KT-2914DF bad width

[MonMotha]

So, my friend's KT-2914DF (Betson multisync, just not from Betson) just keeps getting worse.  Width at maximum adjustment (in service mode) is about half the tube, but the control does function i.e. it does alter the picture width.  It's not really usable now, and I'm going to assume that eventually something will actually break.  Of course, then maybe I'll know what the problem is.

Horizontal timing caps (C702, C705, C709, C710) all test good.  It does this in all modes, 15k-38k, so I don't really think it's any of the FETs that commonly fail.

Ideas?

[grantspain]

if enter factory settings are you able to adjust the h size at all?

[MonMotha]

Yes, but that's as big as it gets.  You can make it very narrow, if you want, though.

It has been continually getting worse over a period of years.  The settings do stay.  There does not appear to be an EEPROM related issue. It's just that a given width setting results in a narrower and narrower picture over time.

[grantspain]

i have seen the flybacks cause the opposite on this chassis whereby the picture goes mental large but not this issue,i imagine you have checked through the fault flowchart in the manual regarding horizontal size issues.On other chassis this would point towards either east/west correction,width control transistor,hv caps in horizontal stage or even flyback

[MonMotha]

I didn't see anything in the flowchart that seemed to pertain to this directly.  The flowchart definitely had an entry for the H. size controls being ineffective, but they seem to work just fine aside from the slow collapse.

As I recall, it used to exhibit a pincushion issue as the width was dropping, but that seems to have gone away.  It's probably the next best thing to check.

HV caps test good, per my original comment, though that doesn't entirely rule them out.  They are a common failure item so one of the first things I checked.

The flybacks on these are indeed notoriously unreliable.  They're also hard to get.  These do have HV generation that's separate from horizontal deflection, though the two are still coupled in some ways.

I at least have a donor parts board if I need it.

[MonMotha]

Anyone happen to know if there are any chassis differences between the F and DF models?  I happen to have a couple known-working F variants (mostly flat tube).  I'd like to swap boardsets between one of them and this DF (totally flat tube) just to make sure the problem is not with the yoke.

I know it's the same board, but I'm not 100% sure that there are no component substitutions or firmware differences.

[grantspain]

i expect df denotes dual focus
i doubt there are any major differences in the chassis except the crt socket,possibly heater resistor

[MonMotha]

Amusingly, it does not.  It means "DynaFlat" aka "actually, honest to truth, flat tube".

They often do have a dual focus flyback on them, but so do the F versions.  In all cases, the second focus line is cut and capped off right at the flyback, it seems.

[MonMotha]

Checked last night: the DF is only single focus.  Like many (maybe all) of my F boards, it has a dual focus flyback, but one of them is capped off right at the top of the flyback, and there's only one focus running to the neckboard.