Replaced CM8833-II monitor flyback - swap the HOT too?

[philpem]

I've just swapped a dead flyback on a Philips CM8833-II variant (the Acorn AKF17).

In general terms, is it recommended to swap the BU2508AF horizontal output transistor (HOT) at the same time as the flyback/line output transformer (LOPT)?

The monitor works fine -- I have two HOTs and two vertical drive ICs on hand (bought to meet the minimum order) so it's a no-cost option for me if there's value to doing it.

I've also got a spare TDA3653B vertical drive, but my thinking is that that's far enough away from the LOPT's blast radius that it won't have seen any damage.

The failure mode was the monitor failing to start up -- power LED stuck off, no HT, and a loud whining sound which changed in pitch when the power was turned off.

Cheers
Phil.

[Ian.M]

Sounds like the bad transformer had developed a shorted turn.  There's no benefit in replacing the horizontal output transistor if its working properly.  However you should check every capacitor associated with the horizontal output section, from the drive chip all the way to the scan coils, especially if there's any linearity distortion or other geometry  defect associated with the horizontal scan.   Considering the age of the beast, any electrolytics associated with either deflection circuit or the PSU (apart from the main primary side reservoir cap that's probably still fine) should probably also be replaced.

[CaptDon]

NAP North American Philips made some real nice color
t.v.'s back in the day of the 'L' chassis. K, L and M were
very similar. They had a nice picture but all suffered from
flyback transformer failures as well as two resistors that
changed value in other non-horizontal related circuits.
Sometimes the H-Output transistor would fail, sometimes
not. Once the flyback was replaced and the two resistors
replaced the sets would play without any additional failures
until the CRT finally faded away and that took about 15 years.
They were one of my favorite t.v.'s to watch and simplicity
of repair.

[philpem]

Thanks for the help, folks.

I've left the original HOT in place -- surprisingly the vertical chip (another common failure on these, after the FBT/LOPT) is fine too.

I recapped it last night and it's absolutely spot-on this afternoon. A little bit of focus/screen size shift as the brightness is adjusted (or when the green-screen switch is turned on), but nothing excessive.

There's a service manual error on these: capacitors 2560 and 2561 were called out as 22uF 35V -- the correct parts are 220uF 35V.

I also found 2268 was a 2u2 63V in my monitor (Acorn AKF17) vs the 1uF the service manual claimed it to be. I've fitted a 2u2.