KX-M270 Sony Trinitron Color Monitor (Repair Complete)

[Amra]

I have begun to delve into a repair of an old CRT Monitor, which I was/am using for my arcade cabinet.   I figured this would be a good place to keep track of my progress, as well as get help from all the great minds on this forum.  Hopefully this will result in a successful repair!

I should preface this by saying that I am familiar with electronics in general, but have not done much in the way of technician work, or troubleshooting in general, so while I can identify all the parts and understand their basic function, I may or may not entirely understand their purpose in circuit. 

I do have the service manual for this TV and I have scanned it into the computer, I will be posting that as it seems pretty important if anyone does get the inkling to offer some assistance.

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The initial symptom on this TV Set was a blown fuse, and it ended up just sitting unused for a couple years.  I decided it is time to repair it, and so I figured the first step is to replace the fuse and power it up to make sure it wasn't caused by a power surge/lighting strike etc.

The fuse didn't blow again, and I could hear the static/charging of the CRT.  I guess that means I have high voltage, so I give it a minute while looking at the back of the set, but noticed no glow from the CRT neck (no heater). That suggests to me that high voltage is working but low voltage is not.

First thing I did was probe the test point (TP9) for the B+ voltage, which according to my manual should be 135v, but its 161v.  That's way too high (~20%). I look for a B+ adjustment, but it doesn't seem like this monitor has an adjustment, it guess it uses an IC that should output 135v (STR3035/NTE15012) with no need for adjustment?

A thought occurring to me at this point. Should I try disconnecting the output of the primary voltage regulator from the rest of the circuit and try retesting the output to see if it goes down to 135?

Still, that shouldn't keep the heater from working, as that should only raise the supply voltage on the low voltage supply side by a few volts and the regulators should be rated to deal with a higher supply voltage anyways. 

So I check the voltage going to the 12v regulator, which appears to be a series regulator setup using a power transistor (2SD774) and a zener diode connected to the base (RD13Eb2) and to the output through a resistor, to clamp the output to 12v. I got 0v at the base.  Just for kicks I check the 9v regulator as well, its a similar configuration, Power Transistor (2SD880) with a zener (RD10EN2).  0v as well. Did the same for the 5v regulator. Same situation. Obviously no low voltage supply at all.

I traced the supplies for these, and they all converge and travel to the flyback, which has an output pin for 12 volts.  I test the output  of the 12v supply from the flyback, and I am also getting 0v. 

I notice that right near the 12 output on the flyback there is a low ohm resistor and inductor in series before it goes off and splits to the different low power circuits, so I disconnect one leg of the resistor to isolate the 12v flyback output completely, and retested the output from the flyback.  I figure this would remove the possibility of anything shorting the output to ground or otherwise pulling the voltage down.  Still 0v output from 12v pin on the flyback. I checked the H1 and H2 pins and got the same voltage as B+, which seems close to what they should be according to the schematic.

As I said, I am pretty green when it comes to working on TV's.  As a matter of course, I pulled the Horizontal Output/Drive Transistor and the Pin Out Transistor and tested them using a diode checker, and they both checked good, no shorts and tested like two diodes facing each other.

That was round 1.  So, Right now I am under the impression I need to purchase a new flyback and a new regulator IC. Does this seem like a reasonable conclusion given the information so far?

I plan to pull out the OScope and test a few points in the circuit using a 10Mohm 10x Probe, but I need to find my isolation transformer first. Ill scour the garage in the next day or two.

Thanks for any tips/tricks and information, if you so choose to throw some knowledge at me. 

Lastly, apologies for any crudity in the schematic image, I had to scan it bit by bit and then stitch it together in photoshop, despite not having used photoshop since college.  I have the block diagram and board layout if those are needed, though I would need to stitch them together as well.. 

[station240]

Try powering a dummy load off the 135V regulator, as it's likely the higher voltage is due to the rest of the set not drawing current.
I'd be suspect about the regulator or its two 10uf 160v caps, as that close to the mains input makes it vulnerable to surges etc.

