27" CRT TV blackened area on PCB near big resistor, not sure the cause? - Page 1
Forum section: electronics / repair

author: CRTguy23
author_rank: Contributor
timestamp: April 22, 2025, 07:45:49 pm
content: I have 27" Panasonic CRT and at the base of one of the big resistors, the PCB is black. The PCB is also blackened around the nearby components. It isn't charred; it's just black in color. The rest of the PCB looks normal. The solder joints in this area looked pretty bad so I re-flowed them. I have the service manual for the TV and the resistor in question does measure at it's correct value of 27ohms. I also pulled out the transistors, diodes and other nearby resistors and tested with a multimeter and I got normal readings. I suspect there's some sort of over voltage problem but I don't know what's causing it because the parts in the blackened area of the board are testing fine. The picture on the TV looks normal. I have included a photo of the darkened area of the PCB as the service manual and a schematic of the resistor in question. I highlighted the resistor in yellow. It is resistor "R629". I can see that this resistor and the transistors next to it are involved in regulating the 5V current but I don't know where to go from there. Any help is appreciated.

author: Gyro
author_rank: Super Contributor
timestamp: April 22, 2025, 08:08:01 pm
content: It's a cheap phenolic PCB. They tend to discolor over time when components run hot - the resistor is stood off the board on its leads in an attempt to minimize this. If the TV is running normally, with no unusual smells, there is most likely no problem, apart from advancing years.Edit: The resistor is dissipating around 1W if Unreg+12V is within reasonable limits and the body doesn't show any signs of discoloration.

author: richnormand
author_rank: Unknown
timestamp: April 22, 2025, 09:05:20 pm
content: Not nice for the adjacent electrolytics for their lifetime depending on the orientation.

author: joel16
author_rank: Contributor
timestamp: April 23, 2025, 02:58:46 pm
content: I agree with Gyro, probably not an issue.You could measure the voltage across it when powered on and calculate the power with ohms law. If it is dissipating more than the rated 3W you know there is a problem.

author: Gyro
author_rank: Super Contributor
timestamp: April 23, 2025, 04:27:18 pm
content: Actually, you'd never get near 3 usable Watts dissipation out of a 3W resistor on a phenolic board in a reasonably still-air environment. The body and lead temperature would go too high and the PCB pads would come off (in fact, be careful, their adhesion might not be what it once was), Panasonic have sensibly de-rated to about 30% but that still means hot.Yes, the way to be sure would be to (carefully) measure the voltage drop across the resistor, it should be in the roughly 4.8V range given that the +12V rail is unregulated.

author: CRTguy23
author_rank: Contributor
timestamp: April 23, 2025, 05:08:01 pm
content: Quote from: Gyro on April 23, 2025, 04:27:18 pmHow would I safely measure the voltage across the resistor while the TV is running? Would I need an isolation transformer? Is this multimeter (the one that I own) sufficient to do this? (https://www.amazon.com/dp/B08DHHJPS1?ref=ppx_yo2ov_dt_b_fed_asin_title)

author: Gyro
author_rank: Super Contributor
timestamp: April 23, 2025, 05:32:31 pm
content: Measurements on CRT TVs can always be hazardous but given the proximity to the RCA jacks, you are in a low voltage area. The safe way to perform the measurement is to attach a couple of clip leads to the resistor legs with the TV turned off and disconnected from the mains, so that they can reach the DMM outside when assembled. The measurement can then be safely made without putting your probes and hands inside the operating TV. Ensure that the correct lead connections are made to the volts jacks on the DMM and the correct range is selected - If you accidentally used the current range in this case, you would effectively short the resistor and expose the downstream circuitry to Unreg+12V, probably with bad results.As I said previously, what you have looks like age related darkening around a hot component(s), so I would regard this test as 'optional'.P.S. You wouldn't need an isolation transformer because the battery powered DMM is floating, not mains ground referenced.

author: CRTguy23
author_rank: Contributor
timestamp: April 23, 2025, 06:40:59 pm
content: Quote from: Gyro on April 23, 2025, 05:32:31 pmIt might be age related but I don't think that's possible to say without doing further testing. The TV has 11,500 hours on as confirmed by the service menu and is on the lower end of hours for a CRT TV because their average life is about 30,000 hours. I've worked on a lot of CRT TV boards with a similar amount of hours that didn't have areas on the board that were blackened. In fact, most CRTs that I've worked on didn't have blackened areas on their boards. I would often see high temp areas of my CRT boards that had a medium to dark brown color but rarely black. It's also odd that the power supply section of this TV's board, which is a high temp area, doesn't have any blackened areas. It just seems unusual that the area of my TV's board got so hot that it blackened the board. It's near the front composite input of the TV and it doesn't seem like that area should get hot.

author: Gyro
author_rank: Super Contributor
timestamp: April 23, 2025, 06:51:10 pm
content: It depends what you mean by a 'power supply section. That resistor is a series dropper to reduce the dissipation of three 5V regulated supply rail pass transistors (Tuner, P.CON and Stereo 5V). I don't know what else Unreg+12V drives, but if it was too far out, I would expect them to be showing signs of stress too.Your TV though, it's a basic enough measurement for somebody who works on TVs to perform, I was assuming that you weren't experienced [Edit: with CRT TVs].

author: beatman
author_rank: Regular Contributor
timestamp: April 24, 2025, 02:24:05 pm
content: Show us the unter side of the pcb.I have seen a lot black spots like this on pcbs on dvd smps psus or AB class amplifiers.Excessive heat cause this.In your position i check the voltage on resistor like the others said i replace the around caps and resolder all solder joints under.Another idea is to add heatsink on the resistor body any peace of small (U) shape aluminium hold with high temp glue for electronics.

author: tom66
author_rank: Super Contributor
timestamp: April 26, 2025, 12:16:04 am
content: Most likely the resistor is used as a pre-dropper for the regulated rails, it drops the majority of the voltage from Unreg+12V and the small NPN transistors then regulate it. If it is dropping around 5V, giving the transistors 2V to work with, then a power dissipation of 1W sounds about right. In free air conditions you would expect such a resistor to heat up to about 70-80C, assuming 30C ambient. They are usually rated up to 3W with an operating temperature of 150C but this is usually only sustainable with ambient temperatures of up to 40C and not without reducing the lifespan of the resistor. I would agree if there is no defect with the TV to leave as is. In theory if the Unreg+12V were too high then the resistor would run hotter than usual but chances are this would cause other rails like B+ to increase too which would cause some noticeable artefacts on the image.

author: CRTguy23
author_rank: Contributor
timestamp: April 26, 2025, 03:04:46 am
content: UPDATE:I was working on the board and the leg of that big resistor (R629) was touching Q608 and when I powered on the TV, it fried Q608. Now the TV powers on for 1-2 seconds and I can hear it running but then it powers off. When the TV powers on, the screen does not appear to turn on. With Q608 having failed, does it make sense that the TV would show these symptoms? Could any other components have been taken out when Q608 failed? I'm concerned that the Micon IC, Vertical output IC and horizontal drive transistor may have been damaged. I can easily order a replacement for Q608 but hopefully it's not any worse than that.