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Author Topic: Zenith CRT TV - black horizontal bars  (Read 830 times)

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romshark

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Zenith CRT TV - black horizontal bars
« on: July 12, 2020, 03:07:41 am »
Ok, I know this forum is more about arcade monitors. However, I'm hoping the general knowledge of CRTs may help me out here.

So my old Zenith TV (that I use with my older video game consoles) developed a problem with black horizontal bars. These seem to show up with brighter colors. They do become less visible if I turn down the brightness of the TV with the on-screen menus, but I'm worried that this may be a problem that gets worse.

Here's a video of the problem. Note the black runs on the Sega logo screen (especially that really dark one near the top), and through the middle of the Sonic logo.



The TV is a Zenith SL2067BT, made in 1994. My parents bought it new from the store, but they never used it much. I used it for gaming from 2001 to late 2006 (when I got an HDTV, and didn't yet realize that my older consoles look bad on an HDTV.) Fast forward to this year, when I got the TV from my parents before they threw it out. It had not really been used since 2006.

Besides a lot of non-use, the only other thing in this TVs history is having to be repaired around 2003. It turned off on me in the middle of a game, and could not turn it back on. My father brought it to someone for service, and we got it fixed.


I've heard replacing electrolytic capacitors solve a variety of problems, so I plan to do that. I've already opened the unit, discharged the CRT (I have experience doing that from my arcade cabinet), vacuumed up a thick layer of dust, documented how the wires are hooked up, and removed the boards. Luckily, no sign of a leaking capacitor.

My plan is to document each capacitors values and part number marked on the PCB in a spreadsheet. Then, I'll mark off the top of the capacitor with a small drop of paint or something. Once I get the replacement capacitors, I can just de-solder any capacitor with paint on it (with my desoldering station), and put in the new capacitor. Luckily, the PCB not only lists the component number (i.e. "C10"), but the proper polarity for mounting the new capacitor.

So...anyone see this problem before? Would re-capping fix it, or do I need to do something else? Or is the TV toast and I need to get another one from a yard sale?

Any recommendations on what capacitors to get? I usually just buy whatever from Jameco.com, but if a more expensive brand of capacitor would be better for my TV, I'll consider it.

Also thought about doing an RGB mod while I have it open, but not sure where to start. Never heard of anyone RGB modding a Zenith. I identified a chip LA7672 as a "Color TV Single-Chip Signal Processor", so that might be a "Jungle chip," but I'm not sure. There's another longer chip on the board, but it's protected by a metal shield that's soldered to the PCB and I can't identity it without desoldering the shield. Although if it's too complicated or I have a high chance of damaging my TV, I'll just stick to changing out the capacitors. Or would it be worth it to ask the folks at the shmups forums?

Attached pics:
1.) The TV in question, prior to disassembly.
2.) The back taken off. The dust buildup is thick.
3.) The mysterious shielded chip.
« Last Edit: July 12, 2020, 03:14:41 am by romshark »

Zebidee

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Re: Zenith CRT TV - black horizontal bars
« Reply #1 on: July 12, 2020, 09:51:18 am »
I'm personally not a fan of "recapping", but in your case you might find it helps, especially in the power supply area.

A better option, especially if you also have arcade monitors, is to get an ESR meter so you can check caps without removing them from the PCB. Though if you have a decent desoldering station changing them out won't be very hard.

Speaking of desoldering stations, that should make it pretty easy to get that metal shield off. If you can't get the nozzle around the leg, just suck up excess solder around it first then poke it through. I wonder if that number printed on top is the chip ref number? However, I wouldn't worry about it. My guess is that it has nothing to do with video inputs as it is near the wires that seem to go off to your TV front panel buttons and remote IR sensor. So probably a microcontroller for those functions (though that may include OSD, or On Screen Display).

LA7672 is your jungle chip (I just googled LA7672 + datasheet), it has RGB inputs for OSD on pins 15, 16 & 17. You should be able to find the OSD chip somewhere, feeding into those pins. Looks like somehow the blanking signal piggy-backs on pin 17 (blue), but don't know how that works exactly. Something for you to figure out once those bars are sorted!
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Re: Zenith CRT TV - black horizontal bars
« Reply #2 on: July 12, 2020, 02:37:08 pm »
Reseat all the connectors.

Rejuvenate the tube?

This forum needs more threads about Arcade 1Up cabinets.

romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #3 on: July 12, 2020, 02:57:39 pm »
A better option, especially if you also have arcade monitors, is to get an ESR meter so you can check caps without removing them from the PCB. Though if you have a decent desoldering station changing them out won't be very hard.

