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SERVICEMAN'S LOG Encounters with a notebook PC Servicing is not simply a matter of fault finding. Very often that’s the easy part; the hard part is finding a replace­ment for the faulty component or, more likely, improvising an adequate substitute. And then there was the Colonel and the General . . . This story started when a customer brought in an AST Ascen­ tia 800N 486SX33 Colour Notebook computer, plaintively complain­ing that, “it simply just stopped. And could you fix it ASAP?” And he wanted a free quote. In greater detail, the unit was about three years old and now out of warranty. Which is fair enough but there is still the worry about spare parts availability. And at a practical level I have worked on a few notebooks and learnt there is nothing cheap or easy about fixing them. I asked him whether it had been dropped or otherwise abused and he assured me it hadn’t. I switched it on in front of him and noticed that various LEDs were flickering and hard disc noises were emanating from inside, suggesting it was trying to boot. But there was no trace of any image. I wasn’t prepared to spend any more free time in diagnosing this, other than to simply guess that his colour LCD display was U/S and would probably be very expensive to replace. The customer wasn’t too happy with this but eventually agreed to pay for an in-depth cost estimate of repair­ ing the unit (say one hour’s worth at least, anyway). But I made it clear that this may still leave him where he was now. He said he would chance that. I set aside an hour that afternoon and my first step was to connect an external monitor to it. This was easy enough, using the outlet socket provided. But making the monitor function was another matter. All I got was a momentary flash on the screen and nothing more. 68  Silicon Chip Fortunately, he had brought in the operating manual. This nominated how to combine the Fn (function) and Esc keys to pres­ent the video setup menus; except that there was no way to dis­play these menus in order to find out how to display them (catch 22!). But the manual did indicate how to toggle between the internal LCD and an external monitor, using the Fn and F12 keys. I also noticed that this notebook had no conventional brightness and contrast controls. Changing these functions in­volved operating the Fn and arrow keys. I switched on and tried all these suggestions but, apart from the momentary flash on the external monitor, nothing was happening. However, I persevered until finally, after it had been booting for a few minutes, the Fn and F12 keys caused the exter­ nal monitor to flash on with a useful image. The reason why this hadn’t happened at switch-on, I sur­mise, was because the processor was initially fully engaged with booting up and was unable to accept commands from the keyboard. Now I could at least display the video setup menu for the LCD and the CRT display and check the computer itself, which now booted OK. At least I had now confirmed that the motherboard and the other basic hardware devices were working, narrowing the fault to the display. It was possible that the LCD driver stages in the video section were faulty but this was not very likely. And that left the LCD itself, the power supplies to it and, possibly, the brightness and contrast control circuits. Getting inside So now I had to get inside the device. The only way to do this is to disassemble the lid assembly which isn’t too easy. I had to find two vital screws and, to do so, prise off two con­ c ealed glued covers near the hinges. Next, one has to remove the clip-on hinge covers before – very carefully – unclipping a plastic mask around the edge of the upper top lid assembly. This all involves a high risk of marking the soft plastic and breaking the clips. However, once inside I could see a small PC board about 15 x 50mm which I quickly established as a minia­ture switchmode power supply for the backlight tube. By removing three more screws, the metalwork, plastic sleeving and the board could be removed. The board had two sockets on it, one at each end. One connected a 2-pin lead to the backlight and the other was a 4-pin input supplying power and data. More than that I could only guess at without acquiring a circuit and there was little hope of that at short notice. Examining the board, I noticed a small 1A “Pico” fuse (these are moulded devices, similar in appearance to a 0.5W resistor and soldered directly into the board). It was near the input socket, and the ohmmeter quickly confirmed that it was open. I worked out that pins 1 and 4 were the 12V battery input, pin 1 being common. Unfortunately, replacing the fuse caused it to blow immediately though there was no obvious short circuit. Most probably, this was the problem area and the easiest solution would be to replace this board. Mrs Serviceman was enlisted to track one down, which she enthusiastically proceeded to do. However, one week later and totally dispirited with broken promises of phone and fax backs, she finally established that this part was unavailable. The best offer was a complete display unit at $522.15, plus tax, plus freight plus six weeks delivery. I telephoned the customer with the bad news and his re­sponse was to ask whether I could actually repair the board. I pointed out that the multilayered board contained several ICs, all the components were surface mount­ ed, and I couldn’t identify many of them. And if the transformer had shorted turns, there would be no hope. However, I very hastily added, I might be able to fix it. My ego was exceeded only by my stupidity. “Look”, he said, “if you can fix it for $300, go ahead; otherwise you can have it for parts in lieu of service charges due so far”. What a challenge! Three-layer board With a three-layer PC board and about 20 SMDs (surface mounted devices), it was going to be very difficult to work out the circuit. And there were no visual clues to show where there was a short circuit. As a starting point, I decided to connect an ammeter across the blown 1A fuse and see what current was actually being drawn. This turned out, in a round