Repair of Samsung LE40M86BD Television (power cycling)

Introduction

Recently I have had cause for complaint with our Samsung LE40M86BD LCD Television.  It had been working just fine, and then all of a sudden it developed a power-cycling problem.  The symptom is as follows:

  • From power-on, the TV works perfectly for around 10 minutes.
  • After a while the TV switches itself to standby, waits a few seconds, and then switches itself back on.
  • Once this starts happening the cycle repeats itself every 30 seconds.

I decided to take a look, and that’s what this blog post is about!

Troubleshooting

A Peak Inside

I took the back off, and it never ceases to amaze me how little there is inside modern TVs.  They are for more complicated than older TVs of course, but all the technology is packed into densely populated embedded systems.

Inside the Samsung TV

In this photo you can see the two main parts of the Television.  Near the centre is an off-white coloured circuit board; that’s the main Power Supply.  To the right of the Power Supply is a similarly sized green circuit board.  This board is the heart of the Television.  It’s basically a custom computer!

Straight away I noticed something suspicious on the Power Supply board; nasty looking electrolytic capacitors! Let’s check them out:

The Power Supply Board

Here’s a photo of the power supply, on the bench.  Now, any time you see electrolytic capacitors mounted right next to a heatsink as they are here, you simply have to be suspicious of them, especially in older equipment.  That big old heatsink pumps heat into those capacitors day in, day out.  And if there’s one thing electrolytic capacitors don’t respond very well to, it’s long term heating.

Two of these capacitors, highlighted in the image above, are showing the classic signs of dielectric degradation.  The top of the cans are bulging at the seams.
Sometimes this type of capacitor will also leak electrolyte, which can be very bad news indeed.  In this case, it’s just the classic bulging.

At this point I decided to replace all the capacitors in the local area, as sometimes an electrolytic capacitor can be bad without displaying any obvious physical signs and they all will have been subjected to the heat pumped out by the nearby components.

More Trouble

After this I was quite hopeful of a quick and easy repair.  But my hopes were dashed when I discovered that the TV was still power-cycling after a few minutes of use.

So, what to do? Well, I decided to be a bit more scientific about it from now on.  I got my ‘scope out and checked each of the power supply rails generated by the PSU board.  I discovered two things:

  • The Power Supply rails were now rock-solid.  They probably weren’t before I changed those nasty capacitors, but they definitely were now.
  • I could run the Power Supply into a load, away from the rest of the TV, and it never power-cycles.

So… the original fault was not on the Power Supply board then.

As a point of interest, I discovered an input control pin on the Power Supply called “ON/OFF”, which is driven from the main system.  I decided to take a capture of it and I discovered that my estimate of ~30 seconds power-cycling was almost spot on:

You can see here that the TV stays on for 28.7 seconds, then switches OFF, and immediately back ON.  The cycle repeats with exactly the same period over and over again.

So, this got me thinking.  The fault is surely on the main circuit board, as this appears to be deliberately instructing the Power Supply to turn OFF at regular intervals.  Let’s take a look!

The Embedded System

Embedded System Main Board

So, at first glance, there isn’t really much for me to get my teeth into here.  There was next to no information about this board on the internet.  I found a schematic, but it was more block-diagram level than anything else.

The photo above shows the board with the screening can removed, revealing the microprocessor underneath.  I took the screening can off because I noticed a bunch of SMD electrolytic capacitors and I wondered if they had been getting a little hot under the collar over the years.

An inspection of all the SMD electrolytic capacitors didn’t reveal anything suspicious; no bulging or evidence of leakage at all.
However… that isn’t particularly definitive.  Let’s see what happens if I try to measure the ESR (Equivalent Series Resistance) on some of these capacitors!

C1104 ESR

So, the ‘scope capture above shows the voltage drop across C1104 when stimulated with a 100kHz square wave at 1V peak-peak (50Ω output impedance).  C1104 is a 100uF capacitor, so the voltage drop at this frequency on a healthy capacitor should be close to zero.  What do we see instead? 286mV!

If you do the math, you’ll find that this comes out as approximately 5.5Ω ESR.  A horrendously bad capacitor!

At this point I went all around the board measuring ESR on the SMD capacitors.  I was able to measure the majority of them in-circuit, and I found a whole bunch of bad caps.  I replaced them all.

Success!

After this, the TV is finally working properly.  The fault has not re-appeared in over a week.

The total components replaced are as follows:

Power Supply Board

CM880 1000uF 25V
CM876 1000uF 25V
CM852 2200uF 10V (evidence of bulging, ESR 2.4Ω)
CM853 2200uF 10V
CB850 1000uF 10V (evidence of bulging, ESR 3.2Ω)

Embedded System Main Board

C1140 100uF 16V (ESR 5.5Ω)
C2267 100uF 16V (ESR 3.2Ω)
C1204 100uF 16V (ESR 5.5Ω)
C1131 100uF 16V (ESR 3.2Ω)
C1104 1uF 50V (ESR 21.5Ω) (!!!)