Never heard of an ESR meter before. Looked it up though, and seems to be a very handy tool. Since I have 2 CRTs (this gaming one and arcade), a number of arcade PCBs with caps, and who knows what else, I went ahead and ordered one on Amazon.

LA7672 is your jungle chip (I just googled LA7672 + datasheet), it has RGB inputs for OSD on pins 15, 16 & 17. You should be able to find the OSD chip somewhere, feeding into those pins. Looks like somehow the blanking signal piggy-backs on pin 17 (blue), but don't know how that works exactly. Something for you to figure out once those bars are sorted!

I looked at those pins on the board. All three go in two directions on the bottom of the PCB. One direction goes though some surface mount resistors to a large ground area under the chip (verified ground with my meter.) The other direction goes away from the chip for a bit, then turns into a solder point. There is nothing on the top of the board in this area, so it must be a multi-layer PCB. Unfortunately, I haven't found where those pins connect to. I tried (with my continuity meter) all the pins on the other chips, and both sides of pretty much every resistor and every jumper. I may try again the future.

Reseat all the connectors.

Rejuvenate the tube?

I could try reseating everything, and see if that works. Could it just be an issue with the way the neck board was on or something? Couldn't hurt to try.

Not sure how to rejuvenate a tube. Sounds like I'd need to pay an expert to do that.

Zebidee

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Re: Zenith CRT TV - black horizontal bars
« Reply #4 on: July 12, 2020, 09:39:00 pm »
Never heard of an ESR meter before. Looked it up though, and seems to be a very handy tool. Since I have 2 CRTs (this gaming one and arcade), a number of arcade PCBs with caps, and who knows what else, I went ahead and ordered one on Amazon.

Oh I should've given you some suggestions/recommendations! Some of the cheap ones I've seen on ebay are rubbish, I assume Amazon isn't much different. Cheap ones might advertise that they can test caps without removing from PCB, but might not be true. Cheapest decent ones I know of are the Anatek "blue" ESR meter and the old Dick Smith ESR meter (which the Anatek one is based on), and they go for around $80-$100. Otherwise you should go for a decent name brand, I see some MESR brand ESR meters on Amazon for pretty cheap that look like they'll do the job. Some of the better ones can also check capacitance.

Quote
Not sure how to rejuvenate a tube. Sounds like I'd need to pay an expert to do that.

I don't think your tube's CRT guns need rejuvenation. Biggest symptom of that is weak, dark image or possibly loss of one or more colours. Rejuvenation is only something that you do fo a dying tube, and it only buys you a bit more life at best. Rejuvenators basically overrun the heaters for the CRT cathodes ("guns") at back of tube to burn the oxidised crud layer off the top. There is some risk of completely FUBAR-ing your tube by doing this, but if your CRT is bad enough that you want to do a rejuve then it probably isn't an issue.
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romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #5 on: July 12, 2020, 09:55:02 pm »
I had gone with the LCR Research Pro1 unit. It got pretty good reviews, so hopefully it'll work for me, and it's in a neat "tweezers" form factor, and do more than just ESR measurement. It wasn't exactly cheap though, but it's already shipped.

I did try putting everything back together and testing it out, to see if there was a loose connector. Same exact problem with the picture. So I'll let it sit and discharge it next weekend, and go from there.

Zebidee

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Re: Zenith CRT TV - black horizontal bars
« Reply #6 on: July 12, 2020, 10:37:08 pm »
That LCR Pro1 unit looks like a nice bit of kit with a lot of useful functions.

Remember to discharge capacitors before testing (put screwdriver/pliers across terminals), although this is really only an issue for the bigger caps and they usually discharge by themselves within a few hours. It is more of a danger to your equipment than to yourself, though some caps can give a nasty jolt. Essentially they are just like the CRT and can hold a charge for a while (another way to look at it is CRTs are basically big capacitors).
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Re: Zenith CRT TV - black horizontal bars
« Reply #7 on: July 12, 2020, 10:57:25 pm »
Well, if youíre lucky Ken Layton will notice this thread.

Doesnít look like a geometry issue to me, but Iíll defer to the expert.

Zeniths, at least in the USA, were not a particularly good brand of television when new.

This forum needs more threads about Arcade 1Up cabinets.

Zebidee

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Re: Zenith CRT TV - black horizontal bars
« Reply #8 on: July 13, 2020, 01:38:24 am »
Well, if youíre lucky Ken Layton will notice this thread.

We're all lucky that Ken hangs out here and pokes his head in from time to time :cheers:

Quote
Doesnít look like a geometry issue to me, but Iíll defer to the expert.

