pulses of full power followed by no power to the magnetron, but much more regular and produces a smoother pattern of heating in the microwave.  A nice big 120W wirewound resistor would be the best idea for setting power level, but that’s prohibitively expensive and I’m just too lazy to look.  This microwave got shorted to full power for the remainder of the time it was under my care.  I’m not sure what happened to it, but I’ve only seen one better than it and that one had digital controls, a temperature probe and a rack!

Inside this microwave I found schematics and testing procedures, as it turns out our magnetron is operating within normal parameters of full power.

Schematic

Manual

LCD monitor repair (again)

OK, so this is one of those days that starts with big plans and gets derailed seemingly because those plans were so well thought out.  I come from a group of people that spend their days just repairing things because they’re there.  We have taken it in stride to develop techniques for repairing commonly thrown out items and apply our experience to new items as we get a hold of them, but we rarely document our work or share our knowledge beyond the confines of our lab.  This is the story of what happened when I tried to document what I figured would be a routine repair on a common model LCD monitor.

Our Patient

The monitor (as you can see) is a Dell 1907FPt 19″ widescreen LCD monitor.  The first thing I recommend when you have a device of any kind to repair is to look up that model (or series) and see if anyone else has encountered problems with it.  Hopefully you’ll run across a helpful blog post of someone who took the time to explain the steps they took to debug and fix theirs, so you can cut down on the time it takes to fix yours.  I, of course, didn’t do that.  I have no idea what the common failure modes of this monitor are and what the commonly accepted fixes are, so I blazed my own trail to some hilarious consequences.  Seriously, do as I say, not as I do.  For every blog entry of a moderately successful hack/repair I have there are a dozen failed ones that could usually be avoided by spending some more time researching and less time breaking things.  In the future I hope to post my failures with notes for future pioneers, but my successes are usually still around after the fact so photos are easier to take (since I almost never take them during the event, but months after).

Now, having opened more than my fair share of Dell monitors I thought I’d itemize just what steps are involved.  First you need to remove any visible screws, this usually just consists of the four phillips head bolts on the back (seen above).  Next you need to insert a spudger (yes that’s a technical term, look it up) into the gap between the monitor back and the bezel.  That separation into two pieces is the entirety of the plastic casing on this model.

step one

tabs exposed

keep going, almost there

and there you go, dis-assembled

Once you get the back off of this monitor you will inevitably hear a clatter as something falls to the floor and hides among the junk there until you figure out what exactly was missing so you know what to look for.  What you are looking for is a small plastic button and spring that retains the base to the monitor.  The button is a Dell thing, they have brackets on their stands that allow you to use your monitor with only that stand and not that of another Dell monitor.  These stands are “tool-less” which is slang for “our main consumer base can’t operate a screwdriver”.  The monitor itself has VESA standard mounting holes, but the stands are mostly useful for the monitors with which they came (although I may explore the hacking I have done on these stands in a later post).

The button and spring, free without the back

carefully remove this ribbon cable and the double-stick tape that holds it to the monitor casing

Now we need to just keep removing screws and remember where they came from.  You can usually ditch the shielding if you don’t care about potentially harmful interference as covered in FCC part 15.

remove one cover over the inverter wires

and the other

remove the inverter wires from one side

and the other, while you’re at it remove the screws that hold the monitor to the controller board housing on both sides

the panel without any controller, power supply, or inverter

now remove the screws that hold the heatsink to the casing

screws seen here (note they are flat, they mount flush with the panel

power supply board has had it’s screws removed

mains plug with screws removed

screws in question

Not pictured (I know, with all these pictures I forgot one) is the removal of the logic board’s retaining screws and the mounting hardware for the d-sub connectors (VGA and DVI, although DVI’s not a true d-sub).

controller board

Here is the controller board.  There were no bad caps on it, which is a shame since that would have been an easy fix (unless the problem caused a chip failure).  On the bottom are the ports exposed to the outside.  DVI, VGA, USB-b, and a dual stack USB-a.  This particular monitor has a four port usb hub built in (which I’ll go into later).  The port on the right side of the board is for a two-usb port daughter board for the hub (only passives if anything).  The ribbon port on the right is for the front panel controls (bonus points to those of you who remembered that ribbon from earlier).  It is noteworthy that different versions of Dell UltraSharp monitors use different pinouts for the cable with different numbers of features, but they all seem to use the same size cable and connector so they are interchangeable as far as that goes (buttons have different functions, or no function at all).  The port on the left is for power input (it could be pinned out, but I didn’t as they’re usually labeled).  The top port is the LVDS signals to the LCD panel.

close up on VGA and DVI port electronics.

