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Author Topic: Fried Transcoders - Cabinet CRT or PC Grounding Issue?  (Read 1490 times)

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DJO_Maverick

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Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« on: June 01, 2023, 05:29:36 pm »
Having fried over $200 worth of sensitive and hard-to-replace electronics in the span of a few hours, I figured it's time to seek outside consult...

I've been in the process of upgrading my cabinet.  It's a converted Pit Fighter cab with a de-cased Samsung slim-fit CRT for a monitor (two prong AC) and previously used a Pi 3B connected via RGB-Pi to a Shinybow 2840, and on into the component.  Power is via an APC PH8 surge protector that's wired via a terminal block directly into the main power toggle.  That one has a ground indicator that's indicating fine, so presume there's no grounding issue with that.  It had been like that for years with no known electrical issues.

I've been in the process of upgrading to be PC/Groovymame based.  Using a partially decased Optiplex 3040, which is using a R7 350 running CRT Emudriver.  For the past couple of months at least, I've had that system hooked up to my console setup via a DVI-I/VGA dongle, retrocables.es VGA to SCART cable (which plugs into USB for power), into my Bandridge switcher and through a Retrotink RGB2COMP on the way to a big JVC.  This setup has caused no electrical issues or problems during those couple of months.

I finally got ready to transfer it all into the cabinet.  Once the PC was in and hooked up to the Shinybow, no signal.  I swapped back to the TV setup and confirmed it was still outputting, then went back to the cab again; still nothing.  Started fiddling with the Shinybow and it grazed the PC case, causing an arc and pop.  Not connecting the dots yet, I assumed that the Shinybow had somehow just died inexplicably from age.  So I decided to move over my RGB2COMP to keep working on it until I could get a replacement for the Shinybow...  Still no signal.  Tried plugging the SNES into the 'tink to see if there's an issue with the Cab TV, notwithstanding the menus working fine.  Only audio.  I then go to plug the scart from the PC back into the 'tink, and it arcs on the connector.  Now, only too late, do I realize the error of my ways.  Both the Shinybow and the RGB2COMP are fried.  Suspect bad things for that 350 as well, but that's easier to source at least...  And as a final issue, I go to unplug things out of the PC to take it out, and I get a significant, numbing zap to my forearm when I first touched it again.

Trying to figure out how to go about troubleshooting this...  and I don't want to risk any more expensive transcoders, whenever I can get them in.  Unclear if it's the PC, or the TV at fault.  Can that kind of voltage leak back upstream through the transcoders into the PC?  If it's the PC, unclear why it caused no issues until in the cab.  I did immediately check the power cord for the PC for continuity and shorts, and it checked out fine...  and it was plugged into that grounded main power strip.

Can anybody suggest how to diagnose and fix this?

Thanks!

bobbyb13

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #1 on: June 02, 2023, 04:41:17 am »
Something is shorted obviously but some pictures will be necessary for any good advice I think.
You may have killed part of the pc too if you shorted power from the Shinybow to the pc case (which should be grounded if plugged in with a 3 prong plug.)

Need to verify ground continuity and then voltage coming into the cabinet first and then go from there.
Relax, all right? My old man is a television repairman, he's got this ultimate set of tools! I can fix it.

Zebidee

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #2 on: June 02, 2023, 06:39:25 am »
Try checking voltages in different places (like PC case, TV video ground, grounds on transcoders, GPU video outputs, cable "shields") to see what is buzzing. Check AC voltage too. To pin it down, then unplug connectors to see if the voltages are still present.

A new PSU may improve things. PCs can leak significant current to earth ground - it depends largely on the power supply. This might not be an issue if all is grounded properly. Is it grounded properly to and through the mains? Some electricians cut corners and don't bother.  I've measured 220v at a PC case before (in Thailand, usually no ground pin available), but measuring ~10-12v and zaps through forearms when using laptops is not unusual.

Maybe your GPU card is shorting somehow? Can you test that too?

I wouldn't rule out the TV, but if the TV is the source, suggests a failure of the DC rectification/SMPS and you would expect to see other problems.

