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SERVICEMAN'S LOG Time to talk about timers Why is there such an unbridgeable gap between one of the video recorder’s main features & the way the public reacts to it. I refer to the program timer, which allows the VCR to record programs in our absence. They can cope with almost any timing requirement, yet hardly anyone uses them. The story is really about the mechanics of a tricky VCR timer problem which, I suspect, may be more widespread than is realised. It may, just possibly, also be age dependent. But it set me thinking about the public’s response to timers. The particular case involved a National NV-370 machine; a very popular model which first appeared some 10 or 12 years ago. And it says something for the quality of these machines that most of them are still giving excellent service. Serious faults have been minimal, with most service work being simply routine; eg, cleaning, replacing worn belts and the occasional head replace­ment for well-used units. In this case, the owner was an elderly lady and her com­plaint was that the timer function was giving trouble, but “only sometimes”. I didn’t like the sound of it because of all the intermittent faults one can think of, a timer function intermit­tent is about the worst imaginable. And had it been anyone else, I would have immediately sus­pected finger trouble; the inability of the user to set up the timer function correctly. For the truth is that the majority of VCR owners are incapable of using this facility – and readily admit it. Some have never even tried. Others have tried a couple of times, fouled it up and given the idea away. And that’s a pity, because it is one of the most valuable features of a VCR, allowing users to capture programs they would not otherwise enjoy. I’m not sure why this is such a problem. Customers often complain that these devices are, to use a glib “in” phrase, not “user-friendly”. In response, the makers have responded by pro­ducing new models which are claimed to be “more user-friendly”. Yet, in reality, the more they try, the more complicated they seem to make them and the more the public shies away. Conversely, many early machines like the NV-370 were relatively simple to set up and most had a good instruction book. Even so, few of my customers seemed to have mastered the simple procedures involved. There are probably many factors involved but I do suspect one: the 24-hour clock which most makers now use but which is foreign to most users. Not that I blame the makers for using it. It is far more logical than the clumsy AM/PM arrangement which is CLOCK ON OFF CLOCK NORMAL PROGRAMME ON OFF DAY HOUR MIN TIMER REC Fig.1: the clock and timer controls for the National NV-370 VCR. The 24-hour clock used in most VCRs confuses many users. 56  Silicon Chip itself wide open to confusion. Unfortunately, program guides are invariably set out in 12-hour times. Perhaps it would help if they could include both time systems with the 24-hour figures in brackets. Well, it’s just an idea. Back to the VCR But I digress – back to the lady’s NV370. I suppose it goes without saying that when I put it up on the bench and checked the timer function, it worked perfectly. But I know the lady well enough to rule out finger trouble. She’s been using this timer function for years and, in a sense, is more familiar with it than I am. So I proceeded on the assumption that there really was a fault. And in order that those not familiar with this machine can follow the story, it will help if I set out the various controls involved and how they are used. The clock and timer controls occupy the right-hand half of the front panel and are normally concealed by a small fold-down flap. And, from the left, they are: Timer Selector, On, Off, Day, Hour, Min-, Min+ and Timer Rec (the latter coloured orange). The Timer Selector is a 3-position slide switch, the three positions being designated (from left) Clock, Normal and Program­me. The other controls are pushbuttons; toggle, hold down or lock, as appropriate. Setting the Timer Selector to the Clock position (left) allows the clock to be set to the correct day and time, simply by holding down the Day, Hour and Min buttons in turn, until the appropriate reading is obtained for each. In practice, the clock is set so that it is slightly ahead of the real time and this reading is held until the real time coincides with it. Then, when the Timer Selector is switched to Normal, the clock will start and keep time. Normally, of course, the clock doesn’t need resetting unless the power has been turned off at the mains. When the Timer selector is set to the Programme position (right), the timer function can be set up. Pressing the On button brings up the clock display, which is then programmed by holding down the Day, Hour and Min buttons, until the required starting time is displayed. The Off button is then