So basic idea is to separate the power supply from the rest of the set.

[SoundTech-LG]

100W incandescent lamp (if you can find)

[Amra]

Try powering a dummy load off the 135V regulator, as it's likely the higher voltage is due to the rest of the set not drawing current.
I'd be suspect about the regulator or its two 10uf 160v caps, as that close to the mains input makes it vulnerable to surges etc.

So basic idea is to separate the power supply from the rest of the set.

Sounds like a good idea, and speaking of the two 10uf 160v caps, I am planning to replace every cap in the unit that is tantalum or electrolytic, ill make sure to check or replace those two as well.

100W incandescent lamp (if you can find)

Are you suggesting that as the dummy load? Sounds like a good idea, thank you.    Will a 120v one be ok, or should I see if I can find a 250v? (I suppose I can just try and see, worst it will do is pop a bulb, lol)
Do you suppose I should also add one in line with the wall to prevent possible damage from shorts or other issues?

----
I went ahead and ordered a replacement flyback and the voltage regulator (STR3035) anyways, ill try adding the dummy load tonight and see if that changes the output voltage of the regulator. If that solves the issue there, then I could just save it as a replacement if it ever goes bad.

Its going to take a couple weeks or so for the flyback to arrive, in the meantime I will get to work on replacing all the caps.

I checked the ohm reading from the 12v output just past L502 to ground with the 12v pin supply from the flyback disconnected at R540 and it reads 14.8ohms, does this seem about right? Its not 0, so I dont believe there are any shorts, but if it is too low, then could that be what burned up the 12v output of the transformer?

I also checked the 26v output from the flyback, and no output there either.

I suppose I shouldn't worry and just wait until I replace all the caps, as the value may change after I replace them and then I can compare to the current value.

Thanks for the tips and suggestions!

[T3sl4co1l]

I don't get how you'd have HV but not 12V or heater (also a FBT winding).  Replace the FBT, I guess?

Trinitrons are great sets, hope it works out.

Tim

[poot36]

Check that the 12V supply coil is open with your multimeter on ohms range.  If it is you can try using a high voltage low current source (eg florescent light transformer for cold cathode lights) to try and reweld the open section of the coil back together.  Make sure to disconnect all coils from any other circuitry as you will cause the flyback transformer to try to transforme the way higher then normal input!  Caution! This can be dangerous if you do not know what you are doing!  I used a florescent light transformer from a old scanner slide illumination add on.  It ran from 6V to 24V DC and I would slowly increase the input voltage until I saw a jump in the current consumption of the light module and then I would stop.  I used this trick on a 1929 vacuum tube radios audio output tube driving transformer and it worked great.

[Amra]

I don't get how you'd have HV but not 12V or heater (also a FBT winding).  Replace the FBT, I guess?

Yeah, that's what I am thinking at this point.  I haven't worked on a lot of TVs (outside of replacing caps on cheap LCDs and computer monitors) so I am mostly guessing though.

I do get HV to the CRT, but no low voltage anywhere on the circuit, and it seems this set is designed where B+ goes into the flyback, and then the flyback provides the low voltage outputs, 12v and 26v. I have to assume this is something common to CRTs, but don't have the experience to say for certain, this is a learning experience for sure.

I have essentially disconnected the low voltage supplies from the rest of the circuit completely and still have no low voltage out of the flyback, but since I get high voltage I know the flyback is *getting* power and 'partially' functioning, so the FBT seems like the most likely culprit to me at this point. 

My biggest concern is whether something caused the flyback to fail, or it just gave up the ghost through old age.  The monitor is from 1985, so I'm inclined to believe age is a driving factor.

Trinitrons are great sets, hope it works out.

Tim

Yeah, I loved this monitor when it was working, and despite being 30 years old the colors on this thing were gorgeous. Thanks for the hope!