How are the transcoders powered? From a separate power brick? These can give very variable performance. It may be better to power directly from PC via USB or molex.

I wonder if your "Bandridge switcher" was providing enough AC coupling and ground isolation or whatever to protect your transcoders.

So, assuming that PC, transcoders and TV all rectify their own local DC power and DC ground levels (this is not the same as earth ground). There can be a significant difference between three DC levels, and where there is a difference in voltage potential you can get problems. If not zaps, you might just get interference or other issues. Powering the transcoder from the PC may reduce this potential variance and associated issues.

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DJO_Maverick

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #3 on: June 02, 2023, 08:03:50 am »
Ok, will go nuts with the multimeter when I get home.

Try checking voltages in different places (like PC case, TV video ground, grounds on transcoders, GPU video outputs, cable "shields") to see what is buzzing. Check AC voltage too. To pin it down, then unplug connectors to see if the voltages are still present.

Forgive one amateur question; when checking things that OUGHT to be ground, but might not, what's the other reference point?  Some earth ground external to the whole system?

A new PSU may improve things. PCs can leak significant current to earth ground - it depends largely on the power supply. This might not be an issue if all is grounded properly. Is it grounded properly to and through the mains? Some electricians cut corners and don't bother.  I've measured 220v at a PC case before (in Thailand, usually no ground pin available), but measuring ~10-12v and zaps through forearms when using laptops is not unusual.

How would I go about testing the PSU other than just seeing if the chassis is, in fact, constantly hot?  I know the cable directly coming out of it is good and plugging into the power system that is at least self-reporting as properly grounded.  I'll check every length of that tonight for shorts as well.

How are the transcoders powered? From a separate power brick? These can give very variable performance. It may be better to power directly from PC via USB or molex.

The Shinybow was getting its power from its own brick.  The RGB2COMP was plugged in to PC USB...  though sometime during The Incident I swapped it to an external power brick because I wondered if it wasn't actually getting power from the PC.  Now I'm not 100% sure whether its arc happened while it was plugged in to the PC or when it had separate power.  Probably when separate, but it was already somehow fried BEFORE I switched it to external power...  as there was already no video coming through it, only audio.

I wonder if your "Bandridge switcher" was providing enough AC coupling and ground isolation or whatever to protect your transcoders.
It's an auto-scart switch, maybe.  One of these: https://videogameperfection.com/2016/10/20/bandridge-selector-review/

Zebidee

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #4 on: June 02, 2023, 07:00:31 pm »
Forgive one amateur question; when checking things that OUGHT to be ground, but might not, what's the other reference point?  Some earth ground external to the whole system?

I usually just use the nearest non-electronic large metal object, bike, table, chisel, hammer...

Quote
How would I go about testing the PSU other than just seeing if the chassis is, in fact, constantly hot?  I know the cable directly coming out of it is good and plugging into the power system that is at least self-reporting as properly grounded.  I'll check every length of that tonight for shorts as well.


If you detect significant voltage at the PC case, the DVI video ground (this is same as DC GND) or the DVI port's shield then you have a problem. If you're using a DVI/VGA adapter, use that and plug in a VGA male-male cable (or a DB15/VGA gender changer). That way you have easy access to the pins for your DMM. Black lead to chisel.

I can tell if a PC case or "full earth ground" is "HOT" by lightly passing over it with the palm of my hand, fingers splayed. Or the inside forearm, feel the "spidersense" tiny hairs ---tallywhacker--- up. You can feel the "humming". Don't do this if you have a heart condition :D  a multimeter is safer and more precise.

There would be YT tutorials on how to check health of an ATX PSU. Given that yours "works", there may be software tools to benchmark performance. I'd probably just swap-in a quality new or known good PSU and see if problems persist. If the original PSU is underperforming you can dispose, recycle or repair(!) thoughtfully.

Can't rule out something else going on, not connected properly or failing.

You should test your R7 350 card separately as well. Try reseating the card. Go over all your cabling and everything else visually and manually to make sure everything is secured correctly and not going into places it shouldn't.

