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SERVICEMAN'S LOG Variety – the spice of life? It has been an assorted month with lots of minor faults. They ranged from a dead garage door controller to a whis­tling TV set, a crook notebook computer and a couple of trouble­ some VCRs – including one that bounced. My first story concerns a Blaupunkt stereo TV set with a dead remote control. The main drama with most remote controls is opening them without breaking or damaging them. Most are clipped together but which way? Is the lower half of the case on the inside or vice versa? And are the clips on the outside or the inside? Having opened this one, I confirmed a flat battery and cleaned away the obvious corrosion due to the coffee (or was it lemonade?) which had been spilt over the unit and which had leaked onto the PC board. After that, I quickly tracked the problem down to a fracture on the crystal leg. While I was on a winning streak, I tackled a Sharp remote control which wouldn’t select programs 1-14, although the rest were OK. I suspected that part of the multilayered PC board had corroded but when I finally opened it, I found it was remarkably clean. It was only by chance that, under a strong light, I caught sight of a fine 50mm-long crack! This surprised me, as I was sure that no-one else had been inside this unit since it was first manufactured some eight years ago. What’s more, there was no indication of any damage to the outside, yet there it was; a 50mm crack cutting off the return path to the IC from these buttons. Not only that, but the fracture was not from one edge to the other but in the middle of the board. I can only surmise some stress had been applied to it during manufacture and it had main­tained continuity until just recently. Anyway, the repair was simple – a little solder over the cracked track soon had the unit going again. Garage door controller My next job was a garage door opener. The unit in question was a B&D Controll-A-Door, and the LED on the remote control transmitter wasn’t even lighting. Access to the inside was via just one screw but unlike the last remote, it was very dirty inside. I brushed out the dust and cleaned the board with metho and a toothbrush. When I tried it again, the LED was just begin­ning to glow intermittently. 28  Silicon Chip I first suspected poor contacts on a plug that’s fitted to set the frequency but they proved to be OK. The problem with small assemblies like these is holding them steady while you attach two meter probes and push a button simultaneously. First, I determined there was continuity from the 9V battery to the circuit board, via the switch. I then checked the board and crystal for dry joints and fractures. I tried freezing the parts and noticed that the intensity of the LED varied but without pinpointing the cause. There was an old style .0068pF styroseal capacitor (a type which has problems with internal connections) so I replaced that but to no avail. Similarly, I made sure that there was no dirt or shorts between the vanes of the preset tuning capacitor. When in doubt, it always a good idea to check the supply rail to various parts of the circuit. To do this, I soldered a pigtail to the negative side of the PC board 9V battery connec­tion and followed the trail with the positive probe. It didn’t take long to find the culprit, there being a substantial voltage drop across the microswitch. A few squirts of contact cleaner between the cracks of the switch shell restored its function completely but I would be a lot happier if I could find a re­placement. I checked that it was actually transmitting by listening for noise in a shortwave radio and it seemed OK. Most similar remote controllers can be checked like this but it is better to use one of the remote checkers available on the market. However, many of these won’t prove that the correct waveform or frequency is being produced. They only prove that the unit is transmitting. Sanyo colour TV set My next customer brought in a Sanyo SS Plus CPP6012-00 TV set (A3A20 chassis series). He described the fault as what amounted to intermittent line tearing and rolling but only on some channels and when the set was cold. I began to worry when he told me that another company had tried to fix it but had given up and suggested he bring it to me. I asked him straight out who this obliging company was and made a mental note to return the favour. However, they did tell him what a “good chap” I was and so, with my ego suitably stroked, I took on the challenge. I began the job by phoning the other service company to find out what they had actually tried, so as not to repeat the exercise and waste time, effort and money. Their general consensus was that the problem was due to the AGC circuits crushing the sync pulses. A large scale integrated IC (IC101, LA7680) which contains all the IF and