Check that the 12V supply coil is open with your multimeter on ohms range.  If it is you can try using a high voltage low current source (eg florescent light transformer for cold cathode lights) to try and reweld the open section of the coil back together.  Make sure to disconnect all coils from any other circuitry as you will cause the flyback transformer to try to transforme the way higher then normal input!  Caution! This can be dangerous if you do not know what you are doing!  I used a florescent light transformer from a old scanner slide illumination add on.  It ran from 6V to 24V DC and I would slowly increase the input voltage until I saw a jump in the current consumption of the light module and then I would stop.  I used this trick on a 1929 vacuum tube radios audio output tube driving transformer and it worked great.

Hmm, this sounded intriguing so I measured the ohm reading from the 12v output on the flyback to ground and it seems to be a dead short. It measures the same ohms as though I had the probes on my multimeter touching. (~000.6 ohms).

Given that my FBT is shorted instead of open, I don't believe this method would avail me, but for those with open coils this may offer a chance of repair instead of replacement, so thanks for sharing!

[T3sl4co1l]

I do get HV to the CRT, but no low voltage anywhere on the circuit, and it seems this set is designed where B+ goes into the flyback, and then the flyback provides the low voltage outputs, 12v and 26v. I have to assume this is something common to CRTs, but don't have the experience to say for certain, this is a learning experience for sure.

Very typical for TVs.  Since horizontal retrace is a constant 15kHz or thereabouts, and HV is always needed, all of it can be rolled together.

Computer monitors don't have the same advantage; my Multiscan Trinitron (60 to 144Hz vertical, 30 to 110kHz horizontal) has an independent FBT, driven by a MOSFET, for the high voltage alone (and maybe some other things).  The deflection circuit (which is horrendously complicated -- variable supply voltage, many chokes, capacitors and switches) has a beefy 30A BJT at its heart.  TV HOTs have nothing on it, of course, but it's still the same idea, refined to a different extent.

I have essentially disconnected the low voltage supplies from the rest of the circuit completely and still have no low voltage out of the flyback, but since I get high voltage I know the flyback is *getting* power and 'partially' functioning, so the FBT seems like the most likely culprit to me at this point. 

Can you scope the waveforms on those low voltage windings?  Check the diodes, resistors, caps and so on which connect to them?

Low ohm windings should be normal; there's probably ten turns of copper wire for one of those secondaries, it doesn't take much.

Tim

[Amra]

Small update today 

I got my caps in the mail, so I went through and replaced most of them. I did them one at a time, putting a dot with a red sharpie on the top of each one as I went, making sure to double check the polarity, voltage and capacity as I installed them. There are a few I haven’t changed, one because I ordered the wrong value (100uf instead of 1000uf), a couple which I ordered are the wrong package dimensions and so they simply won’t fit , and one which *looks* like an electrolytic but is actually non-polarized. I suppose its not really a project if I don’t make a mistake somewhere. 

To check the low voltage side, I connected a flat 12v dc signal to the 12v output pin where the flyback used to reside. Doing this, I checked the test point for the 9v regulator and got 8.9v.

 I checked the 12v test point and got 0v but then remembered the 12v regulator is isolated behind some kind of transformer, so a flat dc signal isn’t going to work. I’ve not worked on a lot of TVs, but I am pretty sure all the voltage signals from the flyback are all DC pulses. Still, the regulator itself should function with a flat dc signal given it uses a zener to clamp the output of a power transistor, so I connected a 15v dc signal after the transformer and got 12.5v from the test point, a good sign I believe. 

To test the transformer itself, I put a 1v AC signal into one side and tested the isolated side and got just under 1v out, so I will assume that is fine for now as well. 

So, all that said, it appears at this point that the low voltage side will function, at least I am getting close to the correct voltage out of the regulators (no loads right now, so not expecting perfection), looking more and more like the flyback at this point. Won’t know for certain until I receive the new one and hook it up.  It would be nice if that were the only problem. 

