ANIKO TV-328 Repair

Fault

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.

Compaq V75 Monitor signal cable spliced

Not really a piece of electronic repair work, but it was necessary to read up and work precisely.

The problem with the monitor was that one of the pins in the 15-pin D-Sub connector came loose and moved back into the connector. At first, it was possible to pull the pin out again with a long nose pliers, sometimes wiggling the cable would make the monitor work again. Finally the pin was totally gone and the monitor would not work any more.

I had an old cable from a different make and model monitor. Even if I would be able to buy the correct new cable (not being a Compaq agent), it would probably have been to expensive since the monitor is old. The header plugs, plugging into the PC board inside the monitor, was of course different between the two monitors. Thus I had to cut off that end of both the cables and splice the Compaq header plug onto the other cable.

First I looked up some references on the internet to find the standard SVGA 15-pin plug pinout. I was worried about the monitor ID pins, as they did not seem all present on both cables. After Googling a bit more I found the following here: Q:-I give up: How does a Sony (FX-140) notebook recognize the presence of an external (VGA/SVGA) monitor?...Any hints? Diodes between pins...resistors...holy water? A:- Monitor ID pins haven't been used for a LONG time.
The ID solution used today is a VESA standard called the Display Data Channel (DDC), which is based on the I2C electrical interface.

So I printed one of the Pinout Tables I found on the internet, and on the same piece of paper started making a list of the wires in each cable, by testing them out with a multimeter on diode test, so it will beep on continuity. I used a short cable with crocodile clips and a bent-open staple to connect to the "header" connector, using the free probe on the 15-pin D-sub.


After re-checking each cable again, I took the final step and cut both cables. In the case of the "new" cable, the ferrite core around it used to be inside the monitor, only secured by two cable ties. Remember to slip it over the long cable before you start splicing! As can be seen, a small bench vice (even if not mounted!) makes it a lot easier than holding everything with your hands. I started with the easy ones, e.g. the red, green and blue plus their returns. It is important to try to keep these returns seperate, I think it might not prevent your monitor from working, but it might influence the display quality. As I was joining the wires, I slipped a short piece of small heat-shring sleaving over each wire. Of course a bigger heat-shrink was already slipped over the entire cable.

At this point I tested the monitor with an old computer, making sure the short pieces of sleeving were all in place. The next day I took the cable with me, borrowed a heat gun and shrunk the sleeves. The "new" cable actually had 3 thin coax cables inside it for the RGB. Thus the sleevings could not entirely cover their returns. I used short pieces of insulation tape on them as well. Then I slipped over the big sleeving and shrunk it as well.


This monitor had a bit of an unusual molding where the calbe enters the monitor. I used a knife (carefully!) to cut it open from the outside corner. Thus I was able to insert the new cable into it.


After fitting this extra little cover over it, it does not matter that this corner moulding is cut open.


The ferrite core was now move close to the monitor and again secured with two cable ties. I does not look as smart as a moulded-in one, but being on the back of the monitor it does not bother anyone.

Job done!

I think it is important for the environment to keep things like computer monitors working for as long as possible. When you finally throw it away, it gets destroyed, some of its parts may pollute the earth, you go to the store and buy a new one, a factory somewhere has to make another one to replace the stock. A lot of pollution takes place making the new one. You might say yes, an LCD monitor is not so expensive now, but hopefully the you still have a long life ahead of you to enjoy LCD monitors. Just try to keep he old one a year or two longer, OK?

SANSUI SV2916 TV Repaired

As I have said in my About page, I did not do any electronic repairs in a long while so it took me a while to fix this TV. It started out with a smaller fault, I tried something which caused the TV to have no picture. Then also it took me much longer than necessary, because of my stubbornness - my mentor suggested I should try something, but I said no, I want to find the fault first.The original problem was that the picture became too wide for the screen, losing some 20% of the picture I'd say. I opened the TV and found the H-Size pot, VR350. It did not make any difference to the picture, neither did VR351 (East-West correction). So I got a schematic. I thought from there on it should be easy.

I do not have an oscilloscope, so I was trying to find the fault by looking at voltages at certain points and trying to see if it agreed with my understanding of how the circuit works. To follow my discussion, get the diagram as SV2916-P1 and SV2916-P2. (Or look right at the bottom)

Obviously I started looking at the circuits around Q350, Q352 and Q353. On Q353 my multimeter showed 0 V on all three terminals. I unsoldered the transistor and tested it, it was OK. In the process of unsoldering it, I broke a PCB track and had to use a short piece of wire to repair it. Now I wasn't sure if everything was OK where I soldered back the transistor . Still 0 V on all three terminals.

I thought OK, if this transistor is 0 V anyway, let me try to isolate this part of the circuit and concentrate on it. I decided the horisontal deflection coil is driven by the LOPT-driver Q402 from the bottom, the top goes via L401 to the collector of Q353 via R365 with a low resistance of 3.9 ohm. I found a jumper between L401 and R365, unsoldered the side going to R365 and earthed it. Swiched on. No picture on the TV.

OK, undid the earthing and soldered back the jumper. Switched on. Still no picture. In addition to having no picture, I now had a new problem: When switching the mains on, the standby LED was on. I could use the RCU or Ch+ button on the front to switch it on, but not off again. So I was in for a completely new round of fault finding.

The so-called "video jungle" chip is a TDA8842, so I started looking at the datasheet of that chip. Somebody told me that the CPU, IC001 (M37221EASP) could be waiting for something after I told it to swich on. I studied it and could not find any inputs to it, except for Hsync an Vsync, which it needs to sychronize the OSD information. I thought it might be possible that the CPU were waiting for these syncs and refused to do anything further until they arrived.