RF stages (including the AGC) had already been changed, so it appeared that a peripheral component was to blame. I removed the covers and connected the set to a colour bar generator. I first fed the signal in as RF and then via the AV sockets at the rear. When the AV input was selected, the picture was sharp and clear. Conversely, when the RF input was selected, the fault was very pronounced when cold but improved when hot. As the temperature seemed critical, I tried freezing and heating and noticed that the fault varied quite dramatically when I was close to Q110. This transistor is an NPN power regulator that supplies 9V to IC101 and to the front end. I measured the rail to find it a little low at 8.8V but not unduly so. A common problem with many manufacturers is that rail vol­ tages are not always accurately shown on the circuit. The circuit – if you are lucky enough to have one – is really only a guide. Anyway, I exhausted my investigations around the AGC cir­cuit and on Sanyo’s advice replaced D801 (1N­ 4148), which comes off the IF AGC line, as well as C115 (0.47µF). This made no difference and so I came back to the 9V rail. This rail starts from pin 15 of the switchmode power supply chopper transformer as B5 (15V) on the cathode of D554. It is then switched by Q554 (2SB764) from the remote microprocessor CPU (IC701) and Q792, Q552 and D562, before going through IC551 to become B6, a 12V rail. This 12V is then applied to the collector of Q110 which produces 9V, courtesy of R110 and D110. June 1998  29 Serviceman’s Log – continued All this was working as expected and there was no unusual noise on the rails themselves, as shown by the CRO. I replaced electrolytic capacitor C117 just in case but was still going nowhere. My next step was to connect an isolated variable power supply to the emitter of Q110 and chassis. When I varied the level, the symptoms were quite obvious below 9V but the set be­haved perfectly at this voltage and above. The ridiculous thing here is that we are only talking about 0.2V but it was enough to make the difference. Since there are only about five components in this simple circuit, I removed and checked each one but could not fault any of them. Finally, I replaced them one at a time with new ones and only when I replaced D110, a 10V zener diode, did the fault clear. Now it might appear that this is what I should have done as soon as I suspected the 9V rail but the spooky thing is that when I tested it across a current limited power supply, the zener worked out exactly on 10V, 30  Silicon Chip as indeed did a new one. So I put it down to a dodgy zener. I don’t have the luxury of unlimited time to make exhaustive tests as to why that part didn’t work properly when cold – it was enough to track it down. Anyway, it fixed the problem and the set was still working when the owner called to pick it up a week later. Both he and I were on cloud nine. Resurrecting Sam The next customer was a lady who complained that her Sam­sung Winner VB306 VCR was dead. And she was right. The mains fuse hadn’t actually failed but to all intents and purposes nothing was working. Fortunately for me, I have a service manual for this model, which was a great help. I decided to start with the power supply but quickly dis­covered that it is very difficult to access the circuit anywhere in this machine to measure voltages. In the end, I removed the switchmode power supply, removed its covers and soldered pigtails to each of the secondary voltage rails, before plugging it back into the motherboard. I soon discovered that the 5V rail was down to 3.5V. By replacing a 470µF capacitor (C3), the 5V was restored and the machine began to operate. However, the panel display was still non-existent. I delved back into the power supply to find that the 5.8V rail which goes to the filament of the fluorescent display was down to only 1V. Replacing a 100µF capacitor (C38) fixed that problem but since two of the electrolytics had failed, I decided to replace all the secondary electro­ lytics to improve reliability. The lady agreed and is now happily reunited with her Winner. The notebook computer For a change of scene, the next customer produced a little notebook computer. It was an early KTX 386SX16 monochrome LCD notebook and not much was happening that gave anyone any con­ fidence. The green power LED would come on and the other LEDs would flicker but there was no display. Worse still, there were thumping noises from inside, probably from the hard disc drive. In short, it rather seemed as though it was “cactus”. Still, I volunteered to look at it, especially as I own the very same model. I proposed to compare them and swap parts if neces­sary, to find the cause of the problem. The weakest part of this early notebook is the 12V battery pack. These never last long due to problems of charging and discharging them