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Computer monitors don't have the same advantage; my Multiscan Trinitron (60 to 144Hz vertical, 30 to 110kHz horizontal) has an independent FBT, driven by a MOSFET, for the high voltage alone (and maybe some other things).  The deflection circuit (which is horrendously complicated -- variable supply voltage, many chokes, capacitors and switches) has a beefy 30A BJT at its heart.  TV HOTs have nothing on it, of course, but it's still the same idea, refined to a different extent.

That makes me wonder, given the primary purpose for the flyback is the high voltage, and that is working, if worst came to worst, could I get a center tapped transformer that outputs ~30v, grab 15v one half to the center tap, do some rectification and drop it into the low voltage sides of the set.  Not currently planning on that of course, but that is where my brain went. 

Can you scope the waveforms on those low voltage windings?  Check the diodes, resistors, caps and so on which connect to them?

I currently have the FBT removed, but I could try putting it back in and running a few more tests.  Is there a clever way to test it out of circuit?

Could I connect the HV Output to the CRT, then connect the ground and the B+ from the board to the flyback using clips, and then test while out of circuit? Is that crazy talk?

Low ohm windings should be normal; there's probably ten turns of copper wire for one of those secondaries, it doesn't take much.

That makes sense, which means the low voltage winding probably should seem like a dead short out of circuit. Hmm.  Seems like I might be reinstalling this FBT later today. 

[Amra]

Small update.

I was able to pull out my oscope and my isolation transformer. I went ahead and reconnected the flyback transformer in circuit in order to run a few tests using the oscope.

As soon as I connected the ground terminal of my oscope probe to the main ground on the tv chassis, the isolation transformer started making a loud buzzing sound.  That seems odd to me, given the board should  be completely isolated.  I double checked the grounds for the isolation transformer and ensured the windings for the transformer weren't shorted or anything. The TV is only two prong so there's no earth ground reference anyways.  I reconnected everything and tried once more, leaving the ground connected for a bit longer, it wasn't blowing the fuse but the windings in the isolation transformer must be getting hot because it started smelling bad, so disconnected the ground from my 10x probe.

Now, I am not sure on why that is happening, maybe my isolation transformer is rated for lower amperage than the TV is pulling? That or I could be doing something complete wrong. That's *always* a possibility.

Still, decided not to risk damaging the scope or the transformer by testing my luck (or stupidity) and decided I would take some measurements using just the 10x probe tip, no ground clip.

Picture 1
This is the signal I got from the B+. Scope was DC coupled, set to 50v per division, and 2ms timebase.

Picture 2
This is the signal I got when connecting the probe pin to the chassis ground, as well as the signal I get from the 12v terminal on the flyback, and the 26v terminal, basically the same signal on all three. Also DC Coupled, 50v per division and 2ms time base.

I thought that B+ signal was strange, I would imagine the B+ signal should be fairly even pulses even without a ground reference. The only things the signal goes through prior to getting to the flyback is a rectifier, the voltage regulator, a transformer and a few passive components. Ive already ordered the power regulator, but I hadn't really tested the rectifier, so decided I would pull it out of the circuit and test it on the bench.

Picture 3
This is the output signal of my variac set to ~10v, using the 10x probe. The scope is DC Coupled, 10v per division, 2ms timebase.

Picture  4
This is the output of the rectifier after being disconnected form the circuit, using the 10x probe with ground connected.  The scope is DC Coupled, 5v per division, 2ms timebase.

I thought perchance the 10x probe is affecting the reading, so I exchanged the 10x probe for a 1x probe, and did the test again.

Picture 5 is the input signal to the rectifier, scope is DC Coupled, 5 volts per division, 2ms time base

Picture 6 is the output signal from the rectifier, scope is DC Coupled, 5 volts per division, 5ms timebase.

Seems I am off to order a bridge rectifier! On the bright side, they are only about 5USD.  Another step in the right direction! 

[Amra]

Now, I am not sure on why that is happening, maybe my isolation transformer is rated for lower amperage than the TV is pulling? That or I could be doing something complete wrong. That's *always* a possibility.

