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It's a long way to trip a ''rarery'' Sorry about that heading but it was the only way I felt I could sum up this month's main story. The fault was a real "rarery"; so rare as to be possibly unique. And tripping it did involve a long path, mainly through a maze of circuit diagrams and board patterns. The story concerns an AW AThorn video tape recorder, model AV-14. This model first appeared about 4 or 5 years ago and carried a 4-year warranty - a warranty that still covered this particular recorder. According to the owner the machine would usually perform when first switched on, after which it might run for half an hour or more and then shut itself down just as if it had been switched off. And if it was then left in that situation, it could just as suddenly come good. On the other hand, it would also sometimes run indefinitely without any problems at all. When the customer first delivered it and described these symptoms, I set it up while he was there and gave it a trial run. Neither of us was particularly surprised when it worked perfectly; according to the customer, that was exactly what he had expected. Hopefully, a longer run would display the fault. I had no manual for this machine, although I had some for earlier and later models, so I rather hoped that the fault would turn out to be fairly obvious. But it refused to show up. I tested the machine many times over the next few days, running it for long periods under varying temperature conditions, but it wouldn't fail. Finally, I rang the customer, explained the situation, and suggested he take it back until the fault became more predictable. But he said he was about to leave on several weeks' holiday and that I might as well hang on to it and keep trying. 0 ACCOROlt\\G i'O 1'\-\£ OWNE.-1<, ,He: N\OCMINE: WOUL-t) USUAU...'-< P6-RF'ORIV' WHe;N l='"IRS1 SWr,C.HE:O ONooo 40 SILICON CHIP So for the next week or so I did just that, all to no avail. Somewhat discouraged , and facing the pressure of other work, I was forced to push it to one side. Time slipped by and the next thing I knew the customer was on the phone, announcing his return from holidays and enquiring about the recorder. Feeling a bit guilty, I told him I had been unable to fault it so far but suggested that he give me a few more days. He agreed quite readily. So I set it up again, removing the main cover this time in the hope that this might provide access to an appropriate check point if the fault showed up. This didn't help much. The main printed board sits pattern side up above the drum and transport system and lifting it out to get at the component side is quite an involved procedure. So, for the moment, I simply switched the machine on and let it play, as I had already done umpteen times. But this time I was lucky; it ran for about an hour then suddenly shut itself down. So now it was time to lift the board out of the chassis. There are several minor boards at the front of the machine: a board holding the timer controls, one holding the tracking controls, one holding the function buttons, and one holding the on/off and timer switches. These boards are all connected to the main board by flat multi-conductor cables and must be removed from the chassis (by unlocking the pressure tabs) before the main board can be lifted. The same routine applies in order to perform a head and drum cleaning operation. It's a real pain in the neck! Anyway, having lifted the main board I was able to identify some components in the power supply section, where I suspected the fault might be. Then, making sure it was - V +30V C"'79 '""" ; IN-.,C?tX' 1sse2 AE C T OUT iSAl/1.S<at>,® :SAl/750,®,© TV12V } PCB-MAIN (TUNER/VJF) IQ903! 2SC2603<at>.® Lh E9 0907 2SC2603<at> jPOWERI ' !_ _ _ _ __ _ - - - - -- --- - --- -- - ----- - - -- -- ---- - BF BO CTEJ !' PCB.TIMER - - - - ---- - - --- - - - - - -- - -- - - - - - - - - - - - - -- ---- - ' -------~ Fig.1: the circuit of the AWA AV-14 VCR power supply, where the first clue to the fault was observed. It generates no less than four supply rails, two of which failed ( + 12V and + 9V) when the fault occurred. safe to run the recorder in its partially disembowelled state, I switched it on again. Again it came on normally but this time it ran for only about 10 minutes before it failed. In fact, its behaviour became fairly predictable from here on. Voltage checks While it was running normally I made some voltage checks around two transistors and one IC which I had nominated as part of the power supply. I was hoping that when it failed, further voltage checks would provide a clue. Unfortunately, these results were inconclusive, with no drastic changes which might have helped. By now it was obvious that this was not a simple fault which was going to show up in a routine voltage check, particularly without the benefit of a manual. I was going to need all the help I could get. I placed an order for the manual $25 worth, incidentally, but that's cheap as VCR manuals go. The manual arrived quite promptly and I turned immediately to the power supply circuit (on the reverse side of a sheet marked 2/3). It was something of a shock to realise that, whereas I had identified two transistors and one IC, this section contained no less that nine transistors and two ICs. It is reproduced here as Fig.1. The power supply generates no less than four supply rails. Q901 is supplied with - 38.8V at its collector (derived via transformer T971 , diode D901 and filter capacitor C902) and delivers - 30V at its emitter. It did this, even in the fault condition. Similarly, Q902 is supplied with + 45V via diode D905 and filter capacitor