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SERVICEMAN'S LOG Symptoms don’t seem to help any more Is it my imagination or is it becoming more & more difficult to relate the symptoms to the fault? Time was when most symptoms gave at least a hint as to the area involved. But in both this month’s stories, they were no help at all. This is the third story in the Contec saga and must be regarded as weirder that most, if only because even the customer’s complaint was weird. And, as I’ve already hinted, the symptoms gave no clues at all. As usual, it was the motel proprietor himself who fronted up to the counter, lugging the offending set with him. And in answer to my query as to the nature of the problem, he replied (somewhat facetiously I now realise), “It’s the remote control system – it only half switches the set off”. 40  Silicon Chip My private reaction was, “Don’t be silly – how can you half switch a set off? It’s either on or it’s off”. However, when I spoke to the customer, I was a little more circumspect: “How do you mean?” He grinned. “Turn it on and I’ll show you”. So we hooked the set up to a power point, connected an anten­na and turned it on. And up came a first class picture. I flicked through all the channels, tried the various controls, and everything seemed fine. So it was over to the customer to explain further. He produced the remote control unit and pressed the OFF button. And I had to admit the his description of what happened wasn’t all that silly after all. There was still a picture on the screen but there was a total loss of colour. There was also some loss of brightness and a noticeable shrinkage in picture size, amounting to about 12mm on all four sides. Pressing the ON command on the remote control unit brought everything back to normal. There is no standby control on the set itself, as with some sets; only a normal ON/OFF switch directly in the mains circuit. And of course, this functioned normally. Well, I had no quick answer to that one. So the set was left with me to tackle when I felt a little stronger. Power supply checks I went first to the power supply and to the HT rail coming from it – see Fig.1. This is derived from pins 5 and 6 of T502, the switch mode transformer, via diode D516, 100µF capacitor C523, and series transistor Q507. It goes out on pin 1 of plug/socket C. This rail is shown as 114.9V – a rather unnecessary order of precision, I felt, but at least we did have a figure. Checking this when the set was operating normally confirmed that it was very close to this value. I then tried to turn the set off via the remote control. This should have reduced the HT rail to zero but it didn’t. That in itself didn’t surprise me but I did imagine that it might drop by some amount. But no; it actually rose very slightly, by about 1.5V. The amount was not significant in itself but the fact that it was the opposite of what one would expect made the whole situation that much more puzzling. With no obvious clues, I decided to trace out the remote control path and check for possible faults in this section. It wasn’t a particularly dif- -31V TO PIN 27 OF IC 801 5V 4 F 3 F 1 F 12V 2 F 1 E 2 E 8 1 D510 C514 47 7 6 240V D511 3 4 2 5 R519 1k IC 510 IC502 330 C515 470 0.1 Q506 T501 Q505 T502 Fig.1: the Contec MSVR-5383 power supply again. The on/ off control signals from pin 27 of IC801 come in on pin 1 of socket F & go to the base of transistor Q509. This transistor then controls Q508 & Q507. When pin 27 goes high, Q507 turns on & delivers the HT voltage to pin 1 of socket C. Conversely, when pin 27 goes low, Q507 turns off & removes the HT rail. ficult exercise. Infrared pulses from the remote control unit are picked up and processed by the in­frared receiver, IC601. The resultant signals come out on pin 2 of this IC and go to pin 35 of IC801, the central processing unit. This, in turn, toggles pin 27 between 4.5V and 0V at each press of the ON/ OFF button on the remote control to switch the HT rail on and off respectively. In fact, this is the only function that this signal performs. Jumping ahead a little, the actual switching is performed by series transistor Q507. This, in turn, is controlled by tran­sistors Q508 and Q509. When the voltage from pin 27 of the CPU goes to +4.5V, Q509 turns on and so Q508 & Q507 also turn on to provide the HT rail. At the same time, Q505 & Q506 also turn on to provide the 12V rail. Conversely, when the voltage on pin 27 of the CPU goes low, Q509, Q508, Q507, Q505 & Q506 all switch off and the HT and +12V rails are removed. At least, that is what should happen. In fact, there was no change of voltage Q507 114.9V C 5 6 D516 C C523 C 1 2 3 Q508 Q509 at the base of Q507 as the on/off function was initiated. I was able to trace the signal to the collector of Q509 and from there to the base of Q508 but no further. There was no change at the collector of Q508 and, of course, none at the base of Q507. So it looked as though either Q508 or Q507 was faulty. I pulled Q508 out and checked it and it appeared to be OK. I then did the same with Q507 and found that it appeared to be a dead short. But I must confess that, as a result of subsequent think­ing, I did wonder whether the fault was more subtle than that. But more on that later. The immediate requirement was for a replacement transistor. I didn’t have the Q807 type (a 2SC3310) or any of its direct equivalents in stock but I finally settled for a 2SD841 which has slightly higher ratings. This, I felt, should be a suitable substitute. And that was the answer. It wouldn’t have surprised me if there had been more than one fault, considering the various symptoms, but the set came good immediately, and responded quite positively to the remote control signals. So, at a practical level, that was the end of the exercise. Puzzling symptoms But I couldn’t help wondering why the set behaved as it did. Why the loss of colour, the reduced brightness and the picture shrinkage? And why the slight increase in HT voltage, in