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SERVICEMAN'S LOG Death rattles: a thorny problem We go south again this month, to commiserate with one J. L., who regularly livens up these notes. Maybe he lives in the Apple Isle, but everything was not apples for several weeks while he wrestled with an obscure fault in a Thorn colour set. This is a tale of woe but with a happy ending. The woe has lasted three weeks or more but ifI had known when it started what I know now, it could still have taken three weeks or more. The set was a 34cm English Thorn, model 9418, now close to 15 years old. The complaint was that it often stopped working, for no reason that the owner could discover. When it was going, it worked well and showed what he believed was an excellent picture. However, it had been getting worse and he was finally forced to have it attended to. I fired it up on the bench before he left and, according to Murphy's Law, it started and kept running. The owner said it would fail within five minutes loss Our -IS your gain - ---, ' I I II ' L ~ -· . · Fracarro's ESVU2. The really low loss VHF/ UHF mixer with areasonable price tag. Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Ad.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 64 SILICON CHIP but half an hour later it was still going. It did the same next morning but just before lunch it gave out a loud clattering noise and quit cold. I whipped the back off the cabinet, then switched it on again. There was another clattering protest from the chassis and nothing more. I located the 115V rail test point and hooked a meter across it. Several start-ups later I had learned nothing more than that the power supply could only deliver about 80V before it shut down. There was nothing to indicate what was causing the shutdown and finding the cause was going to give me some nasty headaches in the weeks ahead. · I had previously serviced the set some years ago for a minor problem but that didn't require a circuit diagram. This time, it was going to be different so I set about my files with a vengeance. Most of the Thorn models were actually an AWA chassis in disguise (or should that be the other way round?) but there was nothing like a 9418 among them. Neither was there anything similar among the uniquely Thorn models. Finally, in desperation, I turned to a list of AWA models that had just then arrived in the mail. The list was helpful, in an unhelpful sort of way. Instead of telling me that the 9418 used a so-and-so chassis, it said simply "Refer to Radio Rentals". Which didn't bode well for me, since the model was so old that the company was now unlikely to have any infor- mation on it in its files. Fortunately, I have a couple of mates who work for RR, so I fronted up and begged for assistance. All they could tell me was that they wished me luck with the TX9, as they called it. There hadn't been one in their workshop for many years, for which they were very thankful. They remembered the chassis as · the very devil to work on and a horrible thing to troubleshoot. In fact, this chassis is an English model and is grossly over-designed, as is their wont. It wasn't hard to see why my friends were glad that it was somebody else who had to fix the soand-so thing. On the credit side, they did have a copy of the circuit diagram somewhere and eventually turned up a much photocopied drawing, many generations from the original. But at least it could show me was how the circuit was arranged, even though many component values and part numbers were almost indecipherable. As a first approach to fault finding, I gave the board a thorough going over with a strong magnifying glass, look- ••• ,w 6:NGL\Slol T'~N-THlii::' COM'PLJ:~1N-r WA.~~ 1-r OFllis-N ~ WORKING, FoR~~0~11tS: OW~Q C.OV'-1:> l>\SCOV'~.. , likely to have a suitable replacement. At this point I put the set aside until I could contact the owner. It seemed that from this point on whatever I did was going to take some considerable tim e , whether I prov ed the transformer faulty or not. I wanted to know if he valued the set enough to pay for a long and involved repair. As it happ ened, he was away for a couple of weeks and I had time to talk to various colleagues about the baulky Thorn. My main interest was the noise it was making as it turned off. .,..,... AN'9 11-\\S O'PEiNE-D The noise could best be described as FIR.S"'r CP\N OF WORM~ "'0-\AT someone play ing 'I: s-r,;r.uc.t<: VJ rr t-\ 11-\,s the castanets . It was a loud, wooden , rating for dry joints or other physical tly sound that sometimes went on problems. This revealed nothing that and on until I switched it off to save could explain the intermittent nature my sanity. Mostly though, it just gave of the trouble. a short rattle and then quit. I had never heard anything like it A can of worms and I tried to find out from others if Next I tried monitoring the collecthey could explain it. More particutor of the line output transistor. The larly, I hoped they could suggest any shape of the pulse at this point can source other than the line output transoften indicate the type of fault that's former. eventually exposed. And this opened Nobody had any more