[Having mentioned this TV repair on my Status page and getting supportive comments from dixonselvan and genebren, I felt perhaps a blog post would help]

 

Background

I was given a nice 22" TV a few years ago as a present. About a year later it stopped working - not even the power light came on. I checked the fuse that was found to be OK. Because it was a gift I didn't want to seem ungrateful by notifying the giver that it had failed so I rang the manufacturer/retailer who's information was on the back hoping that from the serial number and model they would realise that it wasn't too old and would kindly repair it for free (in the UK we have quite good laws relating to product guarantees although it is somewhat a grey area at times). I had a constructive conversation and they were really helpful, likely realising I was another customer with fault number X, but insisted they needed proof of purchase....apparently even the credit card statement would be sufficient. However there was no way I was going to ask the giver of this gift for that. The TV sat on the side for some more months as I hate parting with anything that could be repaired. The label on the back stated 'No User Serviceable Parts Inside."....we'll see about that!

 

First Investigation

I popped the back off and took a visual inspection. There are two boards side by side; the left is the PSU and the right all the TV signal generation.

My thinking was that the fault was PSU related as not even the red LED illuminated. Looking around where the mains voltage came into the PCB I could see a component at the bottom next to the doughnut transformer - it was marked TH1 and had a 'crack' in it. This was a negative temperature resistor/thermistor that is used to limit the initial inrush current to the Switched Mode PSU. If that had gone open circuit then it would be acting like a blown fuse. I decided I would need to find a replacement......but I didn't actually use a multimeter, instead I put the TV back on a shelf and forgot about it.

 

Roll On Another 2 Months

My PC monitor started to have issues (maybe I'll cover this in another post) and being very miserly with buying new things I didn't want to pay for a new one. It was time to look at this TV/monitor again. I went straight to the TH1 component with a multimeter....and found it wasn't open circuit at all. I looked closely at the 'crack' and ran my fingernail over it only for it to come off ! The crack was actually a streak of black paint and you can see the stain left by it below.

In that case it was now 'game on'. My investigation went along the lines of:

 

(1)     Anything burnt, tracks heated or conformal coating looked like it had melted ? Couldn't see anything.

 

(2)     I looked around with an eyeglass - there was plenty of microscopic solder balls which indicates a poorly finished board but probably didn't lead to the failure.

 

(3)     I checked out a couple of the higher power resistors with the DVM - values seemed about correct and they too looked OK.

 

(4)     I visually inspected the capacitors - checking the end cap wasn't bulging or anything had leaked out.

 

(5)     I used the DVM on capacitance setting to check the large electrolytic capacitors. Some were at the very extremes of value to say the least but still had some capacitance (see A Question on Aging Effects on Electrolytic Capacitors by jw0752 ) . They all checked out OK. If your DVM is capable of measuring capacitance, just make sure the lead polarity is correct when testing electrolytics. Also if the TV had been powered up recently then you would need to discharge them first.

 

(6)     On to the the semiconductors. Using my DVM on the diode setting I tested each of the diodes out (both ways) and there were three of the larger ones that showed a short both directions ! Looking at the track side of the PCB I could see they were all in parallel so any one of them could be faulty.

 

New Diode Selection

The fault diode(s) were marked as SB5100. As I only wanted a small order I decided to look at CPC Farnell which is a subsidiary company of Farnell (part of Premier Farnell) but they do free delivery on orders over £5 - so ideal for little bits like this.

 

CharacteristicSB5100MUR420MUR420
Peak Reverse Voltage (V)100200
Average Rectified Current (A)54
Peak Rectified Current (A)150110
Forward Voltage (V)0.80.89
TypeSchottky Barrier - e.g. fast switching

 

As you can see the  MUR420MUR420 is quite close but cannot handle the same peak current however I suspect that the cheap non-branded components fitted to the PSU couldn't manage their rated 5A either plus these MUR420 are quality branded  by OnSemi (Product LinkProduct Link) . I also had another plan. When I looked at the PCB I could see there was unused pads for another parallel diode. The designer must have paralleled up 3-4 diodes to increase the current handling. Then the manufacturer that incorporated the board wanted to save £0.05 per board and asked them to reduce the component count. It was probably that poor decision that lead to the premature failure of my TV (and lost of trust by consumers in purchasing that brand in the future).

 

Swapping The Diodes

My soldering iron is a fixed Magnastat that only gets to the temperature for leaded solder. Therefore I initially heated each diode joint and added a small amount of leaded solder to bring the melting point down before heating them further and pulling the diodes out with needlenose pliers. Don't hold the lead if doing this as you will likely draw all the heat out before the solder melts. Also this works well as the PCB is only single sided. Using this heavy approach on double sided boards will often result in physically pulling the pads off the other side (use desolder braid or cut the component leads would be better).

 

I cleaned the PCB with some flux and solder wick. I bent up the new diodes, with part number facing out, and soldered each back in. Leading plenty of air around each as the initial ones also had. The new diodes are the three on the right-hand side and the diagonal one. The middle one out of the three is an extra one I added. The diode on the far left was OK when tested, but I changed that too for good measure.

{gallery} Swapping Out Diodes

 

 

Reassembly and Testing

I reconnected the internal connectors, clipped the back case on and screwed it together. Plugged in to the mains and the red LED illuminated. I added my HDMI from my computer and "hey presto" it works again

 

Diode Voltages (After Desoldering)

The three suspect diodes measured in forward bias as 0.286v, 0.266v and 0.002v (with my DVM being 2.45v ).

 

Safety

For anyone doing something similar there are a few important points to ensure safety.

 

  • Make sure the unit is unplugged before removing the back.
  • Discharge any capacitors with a small value resistor if the unit has recently been on.
  • Ensure replacement components are equal or better in specification (OK - mine were 4A not 5A )
  • Ensure parts are inserted correctly.
  • Do a soak test while you are nearby - rather than plug in, leave on and go out shopping.

 

 

regards to all and happy mending,

 

Rod