pressed, so that the required finishing time can be similarly set up. Finally, the Timer Rec (orange) button is pressed and locks into the On position. At the same time a small clock symbol appears to confirm that the timer function is set. The Timer Select button is then reset to Normal. It’s not a particularly complex procedure really but mis­takes can still all too easily occur. Common mistakes include setting the wrong 24-hour time, the wrong day or the wrong chan­nel, neglecting to press the orange button, and forgetting to rewind the cassette, to name just a few. Anyway, those were the steps I went through to test the timer, normally setting the starting time a minute or so ahead and the finish time a minute after that. Even so, it takes time, and I tried to fit the tests in during natural breaks. Initially, I couldn’t fault it but persistence eventually paid off. I had set the Timer Selector button to Program, then pressed the On button. But the normal On function did not re­spond. I fiddled with the On button and, after several tries, it came good and I was able to set an On time. I then went to the Off button, only to find that it was reluctant also. At this point, more or less by chance, I happened to touch the Timer Select button, whereupon the clock display flashed off and on again. I wiped that setting and went through the procedure again. And again the On button did not respond but this time I fiddled the Timer Select slide switch button and was rewarded with a most erratic flashing clock display. So, a faulty slide switch? Dirty contacts? Worn contacts? Well, it was something like that. Access requires removal of the top and bottom covers, which then allows the front panel to be unclipped. By then undoing one screw, the timer board can be lifted out for examination. And a point to note here is that the Timer Select slide switch is soldered directly to the underside of this board but is not supported in any other way. The first thing I did was squirt some CRC into the switch and flick it back and forth a couple of times. I then went through the timer sequence again but there appeared to be no improvement and so I turned the machine over to check the under­ side of the board. In particular, I wanted to take a closer look at the soldered joints that secured the switch. At first glance they appeared to be OK but the jeweller’s loupe told a different story. Two of the joints were cracked – not dry joints, but definite fractures. I was puzzled as to what might have caused this but, for the moment, I was more interested in having found a fault (hope­ fully, the fault). Some careful attention with a good hot iron and solder effectively remedied the cracks and proved to be a complete cure. Prolonged testing on the bench and follow up checks with the owner have proved the point; it hasn’t missed a beat since. More to come Well, that was the end of that particular episode but there was more to come. It was the first time I had encountered or heard of such a fault and thus alerted, I decided to make some routine checks as any NV-370s came through the workshop. I have handled several since then, mainly for routine checks and cleaning, and have found one more with the same fault. Which brings me back to the question as to why it happens, remem­bering that we are talking about fractures and not dry joints. The best theory that I can advance is that the soldered joints are not quite adequate, the solder layer being quite thin. While they are undoubtedly adequate electrically, they are simply not strong enough mechanically to with­stand the stresses as the switch is actuated during regular use. And that conclusion leads to a August 1994  57 SERVICEMAN’S LOG – CTD somewhat contradictory thought. For those people who don’t use the timer facility – which, as I have already implied, is the majority – such a weak­ness is not a problem. So the facility will not cause problems, as long as you don’t use it. (There must be something wrong with that line of reasoning some­where)! More realistically, it probably explains why this problem has not surfaced to any extent before this; it has probably taken this long, with regular use, to find the weakness. Colour TV set The “worn-out” VCR And there is one more incident worth relating. One of my customers is a local electrician and, while on a job one day, his customer asked him if he had any use for an old video recorder, adding that it was destined for the tip because, as far as he was concerned, it was “worn out”. The electrician already had a VCR but it so happened that he did have a use for another one – “worn out” or not. His need was for a UHF/VHF down-converter, for use with an old, but still good, VHF-only TV set. And he knew enough to know that the down-converter function should still work, even if the rest of the machine really was “worn out”. So he grabbed it with both hands. I first learned about