Main > Monitor/Video Forum
Zenith CRT TV - black horizontal bars
romshark:
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.
Zebidee:
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!
pbj:
Reseat all the connectors.
Rejuvenate the tube?
romshark:
--- Quote from: Zebidee on July 12, 2020, 09:51:18 am ---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.
--- End quote ---
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.
--- Quote from: Zebidee on July 12, 2020, 09:51:18 am ---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!
--- End quote ---
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.
--- Quote from: pbj on July 12, 2020, 02:37:08 pm ---Reseat all the connectors.
Rejuvenate the tube?
--- End quote ---
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:
--- Quote from: romshark on July 12, 2020, 02:57:39 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.
--- End quote ---
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.
--- End quote ---
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.