about sort of way, to be the best thing I could have done. Before I could even change the range on the multimeter, smoke appeared from under a large 3-terminal active power device – probably an SCR. It was bent over parallel to the board and bending it upright revealed two surface mounted transistors and two surface mounted capacitors. And one of each of these devices was cooked. The overloaded transistor was marked R25A (only just vis­ible under the burnt case) and the capacitor had no markings at all. It looked like a ceramic. I could read no short circuit on either component, in circuit, and as the capacitor was connected to the transistor’s collector, I thought the best course was to remove the capacitor and see what happened. Unfortunately, in the process of desoldering it, the ca­pacitor disintegrated, leaving a black patch under- neath. I sol­dered another Pico fuse in and switched on – not expecting much progress. But I was delightfully surprised to see the screen light up and data appear. Delirious with happiness, I cleaned up the black spot, reassembled everything and put it aside to test. Everything continued to work OK until I switched it off at the end of the day and noticed that the screen was still alight, although there was no image. There was no time left to do anything about it except disconnect the battery. I thought about it overnight and concluded that the burnt transistor must be damaged and would need to be replaced. The next day I measured the transistor again, in circuit, on the x1 ohmmeter range and it read OK (it turned out to be a PNP transis­tor). them but this didn’t help with a 1994 4-digit identifier. My educated guess was that it might be equivalent to a Toshiba 2SA1204 using an X12B case but in any event where would I get one of these? A little lateral thinking led to a scrapped Marantz audio cassette player which used SMDs, and for which I had an excellent service manual. From this, I spent some time looking for the most powerful device used in the power supply circuits with the same case package. Having identified the most likely one, I transplanted it into the power supply and reconnected everything. This time everything worked perfectly. All that remained was to run Scan­disk and other utilities to clean up the hard disk. Both the customer and I were happy with the outcome. The ceramic capacitor would probably have had a value of anywhere up to .001µF but because there was no room over the burnt area, I couldn’t fit a replacement. I was not able to locate the actual cause of the problem as the capacitor had disintegrated on remov­al. Precision walking However, when I removed it from the board and checked it on the x10,000 range, it measured quite leaky. I was now faced with the problem of finding a replacement and I couldn’t find any mention of the R25A in any of my equival­ents books or software. Surface mounted components are not normally considered serviceable and only manufacturers keep specifications. As they have been around for approximately 10 years, the standards for the alphanumeric characters print­ ed on them have changed. I found an early Sharp VR service manual with a section on SMDs using only 2-digit alphanumerics to identify My next story is long way from notebooks. It involves a regular lady customer; kindly, energetic and euphemistically described as “stocky”. But with six offspring to control, she doesn’t take any nonsense. How she and the 52cm TV set she was carrying both fitted through the door was an exercise in precision walking – there was barely a 1mm clearance on either side. She plonked the set on the counter, informing us that it was dead. She also added that if it turned out that one of the kids had done it, she wanted to know. This sort of lady commands respect, if you know what I mean! The set was an Akai CT2007A and was made in China. It was not very old but obviously was rarely, if ever, switched off. The remote control wasn’t supplied with the set but I wasn’t going to ask any questions about that. The circuit of this set is similar to so many different brands and models December 1997  69 Serviceman’s Log – continued that I had a pretty good idea of where to look first. It was no real surprise to find that C917 (100µF) on the main HT line (115V) was about to expire and that R918 (0.68Ω) on the 18V rail was open circuit. I also automatically replaced two 47µF electros (C909 & C911) in the switchmode power supply before going for 12V zener diode ZD401 which was shorted. It also took out the 1A Pico fuse (F401) supplying it. I felt fairly sure I had everything right before switching it on – but nothing happened. A voltage check cleared the 115V rail but the 18V rail was low. At that moment I didn’t put too much significance on this, which was a mistake. The most obvious symptom was the failure of relay RLY901 to activate. This switch­ es the set on and off and is driven by transistor Q905. Q905 is in turn driven by Q621, then by Q605, and this is fed from pin 15 of the CPU (IC801). I suspected some sort of control problem from this CPU. By using the ohmmeter on the x1 range, with the black lead as active, I could bias Q605 on and the set fired up, giving a good picture but no sound. This last observation was the break- through. There was loss of sound, a low 18V rail and now another indicator: R922 was overheating. This feeds Q905 and then pin 2 of IC201, the TDA1904 sound output IC. All of which threw suspicion on this IC. Sure enough, replacing the TDA1904 not only allowed the set to switch on correctly but also restored the sound. I was pleased to timidly report to the customer that the kids probably hadn’t done anything wrong – except perhaps watch too much TV! The Colonel’s General When Colonel Jones came into the shop, mumbling about something wrong with the General in the back of his car, there was some confusion at first. But I quickly realised that he was referring to a TV set rather than to his military superior. Some models stand out among the early colour TV sets sold in Australia and General was one. General made its reputation with cheap, reliable sets that performed well. And the