I kept on looking in the region where I grounded the circuit and found something: C410 (0.022 uF, 400V) turned into a short circuit. I was lucky to borrow one quickly from a monitor awaiting repair work. Soldered it in and hoped to get a picture. Still, no picture, turn on with RCU but not off again. The TV also has A/V-out, so I connected this CVBS output to another monitor and found that the set was actually receiving the channel it was last tuned to. The RCU also would not bring any OSD on this monitor.

By then I realized that the CPU reads certain status bits via I2C from the TDA8842, so I had to look for inputs on the TDA8842. The ones that I really considered as possibilities were EHT-PROTECT (50) and BEAMCURR VGAURD (22). If you trace it out on the circuit diagram, you will find they come from the circuit at Q404 and Q403. I saw that the emitter of Q404 goes via resistors to the bottom of the secondary winding of the flyback transformer. With the base of Q404 on ground, you would need a negative voltage for it to ever turn on. I was reading 0 V on R430 (100 ohm, numbered differently on the board), so I suspected something wrong here. I told my mentor I am reading 78V on the screen voltage, but I suspected the bottom diode in the flyback to be blown. He said no, he did not the that diode could be blown and 78V is a bit low for the screen voltage, suggesting I use Tippex to mark the position of the screen adjustment on the flyback transformer. No, said I, I don't want to go adjusting, I must have blown something else than C410 and wanted to find the fault first.

Fortunately I was able to borrow an oscilloscope at this point and also my mentor told me to make sure, since I could feel static on the screen so we expected EHT to be present, that I get RGB through right to the CRT board. I told him that I already saw 3 V on each of the three going into the CRT board, I could not see any variation. The oscilloscope confirmed: no RGB coming out of the TDA8842. Just to be sure, I also checked for HSync and VSync on the CPU - they were present.

Now I wanted to follow up my theory that a diode was blown in the flyback transformer. I read that people use two multimeters in series to test it, but I only have one. I read that somebody else said, use a DC supply and put the multimeter on volts. I tried this and was able to read 0.9 V when the supply was in the direction to forward bias the diodes in the flyback transformer. But I wasn't very sure, since I was testing it in circuit, I was not sure if the current was maybe going via the resistors in the flyback.

To take this measurement, I of course had to remove the EHT lead from the tube, for which I very safely used resistors tied to a screwdriver to discharge it first. But the excercise almost led to catastrophe! I used long nose pliers to put it back again, but when I switched on, it was not properly hooked up, fell out and started arcing on the aquadag! I cannot stress it enough: Be very carefull, if you need to remove the EHT lead, make sure you know how to safely discharge it, when putting it back, make very sure it has clipped in properly. At least, now I was VERY sure I had EHT!

I now decided to inject a negative voltage on the emitter of Q404. I used the 24V DC supply again, tying it's + to the ground, a 10k pot across it and it's wiper to the junction of R403 and R404, first having made sure of course that the pot is turned on the + side, so it would not pull towards negative. I then started turning it, checking the voltage going negative and also on pin 50, the EHT protect of the TDA8842, which I think initially was over 3 V, pulling it to 1.6 volts or so. But to no avail. No RGB coming out. I started looking for any more inputs on the TDA8842 and found pin 18: BLACK CURRENT IN. Now it dawned upon me. The TDA8842 wants to see a little bit of current passing through the tube before it will switch on the RGB. I measure 0V coming out of the CRT board to this point.

So now I though - maybe the guy is right. I have EHT over the tube. The RGB amps should be biased normally, so if there is no current, why? Could be the screen voltage. So I found Tippex, marked it and started turning clock-wise while watching in the mirror. At first I started seeing a blueish glow on the screen - there is life! Then I started seeing flyback lines with something of a moving picture. I realised I turned it to quickly - it is a fine adjustment. I was then able to fine tune it, so the picture brightness was normall. Now the picture was actually too narrow. I tried the H-SIZE and EW pots. They worked now!

I measured the screen voltage again and found 79V, just 1 V higher! With the picture back now, I noticed that the moment I touch the meter on the grid, the picture dies. Only now did I notice that the moment I put the meter on, there is momentarily a higher reading, I thing I saw 150 V and 125 V, it is a bit difficult to see using a digital meter. Finally my Tippex marking showed a difference of maybe less than a millimeter!

My final conclusion was that C410 was busy developing a problem, causing the picture to go too wide, and it blew completely when I grounded part of the circuit.

I used the H-SISE to adjust the picture just to almost reach the sides. I then used the EW pot to make the sides exactly straight vertical lines. Then the HSIZE again to hide the slight unevenness you get there.

My idea was to take several oscillograms for future reference, but it was late in the evening and after more than 20 days I just wanted the bulky TV off my workbench. So I only took 3, here they are:

All the oscillograms were done with a 10x probe and 0V DC was vertically in the centre.

R420

Any of the R, G or B out of the TDA8842, on R202, R203 or R204.

D201 Cathode.

I used the opportunity to fix something else that I have been putting off for a long while. The set has an AV2 input on the side which was broken. The S-video connector was completely loose, so I resoldered it. I also noticed that it had a hole were it was intended to be mounted with a screw, which the manufacturer chose not to do. It was a bit difficult to mark the hole on the case side, but I did it, drilled a hole and fixed it from outside with a small self-tapping screw.

In this picture you can see the S-video connector has a hole where it was supposed to have been mounted with a screw.

While working on this TV, I noticed something that made me not very happy with Sansui. I picked up several short bits of wire, ends of component leads that have been cut off during manufacture. I took several pictures of the board, finding it easier to look for certain components sitting at the computer than standing over the board. I think I removed about 10 in total. I only noticed later there are about four in this picture! Look around the bottom. I don't know how many of them have remained inside the TV.