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SERVICEMAN'S LOG The customer is always right? In this business, one not only has to deal with technical malfunctions, which by themselves are bad enough, but with the customers as well. I sometimes wonder which are the worst. As I have mentioned before in these notes, one of my regular customers is a local motel owner who has about 30 Samsung TV sets spread amongst his guest rooms. Most of these sets are 50cm CB15F units and were purchased a little over three years ago. It is one of these sets that is the main subject of this story. At the time of purchase, the sets carried a 3-year warranty and the owner used to call me in whenever one gave trouble. There was no contract between us; each job was treated on its merits and the charge depended on the warranty situation. If the set was still under warranty, I simply made the necessary claim, Samsung re-imbursed me, and there was no charge to the customer If, on the other hand, it was a set FRACARRO DELIVERS... .. .best television antenna performance tor less cost. Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 44 SILICON CHIP that was out of warranty, I charged the customer accordingly. I have presented this brief background because it does have a bearing on the sequence of events, although not in a technical sense. Basically, the story involves a complaint by the motel owner to Samsung, with yours truly as the meat in the sandwich. In greater detail, not long after these sets were originally installed, several of them developed the same fault. This fault involved several components in a front corner of the main board: inductor L401, capacitor C414 and resistor RH0l - the latter described as a safety resistor. Any or all of these components could have dry joints to the copper pattern and, as I later found out, this could cause arcing and damage to the board. And to digress here for a moment, the resistor type number (RH0l) caused a good deal of confusion. While it was so designated on the PC board, it did not appear at all on some early circuits and nor did its companion component, diode DHl. And when it eventually did appear, it was as RHl. It might be worth making a note of that. Anyway, after the first two or three encounters, I realised that this was something to watch out for on a routine basis. From then on, as other sets came in for attention over the next two years, I made a point ofresoldering all the connections in this area, regardless of their appearance. In fact, all but one, as I recall, appeared to be quite OK but I wanted to be sure. Eventually, the time came when there was only about six months of warranty period left. I had a full list of serial numbers for these sets and, on checking my own records, I realised that six sets had never been serviced, so this potential problem remained. More to the point, if Murphy had anything to do with it, there would be a faulty one in this group. At the first opportunity, I drew the owner's attention to this, emphasising that these sets should be checked now, while still under warranty. Furthermore, as this was an off-season for the motel, it would be a good time to take them out of service. He said, "Yeah, sure mate", but that was as far as it went. I subsequently prodded him a couple of times but again there was no result. I then went off on a few weeks holiday and by the time I returned, the warranty had virtually run out. I did make one final plea but nothing happened and so that was the end of it. Or so I thought until a couple of months later when the owner ap proached me to have the work done under warranty! Naturally, I had to knock him back. The warranty period had now expired and I was in no position to break the rules. Had I done so, I could have been left holding the financial baby. On the face of it, he seemed to accept the situation and went off to think about it. Call to Samsung What, in fact, he did do was ring Samsung and complain - not about me directly, but about the strictness imposed on the warranty period. To give Samsung their due, they didn't muck about. If this customer was dissatisfied then, regardless as to who might be at fault, they weren't going to haggle over a few weeks beyond the warranty period. I was instructed to go right through the system and make whatever repairs I considered necessary, all under warranty. At the same time, they made it clear aggravating what was already a delicate situation. In this situation, some diplomacy was called for. So, without showing any reaction to the implied aspersion, I simply asked what the problem was. This brought a more conciliatory response; apparently the set was drifting off channel after prolonged use and was also delivering a snowy picture. Well, one thing was certain; I wanted to see these faults in-situ. The snowy picture, at least, could be an antenna distribution problem, so that point had to be cleared first. s 'I,'< 1.0.WS. ~'S, ..,.etJ'f S, . ~e,tJ~"'o,JS 1 e"~•~t',,!1 ,-fllitr~!Q: Ill. f,A N 1111I, " ~~,a.,1111if'S, ,~-··· t-f('(SAALL wr>,Riv. & vott:> Back to the workshop 8E. f'JU~S1\CK'I I\OVl-~l'AA~WT~ t ~P,.W\l ~T"/~ Oil ~ ~,. ~ Well, he was right; it was snowy. A second set plugged into the same outlet cleared the antenna system, so it was back to the workshop with the set. I set it up on the bench, checked the adjustment of each channel, and let it run. This confirmed the snowy picture but I wanted to check out the drift problem first. If it was drifting, a new tuner would be quickest solution, and this might also sw eep away the snow, killing two birds with the one stone. I let it run for a couple of hours but there was no noticeable drift during that time. However, I wasn't going to be caught that easily; I'd been there before and a few comments by the owner had put me on