Maybe OT: Sony TV field colapse

[Grumps]

Sony KVX2142, ae-1c chassis. Picture is severely squashed vertically, with bright band in the centre. Preliminary investigation suggests that the HT is ok (I get a picture), and the horizontal drive is also ok. The 27V for the vertical deflection circuitry sits as 12V; there must be some path from the

12V to the 27V. According to the circuits, the FBT generates the 27V supply via a resistor (0R47), diode, and a elec cap. There would appear to be no volts either end of the res. This would seem to indicate the FBT is kapput, but only the secondary that is used for the 27V bit. Is this possible/likely. Before I splash out £50+ for a replacement, I'm wondering if I could make a separate 27V supply. Comments? Ta.

[Derek ^]

Bad soldered joint on the PCB where the LOPT is mounted ? (not uncommon)

Don't even think about it. ;-)

Try sci.electronics.repair (although it is a US group) you have a set of good observations and measurements, there are people on there who have access to manufacturers CD's of standard faults.

DG

[meow2222]

Easily. Back whenever I sometimes replaced sections with custom lashups. The most memorable was the replacement of a whole Sony smpsu with a lightbulb. Light had to be external cos of the heat it gave off... :) Cant remember how I did the tube heater supply, separate tf probably.

If I were you though I'd check R on the loptf and see where the winding goes and whether its R is ok. if it is then either

- you can fix the rect/filter

- or excessive load is pulling the rail down, in which case a separate psu wont solve it

Sometimes protection circuitry will object, but it sounds like none is acting in your case.

BTW loptfs can be repaired, so if its o/c you may well be able to add an external winding onto the core. Forget spending 50 notes, no need. ISTR finding in the region of 2v/turn on sets, so its not difficult, just make sure a new wind doesnt block loptf cooling, and keep away from the eht - pretty obvious stuff.

The 2v figure is rms, not sure what Vpeak would be as its a peaky non-sine waveform, but over 3v a turn.

The 12v line may supply the 27v via a diode so it scans some during startup, to avoid phosphor damage. Or the loptf might produce 13v thats piggybacked onto the 12 line. PSUs are very inventive in tvs.

Its hard to know what you know and dont, so I may have bored you, but that should cover everything. Except the usual warning that tvs and microwaves have the most ways to die of any household appliance, so should not be mucked with unless competent.

NT

[Grumps]

Thanks. I'll have a look under the LOPT tomorrow.

[Grumps]

I was going to check the R on the LOPT, but an enticing cool beer persuaded me to do it tomorrow. Thanks.

[Grumps]

My mistake. I had not looked under the circuit board, so just assumed which was the FBT feed to the resistor. I got the wrong end! The 0R47 is open. Q is, why did it blow?!

[Mike Tomlinson]

In article , Grumps writes

It acts as a fuse for that circuit. The vertical IC has probably blown: check the diode in the 27v supply very carefully, replace if in any doubt. Also check _all_ electrolytic caps around the vertical circuit and replace if in doubt.

Vertical IC failures in TVs and monitors are very common, probably due to inadequate/borderline heatsinking by manufacturers trying to save a few pennies.

[Grumps]

Diode checked.

The only elec cap seems fine. I will replace, and the above diode.

Indeed. I've replaced one before, in an old b&w set.

I've replaced the 1W 0R47, and it seems to work ok. The 27V is back to 27. Still, if the vert def IC is not blown, then perhaps the res just had an off-day. Or maybe it's linked to our new policy of switching off at the mains and not leaving it in standby. It may also be time to start collecting replacement parts before they go obsolete. I'm determined not to buy a new telly for as long as possible.

Thanks for the reply btw.