C907, and delivers + 30V at its collector. Once again, this value was correct in the fault condition. Which brings us to Q904. This is supplied with + 19V from the bridge rectifier (D908-D911) and is supposed to deliver + 14V at its emitter. And this was the first clue to the fault, because there was no + 14V output in the fault condition. And as I half expected, there was no + 14.4V on the base. Normally, I would have tried to track down the missing base voltage as the next step but I was sidetracked. Q904 is mounted near the edge of the main board and I found that if I exerted pressure on this part of the boa rd, I could sometimes create or cure the fault. This effect was not consistent but it seemed likely that Q904 was faulty. So I pulled it out. It tested OK but that meant very little, so I replaced it. All of which was a furphy; the fault remained. By now, incidentally, the fault was much more in evidence and would usually appear within a few minutes of switch-on. So back to the circuit. The + 14.4V for Q904's base comes from the collector of Q908, the emitter of which is fed from the aforementioned + 19V rail. The + 14.4V rail is also fed to zener SEPTEMBER 1989 41 diode network D917 & D918, which in turn supplies + 12.5V to the base of Q905. The collector of Q905 is fed from the + 19V rail and it delivers + 12V at its emitter. This + 12V rail is fed to the tuner and video IF circuitry and also feeds voltage regulator IC902 which provides a + 9V rail. So absence of the + 14.4V rail shuts down two supply rails: + 12V and + 9V. And if you found all that hard to follow I can assure you that it was even harder to work out in the first place. But the reason for the missing + 14.4V was still a mystery, except that Q908 obviously had to be turned on in order to create it. And equally obviously, Q907 had to be turned on in order to turn on Q908. The base of Q907 is shown as + 0.6V and this voltage was present in the no-fault condition but vanished in the fault condition. Well, we seemed to be making some progress. But it wasn't quite as clear cut as this. When the system failed . the + 0.6V on Q907 didn't simply drop to zero; it dithered its way down and the system continued to function down to around + 0.4V. But at + 0.38V everything collapsed. And in between these two values all sorts of funny things happened to the + 14.4V; it couldn't make up its mind whether to come or go. The next stage down the line was Q906. Unfortunately, there are no voltages given for this stage which meant that I had to go through the routine of measuring them when the system was working and then again when it failed. From this, I established that there was supposed to be + 0.6V on Q906's base but that this also collapsed with the fault. This left only R909 (390k0) and l!>'< NOW ,.,.. w~s o~v,oos w~s 42 -n4~-r "ffi\S NO"r F\. S\N\?l-E. FAUL--,••• SILICON CHIP diode D913 as possible suspects in this area but they were quickly cleared. So the upshot of all this effort was simply to establish that there was nothing wrong with the power supply; it wasn't generating the necessary rail voltages because it wasn't receiving the correct voltage from somewhere else. It was a negative kind of achievement in some ways, although it did mean progress. But little did I realise what kind of a search lay ahead of me. Circuit tracing The first thing to do was to trace the run from the base of Q906. This part of the circuit is on a 6-section foldout sheet portraying, in addition to the power supply, the servo and the mechanism control sections. After getting lost a couple of times, I traced the run from Q906 to pin 20 of the microprocessor, IC5AO, a 64-pin chip at the other end of the sheet. Pin 20 was marked as being at 4.8V and this proved to be correct in the no-fault condition. However, it collapsed in the fault condition so I was still making progress. But where to from here? A faulty microprocessor chip? It could be but I didn't think so and, with 64 pins involved, I didn't want to think so. The alternative was that the microprocessor was not being fed with the correct instruction on its input side. The problem was to identify this circuit. The circuit identifies the pins only by a system of alphanumeric codes and there is no list of these in the manual. I was on my own. I decided on a different approach. Assuming that the missing voltage was an instruction to the power supply to activate the various supply rails, it was most likely to originate at a switch probably the on/off switch. So I decided to take a punt and start at the on/off switch and trace back from there. Does this sound simple? It was anything but. This switch is mounted on one of the sub-boards already mentioned (PCB-SW) and its circuit is on another fold-out sheet marked 3/3. According to this the two active contacts of the on/off switch were connected to pins 6 and 7 of connector F2. And pin 6 was marked "STBY" (standby) and " - 5V". Pin 7 was marked simply "PSW". So was this the voltage that was fed to the microprocessor, to eventually appear as a signal on its pin 20? The minus sign was confusing, because I was looking for a positive voltage. In fact, that sign was a furphy; the voltage was positive and the minus sign on the circuit was simply a draughting error. I began tracing. The mate to F2 was found near the microprocessor, back on sheet 2/3 where I'd started, which looked promising but wasn't. The connection to pin 7 left this board immediately via one of the flat cables (connection 22). After much page shuffling I picked up connection 2 2 again on the back of foldout sheet 1/3, which portrays the timer board. From here it ran to pin 16 of the timer