Nobody here talking about geometry...  :dunno

Quote
Zeniths, at least in the USA, were not a particularly good brand of television when new.

That's because they're a US company (Illinois).
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romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #9 on: July 19, 2020, 08:42:37 pm »
I got my meter on Thursday, and played around with it after work that night (measuring components on other boards laying around, and on some loose capacitors and other components I had). Works nicely, as far as I can tell. This one directly shows the correct units on the display (looking at other meters, I think they give readings that have to be cross-referenced on a chart to get the correct number?)

Discharged the TV again this morning and pulled it all apart again. I checked some of the capacitors, but I'm not sure what I'm looking for. Am I looking for significantly lower values than what's marked on it? Significantly higher? Or maybe it's bad either way.

For example, one capacitor I tested on the TV PCB:
1000 uF Listed.
 890  uF measured in circuit.
 898 uF measured after desoldering and testing loose.

I'm also still getting used to interference from other components. Another capacitor:
2200 uF Listed.
3128 uF measured in circuit.
2111 uF measured after desoldering and testing loose.

What's the threshold for a "bad" capacitor? That first one is off by 10.2%, and the second by 4% (if I'm doing the math right). Or am I looking at this wrong?

Also, the empty spot where that second capacitor was reads 626 uH, so there's a choke or something in that circuit (I wish I had a schematic to find out.) The meter should automatically determine series or parallel circuits, but I may try manually selecting the circuit type and see if that helps.

Sorry for the probably rather basic questions, but I want to make sure I know what I'm doing with this ESR meter. Even if I end up just replacing all the caps on the TV, I should learn to use the ESR meter for other projects and repair jobs.

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Re: Zenith CRT TV - black horizontal bars
« Reply #10 on: July 19, 2020, 10:04:30 pm »
Yeah I really should have given you some recommendations in the first place.

You use the ESR meter to measure Equivalent Series Resistance, not capacitance. All capacitors have some ESR, and it goes up as they age/fail. ESR is actually a better measure for failing electrolytic caps than capacitance.

Here is a decent guide featuring a few different types of ESR meter:
http://www.electronicrepairguide.com/esr-meter.html

This guy (EEVBlog) on Youtube demonstrates how to use an ESR meter to identify some bad caps and fix an LCD monitor he picked out of a dumpster. You'll see how easy the repair was. He checked all the caps but only needed to replace two, others in good condition still.


if long YT videos bore you, you might want to skip to 4 minutes in where he starts talking capacitors, or about 5 minutes in where he starts using the ESR meter. The meter he's using is exactly the same as the one I use (Dick Smith/Silicon Chip/Bob Parker design, built from a kit). I have used mine for about 15 years and the batteries last a very long time.

You'll see there how he takes the ESR reading, then compares value to the chart on front of meter to determine if capacitor is good. The chart values are a little rubbery, but bad caps will be obvious because they'll have wildly wrong values. With a little experience you won't even need to check the chart most of the time.

I actually have a few of those Dick Smith/Silicon Chip/Bob Parker kits (still in shrink wrap) in my shed that I got from Dick Smith (hobby electronics store like Tandy/Radio Shack) in Australia before the company finally went completely broke in the late 2000's. The only thing that is different is the screen-printing on the front. Nobody makes them anymore, though there are alternatives. I can happily sell one to you, except I worry that the postage from Thailand (where I am) to wherever you are might be a bit too much. If you are still interested, please PM me.
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Zebidee

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Re: Zenith CRT TV - black horizontal bars
« Reply #11 on: July 19, 2020, 10:14:18 pm »
What's the threshold for a "bad" capacitor? That first one is off by 10.2%, and the second by 4% (if I'm doing the math right). Or am I looking at this wrong?

It depends on the capacitor, but often they are rated to be +/- %20! But yeah, check the ESR. Capacitance can seem OK when you check it out of circuit, only to fail/falter while under load. A failing capacitor with high ESR will not be able to recharge/discharge as efficiently as before, create more heat and obviously will be putting extra resistance in the circuit where there wasn't anything (worth worrying about) before.
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romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #12 on: July 19, 2020, 11:29:15 pm »
Ok, I think I see what I was doing wrong. Since ESR is in the title for the LCR probe on Amazon, I thought it was using ESR by default when needed in automatic mode. Went into the menu, and set it to ESR mode.

Looking at my TV PCB, it looks like most of my capacitors are from Nichicon (though I did find a couple so far by Rubycon). The problem is, I can't find the manufacturer Impedance values for some of these. The charts online (and on the front of your meter) cover the common ones, but I can't find, for example, the expected value for a 330uF 200V Nichicon capacitor. Can't seem to find it in any datasheets I come across.