So, the three chips here are two i2c EEPROMs and one chip that I assume is the receiver for the VGA signal.  The eight SOT-23 packaged devices to the left are probably FETs being driven by the differential signalling on the DVI port (the same as the monolithic analog receiver for the VGA port.  The EEPROMs are of note because they hold the EDID information for each port, they tell the computer that’s connected to the monitor what resolutions are supported.  The noteworthy fact is that these type of EEPROMs are massively useful as they’re easy to reprogram and use in things like PCI network cards, motherboard BIOS chips, and USB peripherals.  The chip with the little white sticker on it is probabbly the i2c EEPROM that holds the settings for the Genesis main controller, setting what the settings on the LVDS panel are.

This is interesting to me because you can now buy generic LVDS driver boards and inverters to make use of that surplus laptop LCD panel and I have this thought that I could re-purpose one of these drivers to be used on a different panel.  This phenomenon changes the standard answer to the question of “I can hook the composite signal of my n64 to this old laptop panel” from “No.” to “Maybe.”.

USB hub

So, this circuit is interesting, it’s a USB hub.  The USB hub is entirely separate from the controller that drives the LCD except for power.  I have in the past had good results tracing out the power for the circuit seen here and cutting power to the rest of the board to use it as a stand-alone hub.  The thing about powered and un-powered hubs is that in most of the hubs I’ve seen is that the 5 volt power is hooked directly to the input 5 volt line on the USB-b port.  The un-populated chip on the upper right side of the board is probably another one of those ubiquitous i2c EEPROMs that would hold the USB Vid:Pid pair (as seen in that older article on the logic analyzer).

Here is the problem, the backlight doesn’t work.  The symptoms are that when you power it on one of the coils fizzes and then the over-current protection kicks in and kills it.  Presumably the coil had shorted, so I did the inadvisable and replaced it with an inexact replacement coil.  That coil fizzed as well, so I figured there was a problem further upstream.  Probably a broken FET that’s passing too much voltage or something, but I never found that out.  My solution was… very hackish.  I replaced the inverter with a new one.  A new one salvaged from an HP film negative scanner lightbox.  I don’t have pictures of that, but the inverter is exactly the came as one out of one of those blue inverter boxes from the mid 00’s that drove the ever classy blue cold cathode tubes (you know, the ones that are really shitty quality and explode periodically).

crazy solution being tested

In this picture I have removed the coil (no more arc-ing (fizzing)) and powered the inverter from the 12 volt out put designed for the amplified speakers that are an optional extra for this and most model Dell monitors.  The problem is that the backlight still shut off after a few seconds.  Time for some more fast-and-loose hacking.

underside of working inverter

So, without tracing out the circuit here I can see a bunch of diodes and resistors tied to the high voltage section of the transformer.  The thing I can assume from here is that a network like that can be used to sense the voltage (through dividers) and waveform (through dividers or zeners) of the output of the step-up transformer.  This type of closed-loop control is advisable so you can shut down parts of the circuit without blowing anything up.  It is also the enemy of simple hacks like this.  Let’s look at the section I modified.

“modified” board

So, of note here is that I removed pretty much everything.  The red stuff is glue used to hold the components down before they were soldered in (or maybe to take the mechanical stress off the solder).  This part of the board was gutted and I assumed I’d have to feed the signal from the other inverter to the sense circuit for this one.  I didn’t.  It just works.  Who would have figured.

you’ve had some cowboys in here…

The final configuration, complete with plastic sheet for insulation and hot glue for adhesion.

It works!

