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The case of the blowing fuses I'm kicking off this month with a mystery story from J. L., our regular contributor from Tasmania. I've chosen this sequence because, after reading his story, I had a chance to work on a couple of similar chassis and make some further observations. They may help a little. What follows in in J. L.'s words. Here 's how he tells it. This is a story without any real ending. It has had me thinking for a long time and I still don't have acertain answer. I can make a convincing guess as to the cause but I'm not sure. See what you think. The set was a General Electric TC53L2, a 53cm model, fitted with an Hitachi NP6A-A chassis. I don't think that has anything to do with the fault or the cure. It could have been any model of any brand of similar vintage. The set came in for an intermittent "no go". This model offers two possibilities for this fault. One is a faulty joint under the clamping screw for the horizontal output transistor collector. The other is a loose connection to the emitter of this transistor (it uses a slip-on connector which is apt to come loose). Either fault is easy to cure and, in this case, I replaced the slip-on connector with a hard wired lead, soldered to the emitter pin. It was a total cure - for that fault. The customer returns Unfortunately, the customer didn't agree. He was back within a week for what he said was "the same trouble ". It wasn't though; this time it was the main HT fuse, F903, a 1A time delay type, that had gone open circuit. .The fuse had blown gently. It wasn't just broken - as through old age - nor had it blown violently. But it had blown through over-current and I needed to know why. This chassis uses an isolating transformer, T951 , feeding a bridge recti60 SILICON CHTP fier and then a chopper type regulator delivering 125V to the rest of the set. Fuse F903 is between the positive output of the bridge rectifier and the chopper transformer, T902. Apart from the unlikely event of a short to chassis in the chopper transformer, a shorted chopper transistor, TR906, is about the only thing likely to take out the fuse. The only other connections to this rail are C910 (a 4. 7µF 400V electrolytic kickstart capacitor) and the R908/ R935/R909 network which supplies HT to the chopper pre-drive transistor, TR904. Both these connections TETIA TV TIP Hitachi CEP288, CEP289 (PAL3-A chassis). Symptom: Reduced height, about 2cm of black at top ~nd bottom of screen. No colour. The picture can recover to normal after 10 minutes but the fault does not appear to be heat sensitive. Cure: C753 (100µF/25V electro) open circuit. This cap is the input to the filter on the 20V rail and its loss causes the rail voltage to drop, in this case to 14V. The rail shows no unusual ripple, just a lower than normal voltage. 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. are quite high impedance, so a short to chassis is an unlikely result if any of these components breaks down. So, after considering all these points, I decided that it had to be a leaky chopper transistor. An in-circuit test indicated that the transistor was OK but, in the absence of any other indication, I felt that it had to be faulty in some way or other. So out it came and in went another one. I fitted a new fuse and switched on. Up came a perfect picture - for about an hour. The new fuse then failed just as the first one had done. I tried again and again but the fuses lasted from only 15 minutes to an hour before failing. I checked everything I could think of that might be overloading the fuse but every voltage or resistance that I tested appeared to be well within tolerance. Thermal cutout By this time , I was running out of fuses, so I firkled about (good word, that) in the junkbox until I found an old 1.5A thermal cutout, rated to trip at 3A. With this clipped into circuit, the set ran for many hours without any trouble. As far as I could tell , nothing was overheating, there were no excessive voltages, and there was no sign of incorrect picture geometry. The set seemed to be operating perfectly within normal limits. With everything apparently normal, I guessed that it would be OK to refit the correct 1A fuse. But I was wrong. It lasted only 10 minutes. So I fitted a 1.5A fuse and, as far as I know, the set is still going strong. I have racked my brain trying to work out what could have caused the trouble. I'll swear there was nothing wrong with the set, yet it would not work with the correct fuse fitted. The theory I have come up with is this. See if you agree. Most domestic electrical equipment is designed around component val- lation and that he contact me at any sign of overheating. Somehow, I don't think we'll have any trouble. Another explanation ., t.H~~prr WMII-ZZPH (&TC ~r~ The £use had broken. gently. It wasn't just broken-nor had. tt blown vio~Uy.... ....Bu.t it had blown lhrough overcurrent ,...._ and 1 needed to know why ues with a ±20% tolerance. Some parts have a closer tolerance but many are 20% because anything closer would be unnecessarily expensive. The law of averages dictates that the