chip, ICBAO, and then the length of this sheet, through another switch (S8A2, RESV) to pin 17 (RES) of the timer chip, ICBAO. So I had two inputs to the timer chip and no idea where the vital signal voltage would reappear. In fact, the circuit involving switch S8A2 turned out to be a red herring and I never did work out its function. I was grasping at straws now. I went over the timer IC diagram and checked each pin designation (there are 64) until I came to pin 36. It was marked "PWV" and "4.9V". Both rang a bell. The 4.9V was close enough to the 5V I was chasing and PWV was the designation of the lead to the base of Q906, where I started. I followed it. It left the timer board at connection 24 and turned up again at connection 24 on the back of sheet 2/3 where I'd started. From here it ran straight to pin 41 of the microprocessor chip, IC5AO. This was the "input" terminal I had been seeking since I had determined that the output, on pin 20, was failing when the whole system failed. So that was the complete path; at least on paper - literally. It was a place to start but that was all. I had PCB-SW [ill[IT) Fig.2: this section of the circuit shows how the control voltage (PSW) is applied to the main board via F7 and then routed away from the board (to the timer board) via cable connection 22. As shown, it is deceptively simple. The physical path is actually much longer and far more complex. identified the major intermedia te points but the physical path between these points still had to be traced out. This turned out to be a most laborious task; so much so that it is virtually impossible to describe it in any way which would do it justice. Suffice it to say that it took a long time. And a major problem was the need to wait for "go" and "no-go" states in order to establish just where the circuit was failing. Voltage checks I decided to start at the switch and the + 5V supplied to it. If the voltage failed at this point I would have to back track to find its origin. If it didn't I would have to trace forward, initially via the points I had already identified. The situation was quickly clarified; the voltage held on both sides of the switch in the no-go state. So far , so good. The next check point was at pin 7 of connector F on the main board. After some waiting for the no-go state, the system was cleared to this point. From here on it was a different story. As already mentioned, this circuit is shown leaving the main board again (reference point 22) without making any other connections. Fig.2 shows the relevant section of the circuit and this indicates a short run into the board and out again. In fact, point 22 is much further away physically than the circuit implies, while the copper pattern connecting it to connector F is much longer again. More importantly, a check at point 22 indicated that the voltage was failing here when the system failed. So the fault was somewhere along this circuitous copper track. But where, and why? I went back to connector F and, using a pair of stainless steel test prods with very sharp points, I began working along the copper track. And here I encountered the pressure sensitive situation that I mentioned earlier. In fact, connector F is not very far away from the various power supply components (Q901-Q909 etc) where I had experienced this before. Now I found that the pressure I was exerting on the prod, to penetrate the protective lacquer, would sometimes change the state from a no-go to a go. But the effect was not consistent and I couldn't make any real sense out of it. All it indicated was that there was a dicey connection somewhere in the vicinity. In an effort to minimise this effect, which seemed to be hindering rather than TETIA TV TIP Sanyo CTP6631 (BOP Chassis) Symptom: No sound or picture. No output from power supply although all fuses are intact. Collector of chopper transistor is likely to be carrying anything up to full bridge output voltage . Warning: discharge the main filter capacitor (C308) before carrying out any tests around the chopper. Cure: R302 (390k0 ½W) open circuit. This resistor supplies bias to the chopper transistor and without bias the chopper can never start up. Note: any attempt to test the chopper transistor in circuit may lead to its destruction if C308 has not been discharged . TETIA TV Tip is supplied by the Tasmanian branch of The .Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. SEPTEMBER1989 43 SERVICEMAN'S LOG -CTD helping, I decided to work from the other end of this track, starting at reference point 22, where the cable joined the board. From here I made my way along the track, a few centimetres at a time, and this did seem to avoid the pressure sensitive situation. It all took time of course but eventually I reached a point about one third the way along it, where it runs hard against the extreme edge of the board. In fact, it is one of a group of three tracks between the edge of the board and IC5A2. And here I encountered something which was not indicated on either the circuit or the board diagram; a small polyester capacitor connected between pins 1 and 3 of IC5A2. It was fairly obviously a manufacturer's modification - not that additions like this are all that strange in modern equipment. What was strange was the fact that in addition to being soldered into the circuit - which should have provided more than enough support - it had been laid over on its side, across the three tracks just mentioned, and glued into place. Yes, glued; why I can't imagine, but there it was. More importantly, it quickly became evident that this was where the trouble lay, because the voltage on the track on the other side of this component was