alert. I switched to another channel, which I knew was correctly tuned, but the AFC wouldn't pull it in. I then switched back to the first channel - same thing, it wouldn't lock in. I don't know why it happens but it has caught me out before. The set can run OK on a particular channel for hours but, as soon it is switched to another channel, everything goes haywire. The only practical cure is to change the tuner. So I fitted a n ew tuner, set the channels up again and let it run. And that cured the drift problem; the tuner was now as steady as a rock. But it had done nothing for the snowy picture. Based on exp erience, I went first to VR151 , a 10kQ pot on pin 12 of IC101. This pot is the AGC delay adjustment and is a common offender. But not this time; my checks and adjustments achieved nothing. Next I checked the UHF reception against the VHF reception and established that it was just as noisy on either. So, with the tuner cleared, it - ~ ~11'1't ,':,o~Ai '"~-f 'l'y f:: 11-IE. WAR~N1'< HA~ EX? \RED & t WAS IN NO ~ Sl1",0N "'rt> ~RE.A..\<. °™e: ~\JL-E. S ... that this was a on e-off situation, so there could be no misunderstanding. It was not to establish a precedent and that was fair enough .. So I went to work. Apart from the aforementioned routine problem, most of the sets needed little or no attention . A few did, however, require minor adjustments and I did what was necessary to bring the whole system up to first class condition. It thus came as rather a shock when, a few weeks later, the motel owner was back on the phon e, complaining that, "The set in room so-and-so is still playing up". It was the word "still", spoken with some emphasis and with a bit of an edge to it, that upset me somewhat. It implied that I knew about a fault in the set and hadn't fixed it, whereas I could not recall that my attention had been drawn to any particular set or fault. I assumed that the most likely explanation was that it was an intermittent fault and I had no intention of J ANUARY 1993 45 TUOOI ER1U 2.-nc + EC104 .-----+-----,t--(JF 's.~" ED111 ll:2Ui 1N4141 1001( I R211 '-'" SQ10t-S0103 KSA 539 25A 1115 T1t1 PSF 11AT01 ,11:101 ... c,.. f ·" C1i1 1,11 C1H 1.01 1V "" '" iSv 1 l;zss:. '-+---+--.=,c.c.=_ _ _ _ ♦ 12'11-----------R\Di t - - - -- ---+--------+---+---'"""-'"-"'L----~TP-•7_,_,_ _ _ __ _ ____,<at> 11 :~1s10 ~ Fig.1: the tuner & IF circuitry of the Samsung CB15F. The IF output from the tuner (TU001) is fed to IF amplifier stage Q161 and this, in turn, drives SAW filter 2101 & the IF processing chip (IC101) at bottom right. had to be something further down the line. Signal tracing The signal from the tuner goes to the 1st IF amplifier (Q161, 2SC388), then to SAW filter Z101, and subsequently to pins 9 & 10 of IC101. So what to test next? From experience, my best suspect was the SAW filter, because I've had several faulty ones in the past. Fortunately, it is easy to change, involving only five soldered connections, and I had a spare on hand. A few minutes later I had the answer - it wasn't the SAW filter; it was still snowing. OK, so what about Q161? One of the weak points of these Samsung circuits is that they carry no voltage values. So I spent some time checking associated components around this stage, then measuring the voltages on the transistor, and doing some mental gyrations in an - effort to decide whether they were reasonable. In the event, I found no obviously faulty components and finally decided 46 SILICON CHIP that the voltages were reasonable. So, as a last resort I pulled the transistor out and fitted a replacement. Alas, no joy from that either; the picture remained snowy. So where did that leave me? I'd cleared the tuner, 1st IF amplifier and the SAW filter, all by replacement. So the next suspect was chip IC101. I (,4-~""\ \ \ \ \ I \ This photo shows the replacement component pack issued by Samsung for the CB515F/CB515Z TV receivers. The parts involved are C414, L401 & RH01. A detailed instruction sheet accompanies the component pack. didn't fancy replacing it if there was an easier way and so I resorted· to a trick that I have used in the past. As mentioned on a number of occasions, I have a self-contained tuner unit, made up from an old turret tuner and a small power supply assembled from junkbox parts. It has proved invaluable in tricky cases like this, because it delivers a known good IF signal. At this point, the reader should refer to the accompanying drawing, which the SILICON CHIP draughtsman has prepared from my rough sketch see Fig.2. It shows the copper side of the PC board around the tuner, the tuner itself being mounted on the component side. What we are looking at here are four tuner mounting lugs and eight tuner connection pins. For the sake of clarity, the top seven . pins are neither identified nor shown connected. The eighth pin, at the bottom of the drawing and hemmed in by the main copper pattern, is the IF OUT pin. This is connected to a small copper track which forms part of an inverted "T" pattern. Capacitor C161 then connects to the bottom of the inverted "T" and feeds the base of Q161. With me so far? Right. I fired up the substitute tuner and clipped its ouput lead onto C161's pigtail where it was soldered to the copper track. The result: a perfect picture, with not a snowflake in sight. This cleared the IF system, along with the following sections of the set. So what was going on? The external tuner gave a perfect picture whereas the set's internal tuner, which was known to be OK, gave a snowy picture. It just didn't make sense. As far as I could see, the only difference between to two setups was a 10mm length of copper track between the IF OUT pin of the set's tuner and the lead from C161 (which was fed directly by the substitute tuner). There was only one way to resolve the