this when his wife brought their main VCR in for a minor service. After relating the story, she asked whether I thought it would be worthwhile checking it out and possibly restoring it to full operation (they had not even tried it in this role). I suggested she bring it in so I could make some preliminary checks. The machine turned out to be an NV-600, an up-market ver­sion of the NV-370. A preliminary check proved quite promising. I found only one serious mechanical fault – failure of a back ten­sion brake, due to loss of its felt pad. I replaced that, then put the machine through its paces, checking out both the record and replay functions. Would you believe it? – it turned in a first class performance. And it was on its way to the tip! That left only one more test – and I’ll bet you’re way ahead of me. That’s 58  Silicon Chip then the original owner of the NV-600 may not have been far wrong when he used the term, “worn out”, much as I dislike the expression. Personally, I’ve always regard­ed it as a convenient don’t-know, couldn’t-care-less, copout phrase, but in this case, wear may have been a significant fac­tor. Fig.2: this diagram from the Hitachi Fujian 1425B colour TV set shows the tuner (vertical block at top right) & the V164 zener diode (bottom, centre) that’s connected to the collector of transistor V105. right, the timer function. And you’ve prob­ ably also guessed that it was faulty. Right again but more to the point, the symptoms were virtually identical with those on the original NV-370. So it was into the works for a look at the same board. The only snag was that, in this machine, it is a proper swine to get at. But when I did reach the board the jeweller’s loupe told the same story; two fractured joints. Once these had been remade, the timer worked perfectly. So the electrician had scored an up-market recorder, in excellent condition, for no cost other than my routine service charge. It was smiles all round. But one final thought. If my theory as to the cause of these failures is correct, My next story is on quite a different theme. It concerns a Hitachi Fujian colour TV set, model HFC-1425B, which is in many ways similar to the HFC-1421B dealt with in my June 1994 notes. Initially, the exercise seemed like a fairly routine one; so much so that I was not sure whether the story was worth the telling. I finally decided that it was, mainly because the exact nature of the fault was new to me and I thought that it was worth passing on for someone else’s benefit. In the event, it turned out to have an unusual twist. But first I will detail the service exercise just as it happened. The set came in with the complaint that “there’s no picture and no sound”. I interpreted this as meaning a completely dead set, which could mean anything from a blown fuse to an obscure fault in the switchmode power supply. Well, the description was literally true. There was no picture and no sound but the set was still very much alive. It was scanning normally and displaying a beautiful off-channel snow­storm, indicating that the fault was somewhere in the front end. Of course, this kind of fault would normally produce plenty of noise in the speaker but I had momentarily forgotten that this chassis features a muting circuit which kills the sound if there is no signal. This set employs a search and program system for channel selection and this was the first thing I tried. I began by check­ing for any channels that may have already been programmed into it and then, when this revealed nothing, I initiated the search function. But this didn’t work either and, more to the point, there was no change of any kind to the screen image during these checks. Normally, there is some variation as the system searches and encounters odd patches of interference but, in this case, there was just a steady snowstorm. Fortunately, the set is fairly easy to work on, for which I give due credit to the manufacturer. I went first to the tuner, which is shown on the right- had side of the accompanying circuit as a small vertical block with nine terminals – see Fig.2. The IF output terminal is at the bottom, with the +12V supply terminal above it. This was the first one to check and this was OK. The next ones to check were the three marked, re­spectively, BL, BH and BU. These are +12V supply rail terminals for the low (BL) portion of the VHF band, the high (BH) portion of the VHF band, and the UHF (BU) band. These are energised individually, according to the selected channel. So, as the set is put through its programming procedure, each of these terminals should, in turn, go to +12V. Which, in fact, is exactly what happened. So that cleared that part of the system. That didn’t leave much, except terminal VT. This carries the tuning voltage – a variable voltage developed at the collec­tor of transistor V105 in response to the signal fed to its base from pin 1 of IC101. So I should