properly. In this case, the battery was missing so I dug out my old KTX and connected it to his AC power adaptor so that I could see what was supposed to happen on boot-up, with regard to LEDs lighting, etc. To my surprise, my KTX machine misbehaved in exactly the same manner as his. Initially, I suspected that my computer had also given up the ghost. Then it slowly dawned on me that per­haps I was looking at this the wrong way round – it might not be the computer that was at fault but the power supply instead. I couldn’t wait to dig out my power adaptor and plug it into the customer’s notebook. Bingo! – it booted up perfectly. The AC adaptor supplied with the KTX notebook uses a gener­ic “Go Forward” GS-30 A-18 switchmode power supply, designed to deliver 18V at up to 2A. I rigged up a dummy load of 20Ω and measured the voltage across the primary input filter capacitor following the bridge rectifier as 340V, which is correct. On the secondary side of the circuit, however, there was only 9V. A quick inspection revealed a number of electrolytics on the output voltage rail that were beginning to leak. I replaced both 1000µF 35V capacitors with equivalents but this didn’t fix the problem. I then replaced a 100µF 63V unit which restored the voltage completely. I then reworked the soldering and reassembled everything before returning it to the owner with a bill that was much less than expected. and the voltage varies, according to the light, on pins 42 and 43 of IC601 on the central processing unit. These light sensitive devices are notorious for being intermittent and so I fitted two new ones. Interestingly, the replacement PN268 looks like a clear LED while the original is dark violet or black. The two pigtails are designated emitter and collector and not anode and cathode respectively. These devices can be tested on an ohmmeter, on the 10x range (or greater), with the red probe (battery negative) on the long pigtail and the black probe on the short one (ie, the opposite polarity to a LED, which also conducts on the x1 range). The last thing to check is that the chassis return for the mode motor and the sensor is good. This involves tightening the screw which connects the lead to the chassis. It was now all systems go each and every time. The bouncing VCR The next VCR I had to deal with was Mr Young’s Panasonic NV-J11A, which had bounced back with a vengeance. It had come in a few weeks earlier, the customer complaining that it gave a rolling pattern with intermittent noise bars. What was not no­ticed by Mr Young at that time was that there was no counter – or rather, I should say that the counter didn’t count. The cause turned out to be dirt on the ACE head which pre­vented control pulses from reaching the tracking circuits and the real time clock. Cleaning fixed that problem. Complex assembly The next beast The next beast to repair was another VCR, this time an overseas JVC HRD211EM (Middle East Multi System version). This customer’s problem was that the video was intermittently not accepting his tapes properly. First, I checked the 5V and 12V rails from the IC regulator (STK5481) and looked for any lurking brown goo on the PC boards. So far, everything was OK. The cassette housing is controlled by light sensors (PN268R-NC) This time, the complaint was slightly different; it rolled only at the beginning of a tape. Inspection confirmed that the counter was working but the rolling problem was intermittent as described. With the covers off, I could see that the tape was intermittently spilling due to poor take-up torque. A new Play Arm (type VXL1861) underneath the reel pulley fixed this but I noticed that there was too much noise on reverse search and the back tension arm was not being cleared from touch­ing the tape. A slight touch with a pair of long nose pliers bent the lever closer to the plastic lever underneath, so that it pulled away further on review mode. I was beginning to think I was jinxed on this repair when I discovered that, though the noise was much reduced, it was still excessive in the review mode. When I unsoldered the heads (VEH0532) and measured their Q factor (VEH0532), all three read zero out of six on the video head tester. This meant that the heads were far too worn and it was amazing that they were still giving any sort of a picture (new ones read typically five out of six and anything better than three for an old machine is good). I conveyed the bad news to the customer and understandably, it didn’t go down too well. However, he agreed to let me fit new heads. This went off without any hitches and everything worked perfectly afterwards. However, I did have to explain to the now incredulous Mr Young that the first few seconds of recording on any machine have a slight colour patterning, as the tape has to go from the bulk erase head to the ACE head before it is norma­lised. Fig.1: this diagram gives some idea of the compact nature of Orion 10/VR combination VCR/TV set. The VCR “block” sits in the bottom of the cabinet, with the TV “block” above it. The next repair was an Orion 10/VR VCR and TV set combina­tion which was dead. This repair would have been quite routine had it not been for the complex way in which the set was assem­bled. The unit had been dropped and, as a result, the tracking control was now missing and one of the feet was broken on the outside. The major difficulty was coping with the VCR and TV assemblies on the inside; there literally was no access to eith­er. It wasn’t so bad removing both the TV and VCR together from the front June 1998  31 Serviceman’s Log – continued ble the set – you should have seen the fun and games I had getting the loose tracking control sub-assembly back into the front of the case. I then put it aside to soak test while it waited for the PRF-1600-F003 IC protector to arrive. The set was still working a week later but the main thing I learnt from the exercise was to steer clear of this model unless you have plenty of time and patience. Whistling Panasonic shell. The real problem comes after that because the TV motherboard is held in a metal frame above the VCR (which is in a metal box underneath) and both are connected together by a dozen or so leads. The next drama I found from the service manual was that the TV set and VCR had separate mains transformers and bridge recti­ f iers but shared some of the regulators and secondary power supply rails. It was all very tortured and maze like. After a lot of trouble, I discovered that a 12V rail de­rived from the VCR wasn’t reaching the TV set on plug CP7004. This was eventually traced to a fuse F7002 3.15AT in the VCR transformer secondary (T7001). However, there was still no 11.5V on TP501 in the TV section, which was still dead. Gaining access to the TV circuitry meant unplugging it from the VCR and unscrewing it from the metal frame, after which I was able to perform some continuity tests along the PC board. Eventu­ally, I found a 3.9Ω 0.5W fusible resistor (ICP501) that 32  Silicon Chip had gone open circuit. This resistor was wired in series with a 4.2Ω 18W resistor (R501) and connected in parallel with power regulator Q503. I was rather puzzled by this arrangement on two counts. First, connecting a 0.5W resistor in series with one rated at 18W seems rather strange. Second, the fusible resistor was in fact marked in the service manual as a PRF-1600-F003 IC protector, whatever that was (my guess is that this is a circuit protector chip fuse rated at 1.6A but it doesn’t appear in any of my gener­ic parts catalogs). As a temporary measure, I decided to fit another 3.9Ω 0.5W resistor while I waited for the correct replacement part to ar­ rive. Unfortunately, that didn’t cure the problem – the TV set was still dead. It didn’t take long to spot the remaining problem, however. Now that I had the whole TV section in my hot little hands, I could see that the drop had fractured the solder connection to the horizontal output transformer. I repaired this and the set came good. It took quite some time to reassem- My last repair concerns a Panasonic TC-68A61 68cm stereo TV set which came in with a loud whistling vibration from the rear. The owner had finally decided he just couldn’t take it any more. With the rear cover removed, it was obvious that the noise was coming directly from the deflection yoke assembly on the neck of the tube. The question was, what to do about it. A new yoke (TLY15912F) would probably be rather expensive and time-consuming to fit. The “singing” noise was caused by the copper wire vibrating against the ferrite core and the only way to stop this is by securing the winding and the ferrite together. I decided that the cheapest and most effective way was to use superglue and additional rubber wedges. I gingerly emptied the contents of five tubes of glue so that it flowed down the wire and hopefully onto the ferrite. As it happened, this was quite successful and the first application reduced the noise by 75%. The remainder was cured by adding thinner wedges to the ones already fitted (which align the purity and dynamic convergence of the yoke). The aim here was to push the wire harder onto the ferrite former without disturbing the settings. I was relieved when this worked as I didn’t fancy removing the yoke, sealing it and then realigning everything. No-one can be as accurate as the manufacturer in installing modern TV yokes, many of which are actually cemented to the tube. Unfortunately, the cure proved to be short-lived. After about a week of soak testing, the glue broke down under the heat and the whistling returned as bad as ever. So it looks like the repair will be expensive – that’s if the owner decides to go ahead with it. Oh well, you win some, you lose SC some!