TideLog Posts Tagged “PSU”

A customer recently booked her Samsung TV in for repair with us, saying it was clicking, and not actually coming on. I’ve had this problem with an old Toshiba plasma of Kana’s over at White Tiger Martial Arts Academy, it was clicking badly, but actually worked, with visible distortion on dark scenes. In that case though, it turned out to be the plasma panel itself stressing the supply, as we couldn’t source a panel at less cost than the TV it had to be written off.

LCD’s though are much easier to source, and will never actually stress a high voltage supply as they themselves only run at 5v DC on their LVDS bus, so once we’d picked the customer’s TV up from her house, we took off its cover, and took a look. It turned out to be one of our most common problems: Bulging capacitors! Except these were high quality Korean Sanhwa ones. Once you get the cover off (just 12 screws, no plastic clips unlike Vestels!) you’ll see the PSU on the chassis. It uses a massive flyback transformer and opto-isolator for SMPS feedback, to power the backlights. If you thought flyback transformers died with CRT’s, you were wrong!

Samsung LA40R81BD-PSU-removal

Follow this procedure to remove the supply:

CAUTION: WAIT at least 30 minutes if the TV has been plugged in. If you are experienced in electronics you can discharge the main filter capacitor using a resistor, if not, leave it unplugged for a while before continuing, and have a brew, thinking about how you’ll proceed, and make notes. I find cuppa-plan time to be very productive, and it keeps me safe, even as a professional. There are high voltages present that can KILL!

1. Remove all connectors I’ve coloured GREEN. They have tabs on them which you must push as you pull the connector. DO NOT pull them out by the wires, you’ll rip the socket off the board and damage the socket pins, making this cheap repair much more expensive.

2. Unscrew and remove all screws I’ve coloured RED, and put them somewhere safe. Remove the board by lifting it by its EDGES, not by a transformer or capacitor, or any other component. Place the board on a suitable workspace, with plenty of room, and an antistatic mat. SMPS supplies contain surface mount components and controllers which are easily damaged. Simply walking on a carpet generates 70,000V which we can only feel as a slight shock as there’s hardly any amps, but that is more than enough to wipe semiconductors out!

Samsung LA40R81BD PSU capacitors

3. You’ll notice near CN801 there’s a bunch of capacitors, and some of them will likely be bulged, or have actually vented. If any vents have burst, you must clean the electrolyte off as soon as possible, as it’s corrosive to the board and other components. I generally replace all output caps if any have become damaged, as they will have been stressed. On my board there were 2ea 2200uf capacitors that were bulging. Remove the old capacitors ensuring you don’t overheat or damage the copper pads/tracks on the circuit board.

CAUTION: Take care to ensure you install the new capacitors correctly. They are polarity sensitive. The board and capacitors will be clearly marked which way they should be inserted. Shorted or polarity-reversed capaitors can EXPLODE and/or damage other parts of the circuit.

You should check and if necessary replace any adjacent capacitors that are rated 10V as these seem to be the ones more likely to fail. In my case I also replaced the 1000uF capacitor. Capacitors, contrary to misconception do not have to look visibly damaged to be faulty, they can be internally dried out.

NOTE: If replacing the capacitors does not resolve your symptoms you may need to replace the EEPROM chip on the main board as it can be corrupted or damaged by the power cycling. This will need to be done by a professional as the software contained in it can be TV specific.

 

Comments No Comments »

Some manufacturers have inexpensive flat rate service policies for power supplies. If you are not inclined or not interested in doing the diagnosis and repair yourself, it may be worthwhile to look into these. In some cases, £25 will get you a replacement supply regardless of original condition. However, this is probably the exception and replacements could run more than the total original cost of the equipment – especially as in the case of most TVs and many computer monitors, where the power supply is built onto the main circuit board.

Nothing really degrades in a switchmode power supply except possibly the electrolytic capacitors (unless a catastrophic failure resulted in a total meltdown) and these can usually be replaced for a total cost of a few pounds. Therefore, it usually makes sense to repair a faulty supply assuming it can be done reasonably quickly (depending on how much you value your time and the down time of the equipment) and, of course, assuming that the equipment it powers is worth the effort. Most replacement parts are readily available and kits containing common service components are also available for many popular power supplies (such as those found in some terminals, Macintosh and other Apple computers, various brands of video monitors, and some TVs and VCRs).

Where an exact replacement power supply is no longer available or excessively expensive, it may be possible to simply replace the guts if space allows and the mounting arrangement is compatible. For example, for an older full size PC tower, the original power supply may be in a non-standard box but the circuit board itself may use a standard hole configuration such that an inexpensive replacement may be installed in its place.

Alternatively, many surplus electronics distributors have a wide selection of power supplies of all shapes, sizes, output voltages, and current capacities. One of these may make a suitable replacement for your custom supply with a lot less hassle than attempting to repair your undocumented original. It will likely be much newer as well with no end-of-life issues like dried up electrolytic capacitors to worry about. Of course, you must know the voltage and current maximum current requirements of each of the outputs in order to make a selection.

For the specific case of SMPSs for standard computers (PC, Macs, workstations, servers), it often doesn’t make sense to spend much time or money on repair. The cost of replacement of power supplies for PCs in particular is so low, that just buying a new power supply may be the best course of action. Furthermore, the risk of a faulty repair causing expensive or fatal damage to the mainboard and peripherals including total loss of all data stored on disk, makes repair a risk unless thorough testing can be performed before installation. However, it won’t hurt to check for obvious problems like bad connections. Put the dead one aside and considering trying to repair it if there isn’t anything better to do. Realistically, this will be never. 🙂

Whatever your decision, if you’re not confident, always seek professional help!

Comments No Comments »

The following probably account for 95% or more of the common SMPS ailments:

Problem: Supply appears dead, fuse blown.

Cause: Shorted switchmode power transistor and other semiconductors, open fusable resistors, other bad parts. Note: actual cause of failure may be power surge/brownout/lightning strikes, random failure, or primary side electrolytic capacitor(s) with greatly reduced capacity or entirely open – test them before powering up the repaired unit.

Problem: Supply dead, fuse not blown.

Cause: Bad startup circuit (open startup resistors), open fusable resistors (due to shorted semiconductors), bad controller components.

Problem: One or more outputs out of tolerance or with excessive ripple at the line frequency (50/60 Hz) or twice the line frequency (100/120 Hz)

Cause: Dried up main filter capacitor(s) on rectified AC input.

Problem: One or more outputs out of tolerance or with excessive ripple at the switching frequency (10s of kHz typical)

Cause: Dried up or leaky filter capacitors on affected outputs.

Problem: Audible whine with low voltage on one or more outputs

Cause: Shorted semiconductors, faulty regulator circuitry resulting in overvoltage crowbar kicking in, faulty overvoltage sensing circuit or SCR, faulty controller.

Problem: Periodic power cycling, tweet-tweet, flub-flub, blinking power light.

Cause: shorted semiconductors, faulty over voltage or over current sensing components, bad controller. These symptoms are often very evident on many eMachines stock PSU’s, and other cheaply made supplies.

In all cases, bad solder connections are a possibility as well since there are usually large components in these supplies and soldering to their pins may not always be perfect. An excessive load can also result in most of these symptoms or may be the original cause of the failure. And don’t overlook the trivial: a line voltage select switch in the wrong position or between positions (possibly by accident when moving the supply, particularly with PCs), or damaged.

Note that a PSU’s maximum output load should not be held for normally more than 60 seconds, as at full load the circuitry will get hot, and can trip fuses. Excessive overloading will shorten the life of the supply.

Comments No Comments »