tolerances in an average set will be spread evenly between the upper and l_o wer limits. But occasionally there must be a set that gets a preponderance of plus components; or of minus components. Fuses, on the other hand, must always have a positive tolerance. The designer must select a fuse value above the steady state current in the circuit to be protected. Quite obviously, a negative value fuse would blow every time the set was turned on. We can assume that the designer selected a fuse that would have a safe margin over the steady current, but not so much over that it would be too slow to act in the event of an overload. But what would happen if all those 20% component tolerances accumulated in the direction that increased the normal current in the fused cir- cuit? The set would still work normally and the fuse would continue to carry the required current, but with less tolerance to an overload. And, finally, component values change as the set ages. What would happen if these changes accumulated in the direction that added just marginally to the circuit current? The fuse could no longer stand the strain and would pop after only a few minutes. I think this is what happened to the General Electric set that inspired this story. The problem is, what am I going to do about it? The cause is probably the accumulated result of a milliamp or two of extra current in every resistor in the set. Restoring them all would be a prohibitive job. On the other hand, the total current being used by the set does not seem to be enough to raise the temperature of any part of the chassis, so is the slight overload in any way dangerous? I don't think so. I have suggested to the owner that he ensure that the set has good venti- OK., so that's J. L.'s story. In answer to the implied question as to what I think, I'm afraid that, to coin a phrase, "I dunno please". J. L.'s theory is an interesting one but I have some reservations about it. For one thing, I question the 20% tolerance figure. This was true in the bad old days of moulded muck and crude carbon resistors but 5% has been a generally accepted figure for a some time now. And, in most cases, the product is well within this limit. But OK, let's accept 20% for the sake of argument. I held this story for some time after I received it, hoping that a similar model would turn up on my own bench. Sure enough, not one but two came in - a TC53L2 (53cm) model, as above, and a TC63L1 which is a 63cm set using virtually the same chassis. Unfortunately, they didn't help much. The first thing I did, once I had a chassis working properly (more on this later), was to measure the current through F903. This came out as 275mA for a black screen and 370mA for a full white screen. But even the full white value is only a little over one third of the fuse rating, with some margin due to the slow-blow characteristic. On a steady state basis, that doesn't fit in very well with the 20% theory. And that suggests a surge of some kind. Two possibilities come to mind. One is a switch-on surge which, while not quite heavy enough to take the fuse out immediately, weakens it so that it lasts only a few minutes. The other is an intermittent fault perhaps thermally sensitive - which takes the current just above the fuse rating, then clears itself in the time it takes to replace the fuse. Of course, it is easy to propound such theories but quite another to prove them. In practice, of course, few of us have anything like the necessary facilities; nor do we always have the time to tackle subtle faults like this. So thanks for the story J. L., but I say again, "I dunno please". A real swine So what about the two General Electric sets on my own bench? One of AUGUST 1991 61 Australian Made TEST EQUIPMENT YOU CAN TRUST SERVICEMAN'S LOG - CTD SHORTED Tu Built-in Meter to check EHT transformers including split diode type, yokes and drive transformers. $78.00 + $3.00 p&p DEGAUSS Strong magnetic field, larger than usual coil with multicore centre . Double insulated for your safety, also fitted with momentary on/off switch. 240V NC 2.2 amps. Just about as important as having a soldering iron in your toolbox! $75.00 + $10.00 p&p HI-VOL1'A Built-in meter reads positive or negative 0-50kV. For checking EHT and focus as well as any other Hi-tension v.Jltages. $98.00 + $5.00 p&p REMOTE CO T 0 TESTEH (INFRARED OR ULTRASONIC) Designed to test lnfrared or Ultrasonic control units. With the extension lead you can also test infrared units which cannot be placed in front of the testing unit. Requires a 9V battery. Output is via the LED diode and piezo speaker . $85.00 + $4.00 p&p LASER DETE:.C P 0 A new addition to the remote control tester. Comparable with units costing $500 or more. You can test the laser pick-up in compact disc players. $27.00 + $2.00 p&p NEW PRODUCT TEST TUNE Save time and money with this tester. Helps you determine if you have a tuner problem , an AGC problem or a fault in the IF stages. $250.00 (Tax inc.) TUNER IS from $17.00 exchange+ p&p Cheque, Money Order, Bankcard or Mastercard 216 Canterbury Road, Revesby, NSW, AUSTRALIA, 2212 ~ (02) 774 1154 ~ (02) 774 1154 62 SILI CON CIIIP