response to the remote control signal? And the truth is, I’m still wondering. I spent a lot of time going over the circuit – and the set itself, before I re­ turned it – analysing the circuit and making measurements in an effort to rationalise its behaviour. I didn’t make much progress. At one stage, I wondered whether the slight increase in HT voltage was due to some subtle aspect of the transistor failure but I was eventually forced to discard that theory. I am now convinced that Q807 was simply a dead short and was not able to respond to the signal from the CPU in any way. The observed symptoms were due to some other mechanism and had nothing to do with the voltage on pin 27. As I mentioned earlier, the line from January 1995  41 SERVICEMAN’S LOG – CTD pin 27 of the CPU goes only to the base of Q509. Granted, there is another function derived from this. The 12V rail, derived from the 50Hz transform­ er T501, is turned on and off from the HT rail by means of tran­sistors Q505 and Q506. Unfortunately, as far as any explanation goes, it doesn’t help; the 12V rail supplies only the audio IC, IC904. And that apparently leaves only one possible explanation; that the signal coming out on pin 27 performs a number of switch­ing functions within the CPU or comes out on some other pin(s) and controls other external functions. A prime suspect here would be IC701, which handles most of the video and colour processing. Building on this theory is the idea that, if some functions were turned off (colour processing, for example), then there could be a reduction in HT current drain. This could be suffi­cient to cause the extra 1.5V or so which I observed. And the picture shrinkage? Again, this could be tied to the higher HT rail voltage. I had my doubts about this idea initial­ly; doubts that such a small change in HT voltage could have such an obvious affect on picture size. So I pulled a swifty. The HT rail voltage is set by a regulator network within the power supply and, more specifically, by potentiometer VR501. So, with a crosshatch pattern on the screen, I fiddled VR501 to produce an increase of around 1.5V. And the result was a degree of shrinkage very close to that which accompanied the fault. So there it is; these are the best theories I can advance. If anyone is closer to this circuit and can offer a more detailed explanation, then be my guest. That’s enough from me for this month. I’ll now pass you over to my colleague, J. L., from latitude 42 degrees south, or there­abouts. This is how he describes his latest weirdo. Just a stereo amplifier Now here’s one you are going to find hard to believe! Most stereo amplifiers are just that – stereo amplifiers. But not when it’s a Sony STR-AV1070X. 42  Silicon Chip The 1070X is an integrated AM-FM stereo receiver, with no less than six speaker outlets, left and right for front, centre and back! And for inputs, it can accept two phono players, two cassette tape decks, a CD player, a digital audio tape deck, an external equaliser, and three video decks! The entire system can be remote controlled with a “Remote Commander”, featuring buttons to work the radio and amplifier, as well as TV and VCR, CD player and DAT recorder! It can operate Sony equipment with builtin codes and can also learn the codes of other brands if one should be so crass as to connect them to such an elaborate unit. The remote control carries two small slide switches and 80 buttons. Yes, that’s right, 80 function buttons! And its all packed on a handpiece just 200mm long, 70mm wide, and 20mm thick. That’s hardly larger than an ordinary TV/VCR controller! Pardon the exclamation marks, but this amplifier has every­thing!! My customer spent half a day removing all the input and output connections, then staggered into my workshop with the monster cradled in both arms – it weights something like 20kg – and gingerly placed it on the bench. I had been alerted to his approach with what was supposed to be “... just a stereo amplifier”. But I expected nothing like this and I very nearly asked him to take it somewhere else. Later, when I had heard a list of all his problems, I was even more convinced that if he wouldn’t take it elsewhere, then I would. It appeared that he had an intermittent problem. Sometimes when he tried to adjust the volume with the remote control, the process would start normally but then the controller would apparently lose contact with the set and the volume would continue to increase up to maximum. And since this amplifier has an output rating of 250W + 250W into 4-ohms, his neighbours were thinking of taking up a petition. But there was more. At the same time as the volume control took off on its own, the front panel display would go crazy, indicating wrong functions or no functions at all. Similarly, there were times when nothing at all would work and the set had to be switched off at the mains to restore normal operation. On the basis of his description of the symptoms, I suspect­ed that the entire system was microprocessor controlled and that it was this section that was causing the problems. And since it was intermittent, with the system working normally for much of the time, I guessed the fault was going to involve dry joints or something like that. The owner brought along a 44-page user manual for the unit and although it gave lots of detail about operating the set, there was nothing at all of a technical nature. Fortunately, I am on good terms with the local Sony specialists so next day I visited their workshop and was able to consult their service manual. A real eye-opener This was a real eye-opener. It included several 6-page foldout schematic diagrams, as well as minutely detailed PCB diagrams. The “circuit board location” diagram reproduced here – see Fig.2 – gives some idea of the complexity of this amplifier. There are no less than 18 separate PC boards (and I can’t even find one labelled “tuner board”!). Honestly, I’ve seen video cassette recorders with less complexity than