That’s it.  That’s how a simple post on a “standard procedure” repair became a Frankenstein’s monster post.

Cordless headphones revival

So, who here hates bluetooth audio? Now who knows what it is? Good, all the same hands stayed up.  I have an alternative to this crappy option: go old school!

Ok, now I picked up these headphones some time in the past, probably at a garage sale.  They are, specifically, Optimus 33-1145 900 Mhz headphones, FCC ID: CLV-A900T.   Originally these headphones had a Ni-Cd pack that allowed them to be re-charged form the base station, it was long dead.  So, to get these operational I need a power adapter, and a new battery for the headset.  Let’s start with problem 2 first, the battery.  The battery supplied 3 volts, was triangular in shape, and had a charger port on the side of the headphones where it could plug into the base.  I really wanted a cheap solution, so I took the best of my 2xAA battery holders at the time (from an old AV receiver IIRC) and glued it right on the outside.  I removed the charging port and re-located the power LED that the battery covered up now.

the on/charge switch is now the on/off switch

Good, now the headset is converted to replaceable batteries (or, you know rechargeable AAs or whatnot) let’s look at the base station.

with mods applied

Here is the back, it has a DC power jack originally spec’d for 18v, a 3.5mm headphone jack for charging the original battery, a volume control, audio in in a less than useful format for my purposes, and a frequency fine tuning knob.  Now let’s look inside and see what I did.

So, right away two things should stick out: that yellow wire bypassing a voltage regulator and that charging jack doesn’t look stock.  The charging jack was removed, flipped over, and glued back down to be a 3.5mm audio in port (it’s wired in parallel with the other audio jacks on the bottom [with shielded cable even!]).  The regulator was there because the original power brick was apparently very unstable and needed to be cleaned up for audio use.  I did this so long ago I can’t remember why I spec’d the new power supply at 16v, but I trust myself because it’s working.  So now we have a working headphone set, cool! but what other features do we want?

Another headphone jack of course! I could have done this right and used a switched jack, but that wouldn’t allow me to tether a friend to me so we could both listen together.  My motivation for this was mainly that now I can play music (I mainly listen to audio books, but I’m strange like that), walk around the house, and when I get to a place with a set of powered speakers I can settle in, plug the headphones into the speakers and keep listening rather than have to queue up the track on whatever computer was nearby and hooked up to those speakers.  So there you go, replaced a toasted voltage regulator, replaced rechargeable batteries with replaceable, and even added a new feature.  I think this one worked out pretty well.  Full album is here.

Replacement power supply build

I say build, it’s more a mod.  A long ,long time ago in a college town far away I helped a friend move into his frat house.  After I did this one of the housemates gave me an old monitor and power supply simply because he didn’t need it.  I gladly accepted it, but when I checked to make sure it was the right power adapter I noted it was a 12v 1a one, the display called for a 16v 3a one.  Now, I don’t know exactly the audience I have (yes I do, it’s web spiders) but I’ll tell you right now: there’s no way that will work.  I mentioned this at the time, but was assured that that was the adapter he always used for it (no it wasn’t).  My persistence at wanting this thing to work may seem misguided, but here’s the thing: this isn’t a computer monitor, it’s a TV.  Now I know what you’re thinking  that’s worse right? Right.  But in this case I don’t want better, I want more.  This TV (Phillips 20pf5120/28) has a cable tuner (useless), a composite input (expected), an s-video input (expected, but appreciated), a component input (cool! my first component input), and a dvi input (what?).  The DVI input can only do a resolution of 640×480 (and only digital), but it’s a 20″ 4×3 LCD TV, what more can you expect?  Let’s look at the specs for a new power adapter, shall we? 16v DC: that’s not too hard, if it were 12 or lower you could use a linear reg on a computer power supply to get that, but it’s not too bad.  3a: here we have a problem; that much current usually warrants the power supply to be inside the device it’s powering so those are a bit hard to come by, or at least they were.  I say that because we are now in the era of scrap laptops.  Yes, that’s right, laptop computers are being thrown out left and right for all manner of faults: broken screen, dead battery, won’t boot, broken power adapter (more on this later).  The thing about laptops is that they take a lot of power, sometimes upwards of 4.5a.  The problem now becomes the voltage.  Laptops generally run off of somewhere between 18.5v and 20v to charge their lithium ion batteries.  Let’s see what the junk drawer has to offer.