Make sure the TV video input grounds are not hot too, just to be sure.
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DJO_Maverick

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #5 on: June 02, 2023, 11:19:56 pm »
Bit of a progress report...

First off, wall is wired fine and properly grounded.  So checked continuity of mains power on all lines to the wall, checked it for shorts, both upstream and downstream of the main switch, all the way to the terminal block connecting to the surge protector.  No issues there.  Also no issue between protector ground and ground of the wall plug.  And outlet is grounded fine.

Checking continuity between various grounded parts of the PC (chassis sections, video card, PSU ground pin) all the way to the ground of the wall plug shows perfect continuity there.  PC appears to be fully grounded.

Put power to the PC alone...  clear the video card is fried from how its fan is behaving (thankfully have a backup).  That being said, left it running for a little while and tested voltage all around the chassis, the PSU, the video card (corpse)...  no voltage of any kind tonight on it.  Also checked the ground pins and shield from the VGA output as suggested, no voltage there either.

Decided to test the TV alone next.  Plugged it in, turned it on...  and immediately got a flash in the back, and it instantly turned off.  Will not turn on at all now.  I also noticed some odd sticky residue around the middle of the component cables hanging off of it...  and I cannot recall ever having anything glue-like around them or in there, so it left me wondering if perhaps a superficial bit of outer insulation on them had melted at some point during this.

Seem to have an unsolvable chicken and egg problem for the moment...  was something on the CRT failing that somehow backwashed through the video line?  Can that happen, a significant surge back through the video input, enough to cause this stuff?  Or did some phantom voltage from the PC go forward and break something on the TV board?  Nothing visibly popped/broken while mounted, obviously something's wrong.

By a strange coincidence, I do have a fully populated backup mainboard for it that I can swap in tomorrow...  though I worry for it too at this point...

Any thoughts?

Zebidee

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #6 on: June 02, 2023, 11:51:48 pm »
Decided to test the TV alone next.  Plugged it in, turned it on...  and immediately got a flash in the back, and it instantly turned off.  Will not turn on at all now.  I also noticed some odd sticky residue around the middle of the component cables hanging off of it...  and I cannot recall ever having anything glue-like around them or in there, so it left me wondering if perhaps a superficial bit of outer insulation on them had melted at some point during this.

Wow. TV was not actually connected to anything (except component cables) when that happened?

Somehow something wet and sticky, or perhaps acidic, spilled into the TV through the vent holes, shorting high voltage to DC/video ground? That would explain a few things.

the DC/video ground is the only thing that directly connects from TV all the way through to the video card.

Think you need to pull the back off that TV and have a good look over the chassis for burnt and/or sticky components. Sometimes smell can give you a clue, if you investigate fairly quickly. Also check the plastic TV back itself for any sticky residue, maybe there is some around a vent hole.
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DJO_Maverick

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #7 on: June 03, 2023, 12:19:41 am »
TV's fully decased and installed in an arcade cabinet, there's no plastic to take off.  It's open when you open the back of the cab.  Nothing spilled in, there's nowhere for anything to spill in from above.  It's a small sticky spot in the middle of the component cable.  If it's not melted insulation, it must have been residue from tape long ago that I've forgotten about. 

And yeah, it was not connected to anything except power and a set of component cables hanging off the inputs in the air.  No smell after the fact. 

Zebidee

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #8 on: June 03, 2023, 02:06:25 am »
Fair enough, but something has shorted out on the TV. So you'll have to have a good look over the chassis.

If that gunk wipes off and there is no apparent damage to the cables, then it must be just some gunk. Unless it means someone has been spraying gunk around inside your cab.

If the TV does nothing, not even a power LED, it suggests a failure in the power supply. However these things are mostly designed to fail open rather than shorting and ruining your other gear. So still need to find if there is a short somewhere, as that may be what caused the power supply to ---steaming pile of meadow muffin--- itself.

If you have another chassis then lucky you - if you want to try repair you can compare values. Or maybe, if it works, just swap it in. I would still want to know what damaged all the gear though - so it doesn't happen again.