situation. I soldered a short length of wire to the IF OUT pin, clipped the substitute tuner lead to it, and tried again. Result: it was snowing again. So the fault was somewhere on that 10mm length of copper track. I fished out the jeweller's loupe, set EARTHY COPPER PATTERN 1111~----+ • I I \ • TUNER MOUNTING LUGS . . ~ < z :i a: . ~ a: w z ~ -TUNER • • 0161 C1~ 7 Fig.2: this drawing gives some idea of the layout around the tuner. It shows the copper side of the board, with the tuner (shown dotted) on the component side. Note that the connections to seven of the tuner terminals have been omitted for the sake of clarity. Electronics Workbench® up a good light, and moved in close. Sure enough, there it was - the finest of hairline cracks about halfway along the strip. And a continuity check confirmed what was now obvious - the track was open circuit. After that, the cure was simple; a run of solder along the strip put everything back to normal. So the set was returned to the motel owner, with my assurance that this latest fault had now been fixed - under warranty. By now, the atmosphere had thawed somewhat and the owner actually expressed his gratitude for my efforts, and his appreciation of Samsung's cooperation. So everything ended happily. But how did that hairline crack occur iif1cl. why did it show up after the seyhad been running for over three years? Well, I don't think that it was a recent happening. The set had not been dropped or otherwise abused and my bet is that the crack had always been there, from the time the board was made or while it was being fitted with parts. My theory is that, in spite of the crack, there was, initially, some bridge across it; a tiny area perhaps, but ad- EmonaLAB4 Four instruments in one package! The electronics lab in a computer! 0 ... <at>) [l! © t:l A simple, intuitive and very powerful teaching tool, Electronics Workbench lets students and hobbyists design and test both analog and digital electronic circuits, without the delays and expense of a laboratory . 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(002) 34 2233 ]ANUARY 1993 47 TETIA TV TIP Samsung Model VB 711 VCR Symptom: machine will not record sound and a newly recorded pictu re shows flashes of the original ch roma . Any previous ly recorded tapes are unaffected. Cure: bias oscillator not working. This oscil lator not only provides the bias fo r record ing the sound but also powers the fu ll erase head to remove the original picture. The trouble is caused by an intermittent contact at the AC head and is cured by replaceme nt of the fau lty parts and a modification of the AC head connections . TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay 7015. -~ .•r.-- ~-c:::w~~...l. --'=r"'-~ .~ "-#ANO 'tl-\\S M\GH, P.,\...$0 ~W€EP A'WP.."1 1ltE: SNOW, KU..l.1"1G1WO 'B\~t>S W\1'\i ~ ONE. CS1'0~e:.... equate for a signal path. But copper, by its very nature, will eventually succumb to corrosion. So th e tiny contact area was eventually destroye d. Coincidentally, as I started to write these notes, Samsung issued a recall notice con cerning the dry join ts around C414 , 1401 and RHOl. Ap parently I wasn't the only one to rep ort this problem. As a result, I and oth er accredited service organisations have been issued w ith a detailed instruction sheet and kits containing replacements for all three compo nents . Samsung is not taking any chances. Computer repairs Now for a ch ange of scene. In the December 1992 issue, I detailed how I was conned into repairing a computer - or, more accurately, a computer monitor. As it turned out, the fault was relatively simple but , even so, I expressed the hope that it would be my last computer repair. 48 SILICON CHIP It was to be a forlorn h ope because it wasn't long before another customer, on e of my regulars , fronted up with another computer monitor. This time there was no image at all and the customer, rightly, thought that the monitor was at fault. This w as a different brand; an Acer, mo del MM211 , marketed by Dick Smith Electronics. This was a helpful aspect; it's always reassuring to know that there is someone to turn to for spare parts or technical advice , if n eeded. Undoing four screws released the p lastic cover and revealed a very neatly laid out board. It also revealed th e immediate cause of the failure; a blown fuse. And, significantly, there w as a second fuse nearby which was still intact. So where did they both fit in? By pulling off some plugs fitted to the board and removing some screws , I w as able to juggle the board clear and turn it over. This enabled me to make a qui ck assessment of the power supply circuit and even to make a rough draw ing of it. In many respects, the circuit w as very similar to that use d in the previously mentioned MAGIC model. It consisted of a transformer, w ith a fuse in the primary circuit, that delivered about ZOVAC to a bridge rectifier. This was then followed by another fus e, a filter capacitor, and a 12V voltage regulator - just about as ordinary as one could get. It was the fuses that pinpointed the culprit. The blown fuse was in the transformer primary circuit, while the one following the bridge rectifier was intact. That indicated that either the rectifier or the transformer was at fault. It took only a few minutes of checking with the ohmmeter to find a dead short across one of the 1N5401 bridge rectifier diodes; and only a few minutes more to clear the small capacitor across it. So that was it; whereas the MAGIC model had an open circuit diode , this unit had a diode that was shorted. As before, a new diode and fuse restored everything to normal. The biggest job had been getting things apart and finding my way around the innards. There's nothing to this computer business; so far, it's been-a snack. But don't tell anybody. SC