Colonel’s 1980 GC161, a 42cm portable, is one of which I am quite fond. The Colonel’s General was quite dead but the Colonel as­sured me that it would sometimes come on. The first fiddly bit with all these portables is removing and replacing the back, as the telescopic aerials always get in the way. There is also the problem of aligning the chassis with the rear and front shells and the front control knobs. Because their reliability has kept them in the field for so long, most of my colleagues have acquired considerable skill and experience in dealing with them. They know just where to go to find the most common faults. And the Colonel’s General provided an opportunity to recall some of these. The first line of attack is to solder the dry joints on the motherboard, particularly along the edge connectors of the vari­ous modules and on the horizontal drive transformer T602, pin cushion transformer T603, and the horizontal linearity coil L608. Then the modules themselves need reworking, especially the power supply. In this instance, none of this fixed the problem but when I measured the three power supply voltage rails, I found that the 15V rail was down to less than 10V. Replacing C642 (47µF 25V) fixed the problem and restored the sound and picture. This capacitor can also cause lack of height, no colour and a dark picture with low sound, depending on what stage of failure it has reached. If the power supply pulsates, the culprit is invariably the X807 (CV12B) over­ voltage protector and one would be advised to replace all the electros in Fig.1: the power supply circuit for the Akai CT2007A. Relay RLY901 is towards top right and is driven by transistor Q905 (top righthand corner). This in turn is driven by transis­tors Q621 and Q605 at top left, with Q605’s base fed from pin 15 of CPU IC601 (not shown here). 70  Silicon Chip the power supply, espe­ cially C802 (10µF). Retrace lines and an excessively bright picture are due to R418 going high or the screen potentiometer itself (VR406). No picture or a very dark picture can be R419 going high. The clas­sic fault for sets near the beaches is failure of the 22MΩ focus control (VR201), which sometimes sounds like a machine gun due to internal sparking. If the horizontal output transformer fails, it really means the end of the set’s life because it is too expensive. The picture tube rarely fails and most are still good 17 years later. The only thing left with the Colonel’s General – I must stop saying that – was the UHF tuner, which was seized. To fix this, I removed the two knobs and the circlip, then using pliers, cutters and CRC 2-26, carefully removed the plastic sleeve con­ trol shaft and cleaned and lubricated it before refitting. There is no need to refit the circlip, as the tuning knob will keep the whole thing in place. A little judicial greyscale setting completed the repair and it was back in service and returned to the front line with a happy Colonel Jones. A puzzling Toshiba And finally, a rather puzzling story about a Toshiba 259X7M 52cm TV set. This set has an unusual power supply, which is designed to adapt itself automatically to the supply voltage; approximately 240V for Europe and Australasia, or 110V for Japan and the Americas. More exactly, it looks like a 110V circuit, modified to 240V by using an additional module – U801 Power-2 Board PW6004. But that is only general background. The complaint was straightforward enough; it was dead and blowing the mains fuse. Fuse F801 was open, as was expected, but there was more to it than that. More to the point, I hate blown fuses. Whenever I encounter one, my natural reaction is to ask why. What caused it to blow? Is the fault still present? And, if not, is it intermit­tent? And so on. And I found a lot of “whys” in this case. Capacitor C835 was short circuit, as were transistors Q801 and Q802. And C816 and C447 also needed replacing. That was all that was obvious but there could still be more subtle faults elsewhere and one needs to proceed carefully in such cases. In place of fuse F801 (3.15A), I substituted a 200W 240V globe and switched on. The globe lit up very brightly, implying that there was still a major short. I began by disconnecting various circuits, starting with the 145V rail via plug M801 and fuse F802. This produced no change and it still glowed after I removed the degaussing coils. But was it still as bright? I couldn’t be sure and I was thrown off the scent further by the globe intermittently dimming and brightening after a few minutes. I could not find any explanation for this. With the 200W globe still in circuit, I noticed that there were now slight signs of a raster or picture on the screen, which implied that the 145V and 15V rails were probably OK. After checking the bridge rectifier (D831-D834) for shorts, I decided to risk trying another fuse in F801. At switch-on, the sound and picture were completely re­stored. So far, so good but the degaussing coils were still unconnected. I reconnected them and – splat! – the fuse blew again. There isn’t much that can go wrong with degaussing coils but the thermistor network that’s used to control the degaussing cycle can give trouble. Basically, this network consists of two major components; a positive temperature coefficient thermistor in series with the coils and a negative temperature unit in parallel with the coils. This arrangement may use two separate thermistors or, more com­ monly these days, a single package containing both devices. In this case, there was a single package designated as a PTC/PTH dual posistor (R890). Because it was the number one suspect, I reefed it out, noted that something rattled inside and tossed it. I fitted a new one and switched on. The fuse remained intact and after testing it for a few days, I pronounced it reliable enough to go back to the customer. But with so many faulty parts involved, the logical question is which failed first? I can’t answer that; your guess is as good as mine. All I know is this: I still hate blown fuses. One can never be sure what has blown them and it can take a lot of effort trying to find out, not always successfully. SC December 1997  71