You were using that USB powered VGA-SCART cable with all this? I don't mean to cast aspesions, but wonder what's really in that. People do all kinds of things. For example sync and blanking signals that are over-spec and weird grounding. You shouldn't even need USB power to that cable if it is just being plugged into a transcoder or a switcher, as there is no RGB or AV switching to worry about. But then, some designs won't work at all without power. Some of these cables are only really suitable for direct connection to TV, not for use with a transcoder.

For these reasons I always make my own VGA-SCART cables, so I know whose butt to kick if it goes wrong.

Fortunately GreenAntz comes in a VGA version, so it just needs standard VGA male-male, USB-C charge and component cables that you can buy at reasonable prices anywhere. No mystery box cables required.
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DJO_Maverick

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #9 on: June 03, 2023, 01:13:39 pm »
I'm certainly open to the idea that the cable might be the problem (that would be the easiest thing to blame and deal with), though it's a reputable maker that people swear by...  Any idea for how to test/verify that, specifically, as somehow causing this series of unfortunate events?

Nothing obviously visibly wrong with the current TV chassis, yet, other than it blew the fuse...  that's clearly what happened last night.  And the point at which it blew the fuse, it was the only thing plugged in.  There's a single 25v cap near the flyback that is ever-so-slightly bulged, but it's been that way (even the backup needs recapped at that specific spot).

DJO_Maverick

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #10 on: June 03, 2023, 01:29:18 pm »
Well, one other probably significant observation: the ground spring is hanging loose.  It's connected to the ground wire, which seems to be on the tube good, but then the wire and spring are just tucked behind a plastic hook on the chassis holder and don't appear to go anywhere.  I pulled out old photos from as far back as I've had the cab, and apparently it has always been this way since I got it (I picked it up empty except for the CRT already installed).  I have to imagine that spring is supposed to be connected somewhere, but no clue where.  Maybe related, maybe not...  again, apparently been that way with it working for a good ~4 years or so.



And wider shot of the board in position, you can see the toasty fuse at the top.


Zebidee

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #11 on: June 03, 2023, 06:49:22 pm »
That spring if fine if not touching anything - it is just for tension, to keep CRT grounding braid against the aquadag on back of screen.

So the TV blew a fuse - but likely there is another problem that caused the fuse to blow.

I guess it must be power supply - that is probably good news as mostly easier to fix. It may be what was causing your issues all along.

There's a bunch of things I'd check but, unless you know what you're doing, I think your TV needs a tech.

 
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DJO_Maverick

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #12 on: June 04, 2023, 04:16:09 pm »
I guess it must be power supply - that is probably good news as mostly easier to fix. It may be what was causing your issues all along.

There's a bunch of things I'd check but, unless you know what you're doing, I think your TV needs a tech.

Feeling pretty confident from that it was the TV power supply?  Would the HOT or bridge diode going cause it to short out to the video ground or otherwise explain it flowing all the way back up the inputs to the PC?

CRT TV techs are in short supply these days...  I know about enough to get myself in trouble but am a quick study.  Haven't had time to take the old board out yet, planning to check the HOT and bridge diode first, and check its power cable for shorts.  Not sure how to check the posistor, and really unclear how to effectively check the flyback.  Though, I can't seem to quite figure out what is the HOT or equivalent on this board...  the service manual suggests going from the fuse, to the bridge, and then to IC801S, which is a W6750F...  which may be what is serving the function of the HOT, from my limited understanding...?  Though with its more complicated pin-out, I don't know how to test that.

Service manual: https://elektrotanya.com/samsung_txt2782qx-xaa_chassis_k64c_sm.rar/download.html#dl

Zebidee

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Re: Fried Transcoders - Cabinet CRT or PC Grounding Issue?
« Reply #13 on: June 04, 2023, 08:53:39 pm »
Feeling pretty confident from that it was the TV power supply?  Would the HOT or bridge diode going cause it to short out to the video ground or otherwise explain it flowing all the way back up the inputs to the PC?

