Trying to repair Samsung CRT TV CW21Z453N - does not seem to "boot" - Page 1
Forum section: electronics / repair

author: Pentium100
author_rank: Frequent Contributor
timestamp: May 12, 2025, 02:32:59 pm
content: HiI have this TV and it started showing signs of bad capacitors - there were horizontal color lines on the screen and also the picture became unstable (as if there was a bad connection, hitting the TV would make the pircture a bit more stable for a while).So, I took it apart, found 5 bad capacitors, replaced them and now I have a new problem - no image and no sound. The TV is based on a TDA12025PQ chip (I could not find its datasheet) and it seems that the chip works at least partially.Here's what I know so far:1. I have checked and rechecked the capacitors I replaced, they are correct and soldered in the correct polarity.2. All power supply voltages seem to be OK, all voltages supplied by the flyback, also seem to be OK.3. I hear horizontal and vertical deflection working and I also hear EHT when I turn the set on or off.4. The set responds to remote control (turn on, turn off etc).5. CRT heater works, I can see it glowing, also its voltage is correct.6. I connected a VCR to the TV, when I start or stop playing a tape I hear that the deflection sound changes a bit, so I think the TV can sync to the signal.7. There is no sound on the speakers (I tried pressing mute several times and also increasing the volume), but the TV outputs the sound signal on its SCART sockets.8. However, when I turn the TV on it produces a startup chime though the speakers, so the audio amp has to work.9. The chip produces a small pulse on its R, G and B outputs about once every vertical sync period. This is probably the lines I saw earlier - I guess they are normally supposed to be outside the visible area, but a bad cap made them come down. They seem to be "black current calibration".10. I can see those pulses on the cathodes of the tube.11. When turning on, the LED blinks a few times then turns off. The LED is on when the TV is off it also blinks when receiving a signal from the remote.It seems to me that everything is working, but, well, it is not working. I remember that when I would turn on the TV (when it worked), it was dark for a while until the tube warmed up, then the image and sound appeared at the same time. So, I'm thinking that the chip is waiting for something before it shows the picture and enables the sound. The problem is that I do not know what the chip is waiting for.The signals look OK, the only one that deviates from what the graph on the circuit diagram is the H-SYNC signal on the chip pin 72. It should have a flat top, but in reality it has a narrow peak on top of the "flat top". There is a 12K resistor in series with the chip pin - the signal looks good on the other end of the resistor.Could anyone help me troubleshoot and fix this TV? It's probably something obvious, but I have been stuck with this problem for a few days now and can't figure out how to fix it.

author: Pentium100
author_rank: Frequent Contributor
timestamp: May 14, 2025, 09:43:37 am
content: I figured it out - G2 voltage was too low, so the chip thought that the tube was still warming up. Increasing G2 fixed the problem. There was also a bad connection somewhere because the image would still "wobble", but I fixed that by resoldring a bunch of solder joints in the horizontal circuit.