ideas than I the first can of worms that I struck had and mostly they just shook their heads and murmured "you poor-----" with this set. Back at the bench, I needed to devise The line output collector waveform a way to check the line output transis usually a single spike of anything between 800 and 1500V, at line freformer. It was one of the very early quency. In this set the pulse was there diode-split types and I have not found but at many different amplitudes and the usual LOPT testing routines to be at many different frequencies. In fact, very helpful for checking these transI couldn't get the CRO to resolve anyformers. thing but a jumbled mess in the few It seemed as though I had two seconds before the power supply shut choices. One was to power the set down. from an external 115V source. The I didn't like the look of the way this other was to isolate the set from its job was unfolding, since the loud clatown power supply and see if the lattering noise seemed to be coming from ter would run on a dummy load. the line output transformer. If that The first idea was the easier of the proved to be faulty, the set might well two and would have be en implebe a write-off, since it was unique to mented ifmy high voltage power supRadio Rentals and nobody else was ply had not chosen to kark it. The °™~ 5~,... supply is a simp le DC source that I drive from a Variac. The output is filtered but unregulated and does a good job in cas es like this. As far as I could tell, the only trouble w as that the smoke had escaped fro m one of the diodes but I wasn't in the mood to try to replace it at that time. So that left me with option two - to get the set's own supply running. And this, at last, pointed m e in the right direction. The circuit diagram (see Fig.1) shows a link (extrem e right) th at can be used to isolate the power sup ply. This implied that the supp ly should run indep endently of the line output stage and that it was not one of those fi endish designs needing a line pulse to keep it running. But w hat kind of power supply was it? Most ofmy coll eagues referred to it a switched mo de supply but I had some doubts about this terminology; p erhaps it de pends on on e's definition of a switched mode supply. I w as more inclined to regard it as a simple transformerless arrangement w ith a switching functio n to facilitate phase controlled voltage regulation. Jose's Spanish Dancers Anyw ay, terminology aside, I fo und the link on the board and took to it w ith a pair of sidecutters. I th en wired a dummy load (a 60W light bu lb) across the supply to simu late th e absent receiver load. An d in next to no time I h ad the supp ly u p and rattling like Jose's Spanish Dancers. If anything , the n oise was now even more like the rattle of castan ets. With the line stage disconnected, it was apparent th at the rattle was coming from a large w ind ing n ear the back edge of the chassis. This turned out to be L65 (top centre), which appears to be a simple filt er choke. As m entioned earlier, this set was grossly over-designed, as is so much English equipment. The power supply alone employs three SCRs, three tran sistors, 16 diodes and innumerable res istors and capacitors . Finding out w hich of that lot was fau lty was not going to be easy. By sw itching the set on re peatedly, I learned that it h ad two m odes of failure. On e was immediate in that it started clattering straight away. The other allowed the supply to get up to sp eed but then run with the nasty rattle already described. SEPTEMBER1992 65 SERVICEMAN'S LOG - CTD By monitoring the output, I found that the first type of failure caused the supp ly to shut down as the output reached about 80V. With the other failure, it ran long enough to actually reach 115V. However, this was varying wildly, swinging some l0V either way. At other times, it would swing even higher and the supply would shut down. So that was what I was faced with and somehow I had to sort out the trouble. pacitor C147 (bottom right with C148). So this implied that all current before D82 had to be very lumpy DC. That was the start. Next, CSR2 (right of centre) was obviously wired as a crowbar protection circuit, driven either by transistor TR66 (extreme right) in the overcurrent mode or via a 130V zener D85 (below TR66) in the over-voltage mode. This circuit is designed to blow the main fuse in the event of an overload. Since this wasn't happening, I felt that I could ignore this part of the circuit, at least for the time being. CSR3 (left of centre) is another small thyristor and this triggers the gate of CSRl via transformer Tl. From here, I theorised that since the base of TR65 (centre) was linked to the slider of a trimpot marked "set 115V", this transistor had to be the error amplifier (its collector drives CSR3's gate). And because the trimpot was in a circuit across the first main filter capacitor (C147), the transistor could only be there to set a DC level on the gate of CSR3. Switching pulses for CSR3 are derived from the output of the bridge rectifier via TR62, D72 and a filter network consisting ofC138 and R168. Transistor TR62 is fed from a divider across the rectifier output and pro- How it works My first problem was to work out how the system was supposed to function. With unknown circuits like this one, I begin by redrawing the circuit diagram, omitting everything except the transistors and the main capacitors and inductors. This leaves me with the bare-bones outline of the circuit and I can imagine what needs to happen to produce the desired outcome. In this case, since there was no input filter capacitor after the bridge rectifier, we had a raw 100Hz DC supply on the anode of the main switching thyristor CSRl (top left) . The current out of CSRl passes through L65 and diode D82 (to the right of L65) before reaching filter ea- C ~ .. "',. '°" . ,.,.._. " " ' ::: ~ '., 071 u II .._ __ 11U1 111('),l(t-' \.