No sound. It took a fall a few years back, I remember having opened it to see if I could find the fault, expecting something like a cracked PCB. I remember it was difficult to work on, but I can't even remember what the symptom was, I thought it was dead, but when I took it out, the only problem was that there is no sound.
Description
This is a very small (5.5 ") black & white TV, very handy to have sometimes when there is something you just don't want to miss and colour and quality does not matter.
As you can see in this picture, there are a lot of wires connected from the rear part of the case, and from the CRT to the board, all of which does not plug out, making it quite difficult to work on the TV, I was afraid the neck of the tube could get damaged as it keeps wanting to fall over backwards. Note the tennis ball holding the neck seperate from the board. I don't play tennis, the ball is one of a pack that was bought for the dog to play with - this one, unused so far - is now part of my toolkit!
The audio amplifier is a D7368GS, for which I found the data sheet of an equivalent, YD7368. I also found another alternative, TA7368P. It is a 9-PIN SIP. What complicates debugging the audio is that the TV has RCA video and audio inputs at the back, with a switch next to them to switch between auxiliary and normal TV. I also tried to look up the main TV chip, a D2015CP for which I could not find a datasheet after long hours of searching. I found it mentioned in a quiestion on a forum in some unidentified language, and in one of the answers was mentioned AN5151.
The on/off switch is a 3-way switch marked Radio-Off-TV, thus the audio also passes through this switch. There are two wires running across the board from near the On/Off switch, to the AV switch at the back, marked A1 (Yellow) and A2 (Red). I decided to check for audio here with my oscilloscope . I thought I saw audio, then tested at the volume control and also found it, then at the amplifier IC but could not find anything. Then when I checked on the two wire again, it just looked like noise. The baffled me for quite a while. I thought maybe the TV chip is faulty and maybe does not give out audio when it gets hot. I looked at pin 11 on it (according to the AN5151 datasheet) and found the same kind of noise.
I switched the TV off to let it cool down and tried again. I found exactly the same kind of noise. At some point I decided to try to tune the TV better to the signal. I was getting audio again! I then remembered when you tune an analog TV you also use the sound to help you. Since this is black and white, there is a wide range in which the picture looks equally good, but the audio is quite sensitive. I found that using the oscilloscope, I was able to tune to that I could see audio. I also tried it on radio and was definitely able to tune to a station using only the oscilloscope.
Interestingly, I also noticed a TDA7231 on the board. I looked up the datasheet - another audio power amp! I could not understand this, as the audio connections I have traced does not go through this chip. I checked the datasheet to see where it's output is, tested there with the scope and found it looks like a vertical deflection sawtooth! Using the vertical hold pot I confirmed that this chips has something to do with the vertical deflection.
After using the oscilloscope to look for audio, I also came on the idea to use powered PC speakers to see if I have audio on the input of the audio amplifier. I connected it with using test leads with crocodile clips and could hear audio. I decided I will look for an old pair of powered speakers to keep in my workshop permanently for this purpose.
Solution
Replaced the D7368GS with a TA7368P which was very cheap.
The TA7368P soldered in place. Note that I did not push the pins all the way in, it helps a bit with heat dissipation, hopefully it will last longer than the previous chip.
