Going through my repair books, I came across an interesting repair that I did in June 2011. It is now time to “retell.it“. I had purchased a Samsung HT-TZ315 Home Theatre System a few years earlier – it was a good price, and performed well, however I noted at the time that the display was a little dim. I didn’t think much of it at the time since everything was working fine. Fast forward to June 2011 – one day, we went to use the home theatre and the blue power light came on, but nothing from the display. The display just would not light up – but we managed to use the remote control shortcut buttons to prove that the home theatre system was indeed working – just nothing being displayed.
Ok, most faults like this are usually power supply related – I thought. The display panel uses a VFD, a Vacuum Fluorescent Display, so most likely the power to the VFD on the front panel must not be working. I removed the home theatre from the tv cabinet, disconnecting all five speaker cables, plus the optical and hdmi cable etc. After opening the box, I looked at the power supply and fortunately, the voltages are screen printed on the pcb – fantastic.
I used my multimeter to check the voltages on each pin concentrating on the vfd power – and all pins had voltage on them. Although the vfd pins were not quite +4.3V and -4.3V but were +3.2V and -30.1V – I thought that it might be fine.
I also checked the ESR of the capacitors and mostly were ok, with a few that I consider a little borderline. I proceeded to obtain replacements for those that I did not have in stock, then replaced them. After doing this, I retest the home theatre and still no display – sound was working, volume was working, just no display. From a search on the internet, it seems that problems like this require replacing the front panel pcb – AH41-01100A. From what I could find, the only replacements at a reasonable price was from the UK at around 40 pounds, ouch – not cheap when converted to AUD.
Ok, why not check the front panel pcb, maybe there is a broken track somewhere – a long shot because a broken track usually means something burning out, which is usually accompanied by other problems that would be visible. Anyway, removing the front panel pcb was eventually accomplished – need to remove a few other cables in order to get it out.
Tracing the power connections around, I eventually got to the VFD and interestingly, could not measure the VFD voltage – so there was a broken track or a dry joint. I used a 10x magnifier and went through all of the pins attached to the power connector and where they go to, no dry joints seen. Turning it over, I could trace connectivity from the power sockets to all of the pins – what gives. After scratching my head a while, and turning the board over and over, I eventually noticed something that was slightly unusual. Two pins on the VFD were a little bent – ok, most pins should be straight since these are inserted in the factory during manufacture, but two pins, which coincidentally were connecting to one of the power circuits – the -VFD pin. When I put the multimeter probe on the pin, of course I push down, and it makes contact.
What seems to have happened is that because the pins were slightly bent when the VFD was inserted during manufacture, over time, together with transport, the solder pads on these two pins had lifted from the circuit board. Looking from the bottom, all the joints look fine, but using a 10x magnifier, I could see a gap between the soldered pin and the circuit board. I took a photo of it to show that I was not dreaming.
Here is a magnified image, you can see the right two pins have a slight dark area under the solder, this is the gap – compare with the other pins on the left where the solder pad merges well with the board. Resoldering the two pins, and reconnecting the front panel pcb – and voila! I get a display and had saved ordering a replacement board from the UK. It is also brighter than it had been originally. The lifting solder pad had produced a dry joint that was high in resistance, but enough so that it would pass inspection since it was lit, but eventually the resistance increased due to the pad continuing to lift, until it went open circuit.
Now, three and a half years later, this same home theatre system is still working well. You may also think, how could this happen. These home theatre system can get reasonably hot when driving all the speakers. This heating and cooling cycling will cause metals and plastics to expand and contract at different rates – because the pins were bent, it acts like a little spring that pushes and pulls each time the system in on and off, eventually the solder pad lifted enough that no power was going to the VFD. The forces involved are very small, but over a period of time, like 2-3 years, it all adds up. In addition, with a sub-woofer in close proximity, this may also cause the display to vibrate, and this would just make it worse.