have been able to detect a voltage on this pin, ranging from about 0V to (typically) about 30V during a search function. Alternatively, it should show a fixed voltage if a particular channel is selected. However, there was no voltage of any kind on this pin, regardless of which tuning function was initiated. Well that seemed fairly straightforward. Pin VT connects to the collector of transistor V105 via three resistors: R167, R168, and R169. These were easily checked and cleared. From the collec­ tor of V105, the circuit goes (down) to a voltage regulator cir­cuit; the kind of setup commonly found in tuner supply systems and designed to minimise any drift in tuning voltage. It consist­ ed of R182 (15kΩ), C175 (270pF) and zener diode V164 which, I assumed from its type number, operated at around 33V. This cir­cuit is fed, in turn, from the main 113V rail. And the solution was simple; resistor R182 was open cir­cuit. Well, that was a new one in a Hitachi Fujian, although I have had trouble with a similar resistor in a Samsung and also in a Philips. In the latter two sets, the resistor went high resist­ance, which restricted the tuning range to the low end of each band. Anyway I fitted a new resistor and the set came back to life. Nevertheless, it was necessary to check that the full tuning voltage range was available and that all three bands could be programmed correctly. In fact, everything checked out 100% and, after a routine check of overall performance and a few minor adjustments, the job was finished. Never-ending story End of story? Not quite. And I wonder if any reader has spotted what appears to be a contradiction in the circuit as I described it above. Well, take heart if you didn’t because I didn’t either; at least, not initially. The truth is that, in practical servicing, one cannot afford the luxury of analysing every part of a circuit as one works on it, particularly when it is a simple operation like looking for a lost voltage. So, initially, I simply took the circuit at face value. But then I decided to write this story; and I began making notes and mentally organising how I should present it. And, of course, it August 1994  59 SERVICEMAN’S LOG – CTD was obvious that, in order to put the reader in the picture, I would have to describe the circuit, just as I have done. Then suddenly it struck me. How can we have a variable voltage developed at the collector of V105, when that collector is fed directly from a zener diode voltage regulator circuit? And the simple answer is, we can’t. So how does the system work? I pondered over this at some length and was still pondering over it in the workshop when a colleague walked in. Glad of an opportunity to pick someone else’s brains, I immediately buttonholed him. When I filled him in, he readily agreed that it didn’t seem to make sense. His suggestion was that there was an error in the circuit; that there should be a collector load resistor between the regulator circuit and V105’s collector. In fact, he felt sure he had seen such an arrangement in another circuit for a differ­ ent model of the On Sale Now At Selected Newsagents Or buy direct from SILICON CHIP Price: $7.95 (plus $3 for postage if ordering from Silicon Chip). Order today by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or send cheque, money order or credit card details to PO Box 139, Collaroy, NSW 2097. 60  Silicon Chip same make of set, only he wasn’t sure which one. At that point, he left me to ponder some more. But I was through pondering. If his theory was correct, it was easy enough to prove. The set was still in the workshop, the owner having left it with me before going on holidays. At the first opportunity, I pulled the back off and traced out the circuit. But there was no sign of any extra resistor; the set was exactly the same as the circuit. So what’s the explanation? I made some more careful meas­urements of the voltages appearing on terminal VT and, while they can range up into the 20V plus range, they went nowhere near the 30V or so of the zener rating. So, in fact, the zener wasn’t functioning. So why is it there? The best explanation I can offer is that the 15kΩ resistor, R182, is the collector load resistor and the 113V rail, from which it operates, is regarded as being sufficiently well regu­ l ated as not to need further regulation. But that still doesn’t explain the role of the dormant zener, V164. My guess is that it is purely a protective device, designed to protect the tuner in the event of a failure of, say, V105. It this went open circuit, or ceased to draw current for any other reason, then close to the full 113V of the main HT rail would be applied to the VT terminal – and I imagine the tuner may not like that. Well, that’s my theory, and it looks like I’m stuck with it. That is, unless someone out there is on better terms with Hitachi Fujian sets and can come SC up with another explana­tion.