these, the TC63Ll, turned out to have a real swine of an intermittent. It lead me µp several garden paths and was finally cured more on a brute force than scientific basis. Nevertheless, some of test routines are worth noting for reference. The customer used the set in a holiday cottage up the coast, and his complaint was that "it stops now and again". In greater detail, it turned out that the failure was fairly infrequent; it would run perfectly for days, or even weeks. And, when it did fail, normal operation could be restored by simply flicking the on-off switch. This had been going on for some considerable time but, while ever the set responded to this simple treatment , the owner was prepared to live with it. However, the day came when it didn't respond, at least not immediately, and a goodly chunk of an interesting program was lost. That was when the owner decided that something had to be done and it landed on my bench. Of course, it started as soon as I switched it on and ran for several days. And when it finally did fail, and on subsequent occasions, I found it difficult to make any worthwhile checks. It would come good at the slightest touch but I did manage to establish that there was no 125V HT rail out of the power supply when the fault appeared. I had no doubts that the fault was a dry joint; this chassis is notorious for them. These dry joints are found mainly on the power supply board and on the horizontal section of the deflection boards. And this is what makes it hard. The chopper/regulator system in the power supply board (TR903, TR904, TR905 & TR906) is driven with puls es from the horizontal output stage. But the horizontal system can't deliver these pulses until the power supply delivers voltage to the HT rail. And the power supply can't deliver this voltage until it receives pulses from the horizontal system. In practice, this deadlock is broken with a kick-start system; a multivibrator circuit consisting of TR901 & TR902 which is activated briefly from the bridge rectifier via C910. At switch on, it delivers a few pulses to the "pre-drive" transistor, TR904, to get things started. This is a fairly universal technique and is all very clever. But when the system collapses and there is no HT rail , there is nothing to indicate whether the fault is in the power supply or in the horizontal system. After several abortive attempts to get any kind of a lead, I settled for a routine search for obvious dry joints. I concentrated mainly on the power supply board and, in particular, a number of 2W and 5W resistors, such as R908 , 909 , 935 , 928 and 924. In order to aid heat dissipation, these resistors are mounted clear of the board, supported by short metal tubes which go through the board to the copper side, where they are soldered to the copper pattern. The resistor pigtails go down these tubes and are, supposedly, soldered to the them during the flow soldering process. Unfortunately, this doesn't always work. One problem is that the pigtails are sometimes cut short and the solder doesn't reach them. But even when the pigtails are full length, the bond between them and the tube, or between the tube and the copper pattern, is often poor. So one of the routine jobs with faults like this is to go over all these joints and resolder them. Having done this, I checked the rest of the board and resoldered a few other suspicious looking joints just to be sure. I ran the set for several days and it behaved perfectly. But it had done this many times before and I needed more proof than that. By this stage, however, some six weeks had elapsed and the owner came in to check on my progress. More specifically, he wanted to see whether it would be available for another stint up the coast. I explained the situation and emphasised that I could make no claim to having cured the fault. Nevertheless, he was keen to take it and give it a try, so I said, "OK, but be warned". It's not cured That was the last I saw of it for several months. Then, suddenly, the owner turned up with it again. It was TR903 2SC458rRI PHASE AMP C917 0.0068 1 R914 1 180 1 J2 R940 220K D •;;:z.(H'r:-<r"U>- ·c&t', ' i. . J Fig.I: the power supply board for the GE-TC63Ll & GE-TC53L2 TV sets. The output from the bridge rectifier is applied via fuse F903 (lower left) to isolating transformer T902 (top) which is switched by chopper transistor TR906. TR901 & TR902 form a multivibrator circuit which is briefly powered up at switch-on to deliver pulses to TR904 to kick-start the chopper circuit. TR908 & TR909 provide over-voltage protection & this circuit can be disabled by disconnecting R941. the same old story; initially, it would run for a long period, fail , respond to the on-off routine, and run for another long period. It had now again reached the stage where it failed to respond to this treatment, even after repeated tries. I switched it on while the owner was there. And, yes, it was completely dead. So I felt that, at last, I might get to grips with it. Another bonus was that it was not likely to be needed again for several months and I could take m y time. So I put it aside for several days due to pressure of