normal. Somewhere under that capacitor the copper track was damaged; and damaged so delicately that it was, in effect, a high resistance joint. The cure I took the easy way out and bypassed the faulty section of track with a short length of tinned copper wire, routed over the top of the capacitor. And that fixed it; it was almost an anti-climax after all the sweat and bother of tracking down the fault. Unfortunately, a few questions remain unanswered. Apart from querying why the capacitor was glued in the first place, it is reasonable to ask what caused the track to fail. Was there a defect at that point initially, which was aggravated by whoever mounted the capacitor'? Or, more likely I suspect, did the glue contain some corrosive component which attacked the copper'? If the latter is true, what are the chances of the two remaining tracks surviving'? Since this is all pure speculation I'm afraid that that's a risk that will have to be taken. At least, if I strike more trouble in this machine, I'll know where to look first. So there it is; one of the rarest faults I have ever encountered. And how did I come out of it financially'? Not very well I'm afraid, though it could have been worse. Bar codes Tu~ 4-·9V WA.'S c1--ose:. 6NOO~ -ro 111e: 5V t w~~ Cl-l~~•~G & -PWV w~s '1\-\e: DE.S\GNJ.\-rtOI\\ OF "tl\~ t-eAt> TO ..-Hf; "B~'S~. 44 SILICON CHIP 1: F " O ~ \,. To finish off this month, I have to acknowledge a letter from P. Dunford, of Christchurch, NZ, which appeared in the July 1989 issue, page 5. This was prompted by comments in the April issue concerning VCR digital scanners. I questioned the usefulness of these devices, inasmuch as I had yet to see any programs with bar codes. I now understand that one Sydney newspaper, the Sunday Telegraph, provides this service with its TV guide. Even so, I still have my reservations. As my correspondent points out, some people have difficulty using the scanner but there is a more serious limitation. It's not the fault of the system as such but rather problems caused by incorrect transmission times. Late night programs in particular can be early or late by anything up to 30 minutes, with the indicated running time also sometimes in error. Having been caught myself several times I now straddle such 2 3t .1 20 ,a , __________._ ·_·1_·_·_·_ · _·_·_ _· _·_·_· ~ - - - ~ ~ r t l Fig.3: this diagram shows the physical layout of the relevant circuitry on the main board, simplified for ease of presentation. Connector F is to the lower right and the copper track from its pin 7 runs off to the left, via a couple of bridges, to the extreme left of the board. From there it runs vertically past IC5A2, then turns right and runs along the top of the board to cable point 22. programs with at least 30 minutes at either end, using a 180 tape for a 2-hour program or a 240 tape for anything longer. But there is no way to do this using published bar codes, thus largely negating their value. Mr Dunford also comments on my query as to why some machines now load the tape against the control head immediately the cassette is inserted. In fact, I followed up this point after I had raised the matter and came up with the same explanation that he so clearly set out. I refer you to it. Indexing gimmicks Later I was able to pursue this further at a 2-day National Panasonic seminar in Canberra. A whole range of topics was covered, some of which I may deal with in later notes, but for the moment here are some details on the latest indexing gimmicks (models NVD-38, NVD-48, F-70, H-75 and D-80). The system provides for up to 20 indexing signals to be entered onto a tape while recording. An indexing signal is entered automatically when the record button is pressed, either to start recording or during recording to index a particular segment. When such a tape is re-inserted for replay, it may be run in the normal way by pressing the play button. If the play button is not pressed, it will immediately go into the fast forward mode. Then, as each indexing signal is encountered, it will generate a 1/9 size picturewithin-picture of the following 10-second segment in the top right corner of the screen. If the play button is then pressed during this period, normal play results. If the play button is not pressed, the last frame of the 10-second segment is frozen in the 1/9 area frame and fast forward continues until the next index signal is encountered. This generates another 1/9 area frame below the first one and, at the next signal, a third one below that. A fourth signal replaces the first image, and so on. As each new frame is presented, the index number is also displayed, there being a facility to recall a segment by feeding in this number. A segment can also be recalled be feeding in the elapsed time from the beginning of the tape, assuming this is known. Apart from these indexing facilities, there are several other features, such as a picture-withinpicture facility to enable monitoring of one channel while another is by ing watched, and so on. It's all very clever of course, but I do wonder whether we really need all these facilities, or how many people will use them. Still, the makers say we need them, so I suppose we must. [lEI FIX-A-KIT Kit Repairs - $1 5 per hour. Normal Rate - $35 per hour. No charge for kits that can't be repaired . 3 month Warranty on repairs . Construction - fixed or hourly 1 2 Month Warranty on Manufactured Kits. Custom Designing, Manufacturing, Large or Small Quantitites. HYCAL INSTRUMENTS Design, Manufacture, Repair of Electronic Equipment. (02) 633 5897 Unit 4, 62 Great Western Highway, Parramatta, NSW 2150. Trading hours: 8am to 3pm Monday to Friday. SEPTEMBER1989 45