Sony KV-27S42 White Balance Failure

[MarkFromCA]

Hi, First time poster here.

I have a Sony KV-27S42 with a picture issue. The image shows up blue with retrace lines (see picture). It's been an intermittent issue since I've first had it and from what I can see in the service manual it might be a white balance issue.

There are 5 fast blinks on the status light followed by 1 slow blink. According to the manual either G2 needs adjusting, there's a problem with Q306-308 transistors (I'm guessing these are rgb input amplifiers?) or IC301 (jungle chip).

service manual : https://ia601906.us.archive.org/28/i...670/105670.pdf

I tried to rule out physical issues before I concluded that it was any of the solid state components. Not that it's impossible but I figured age/wear and tear of me moving boards about might have stressed something.
I can't find any issues of cracked vias or traces or otherwise anything that would concern me on the chassis (A) board. (see the photo q306-308_and_cn301)

So I looked at the C-board (neck board). I tested the rgb input from the a-board to the c-board to see if there was any issue with a faulty connector. The output from the rgb transistors matched the input voltage on the c-board.

When I checked the connector r,g,b input on the C-Board I noticed that the errored picture would blink back to "video 1" - a proper output screen. So I thought, ok, maybe i have a short somewhere.

Sure enough there were two points where the socket is soldered to the c-board that had cracked joints. However, I realized that it wasn't so much I was making an electrical connection with those points than it was a gentle board flex. I could nudge it with a non-conductive item and achieve the same effect.

It seems to only happen on the side of the board with the CN1752 connection going to the CN301 connector on the chassis board.

So it "seems" like some sort of mechanical failure on the C-Board. However this isn't exactly what the 5-blink error lead me to believe (its jungle chip or the rgb transistors). So I'm thinking it must all be related since the AKB (or IK in sony's case) is relaying some sort of failure from the c-board back to the jungle chip's IK in. I guess.

I'm thinking my next best step is to remove the neck board fully and rework all the joints. At some point along the way those points may have cracked under stress or age.

I'm very much a novice when it comes to tv repair so please pardon my lack of knowledgeable communication! I provided some photos which I hope might give some insight into the areas that I'm troubleshooting.

Thanks!

[vol.2]

The solder joints do not have to look cracked to be cracked.

Most cold solder joints are invisible fractures that develop a layer of corrosion inside of the cracks and make intermittent connection until they heat up.

Step one is to go over all of the PCBs and reflow the solder on anything heavy, anything hot and anything that undergoes physical stress. That means big items like transformers and coils, hot stuff like power resistors and big transistors, every connector in the set, all the pots, switches etc.

See what you have after you do that.

[Username1]

.

Hi - Well it looks like a shorted cathode to G1 inside the picture tube, or shorted driver
transistor for the blue gun. If the transistor & other drive voltages look ok, and you
have re-flowed solder on any connections that look like they need it, the next step
is to get a nice wooden dowel and start hitting the neck of the picture tube and
see if it's sensitive to being whacked. Next is to do the same with the neck
board, and then on the main board. On the main board you can also push
on different points on the board, "point pressure" to see if that's a point
where you need to inspect solder joints.

Now when you hit the picture tube, I mean use the point, and hit the
tube with the point directly imparting the force at a specific point on
the neck. Impact it with light force, then more, don't hit it side-stick.
Tapping = hitting

To find intermittent connections, hitting stuff and finding the smallest
sensitive area is a good way to localize it. Good Luck.

PS Also you said it was going on for as long as you had it. Is it a time
sensitive thing, "been on for 30 minutes then it starts", etc., or is
it sensitive to being moved? that's a bad connection.

.

[MarkFromCA]

Hi, Thanks for the reply.

The "going for as long as I've had it" meant that it's been a persistent issue since I found the set.
I've done a semi-thorough pressure test on the chassis board but I haven't ruled out issues there.
The neckboard really does seem like the culprit. I already found cracked joints on a couple ground pins on the socket which I reflowed. I really should reflow off the tube just to make sure I did a proper job. A pressure test near the socket doesn't seem to cause a change too much. It's only around the areas of the connector to the chassis board and the crt drive IC (IC1751) where a pressure test seems to have an effect.
I'll try tapping the tube as well. I did a little exploratory tapping there and I didn't notice much.

[MarkFromCA]

After reflow of the neck board everything started up just fine. It was very promising.
To confirm I did a little bit of gentle non-conductive tapping on the C-Board. No flicker.
After a minute or two of letting things burn in I started to notice "video 1" drifting upward toward the top of the screen, then, Black screen.

