CRT Troubleshooting - Page 1
Forum section: electronics / repair

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 12:44:07 pm
content: HelloI have an old Philips GR1AX CRT TV that used to work normally. I was probing around the CVBS data path (wanted to solder RCA connector). At one point, I moved my probe a tiny bit and the CRT shut down, with only the power LED staying on.If I created a short as I suspect, it would be between the point that I marked with a yellow arrow (CVBS output of the TDA8305A) and ground.Now, when I turn it on I hear a very very brief HV hum that stops completely after a second, contrary to the constant hum that I had before.No light on the screen.The anode cap does discharge, but makes a very quiet pop compared to before.While probing the the wires going from the flyback to the neck board, the voltage rises very very briefly when powering on but falls to zero. The voltage at the 160V capacitor (2445) starts at 100+ V and slowly discharges into my multimeter. Whether power is on or not, this point will discharge and never charge again, until the next power on. It is marked in a red circle.The power LED stays on continuously.TLDR: I get power only for a very short amount of time when I plug it in and then it goes offAny help is appreciatedSchematic:https://imgur.com/a/wV36ck6

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 01:26:38 pm
content: That looks like, you have shorted the output stage of the IF IC TDA8305 and now a protection function disables the PSU.It gets his power (+12V) at pin 7 and i suspect the rest of the chassis should work without the IF. So you can try to disconnect pin 7 for testing purposes

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 01:35:54 pm
content: Caution! That tv set is not galvanic isolated from mains, so your retrofit video input can be life hazardous.

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 04:27:19 pm
content: Disconnected pin 7. Now the CRT doesn't make a HV hum at all. The 12V going to the now disconnected pin 7, climb up to 3V momentarily and then fall to 0. Could it be that the flyback is fried?

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 04:29:48 pm
content: Thanks for bringing this to my attention, I even got zapped touching a ground wire while sitting on the floor

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 05:07:48 pm
content: There are five 12V rails all derived from horizontal output transformer. 4 with (safety) resistor and one of them with an additional fuse 160mA.Diode D6542 (BYD33D) is rectifier for all.Also check the +95V horizontal supply, if it also disappears after a short time, the main switch mode supply is in shut down

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 05:28:06 pm
content: Turns out I accidentally open-circuited part of my 12V rail while desoldering that pin 7. With that fixed and the pin 7 still disconnected, the short hum returned, though nothing has changed. I am going to test the rest of the rails now as you suggested and post my findings. Also I cant thank you enough for your time and patience, it means a lot

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 06:14:45 pm
content: 95V rail goes up to 90 and discharges to 0, 12v rail(s) goes up to to 6 and discharges to 0.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 06:16:33 pm
content: Oh shit - have overseen, that the IF is only a tiny part of the TDA8305. The complete sync generator for H & V and Audio IF, Audio Preamp, X-ray protect and other stuff is integrated. So the horizontal stage won't work without.I see only a repair chance in replacing it.

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 06:22:18 pm
content: So I suppose that the shut down mode is enabled somewhere outside of the TDA8305?, since I have the same result whether it gets power or not. I am asking to understand if the flyback works normally, but it is just that the system sees the absence of the 3805 and tells itself to shutdown, or if the problem is with the flyback itself

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 06:35:38 pm
content: Not necessarily. Depends on, what the short has internally destroyed. If the whole circuit isn't working any longer, there will be no horizontal drive. So the H-deflection and of course transformers secondary voltages are not presentEdit:Found an absolute reliable source:Henninger Systemservice (we got most tv related spareparts from, and thats 40yrs ago!)https://www.ebay.de/itm/300938748251?

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 06:47:28 pm
content: Ok got it. I am going to buy a new TDA8305A and see what happens. Thank you, I learned a lot of things today

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 06:56:04 pm
content: p.S.: the short hum at startup is the sound from demagnetizing coil wrapped aound the CRT. Starts full with fast decreasing to zero

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 07:01:57 pm
content: Until now I thought it was the deflection coil. Thank you for the info

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 30, 2025, 07:22:49 pm
content: You may also consider in purchasing a 230:230V isolating transformer as a life insurance at measuring in mains powered equipment. Often one can fetch up such parts from automation tech as recycling good for cheap. Bought one additional this year, 635VA as new and paid 28€ + shipping

author: mariosgr
author_rank: Newbie
timestamp: September 30, 2025, 07:35:27 pm
content: You're right, I will. Not becoming toasted is worth way more than 28 euros