I decided to take the isolation transformer apart to investigate, and found the issue.  It looks like this isolation transformer was originally designed for consumer audio gear/medical use, in that it decouples the output from the mains, but then has an added wire connected in with the secondary winding that goes to earth ground in order to help eliminate noise on the output. It was sold to me as an isolation transformer usable for tech work, but I failed to check for myself.  I disconnected the additional ground from the secondary, sealed it, and we should be good to go.  Ill retry those signals tonight, this time without the potentially dangerous situation, to either equipment or myself, that I had before.

Figured I would post this just in case anyone else ever runs into a similar situation, not all isolation transformers are designed for tech service, and it only takes a few minutes to check and fix. Lesson learned.

[Amra]

Quick update after spending the week waiting for parts.

Received the HV Regulator as well as the Rectifier and installed both components.  After installing these components, the set came to life, briefly.  Shortly after giving it the go juice I began hearing some popping/cracking sounds, I pinpointed the sound to the area around the FBT and about that time the fuse popped. Inspecting the flyback I discovered the side was quite warm, and found a couple cracks along the side of the unit, difficult to see unless you pry on them a bit, of course then they become quite glaring. I had already planned to change the flyback, but this pretty much verified the flyback was bad. (Photo 1)

Unfortunately, the original flyback I had purchased from one supplier sent a refund and an email saying the part was no longer available, so I had to find a new supplier.  Oddly enough, the flyback that was in the unit was branded with a slightly different part number than the one listed in the service manual, and while the difference in part number was only one digit (1-439-372-12 vs 1-439-372-11) that allowed me to find several additional suppliers that all claimed to have the part in stock, so I reordered and should be getting it soon.

In the meantime, I have a quick question.  This unit has a HV Resistor in line with the FBT before it gets to the CRT, part number 1-230-940-11 (Photo 2). Im sure this part is fine, but my question is this: I suspect the new FBT will not come with this part, and the new FBT will come with a length of wire and maybe a suction cup. Is there a trick to removing the old wire from the HV resistor (I suspect a long pin might work a treat)? Also, if the new one comes with a suction cup attached, are there any tricks I need to do to make it work, or can I just cut and strip? I currently expect it is similar to how spark plugs for automobiles are made, which I have done, but my expectations and reality aren't always best of friends.

[T3sl4co1l]

Alas poor FBT, we knew him not. 

If you're lucky, it comes with the ends, and you just need to figure out how to shove 'em in there...

If not, might need a splice like this? https://www.eevblog.com/forum/projects/how-to-insulate-50kv-cable-joint/

It ain't gonna be a wire-nut deal, that's for sure; but you'll probably have pretty good luck with anything you can make smoothed over (at the metal surface) and slathered in (non corrosive) insulating goo.  Preferably in redundant layers.

Tim

[Amra]

A circuit of infinite jest, of most excellent fancy. 


Thanks for the tips and the link, I will check it out. Just a waiting game till the post arrives now!

[Amra]

Progress, though not there yet.

I finally received the FBT and installed it.  It came with the proper connector and was pretty easy to install into the HV Resistor, simply push in and twist slightly to remove the old one. I love engineers who plan on things being taken apart!

The set is now alive, but I've run into a new/additional issue that I am hoping to get some direction on.

Unfortunately though, my isolation transformer doesn't seem capable of supplying enough wattage, the set runs on the isolation xformer, but the B+ is very low (~120v).  When I connect to the wall directly the B+ is spot on at 135v. I will probably need the oscope to trace the signal properly, but since Im not getting proper voltages with the isolation transformer I am not sure if I should proceed that way. I don't want to damage the set after just resurrecting it.