No picture, no nothing except for the status light: four quick flashes and then one long one. At least it wasn't 5 flashes anymore!

According to the service manual this could mean: +13V is not supplied. (A board) or IC 541 is faulty (A board)

Effectively I think I'm in a permanent standby state.

for the remaining post I'm referencing this schematic - https://imgur.com/q0Poaqp

I want to point out exactly what happens now. The tv effectively never shuts "off". I power down and the relays click like it power cycles but it never goes off. As long as its plugged in the 4-blink state persists.
If I unplug/plug back in there's a relay click and then 5-7 seconds later a second relay click.

All further voltage/continuity tests are relative to chassis ground (tuner-can).
The first thing I did was test TP 502. This is the +13v test point going to IC541 pin 2. Sure enough I'm getting basically 0v.
I continued with other test points. TP 504 (180V). At first relay click it goes to about 8v. At second relay click it goes down to 0v.
+9v (TP601) - ~ .7v
+5v (TP602) - ~.2v
+B (TP600) - should be 135v. I get ~3v peak.

Most of these show a rise at first relay click and a drop at second relay click.
It may rise to expected voltage but my meter isn't fast enough to capture that.

The only thing that is consistently powered is standby +5v which stays at a steady 4.92v.

At this point I'm unsure where to proceed. I'm definitely not getting power but I can't figure out if its a faulty part or somethings tripping the protection circuit to bring voltages low.

This is literally the first time I've seen this happen. It seems awful coincidental that it happened after the neck board reflow. I can't fully rule it out but I'm at a loss as to how it would cause such a catastrophic power failure.

I don't want to make assumptions here. (its the flyback, its the power supply board, etc). I've checked and re-checked for cracked joints around the a-board power areas. I've rechecked my solder job on the c-board. I'm honestly not sure where to go from here.

[vol.2]

Originally Posted by MarkFromCA

I've checked and re-checked for cracked joints around the a-board power areas. I've rechecked my solder job on the c-board. I'm honestly not sure where to go from here.

If resoldering the joints on the neck board led to the issue then one of two things is true. Either you dropped a solder blob somewhere when you were doing the reflow, or there was a problem that was always there and it only showed up after you did the reflow. If it's a solder blob, it could be anywhere below the area you were working on. If it's a preexisting condition, then it could be either a short or a wrong/missing voltage.

Since your B+ is missing, that's good information. That's something you can trace out.

first thing to do though is to check for shorts across the rails. There shouldn't be any really low resistances between ground and any of the power source points. If you see anything that's like 10 ohms or less, be very suspicious. Really anything lower than hundreds of ohms is a little suspicious on any of the rails, and you would want to verify that on the schematics.

If you find a short, figure out what's shorted.

You can also trace power from the plug all the way down the line and see where is disappears. That's important to do, but it's likely that there's something amiss somewhere further down the circuit as well if one of the main power rectification parts is blown. For example, if the main power transistor is blown, that was probably a result of something else.

[MarkFromCA]

Solder blobs on the chassis board seem unlikely. I had the neck board off the tube while I was working a fair distance from the chassis board.
Not impossible but unlikely. My gut feeling is that the gentle prod on the neck board was coincidental to the failing vertical deflection. At least that's been my response in terms of troubleshooting.

As for low resistance, I've been systematically going through and looking for low resistance/continuity across many points.
This is what I've found so far:
IC541 (V-out)
pin 2 vcc+ -- 1kohm
pin 4 vcc- -- ~200kohm
pin 5 out -- 10ohm
TP600 (+B) -- 3ohm
standby +5v -- 14kohm


Test point 600 stands out at 3ohm. However, looking at the schematic it may simply be its easiest path to ground. Not sure. Seems a bit unlikely?

Flyback transformer. I'm not sure how these readings should be interpreted. This may be completely normal but almost all pins show continuity to ground and have very low resistance. Here's the board layout - https://imgur.com/JjXXuVc

pin 11 (ABL) - non-continuous/infinite resistance
*The remaining pins tested rise to about 100ohm and settle to about 1ohm
pin 9 (+13v)
pin 7 (-13v)
pin 6 (heater)
pin 4 (200v)
the remaining pins are either NC or not even there (like pin 3) but they all are continuous ie under 100ohm.

If this isn't usually the case for flybacks then maybe that's the issue. What would cause a flyback to fail? Would all test points show such low resistance to ground?