- . ..0 ~. 0 • .0-~ ~/~\C ~~~ MJ,. 66 SILICON CHIP ~•..t\ &Clll ·~-~ :.a 0 8 UO'll't ! " . u,u DC )Of Fig.1 (below): as can be seen, the only available Thorn 9418 circuit was pretty grotty. Our draughtsman has fitted fresh labels to help identify those components mentioned in the text. Basically, the circuit uses CSRl to phase control unfiltered DC from the bridge rectifier. The output from CSRl is then filtered by L65, C147 and C148. TR65 is the error amplifier. s . ~ .. "'"°,f duces a 100Hz square wave on its collector (point 2). This is then fed via D72 to the filter network to produce a sawtooth waveform [point 3) which is in phase with the 100Hz unfiltered DC. Thus, CSR3 triggers earlier or later in the cycle, depending on the voltage on its gate. As can be seen, there are an awful lot of components additional to those mentioned and any one of them could have been causing the trouble. What I needed to do was to isolate the various parts of the circuit to see ifl could find out just where the trouble was occurring. Although the circuit diagram shows four oscillogram numbers (in circles), I didn't have copies of these. All I could do was check these waveforms and try to make sense of what I found. Numbers 1 and 2 made perfect sense. No.1 was the l00Hz ripple one would expect out of a bridge rectifier. And No. 2 was a steady square wave _ ,w:r 11.11" ec llt o· I C~I - H ~, _,. __. ~cao..... ,.,,._ --E:: ~JC\ A( .... . 111 ...&1.1 'fl{'"W{O nlOw ~~~ "lS.S IOII derived from the ripple. So far, so good. Waveform number 3 was a different story altogether. Although it still bore a similarity to the waveform at 2, it was jumping about all over the place, changing both amplitude and frequency in a completely random manner. And the waveform at 4 was following 3. My problem was to find out whether the irregularities at 3 and 4 were the cause or the effect of whatever the fault turned out to be. I began my attack by changing each of the semiconductors in turn. And while each was out of circuit, I tested the resistors around it for value. After this exercise produced nothing of any note, I started on the capacitors, removing each and testing for capacitance and leakage. Again , I drew a blank. The only thing I couldn't test was the 130V zener, D85 . But, as previously mentioned, the crowbar circuit didn't seem to be affected by the fault, so testing D85 had a very low priority. Doing it the hard way Eventually, it got to the stage where TETIA TV TIP AWA C2341 TV set Symptom: only even numbered channels can be selected from the front panel controls. There is no problem with the remote control and all channels are readily available. Cure: this was caused by a fault in the control microprocessor. The front panel selector buttons communicate with the chip on different pins to the remote control receiver, hence the faultless performance of the handpiece. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay 7015. I had tested virtually everything on the board. There was absolutely nothing that showed up faulty under static tests. That meant that I had to start substituting for suspect components, AUSTRALIAN MADE TV TEST EQUIPMENT 10% discount on all test equipment. 15% discount on two or more items. SHORTED TURNS TESTER Built-in meter to check EHT transformers including split diode type , yokes and drive transformers . $78.00 + $4.00 p&p since some faults only show up under operating conditions (ie, with voltages applied). First, I replaced the three thyristors and the three transistors but this gave me no joy. Next, because there were less capacitors than diodes, I elected to replace the capacitors. And it was here that I solved the problem, although I still don't know exactly what the problem was. I began with the electrolytic capacitors: C127, C143, C144, C146, C147 and C148. The smaller of these I simply replaced but the last two were combined in one can and could only be tested for capacity and leakage at working voltage. To do this, I had to resurrect my high voltage supply but the effort was pointless. None of the electros showed the slightest sign of trouble. Next, I started on the ceramic capacitors. I ignored C136 and those around the bridge rectifier on the grounds that there was no sign of trouble on that side of CSR1/TR62. The next capacitor was the previously mentioned C138. It's a 150nF ceramic rated at 100V. I don't have ANTRIM TOROIDAL TRANSFORMERS HIGH-VOLTAGE PROBE Built-in meter reads positive or negative voltages from 0-50kV. For checking EHT and other HT voltages. $98.00 + $5.00 p&p DEGAUSSING WAND Strong magnetic field. Double insulated for your safety with momentary switch operation. 