this stereo amplifier. And a colour TV set is a snack after this thing! A quick glance through the manual convinced me that I was going to have to study the circuitry long and hard before I could ever come to grips with it. So I made arrangements to borrow the manual for the weekend and was about to leave for home when their audio technician entered the work- shop. After exchanging greetings and other pleasantries, I asked him if he had ever had to work on an AV1070X. “Not often”, was his reply, “but what’s wrong with yours?” I started to explain that the volume control ran up to full and he continued “... when you use the remote commander, and the displays go crazy and the set has to be turned off to reset everything?” It was almost word for word as my customer had explained it. Quite obviously, it was a fault that had been around for a while. The 1070X is not a common stereo amplifier, so January 1995  43 TRANSFORMER-3 BOARD (1070X : E MODEL) SPEAKER OUT BOARD VOLTAGE SELECTOR BOARD (1070X) : E MODEL POWER BOARD TRANSFORMER-1 BOARD (970X0) TRANSFORMER-2 BOARD (1070X) REAR BOARD SURROUND SPEAKER BOARD HEADPHONE BOARD VIDEO BOARD SURROUND BOARD DISPLAY BOARD MAIN BOARD POWER IC BOARD EQUALISER BOARD MUTE BOARD RELAY BOARD BALANCE VR BOARD VOLUME BOARD Fig.2: this “Circuit Boards Location” diagram for the Sony STR-AV1070X audio/video amplifier gives some indication of the physi­cal complexity of the assembly. The main board, identified on the right, is underneath everything else. it’s not surprising that only the Sony specialists knew about the fault. What is surprising is that the fault should be so common as to be instantly recognisable, even to a specialist. Washers & wire For all that, my colleague’s next statement was probably the most unexpected thing I’ve ever heard. He said, “all you need to cure the problem is five spring washers and a piece of wire!” He went to a drawer and took out an A4-sized plastic bag containing, as far as I could see, one sheet of paper. The paper carried a Sony heading and outlined a list of symptoms, just like those my customer had reported. The document gave no indication as to what causes the problem but it proceeded to give details of where and how to fit five spring washers and a short length of wire. I asked if I could have a photocopy of the paper and was told I could have that one. Then I started to remove it from the plastic bag, only to be told that I would “... need that because the washers are special!” It was only then that I noticed the five tiny internal star washers in one corner of the bag. And on the other side of the paper was a 150mm length of wire to 44  Silicon Chip complete what must be the simplest modification kit I’ve ever seen. I thanked my friend for the information and the kit, then returned to my workshop to see if I could complete the job before lunch. The five spring washers were to be fitted under the heads of two of the self-tapping screws that secured the main circuit board to the chassis and between the circuit board and the chas­ sis where it was secured by three more self-tapping screws. The length of wire was to link an earth track on the front display panel to the main earth track near the microprocessor. That sounds easy enough but when I pulled the cover off the chassis, my heart sank. One end of the frame was occupied by an enormous power transformer and the rest of it was filled with circuit boards and massive finned heatsinks. The main board, the one secured by the aforementioned screws, was at the bottom of all that lot and virtually inaccessible. There is no point in trying to tell exactly how I did the job. Suffice to say that I had to remove three of the power transistors and their associated heatsink to get at two of the screws. The other three were almost as difficult, since the washers had to go between the board and the chassis – pushed into position through a small hole in the side panel of the chassis. Lining them up was a nightmare but the job was eventually done and the screws retightened. I then had to remove the front panel to get at the track that was to be connected to one end of the length of wire. The points to be joined by the wire were only 30-40mm apart and I wondered why Sony had packed a 150mm length of wire in the modification kit. I soon found out! With the wire attached to the front panel and the panel replaced on the chassis, there was absolutely no way I could reach the wire to pull it through to the main board. I had to remove the front panel, thread the wire through a gap some dis­tance from the actual earth points, and then replace everything and pull the wire up to reach the second point. I could then cut off the surplus and make the joint. Fitting three of the spring washers and the wire link had taken me something over an hour. I then had to test the thing! Fortunately for my neighbours, I don’t have six large loud­speakers, so I couldn’t do a dynamic test. However, the owner had suggested that the intermittent front panel display always accom­ panied the volume problem and, since the volume control was motor driven and one could see if it was operating correctly, I was happy enough to sit there pressing the remote control buttons and watching for any signs of distress. No more malfunctions Fortunately, there were no malfunctions and the customer has reported no problems in the three months since he took the monster home again. So I’d say it’s cured. But what a tale of woe about something as supposedly simple as a stereo amplifier! I still don’t know exactly what was wrong with the unit but it was obviously something to do with ineffec­tive chassis connections for the circuitry associated with the microprocessor. And what an awkward, fiddly process it proved to be to fit such a simple modification kit. Thanks J. L. Your story only confirms my impression that the game is getting harder all the time. And, of course, the hills are getting steeper, the print’s getting smaller, and everySC body’s mumbling!