Another perfectly good piece of kit saved from the scrap heap.

Good luck!

Panasonic AE700 Projector Diagnosis & Repair

Panasonic AE700

In the tinkering cave this week we have a Panasonic AE700 HD Projector. These units are pretty old now, but they come highly recommended in many online reviews and the HDMI support means they remain a useful home cinema option.

The unit was first presented to me by a work colleague who complained that the unit would not illuminate. It powers on just fine, but returns to standby after about 30 seconds without displaying any picture. My colleague had already completed some diagnosis of his own and had determined, correctly as it would turn out, that the power supply is not generating a 15V rail – which is required for the lamp circuitry to work.

Dismantling

IMG_20150208_133305With the cover removed, it can be seen that this unit is a bit of a beast! It’s packed with a mixture of high-tech electronics, complicated optics, and extensive cooling components.  Dismantling is slightly tricky; you have to remove the top sensor board first (the one with the buttons on it) and then you need to very carefully remove the dark grey cowling.  The power supply sits underneath; which is why the cowling must be removed for further diagnosis to take place.  There is a wiring loom which connects the PSU to the main board, and this will need to be disconnected temporarily (at the main board end) because it feeds through an opening in the cowling.  You can connect it back after the cowling has been removed.
IMG_20150208_133535With the cowling removed,  you can now take a look at the PSU.  “Let the dog see the rabbit”, as they say.

The first rule of electronics diagnosis is “Thou shall inspect!” In this case there were no signs of explosion, trauma, or electronic stress of any kind to be found on the board.  So it’s time to get the schematic out.  Fortunately, the service manual for this unit was ‘freely’ available on the interweb.  You can download my local copy of the service manual here.  You might see a warning about “untrusted connection” – that’s because I can’t justify the expense of an SSL certificate.  But I assure you there are no harmful files in my repository!

PSU
PSU Schematic

The schematic for the power supply is a little bit limited; it shows some discrete components but other circuitry is presented in block diagram form.  Still, there’s enough information to start a line of diagnosis.  The circuit I am interested in is shown to the left; click on it to open up a larger copy.

The second rule of electronics diagnosis is: “Thou shall measure voltages!” The circuit diagram shows P3 as the main connector – this is the wiring loom that had to be disconnected to remove the cowling.  With the wiring loom connected back in, I measured the following voltages on P3:

[table id=1 /]

Nothing to be too concerned about there.  Here’s the voltage measurements for P2:

[table id=2 /]

As can be seen – no 15V! Looking at the circuit, we have an output from the photoisolator which drives transistor Q107.  The transistor is switching a voltage (which I measured to be 18V) to IC102 which is a 15V regulator.

One thing that I found really suspicious about this, is that we have a tiny transistor (only a small signal device, as it turned out) driving a much larger linear regulator.  Going by the circuit we are looking at, the small transistor has to pass the same current as the linear regulator.  I don’t know what current the linear regulator is supposed to drive, but it definitely has a much larger current carrying capability that the transistor which supplies it!!! It stinks of poor design choices to me.  And Q107 has now become prime suspect for this failure.

And, sure enough, a quick measure of transistor Q107 revealed that it had indeed succumbed to its (inevitable?) destruction.  18V in, 0V out.

Q107 mod
Q107 mod

To remedy this, I replaced the component with one I had in my junk bin.  The choice of transistor is not too critical here; it’s just switching a voltage.  So I chose a ‘beefier’ device, which will happily support the current carrying requirements of the linear regulator – hopefully for the rest of this projector’s life.

 

 

 

A brief triumph – and a gift!

The projector worked after this, and I handed it back to its owner.  Unfortunately it was returned to me a few weeks later with the same complaint.  I was told that I could have the projector to play with, and keep if I could get it working! 🙂

I re-visited the power supply expecting a repeat problem, but everything checked out.  Since the 15V supplies a H.T. power supply circuit for the lamp, and the symptom was that the lamp doesn’t illuminate, I began to wonder if the H.T. power supply needed some attention.  Unfortunately this supply is cocooned inside metal shielding which does not seem to want to come apart very easily.  As I was poking around with my screwdriver, I noticed that the projector had suddenly begun to illuminate, but she only fired up for a few seconds before there was an arcing sound and the projector switched back off.  At the time I had been poking around a flat-cable signal loom which comes from the H.T. power supply and connects to the main board.  I noticed that this loom was routed inside the H.T. leads for the lamp! This is almost certainly where the arcing had occurred!

Nailed it!