CRT TV techs are in short supply these days...  I know about enough to get myself in trouble but am a quick study.  Haven't had time to take the old board out yet, planning to check the HOT and bridge diode first, and check its power cable for shorts.  Not sure how to check the posistor, and really unclear how to effectively check the flyback.  Though, I can't seem to quite figure out what is the HOT or equivalent on this board...  the service manual suggests going from the fuse, to the bridge, and then to IC801S, which is a W6750F...  which may be what is serving the function of the HOT, from my limited understanding...?  Though with its more complicated pin-out, I don't know how to test that.

Service manual: https://elektrotanya.com/samsung_txt2782qx-xaa_chassis_k64c_sm.rar/download.html#dl


CRT TV techs certainly are hard to find these days. I reasoned this would be the case about a dozen years ago, and have started teaching myself TV & electronic repair, slowly at first. I have learned a lot about CRT TVs and arcade monitors, maybe more than I expected, and other things too. That out of the way, I said "your TV needs a tech" because I don't know everything and its easier if you can test and intuit some things directly. I'll try to help but that is my disclaimer.

Your initial analysis and approach to finding the issues seems about right. There is a pic in the troubleshooting guide with parts marked, but some of the labels are confusing and the boxes marking where things are is wonky. So I've attached my own pic below.

The "diode bridge" D801S is the bridge rectifier chip, which is in early stage of SMPS. It is really 4 diodes bundles together into a single chip. Looks like a long rectangle with a corner cut off and has four legs. I've marked the DIODE BRIDGE/BRIDGE RECTIFIER in yellow YELLOW.

IC801S is your SMPS main transistor - this is where the "switching" in "switch-mode power supply SMPS" comes from. It is some kind of MOSFET, fast-switching power transistor.  IC801S is in red, on that big heat sink. You can test it by connecting an oscilloscope to pin 1 and comparing the waveform to that marked in the troubleshooting guide (TP01).

The horizontal output transistor (HOT) is always close to the flyback and the horizontal deflection. I've marked the HOT in blue. A failing HOT *could* cause the ground leakage issues you saw, I guess. But other than that I would be mostly looking at the SMPS. Testing the HOT is not hard but tricky, can get into that later.

Another thing that I'd look at, maybe first because they are easy to test, is the small bunch of main power diodes I've marked in turquoise.  The main power diodes failing is a common problem. It is also a good place to look for testing DC secondary voltages including the B+, as this is where they all come out from the main transformer. Even the flyback and the HOT rely on these voltages being right.

EDIT: You can measure the B+ at cathode end of D811 above (more convenient), or flyback pin 3.

So that is why I think your problem is most likely in the SMPS, and why I'd look there first. I'd be tempted to check and/or replace one or both of the D801S and IC801S. I'd also go and check all the smaller diodes around them because it is pretty easy.

Ideally you'd use an isolation transformer (for safety) and a variac (variable voltage transformer). Then you could replace fuse and use the light bulb in series trick while slowly raising the power. With power applied you could test voltages and hopefully see what parts are failing. Or you could try just replacing the fuse and test a few voltages to see where the problem is, but a little risky and you never know when it might blow again (or worse).

If you want to get into this repair stuff it helps if you have a bunch of "jellybean" parts like the bridge rectifiers, diodes, capacitors, common ICs, HOTs etc & so on to swap in/out. A lot of pro repairers fix thing by just identifying problem areas and replacing suspect parts, rather than testing everything. ICs for example can be hard to test properly unless they exhibit dead shorts. Mostly you just guess it has failed because other bits relying on it aren't working.

I'd also do a quick check of the electro caps health with an inspection and an ESR meter (Dick Smith). For checking flybacks I use a LOPT/flyback/rings tester (Dick Smith). I got a few of these two meters, in kit form, from Dick Smith before the company folded in mid-2000's. Still have some here, "new-in-box" and plastic wrap. Let me know if you are interested.
« Last Edit: June 04, 2023, 09:49:44 pm by Zebidee »
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