Anyhow, the new symptom is random flickering red bars on the screen, or the occasional complete erratic red washout, and then back to normal.  It might stay normal for a few seconds, or a whole minute before flickering again. I am adding an animated GIF to show what its doing, well, one of the more extreme examples of what it is doing.  (see pic 1)

Some things I've done to help eliminate possible causes and give me a general direction to look in:
1. turned the brightness completely down, even with a black screen it still does it, so I dont think its in the brightness control circuitry.
2. fiddled with the controls and they all seem to function (tint/color/sharpness/contrast) so I don't think its from the jungle ic. Note also, changes to focus/tint/brightness/color etc appear to have *no* effect on the symptom, it is the same color/brightness regardless (this would probably be a big clue to me if I knew more about TVs)
3.Tested my low voltages, the test point for 12 volts starts at around 8-9v when the tv is cold and gets to 20-30v when the set heats up.
4. 12v supply to the blanking board reads 14.7v, a little high but not sure if this is meant to fluctuate as temp goes up.
5. 26v test point is around 28v, a little high but not bad.
6. 9v is around 8.7v

Those checks were with set plugged into the wall with a proper 135v B+.

So, are there any guru's who know what direction I should focus my attention for this problem?

My thought right now is to start at the neck board with the red and disconnect it from the tube to see if the problem goes away with the red gone, if it does I could start following the red circuit back with my oscope and just deal with having a low B+ from using the iso xfrmer. Unfortunately I am rapidly approaching the scope of my knowledge of TVs, it seems the horizontal is working, the vertical is working, the sync is working, the jungle is working, the heater is working, the signal input is working, so I'm just not sure which direction is the most likely culprit.

Thanks in advance if anyone wants to offer a suggestion!

[T3sl4co1l]

Arcing, breakdown or loose solder joints in the red video amp path?  Does it change with mechanical tapping?

Tim

[Amra]

I actually did try tapping a lot of different places, and I did not get any lines appearing in cadence with the tapping, though it was hard to tell given the random nature of the issue (sometimes when I tapped a line would appear, but then additional taps don''t seem to have an effect). 

Still, your hint was a huge help, and makes total sense.  I was still getting the issue with the set plugged into the iso transformer (though I get some curling on the screen due to the low B+), and checking the pin on the neckboard connected to the CRT I could see the noise when it went into the gun, but when I checked the red signal coming from the main board I didn't see the noise there, which suggested the issue was probably on the neck board. So, I decided to reflow every joint in the red path on the neck-board just to be sure, no change.

So I started just tracing back the noise using the oscope, I was still getting the noise into the drive transistors, so went back even further. I was even getting noise on the potentiometer pin connected to the drive transistors, but oddly enough wasn't seeing much noise on the inputs to the pot (and the lack of issue with the other guns would suggest the noise isnt there anyways). So, I blew the pot out and wiped it back and forth a few times, and re-positioned it near where it was previous, and that seemed to help a lot.  Its been about 5 minutes and I haven't seen a red flashing streak yet.  I did notice that when I turned the pot all the way one direction, the red arcing/flashes went coo coo for cocoa puffs, so perhaps we found the culprit. 

Ill keep watching, and I think I will buy some electrical contact cleaner and spray the board down just in case there is some contaminants/dust that was causing that issue.  I don't want to speak too soon, but outlook is promising, Im going to play some super nes for a little bit and try to keep an eye on it to see if it happens again.

I just want to say in case this is fixed, thanks to everyone for all the help, I couldn't have done it without you! A special thanks to you Tesla, your continued interest and insight was extremely helpful!

[T3sl4co1l]

Burnt pot, anyone?

You're welcome,

Tim

[Amra]

That seemed to do it, I played FFIII for a couple hours last night and display was nice and bright, no additional issues and no streaks. I think we can call this job essentially complete, so ill mark the first post as repaired. 

I will order some pots and just replace all the pots on the neckboard given they are all about the same age, but there's no need to document that. 

Parts replaced:
1. B+ Power Regulator (~10 USD)
2. Diode Rectifier (though after thinking about it, the off waveform I got could have been a grounding issue, I don't think the output of the AC supply I used was/is isolated) (~6 USD)
3. Every Electrolytic Cap (~25 USD)
4. Flyback Transformer (~25 USD)

Total Cost of repair: ~75 (inc. cost of pots).

Learning a lot about analog TV's: Priceless

Thanks again everyone!