TideLog Posts Tagged “backlight flickers and goes out”

Some Vestel TV’s have combined PSU and inverter in the same board, as did a Polaroid branded ProView chassis I did a year ago. Common faults include:

  • Red tinge on screen, which gradually goes to normal white, but then the backlight(s) go out. Known-good backlights connected up work fine
  • Backlight(s) flickers briefly and goes out again, but TV still operates with dim picture under bright light, known good backlights do the same
  • Backlight(s) shows no life at all, neither does a known good one connected up, but TV stays running

The red tinge issue is mostly a failing backlight(s), because the anodes of the lamp are wearing out it doesn’t warm up fast enough, causing the inverter protection circuit to shut it off. The other two are faults with the inverter section of the supply itself. On a combined PSU & inverter unit the backlight supply is fed with 24v directly from the secondary side of the SMPS circuit, where it is stepped up to 1000v lamp start current by the inverter transformer. Once the lamp has lit (usually within a few milliseconds for a good lamp) the voltage drops to between 300 and 800v depending on the brightness. One such PSU I recently reconditioned was a Vestel 17IPS01, we often replace faulty supplies with new ones, then recondition the old ones for re-use.

The backlight flickered momentarily and went out. We connected two known-working laptop screen backlights to it, and the same occurred. Note that on a two lamp inverter two lamps MUST be connected, if only one is used this isn’t enough of a load and the protection circuit will trigger. On this supply it turned out to be the output capacitors directly before the backlight output sockets. These act as soft-start filters to prevent damage to the backlights from sudden current inrush as they light, if the 1000v ignition current was suddenly applied without a few milliseconds delay the anodes would be damaged:

Vestel-17IPS01-2-high-voltage-capacitors-faulty

To fix the issue, replace the three caps C355, C356 (12pf 3kv) and C354 (4.7pf 3kv) that I’ve labelled above. The capacitors will check out as normal with a multimeter when power is off, however they break down when put under load, hence the reason why they come on then go off again as the inverter protection circuit shuts the circuit down. Faulty backlight lamps can also stress them out causing them to fail. This repair is good for most Vestel and other make combined PSU/Inverter boards.

Unlike TV’s with separate inverter boards, a TV with combined PSU/Inverter supply will always boot up and work even if the inverter section shuts down, there is no fault feedback to the main TV processor in these variants. This makes diagnosing faults with them easier, whereas a TV with independent inverter will often cause the TV to not start correctly in the event of a fault, sometimes causing confusion with inexperienced people.

 

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