pentranic/makvision ch288 - 2425 chassis mod

[lilshawn]

have an issue with this Ch288 chassis out of a former pentanic and a makvision 2425. i'll explain....

the original 2425 chassis vert folded over so i dragged it back to the shop and fired it up... the vertical deflection had totally collapsed. i swapped out some of the caps around the IC3 vert deflection chip and tried to power it up. burned up IC3.

i had an unknown ch-288 chassis from an old pentranic sitting around. I don't recall what the issue was with it, but all the caps have been replaced as well as some of the resistors and the vertical control IC's (IC3/IC2). There was a note sheet tucked into the chassis with the list of components to change it from the 25" tube to a 27" tube. it doesn't appear these changes have been made.

powered up the chassis and it's not bad, but it's folded over... top 1/4 of screen is folded over...screen doesn't extend to that top portion...looks like the scan starts about halfway up the screen, get's to the 3/4 mark and scans down normally. R399 is 386 ohms (390 ohms spec'd) so that's good.

now, to the issue at hand,

these chassis have a mod regarding R318. the resistor only has one leg in the board, the other is tied with a wire to D506.

Iv'e found the working chassis has a 1/4 watt resistor that reads 0.81k ohm but as far as i can tell the color code says it should be 820 ohms. it looks fine. not burned or dark at all.

The non working chassis has a blue 1/2 watt resistor that reads 0.68k and it's color code says it should be 680 ohms, it's been visibly hot and it's blue color is on the brown side. I would certainly suspect this resistor.

what is the supposed value of this resistor?? the 2 chassis I have, appear to have different resistors between them. but i can't tell if maybe the heat has caused the colors to skew on them so i can't be 100% on the value. not sure if this is my issue, but it certainly can't be helping as it (seems to be) feeding power to IC2.

[Rickn]

That R318 mod was made to change the source for VCC to the vertical circuit. Prior to that the supply came from the flyback, if the monitor was used with a gun game, each time the gun was shot the high white flash would cause the highvoltage to jump. This in turn would cause the vertical VCC to jump and result in a jumpy picture vertically. The resistor from D306 to the empty hole ar R318 should be 680 ohms. I developed the mod and have a rough sketch if you need it e-mail me, I will scan it and e-mail it back to you.
Rick@niemandisplays.com
As far as the vertical foldover goes, change C318, it causes weird stuff when it is off value, everything from foldover, jitter and collapse.

Hope this helps.
Rick

[lilshawn]

Thanks Rick, I thought you might have something to do with this. Great explanation as to why too. Makes sense, getting voltage straight from the SMPS where the power is a little more stable.

I'll pick up a replacement resistor for the modded R318 just to be extra sure.

C318 is a 2.2uf - 16 volt Tantalum Capacitor. This cap is tied to the vertical oscillator of the HA11235 IC and controls the vertical oscillation frequency by the discharge time of the capacitor.

I don't have any tantalum caps in stock anywhere around here so i'll have to order it. I'll update the thread once I get back to this monitor.