240V AC 2.2 amps. As important as a soldering iron! $75.00 + $10.00 p&p REMOTE CONTROL TESTER Designed to test infrared or ultrasonic control units. Supplied with extension infrared detector lead. Output is via a LED and piezo speaker. $84.00 + $4.00 p&p From $19 exchange plus post & pack Z'Ji I -~- T~NE~-~ Cheque, Money Order, Visa, Bankcard or Mastercard Phone (02) 77 4 1154 216 Canterbury Rd, Reve~bJ~t~;i:. 2212, Fax (02) 77 4 1154 • • • • Now made in Australia Standard range 15VA to 2kVA Large ex-stock quantities available Competitive pricing available for OE Ms & resellers • Simply the best transformers all round HARBUCH ELECTRONICS PTY LTD 90 George St, Hornsby, NSW 2077. Phone (02) 476 5854 Fax (02) 476 3231 SEPTEMBER1992 67 c.EN-rRe:) CSR~ ("R\G-H"r OF W~'S OSV\005\..V W\R&t:> AS ~ CROW6AR ?'R0"t'"e.c,...,-1ON ••• .. o any 150nF capacitors in my stock. In fact, I don't know anybody who does. But I do have a good supply of 0.15µF capacitors and I wasted no time fitting one. And that was all it took. At next switch-on, the supply came up to 115V and sat there without a flicker. At last the clattering noise that had so bedevilled the workshop had been eliminated. I reconnected the link to the line output stage and the set came good with an excellent though somewhat blue picture. A brief tweak on the screen pots soon put that right and the set is now producing a really firstclass result. Considering its age , it's quite remarkable that it works so well. Naturally, the owner was delighted . that it proved not to be the expensive transformer that I had originally suspected and was happy to pay my larger than usual bill. And I don 't know what was wrong with the capacitor. But it certainly made a mess of an otherwise nice little TV set. Thanks J. L. I sympathise concerning what must have been a really nasty 68 SILICON CHIP one. A colleague to whom I showed the circuit commented that it looked like the Thorn 8000, which was the basis for the early AWA colour sets. But he agreed that the similarity was largely superficial. The things I'm asked And from my own notes, a brief one to finish off. As I imagine most of my colleagues will testify, servicemen get asked some very funny questions at times - funny peculiar, that is. It happened to me again quite recently. I answered the phone and the caller introduced himself as a friend of one of my customers, who had assured him that I could help. And the problem? How to receive interstate country radio stations, particularly in Queensland and Victoria . And it was quickly evident that the caller hadn't a clue. He fondly believed that it was only a matter of buying a sufficiently sensitive receiver and the problem would be solved. His query set me back a bit. I suddenly realised that I had completely lost touch with such activities. It was all the rage when I was a teenager (yes, I was one once!) and advertisements for top of the line receivers always boasted that interstate reception was guaranteed. And many a tall story was told about the number of distant stations logged in one night. But all that had died out decades ago. So why the sudden interest? And what was the technical situation now? Sure, transmitter powers have increased markedly but there are also many more stations on the air, many of them sharing frequencies. I summarised the situation, as best I could, on the basis that there was little future in it. Sure, some stations might be heard at night, but not very reliably. Fading would be a problem, accompanied by distortion and adjacent channel interference. If he simply wanted to log a station, for the fun ofit, OK. But ifhe wanted to seriously follow a particular program for any length of time, well, forget it. Which prompted the logical question: why did he want to do it anyway? This caused a pause at the other end of the line . "Well, I'll tell you the truth. The TAB has been taking a lot of money from me and not giving anything back. And I've worked out a system which I reckon will beat them." (I'm afraid I laughed outright at the word "system", but he was obviously deadly serious). "The trouble is, the local radio stations are not allowed to broadcast the result of interstate races for half an hour after the · race. But if I could listen to the interstate stations I reckon my system would work". I said, "Well, I'm sorry mate but I'm afraid you're flogging a dead horse: (no pun intended). "There's just no way you can receive signals on this basis." · So he thanked me for the advice and added, "At least you've saved me spending a couple of grand for nothing". A couple of grand? Just to beat the TAB! So .what was this "system"? I'm no follower of the Sport of Kings. I recall putting a couple of bob into an office Melbourne Cup sweep many years ago but it must have been rigged because I didn't get anything back either. So I'm not competent to comment. But a couple of grand to beat the TAB? Oh well, it takes all sorts I s'pose. SC