Is there some massive chart somewhere that I could use? Even if it's not exact to manufacturer, I know now that if the ESR is higher, then it's probably bad.

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Re: Zenith CRT TV - black horizontal bars
« Reply #13 on: July 20, 2020, 12:06:51 am »
The problem is, I can't find the manufacturer Impedance values for some of these. The charts online (and on the front of your meter) cover the common ones, but I can't find, for example, the expected value for a 330uF 200V Nichicon capacitor. Can't seem to find it in any datasheets I come across.

Is there some massive chart somewhere that I could use? Even if it's not exact to manufacturer, I know now that if the ESR is higher, then it's probably bad.

Don't worry too much about the datasheets.

For those edge cases you need to guestimate a little. Taking your 330uF 200v example, that's roughly halfway between 220uF and 470uF at 250v, and their best (worst) ESR values are 0.5 and 0.3 respectively. Best/worst ESR values for those same caps at 100v is 0.05 and 0.09 respectively, quite a bit lower. Anyway, I'd say that any ESR reading over 0.5 would definitely be suspicious. That estimate is maybe a little on the high side, especially for a Nichicon, but when caps go bad you'll normally see ESR go much higher/ out of normal range, often by factors of 10 or more. If you have potential replacement caps that you assume are good, you might compare.

Always try to use "low ESR" caps if available, especially in the power supply area.
« Last Edit: July 20, 2020, 12:12:50 am by Zebidee »
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romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #14 on: July 26, 2020, 07:18:05 pm »
Sorry for the lack of updates, but I've only been able to work on this on Sundays (had to do overtime at work.)

I spent time measuring capacitors with my meter in ESR mode (100Khz). After some hunting, I found one reading 1.38K Ohms. Had to desolder it just to find the values (1.5uF 50V).

So, I found one possible issue. I don't have a spare of that value, so I'll have to order it, but I'm going to continue searching for more capacitors with bad ESR first. I may replace some of the other capacitors anyway (where the top looks slightly convex. Not anything like the above video, but still worth replacing. Also the ones that I desoldered last week during my testing.)

My spirits are up. I might be able to save this TV after all.

Edit: Found more with high ESR. It seems to be mostly the 1uF 50V Rubycon ones. I guess the smaller ones dried out first, which I guess makes sense. At least I can probably just order that one in a larger bulk.
« Last Edit: July 26, 2020, 08:07:24 pm by romshark »

Zebidee

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Re: Zenith CRT TV - black horizontal bars
« Reply #15 on: July 26, 2020, 09:10:22 pm »
The 1uF caps have pretty high ESR anyway, even when new. According to the tables up to ~5 ohms is good. So probably best you donít rip them all out. Try comparing esr with your replacement caps. On that same basis, the 1.5uF cap is probably ok too.
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romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #16 on: July 26, 2020, 09:36:00 pm »
~5 Ohms may be OK, but the one I read (and posted a pic of) was 1.38 Kilo Ohms = 1385 Ohms.

Took me a while to get my head wrapped around the metric units part. This meter usually shows good caps in milli-Ohms (so mΩ on the display).

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Re: Zenith CRT TV - black horizontal bars
« Reply #17 on: July 26, 2020, 09:49:06 pm »
Lol wow I was reading from phone, didnít look close enough to mentally register the ďkĒ. Yeah 1.38k ohms is high by any standard! Anyway, yes thatís obviously a bad cap.


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Re: Zenith CRT TV - black horizontal bars
« Reply #18 on: July 30, 2020, 09:32:11 pm »
So the capacitors came in today from Digikey, and I spent the night replacing the suspect ones, and some of the other ones too.
Got everything hooked up and...still the same exact problem. At least I didn't blow anything up with a backwards capacitor.
So I'll wait till the weekend, discharge it, and take a look at the board again.

Did some further testing while it was hooked up:
Before I said the bars were based on the brightness setting. I was wrong; it's actually the contrast setting. Turning it higher causes the bars to darken compared to the rest of the screen. Turn it low enough and the bars disappear. The picture is a lot darker than I'd like when the contrast is set that low though (definitely darker than I played back in the day.)

Also tested my Sega on an RF connection (I had done all my previous testing with AV cables.) My thinking was that, if the image is normal through RF, then it may be one of those many capacitors near the AV in. However, the problem looked just the same on both connections.

