On Fri, 27 Sep 2019 06:55:28 -0700 (PDT), John-Del
wrote:

On Friday, September 27, 2019 at 8:44:05 AM UTC-4, legg wrote:
Somebody brought in an RCA RLDED5098-D-UHD widescreen TV the other
day.

Damned if I could find service info or any spares on offer.

Though it looked like a small post-regulator wasn't running, to
prevent operation, there was no way to find out if it was supposed to
be off during a protective fault, or what the replacement parts were,
from smd coding alone.

It looked like the only alternative to the owner, for repair, was a
box distributor in the sticks of the greater Toronto area. This would
be complicated by a apparent lack of a serial number on the thing.

This is the first time I've run into this situation. From a service
manual information, I can generally do something.

RL

You won't get any help from "RCA"..

Try disconnecting the TCON, the LED harness, and any remote and wifi boards and see if it responds differently.

Many of these use a particular start (ping-pong) sequence:

On plug in, the power supply standby supply turns on which feeds the standby voltage to the main board. The main board starts loading its software - this usually takes about 5 seconds on newer TVs. When the software is loaded, the main board then turns on the "run" supply of the power supply. The power supply turns on and feeds the run supply (typically about 12V) back to the main. The main detects the run voltage, shuts off the power supply and goes into standby to wait for a remote signal or the power contact to be pushed.

So if you see the "ps-on" line high and no 12V (just the standby 3.3V or 5V), the power supply is the problem, not the main. If the power supply ps-on line never goes high at any point, the main is bad or being locked up by a peripheral board.

It seems that there are two versions of that model; one has a separate power supply and main, and the other has them on one board. So if yours is a really low end toilet, the power supply is combined with the main board, so identifying control lines is a real problem. If you have an all in one it's best to troll the internet and look for a donor.

This had one main board with power and secondary-side signal
processing/communications/audio etc, with a smaller board interfacing
the led strings.

The PFC section (400v boost) was enabled, with four secondary-side
supplies ID'd as running (12v, 5v, 3v3, and 1v2).

UD3, marked GH6K, seemed to be the non-functioning postregulator - I
assume it would supply something between 5v and 1v2, considering the
lack of designer discipline in the selection of chipsb but couldn't be
sure it wasn't disabled internally.

As I've said, there's no sign of parts on offer for this thing, or a
manual.

RL

After a long power withdrawal, the unit could be convinced to give a
single lamp burst after start-up, but this was not evident in response
to soft on/off cycles. I assume that's latching protection of some
sort.