TDA3567 NTSC Decoder question - Page 1
Forum section: electronics / beginners

author: mjrossit
author_rank: Newbie
timestamp: October 10, 2023, 04:25:22 pm
content: Hi Everyone,I'm trying to diagnose a Radio Shack 5" Portavision TV - Cat No. 16-125B. Can't find a schematic anywhere. It powers up and if you turn the screen setting up I get an all white picture. When I'm poking around I found that there is a 3.58Mhz crystal connected to pin 16 of a TDA3567 NTSC Decoder but I don't see any clock. It just floats at around 3.1 volts. The chip is getting supplied about 11v on pin 1. I do see a sandcastle pulse on pin 7 that looks like the datasheet.I replaced the 3.58Mhz crystal and it is connected to a 12pf capacitor and ground.Any thoughts on why I can't see any clock signal?https://www.alldatasheet.com/datasheet-pdf/pdf/19271/PHILIPS/TDA3567.htmlI attached a pinout of the chip

author: CaptDon
author_rank: Super Contributor
timestamp: October 10, 2023, 04:55:31 pm
content: First guess is that it isn't a 'clock' signal but is a phase locked reference to the color burst. 2nd guess is your scope probe kills the signal through capacitive loading.

author: jmelson
author_rank: Super Contributor
timestamp: October 10, 2023, 05:27:18 pm
content: is the set receiving a good-quality NTSC over-the-air broadcast signal? If not, it will not have a color burst to lock onto.Jon

author: mjrossit
author_rank: Newbie
timestamp: October 10, 2023, 05:42:05 pm
content: I just have an NTSC signal generator putting test patterns in through the composite input.Here is what the chip datasheet says:Oscillator and a.c.c. circuitThe 3,58 MHz reference oscillator operates at the subcarrier frequency. The crystal must be connected between pin 16and ground. The oscillator does not require adjustment due to the small spreads of the IC.-->The free running frequency of the oscillator can be checked by connecting the saturation control (pin 5) to the positive supply line. Then the loop isopened, so that the frequency can be measured.I tried this but apparently I don't know what I'm doing because I didn't get any read.The chip is new so I may be just chasing the wrong thing, but wanted to confirm.The oscillator has an internal gain limiting stage which controls the gainto unity, so that internal signals are sinusoidal. This prevents the generation of higher harmonics of the subcarrier signals.The burst signal is compared to a 0° reference signal by the burst amplitude detector and is then amplified and fed to apeak detector for a.c.c. and to a sample and hold circuit which drives the colour killer circuit. The reference signal for theburst phase detector is provided by the 90° phase shifted signal. An RC network is used to obtain the required catchingrange and noise immunity for the output voltage of the burst phase detector.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 11, 2023, 07:25:37 am
content: Do you have luminance signal going in on pin 8?You should have posted in the Repair section.

author: mjrossit
author_rank: Newbie
timestamp: October 11, 2023, 10:12:56 pm
content: The Pin 8 Luminance signal is floating a little above ground. I agree, this probably should move over to repair. I was thinking the original clock signal question would be something beginnerish.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 12, 2023, 10:39:37 am
content: Quote from: mjrossit on October 11, 2023, 10:12:56 pmEven if the oscilator and its PLL are U/S, if the luminance input and the sandcastle input are correct, there should be signals at the R, G and B outputs.

author: mjrossit
author_rank: Newbie
timestamp: October 12, 2023, 07:20:28 pm
content: I checked and I do get outputs on r g and b. While I was fiddling, I did notice a picture coming in but I really needed to fine tune the screen, focus and brightness settings to get anything worth looking at. Could this be an issue with the tube? Or something in the circuit? See picsBTW, this is with new Nichicon caps installed.

author: mjrossit
author_rank: Newbie
timestamp: October 13, 2023, 07:37:27 am
content: okay. a little more investigation and I found a 68000pf 250v film cap near the flyback with one leg that wasn't soldered in. Soldered it back in and the picture looks good as new. I'm guessing a prior botched repair job. I put clean new solder around the flyback pins just in case there were any solder cracks. I found one ground pin that looked suspicious.