IMG_20150208_165054I re-routed the small signal loom so that it gives the H.T. leads a wide-berth, and tucked it into the scart PCB.  I imagine that the cable had become disturbed through repeated dismantling of the unit and had found itself tangled in with the H.T. leads somehow.  You know cables, right? They tangle themselves up when you’re not looking!

After this I switched on the projector and voila! We have illumination.  I took this opportunity to play Karate Kid: Classic.

Repaired
Projector Working!

Result! One working projector, saved from the scrap-heap.

Good luck!

JVC TH-S5 Diagnosis and Repair

Symptoms

I’ve owned the JVC TH-S5 home theatre system for many years.  It could be as much as 8 or 9 years.  In all that time it’s been a reliable machine, although to be fair it’s also had quite an easy life; I don’t watch much TV (what self-respecting Engineer and tinkerer has time for that?) and even when I do I rarely give the system a run for its money.

About 6 months ago the system developed a problem.  When started from cold the base unit (i.e. the DVD player and system controller) would fail to power on.  I also noticed that if I listened very carefully I could hear a [tick-tick-tick-tick] sound coming from the unit.  I recognised this straight away – it’s the switch mode PSU trying (and failing) to start.  I was about to take the unit away to my tinkering cave for some diagnosis but I further noticed that the [tick-tick-tick] sound would gather pace, getting faster and faster, until eventually it was possible to bring the unit into life after a power-cycle.
The unit would work perfectly from that point onwards, provided that it was left connected to the mains.

Of course, I always knew this was a temporary solution.  This problem was not going to go away.  It was going to deteriorate for sure, and eventually I would be faced with a completely dead system.  And sure enough, 6 months down the line, that’s what I’m faced with now!

An Inspection

First thing’s first, I needed to get the cover off for an inspection.  With an old system like this I already had some preconceptions about what I thought I’d find.  Bad electrolytic capacitors were absolute top of my list for this kind of symptom.  Dry joints were a close second.

IMG_20141011_103310-3600x2700
JVC-THS5 base unit with cover off

As can be seen here, the power supply is self-contained on the right hand side of the unit.  This is where the focus of the attention should be.  An initial inspection yielded a disappointing result.  With bad electrolytics at the height of my suspicion, I was hoping to spot one or two displaying the classic physical symptoms of bulging or weeping.  This would be a sure-sign of trouble, and usually an easy fix.  The capacitors all looked physically healthy, though.  Time to whip the board out of there for a closer look.

PSU Board removed
The PSU board removed

Looking more closely at the PSU board itself, there were no obvious (physical) causes for concern.  I didn’t spot any dry joints on the underneath of the board either.  The focus of this investigation should be the primary side of the PSU because the symptom is that the PSU completely fails to start.  The primary side is marked clearly on the board and consists of everything to the left of the yellow-banded transformer.  The transformer bridges the electrical gap between primary and secondary sides of the PSU.  It’s an electrical gap because there is actually no direct electrical connection between primary and secondary sides.  The two sides are said to be electrically ‘isolated’.  This is important because we have dangerous high voltage mains A.C. on one side of the transformer, and then low voltage rectified D.C. on the other side of the transformer.  Never the twain shall meet!!!

Anyway, back to the fault diagnosis.  Since I am concentrating for now on the primary side of the PSU, and I have electrolytic capacitors as #1 on my suspicion list, it makes sense to have a look for some.  There’s only two of these on the primary side of the board; there’s the big fat one, which is a reservoir capacitor (and certainly not the cause of my trouble) and then there’s a small skinny one next to the chopper transistor heatsink.  The chopper transistor drives the transformer.  It switches high voltage, high frequency A.C. and dissipates significant power.  If you look at the surrounding board, it’s darkened brown from the heat that is generated by these components.  This is where all the electrical stress is to be found on this circuit.

Our suspect capacitor
Our suspect capacitor

Since the skinny electrolytic capacitor is mounted close to the chopper transistor heatsink, it will have been subject to the heat that has been pumped out of the high energy part of the circuitry over the last 8 years or so.  Electrolytic capacitors do not respond well to heat.  Often they bulge and weep, effectively holding their hands up to say “I’m faulty!!!”.  Other times they violently explode.  And other times still, they just die quietly without any fuss or obvious signs of defect.  They’re a bit like vampires in this regard; many different dying behaviours.
So the fact that it’s not showing signs of death doesn’t rule it out of my suspicion.  I’m going to whip that little guy out of there and subject it to some electrical tests which will reveal once and or all whether it’s healthy or not.