I'm going to give it some more work, but I may eventually cut my losses, and check the neighborhood forums (yes, we have that) for a CRT TV that someone may want to part with. At this point, I'm thinking it may not be a capacitor-related problem after all...

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Re: Zenith CRT TV - black horizontal bars
« Reply #19 on: July 31, 2020, 12:00:23 am »
Sorry if this is an obvious question, but have you tried tweaking the screen voltage knob (usually on flyback) at all?
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Re: Zenith CRT TV - black horizontal bars
« Reply #20 on: July 31, 2020, 03:40:58 am »
I didn't look into any of those controls. I'll look it up and hopefully try adjusting it after work.

edit: took a quick look. The flyback has 2 controls. One is marked focus, and the other is unmarked. I'll try tweaking that second one (which should be voltage or brightness) later.
« Last Edit: July 31, 2020, 04:58:11 am by romshark »

romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #21 on: July 31, 2020, 04:18:15 pm »
So I gave it a shot. The other control on the flyback made the picture go more red or more blue/green. I did try the focus control too, but neither one affected the dark bars.

The search continues.

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Re: Zenith CRT TV - black horizontal bars
« Reply #22 on: July 31, 2020, 11:03:28 pm »
So I've come across 2 different things.


First, the inside of the back case has a diagram of the controls. I uploaded a picture accordingly. Unfortunately, it states right on the label "High Voltage Not Adjustable". I think that's the screen voltage (please correct me if I'm wrong). Anything else here look like it might be useful?


Second, I believe I have tracked down a PDF schematic of the chassis board. Uploaded that too, of course (available at the bottom of this post). The site had my model listed, though the schematic itself lists "2767SL" along with similar numbers. I think it's for a different country, but still close enough to my TV.
Not sure if it'll help for fixing the dark bar issues. If that can be fixed though, it may help with RGB modding.

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Re: Zenith CRT TV - black horizontal bars
« Reply #23 on: July 31, 2020, 11:54:47 pm »
I don't think that "High Voltage Not Adjustable" means that you can't adjust the screen voltage, it is just a general warning to not try to tamper. Of course, you will ignore that!

Having said that I am stumped why adjusting screen voltage on flyback is just altering your colour mix/purity! It should affect overall screen brightness. With brightness in neutral position and a dark screen, you adjust it up until you can see the flyback retrace lines (diagonal, near top), then adjust it back down until lines are gone. Then adjust focus as necessary.

I'm guessing that your black bars come from a power supply issue, but have no idea what specifically and don't know your TV.

You might need to expand your net to find help for this one. This forum branch is more focused on arcade monitors. There are a couple of FB groups that you might find worthwhile, "Vintage CRT Troubleshooting and Repair" (fairly good, not bad) and "The CRT Collective" (not as good, but some are knowledgeable). If they can't help directly, maybe at least they can point you to a local repair place.

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Re: Zenith CRT TV - black horizontal bars
« Reply #24 on: August 01, 2020, 10:32:21 am »
Drive around your neighborhood and find another CRT on the curb. Or go to Craigslist. They are free or at the very least cheaper than you time and aggravation are worth.

It is a ---smurfing--- Zenith TV. Let it die.

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Re: Zenith CRT TV - black horizontal bars
« Reply #25 on: August 01, 2020, 10:41:28 am »
It is a ---smurfing--- Zenith TV. Let it die.

I suspect romshark is just using it to get some experience, in which case killing a Zenith before killing something else might not be a bad idea  :o
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Re: Zenith CRT TV - black horizontal bars
« Reply #26 on: August 01, 2020, 10:44:40 am »
Sorry, but I think this whole thread is unproductive. Jumping into CRT repair so you can play your console games is a giant waste of time.

Unless he plans on repairing them with regularity, he is way better off with a TV that works.

romshark

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Re: Zenith CRT TV - black horizontal bars
« Reply #27 on: August 01, 2020, 12:05:47 pm »
Yeah, it probably is at the point to give up trying to repair this Zenith. I was just hoping to fix it easily enough and keep it going for longer. Plus, it fit perfectly in my entertainment center. I'll see if I can find an equivalent size with AV in. Maybe even S-Video.

It's still usable, just has an annoying brightness problem on solid bright backgrounds. I'm going to try to get a replacement before everyone throws their CRTs away, but I can manage with this one till then.

I suspect romshark is just using it to get some experience.

Can't argue with that. I think the whole capacitor / ESR meter will help me fix other electronics. In particular, I have a Neo-Geo board with sound issues (yes, the speakers are hooked up correctly. Not using the JAMMA standard).
« Last Edit: August 02, 2020, 10:17:40 am by romshark »