Sharp Image SI-134 / Kortec KTM-F33 Vertical Collapse

[Esom]

Working on a Sharp Image SI-134/ KTM-F33 Chassis with vertical collapse and am at my limit.

Repair log thus far:
Following the schematic, in order from the point that sync enters -
R524 1 ohm in circuit (10k ohm out of circuit which is what it's listed as, seems ok. No idea what it should read, as I don't have a working chassis to compare)
IC501 no shorts, all pins have continuity to the next pad they lead to
C523 This one stumps me. I have a BSIDE ESR02 ESR meter I use to test caps. This cap comes up as two diodes with a reading of "uF=2.37v 2.09v." Ordered replacement to see what happens
C524 Same thing, except the reading is "uF=3.55v 4.48v." No idea what is going on here. Ordered replacement


Outside of that section I found some other components not reading correctly and replaced them:
R903 reading 1k ohm in and out of circuit, should be 15. Replaced
Filter cap read low, replaced
C205 read low, replaced
IC201 It was recommended this be replaced in a troubleshooting guide I found about this model. This was replaced by someone before me, and they lifted SO MANY PADS. I replaced the IC with a new one and was able to get continuity to each pin. Pin 11 seems to lead nowhere, though, so can't test that one.

So, while waiting on shipping to replace the two other two caps (C523, C524), I put the chassis back in to see if any of my fixes made a difference. Well, it did, sort of. I still have vertical collapse, but it's slightly higher and now there is a faint but noticeably new high pitched squeal. The squeal changes pitch when the screen pot is adjusted on the flyback, and is present when the pot is turned all the way down.

So... given all that: what's next? I'm at the limit of my knowledge and abilities for now. Hope this makes sense to someone else.

 

[trashedcabs]

Obvious dumb first question: you mentioned a few caps, but have you changed all the electrolytics? At this point I don't even look at a chassis until I do at least the power supply and the problem circuit.

Yes IC201 is the vertical IC and would be the first suspect as they commonly blow. Do you have supply voltage at pin 14? The datasheet says max of 35vdc input, not sure what that power supply rail is supposed to be for that chassis but that would be something to check.

 

[Esom]

Not all of them, but I have ordered them now that the initial troubleshooting has been fruitless.

I haven't checked anything while it was running. I'm not sure how to safely do that.

 

[Esom]

Well, now they're all replaced.

Side adventure: I was following a troubleshooting guide for a similar Sharp image chassis where R903 was listed as 15ohm 2w. I accidentally replaced my R903 with that, instead of the 1.5ohm 2w it called for. Realized my mistake after turning it on and finding it still didn't work. But when I turned it on again with the correct resistor, R901 decided to get red hot and start smoking. Well,I removed R901 now, but dont have the correct 3.9 it calls for... so im waiting on that now. No lucky breaks for me, I guess.

 

[Esom]

Still working on this chassis. Further diagnostics reveal: pin 7 from the flyback leads to R901, and is reading 3.7v. Supposed to be 12v. Still burning up. Pin 8 I think is the input side, reads 120v. But I think B+ voltage is supposed to be 133v. Every component in the area seems to read within tolerance, in and out of circuit.

Thoughts?

 

[Esom]

Do you have supply voltage at pin 14? The datasheet says max of 35vdc input, not sure what that power supply rail is supposed to be for that chassis but that would be something to check.

Someone told me the procedure to isolate pin 14 and check its voltage. 29v, which I think is correct for the 33" model. With L602 removed and J8 disconnected, I was able to do a light bulb test. R901 didn't burst in to flames while the light bulb was on, as it does when the connections are present. Any thoughts?

 

[trashedcabs]

Yeah 12v is generated from a secondary winding on the flyback, and by disconnecting the HOT (and therefore the flyback) from B+ it's no longer being generated and therefore won't burn up R901. To me it sounds like something on the 12v line got shorted causing it to burn up that resistor like a fuse. You could try lifting R901 to isolate the circuit and checking for a dead short between the 12v circuit and ground.

I'm assuming this is the schematic you're using? http://slot-tech.com/amusements/monitor_schematic_diagrams/sharp_image_si134.jpg

 

[Esom]

Yeah, that's what I'm going by, except I later found a list of parts changes from the base model to the 33" model. Lo and behold, R903 was intentionally changed and all of this may have been avoidable.

Anyway, so hunt for a ground short? Starting from R901 and going away from the flyback, or the other direction through the flyback?

I did check every cap's positive leg on the board, thinking I may have bridged something. C903 wasn't shorted, but the positive leg to ground was 0.5ohm. Not sure what the expected value is.