image taken post mod

We have a nice 19v 3.25a laptop power supply: perfect! Ok, there’s still that voltage problem; there are two ways to tackle this: a switching regulator, or a linear regulator. The switching regulator is a much more efficient design, but requires parts I did not have on hand.  The linear regulators could not pass enough current (1.5a max) but that’s ok, we’ll just put 2 of them in parallel to get that extra current through.

The design of the linear regulator circuit is simple, we use a fixed voltage linear regulator and a voltage divider made out of two specially chosen resistors to set the voltage to whatever we want.  For this one I just pulled out my phone and used electrodroid, but the calculation is simple to do if you want.  The heatsinks are mandatory since I’m running these regulators at the ragged edge of their tolerances.  There you go, a 16v 3a dc power supply that will give you second degree burns if you handle it wrong.

I swear I build these things better when they aren’t just going to be used by me

Gynecology table repair

I didn’t break this one.  Ok, I just felt like I’d open with that, I don’t get to say that too often.  This also differs from my normal electronics hacking, but I really have this motto: “if someone else can do it, I certainly must be able to” and an addendum: “if no one else can do it, it’s only because they aren’t as good at it as me”.  That’s mostly sarcastic, but it is a bit more honest than “if at first you don’t succeed…”.

This time I’m repairing a gynecology table.  I’ll let that sink in for a minute.  Ok?  Good, let’s move on to the story.  This table was purchased by a very good friend’s uncle for use at his tattoo parlor.  I believe the intended use was/is as a piercing table, and that there was a story that went along with this table that sorta motivated us to get it fixed.  As you can see, this is a vintage table, so repairing it was even more interesting and important.  The problem with it was that when it was moved someone decided to lift it by the tilting part that you see on the right and broke some welds.  I think the original construction method was a series of holes in one sheet of metal, and pegs sticking out of the other so welds could be made along the center of the hinge rather than the edge.

Onto the fix! The easy way to fix this was to drill out the pegs and replace them with nuts and bolts.  Luckily the hinge was bolted onto the seat so all we had to do was unbolt that to take it somewhere else to work on it.  A quick run to Home Depot got us the right parts (and locktite of course), but where could we get a drill? Oh, there’s a metal shop in the basement of a tattoo parlor.  Yup.  Drill press and all.  That was easy.  Some lubricating oil later we had cleanly drilled holes, brought it back and installed it, very easy.

I don’t know what model this was since I’m bad at pictures, but I know that of the things it came with it’s latest patent date is 1943.  So, this has been a tutorial on how to repair a broken post-1943 Hamilton brand gynecology table.  This was the way I spent my last day visiting in Seattle, and it was probably the best way I can think to have spent it.  Full set of pictures here.

LED backlighting a 22″ lcd monitor

This one’s kinda messy, but I was visiting my parents so I only brought my soldering iron and basic tools with me.  I had a friend mention that their lcd monitor was broken, of course the first thing out of my mouth was “I can fix it no problem”.  It was a little more complicated than I thought.  Let’s review the most common failure mode of electronics this decade: bad caps.  What I mean by that is capacitors that have failed, exploded, had their ESR go up, heated up due to that and then exploded, or bulged, or just plain leaked.  The reasons are numerous: there could be an unexpected cooling failure causing the caps to heat beyond their tolerances; there could be too high an ESR as I mentioned; they could even just be, gasp, old! But by far the most common reason for failure is espionage.  It’s a little more mundane than it sounds, there are news articles, wiki entries, and even a website and forum dedicated to it.  I’m just hereto say it has been a godsend for those of us that can open consumer electronics and solder reasonably well.  When you have people throwing out devices worth several hundred dollars each because of a few cents in parts that are fantastically easy to identify and fix… well, let’s just say I know some people who could major in monitor repair and minor in general debugging.