other jobs. And that was a mistake; when I did finally switch it on, it came good immediately. Back to square one. This time I decided to try a different approach. The trick is to bypass the chopper/regulator section of the power supply and run the set directly from the bridge rectifier. If it fails in this configuration, the fault is almost certainly in the horizontal system and, in any case, with power still applied, one has a better chance to track it down. It's a simple trick. The set is fed from a Variac and a jumper lead is used to connect fuse F903 to the cath- ode side of diode CR908 (top right of circuit, n ear T902). The Variac is then wound up until the voltage at this point reaches 125V - or a little less to provide a safety margin. I set everything up to enable me to quickly do this and then left the set running, waiting for it to fail. Eventually, it did and, what's more, it refused to start using the off/on technique. This was the ideal opportunity for the bypass trick - I quickly connected the jumper lead and wound the Variac up, whereupon the set leapt into life. And it kept on going. From this I deduced that the fault AUGUST 1991 63 SERVICEMAN'S LOG - CTD And. yes ... it was completely dead ... was in the power supply and spent some time going over the board again, looking for an elusive dry joint which I might have missed the first time. I drew a blank. There are a couple of other tricks one can try in this situation. First, by bridging capacitor C910 with a ·1kQ resistor, the multi vibrator can be made to run continuously, regardless of the condition of the horizontal stage. This allows the power supply to be checked stage by stage until the fault is located. But there is a point to watch here. If the rest of the set is not drawing current, the over-voltage network consisting of TR908 & TR909 will shut everything down. This can be prevented by temporarily disconnecting R941. A simpler trick is to repeatedly switch the set on and off, to activate the kick-start system, and use a CRO (ideally a storage type) to check for the pulses, stage by stage, up to TR906. 64 SILICON CHIP This was what I did and I managed to confirm that TR906 was indeed receiving these pulses. Murphy's lunch By now, of course, the set was running again. The next time it failed, I again reverted to the jumper lead/ Variac setup. And that clinched it; the set refused to run. The fault was not in the power supply. (At long last I had caught Murphy out at lunch). I restored the power supply to its normal configuration, then moved over to the horizontal system. Unfortunately, we don 't have sufficient space to reproduce the circuit, which is quite extensive. It involves two separate PC boards; the Deflection Chassis Board and the Deflection Output Chassis Board. The horizontal oscillator (TR703) is on the first board, while the driver stage (TR704) and the output stage (TR707) are on the second board. One of the nasty aspects of this part of the set is the mechanical setup. The Deflection Output Board is mounted vertically on the r-ight hand side of the set, copper side out. And mounted on the copper side, supported on spacers, is a large heatsink carrying the output transistor, TR707. This creates a problem because all solder joints under this heatsink are completely inaccessible. And at least two components in this area, R721 and R722 in the base circuit of the horizontal driver transistor (TR704), have a reputation for dry joints. Another item obscured by this heatsink is a wire-wrap pin inserted from the component side and soldered to the copper pattern. The wire-wrap lead from it runs to a similar wirewrap pin on the Deflection Chassis Board. From here, the circuit runs to the collector ofTR703, the horizontal oscillator. All of which is by way of background. Having exonerated the power supply, I connected one CRO lead to the collector of the horizontal oscillator (TR903) and another to the base of the driver transistor (TR704). I then switched the set on several times to active the kick-start circuit. This produced a brief burst of voltage on the HT rail, sufficient to active the horizontal oscillator and produce a short burst of oscillation. So far so good. But there was nothing at the base of TR704; the fault was between these two points. I was getting closer but the exact cause still had to be found. And considering its intermittent nature, it still looked like there was a long haul ahead. I lifted the heatsink clear of the board and made a visual check of the path, joint by joint: Each appeared to be perfect but since this was obviously not the case, I went over each one and resoldered it. That done, I did the same around the oscillator section, involving TR703. And that was it; the set hasn't missed a beat since, which was something of an anti-climax. Which joint was it7 I can't be sure but I strongly suspect one or both of the wire-wrap pin joints; mainly because the solder seemed to come away from these much too readily. And the TC53L2 set? It had a very common fault involving the over-voltage protection circuit, TR908 and TR909. I replaced both transistors and that was it. SC