Capacitor ESR Test

One thing you can measure on an electrolytic capacitor, which generally gives a good indication of its health, is its ‘ESR’.  This stands for “equivalent series resistance”.  An ideal capacitor would have properties of capacitance only, with no ESR and no inductance.  But it’s impossible to manufacture the ideal capacitor.  In practice all capacitors have some small amount of ESR and inductance.  Electrolytic capacitors have relatively high ESR compared to some other types of capacitor, but when it is healthy the ESR is still pretty low.  When it is unhealthy, however, the ESR increases significantly.  And then we end up with a very poorly performing capacitor indeed, and that’s when they begin to prevent SMPS power supplies from starting.
The test I am going to perform here is going to tell me approximately what this capacitor’s ESR measurement is.  I don’t actually own an ESR instrument, so I’m going to have to measure it in another way, using SCIENCE. 🙂

Here’s the circuit which shows how I will complete the measurement.  Click on it for a larger version.

ESR-Measurement-600x308

 

 

So what I’ve got here is as follows:

A signal generator which I will use to apply a measurement stimuli to the capacitor under test.  I will be applying a 100KHz square wave @1V amplitude.  At 100KHz most electrolytic capacitors appear as close to a short circuit, provided that their ESR is low.  This particular device has a capacitance of 39μF.  If you do the math, the impedance that it should prevent to a 100KHz signal is:

Impedance Calculation
Impedance Calculation

The 100Khz signal is injected via a known output impedance of 50Ω.  This is the output impedance of the signal generator.  I then measure the resulting signal that appears across the capacitor under test with an oscilloscope.  The 50Ω signal generator and electrolytic capacitor effectively form a ‘potential divider’.  The capacitor becomes ‘R2’ in the equation:

Potential Divider Formula
Potential Divider Formula
Potential Divider Circuit
Potential Divider Circuit

where Vout is the voltage measured across the capacitor with the oscillpscope, Vs is the supply voltage (1V in this case), R2 is the capacitor’s impedance, and R1 is the signal generator output impedance.
If the capacitor’s ESR is low, then at 100KHz it will only present 0.04Ω resistance.  If you plug that into the formula above, you’ll see that we should only expect a miniscule voltage at 100KHz.
If, however, the capacitor’s ESR is high, then the resistance it presents will be far far greater than 0.04Ω and in that case we would see a fraction of the 1V square wave voltage being developed across it.  From this measurement we can solve the equation for R2, effectively providing us with a measurement of the capacitor’s ESR.

First let me do some measurements without the bad capacitor fitted, so that I can show you what to expect to see in a good scenario

Measurment with no capacitor fitted.

With no capacitor fitted at all, the resistance of the capacitor (R2 in the potential divider equation) is effectively infinite, so the voltage dropped across it will be the open-circuit voltage.  Basically the off-load output of the signal generator.  We should therefore expect to see the full 1V, 100KHz output as shown below:

Off load signal generator output
Off load signal generator output

Measurement with brand new (good) electrolytic capacitor fitted:

Here’s what happens when you insert a good electrolytic capacitor into the circuit.  It presents close to 0Ω resistance at 100KHz, so the fraction of voltage that appears at the oscilloscope is also close to zero:

Output with good capacitor fitted:
Output with good capacitor fitted:

If you look closely here you can just about see a square wave appearing on the flat-line.  If I zoomed into this I could calculate the capacitor’s ESR, which would be small.  But it does have some ESR.
There are also some rather large spikes.  This is because the step change from 0V to 1V is relatively high speed, and this signal is reflecting back down my cabling as a result.  Suffice to say I don’t need to concern myself with the spikes – they are interesting, but they are not related at all to this measurement.

Measurement with capacitor removed from system

Measurement with 'bad' capacitor
Measurement with ‘bad’ capacitor

Here is what happens if I put the capacitor I removed from the system in the circuit.  You can see that a very large square wave is being developed across it! Almost 500mV, in fact.  I could do the math here, but I really don’t need to.  If you see a square wave of this sort of amplitude being developed at 100KHz, then it’s clear that the capacitor has excessive ESR and is no longer performing very well.  As a quick mental approximation, we’re seeing a fraction of the 1V square wave being developed which is almost equal to 0.5.  So this means that the impedance of the capacitor must be close to 50Ω, since a potential divider with the same value resistance for R1 and R2 would give us an output of 0.5Vs (or 500mV in this case).

Replace the capacitor!

With this in mind it’s definitely time to replace the capacitor.  I didn’t have any 39μF capacitors in my home stash, but I had a 47μF so I decided to try that.  It did in fact work fine, and my system is now up and running again.

Conclusion

If you suffer a power supply problem with an aged piece of electronics, electrolytic capacitors are a good place to start.  They can fail on the primary or the secondary side, but will show different fault symptoms in the product depending on their function in the circuit.  Sometimes it is plainly obvious which ones are faulty; they’ve either exploded or they’re bulging at the top or leaking fluid.  Other times, as I’ve just shown here, they look okay physically but are hiding a nasty decline in capacitance performance internally.  In that case you can use this method to gauge whether they require replacement or not.  You can actually do this test in circuit, but it’s not recommended.  It’s always better to remove them first if possible.

Enjoy!