the whole setup, mountain dew and all

This problem monitor happened to be a Sceptre 22″ lcd monitor.  Now, due to the rapidity that people are throwing out monitors I tend to have a large backlog from which I can pull parts (usually entire power or logic boards, sometimes button panels, etc…) but I don’t have any spare dead monitors this big.  Once open (although I should have done this first) it was obvious that the LCD worked, the logic board worked, and the main part of the power supply worked.  I could tell because the only thing that didn’t work was the back-light   This doesn’t preclude it from being bad caps, but it lowers the likelihood   In back-lights I tend to see one of four things go wrong.  First there is the simple bad caps on the inverter input, this wasn’t the case here.  Next there’s the blown PWM chip or blown FET driver, usually SOT-8 or DIP-8, can be blown by heat, time, or voltage fluctuations (sometimes traced to caps). Another potential failure mode I know is shorted transformers; I haven’t ever gotten a monitor I cared enough to diagnose this far, but the PWM chips create a square wave that gets stepped up in the transformers to drive the high voltage tubes.  The problem this time was bad back-light tubes.  These tubes were seriously blown, though I have come across ones that glow red briefly and then the over-current protection kills the back-light inverter.  Well, I don’t have any spare tubes this size, what about LED strip lighting? that should work, right?

assembled with strips installed

I ordered a 12v LED strip off of amazon and it is pretty nice stuff.  The stuff I bought had a rubberized coating on it meant to be used outside, which didn’t really help, but oh well.  The rest of the mod went as expected, I pulled the tubes, carefully disassembled the panel and fitted the new lights, ran power wires through a switch because I didn’t feel like working out the back-light auto-on feature and powered it off of the 12v tap on the power board.  That simple, I could have done with some better diffuser  but I didn’t have much time to work on this.

monitor off, back-light on, diffusion problem

working. Next patient!

entire disassembly album

Repairing Rock Band Drums

Continuing in my trivial repair series I bring you the drums.  One day (about a month ago) I was out driving around at night through my friend’s neighborhood the day before trash day when we come upon something interesting.  It turned out to be an xbox 360 set of rock band drums, complete except one of the far pads was completely broken off.  After I got it home it was trivial to replace the broken wire with another one (a twisted pair from a bit of ethernet cable) however the hardware repair proved to be a bit harder.  My normal approach is to use hot-melt glue or fiberglass resin to just put it back together, but with the forces that would be torquing the joint would shear off any adhesive quite quickly.  What I decided was that I needed was a rigid piece of material to bridge the gap and I needed to permanently affix the new material to the existing body.  For the material to act as a bandage I used some bits of a computer case (one drive bay cover and two strips cut from the case of a power supply) and for the adhesive I used pop rivets, they are cheap, fast, and easier than a nut and bolt.  Since the cable didn’t come with a USB connector on the end I ended up using a female USB b port salvaged from an old printer, insulated with some medical tape since we were out of electrical tape (we now have five rolls since I got fed up with not having any and went out to get some today).

Bottom Brace

 

Back Brace

Front Brace

 

USB Connection

I promise my posts will get more substantial as time goes by, trying to archive my own minor fixes I do on a regular basis (I do have some actual mods and hacks coming up soon).

Repairing Rock Band Guitar

Recently I went to visit a friend living in Mt Pleasant and while I was there he gave me this guitar that he said was his housemate’s, and it was broken.  Well, being me I naturally assumed that I could fix it no problem, and I was right.  Upon close inspection one of the 90° .1″ headers had all three pins broken off.  Not having a three pin 90° .1″ header I used what I had, in this case a 3 pin .1″ header salvaged from an old broken motherboard (probably a header for either a turbo button or a cmos reset jumper).  All I did was solder the header to the remaining pins to make a 90° header.  I have pictures of the entire board to document the orientation each connector has to be in for it to work properly.

The Patient

The Guts

The Opperation

Oh, in addition to that the whammy bar had the spring broken off, I assume this is why it was dis-assembled in the first place, then the header got broken while it was open, and I fixed that by bending a little more of the spring into a loop and re-attaching it.