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A Sharp in Pye clothing What's in a name? So goes the popular adage and the implication is: not much. Even the Bard was moved to suggest that "a rose by any other name would smell as sweet". Maybe so, but a TV chassis made under one name and sold under another can smell sour indeed if the original name is not immediately apparent. This story started when one of my regular customers called into the shop and announced that his TV set had finally packed it in. This was no surprise really because he had been complaining for some time that it was "getting a bit cranky", although the fault never seemed to be definite enough to justify his bringing the set in. Now it had stopped completely so at least I would have something tangible to look for. When he brought the set in it turned out to be a 34cm Pye Model 14G4. This was a set which, as far as I could recall, I had not encountered before. What was more, I doubted whether I had a manual for it and a search through my Pye manuals confirmed this, the closest one being for a Pye 14G 1. This didn't help much. The 14G1 is actually a Toshiba chassis which was also marketed under the Precedent label. More to the point, it was obviously nothing like the set in front of me. There was little doubt in my mind that the chassis had come from another manufacturer but, for the life of me, I couldn't identify it. Stop laughing Such a situation can be serious. If the fault is relatively simple it is sometimes possible to muddle through on the basis of one's general knowledge. But if it's a sticky one, it may be wiser to knock it back before becoming involved. 58 SILICON CHIP Time is money and such sets are best left to someone who has more experience with that particular model. But I wasn't going to give up immediately. I decided to give the set a quick once over for anything obvious and, if this showed promise, approach some of my colleages in the hope that I could borrow a manual. Time enough to panic when all else failed, I reckoned. In the meantime, I felt sure that I should be able to recognise the chassis, if only I could tap into the right memory bank. Hopefully, working on the chassis might provide a clue. A preliminary check revealed an immediate reason why the set would not work - both mains fuses, 2A delay types, were blown. From here I moved to the power TETIA TV TIP National CP2000 Symptom: Hum bars. Vertical lines in the picture waver from side to side and sometimes black patches can be seen at the edges of the screen . Only 1V difference between collector of TR806 and TP-S1, instead of 8V. TR851 (2SC64 7) leaky or short circuited. Cure: Replace TR851 and also TR806 (2SC582), 0814 (EQB01-15) and 0812 (SV04) . Any or all of these components can break down under load and destroy TR851 . supply, a fairly conventional arrangement consisting of a bridge rectifier, main filter capacitor and a 2SC2365 chopper-cum-regulator transistor. The latter was an obvious suspect and sure enough, it was a top-quality short circuit. Based on experience, my next suspect was the horizontal output transistor, a 2SD869. Right again. Which was all very gratifying up to a point but gave little indication as to what had caused all this; which transistor had failed first and taken out the other one? I decided to replace both transistors, then sneak up on the set and try to find any other faults without doing any more damage. I replaced the chopper transistor with a 2SD380 and the horizontal output transistor with a 2SD898. Sneaking up With two new fuses in place it was time to start my sneaking up procedure. I connected the set to a Variac and gradually increased the input voltage while monitoring what was fairly obviously the main HT rail. Nothing happened - good or bad. There was no sign of any HT voltage, nor were there any signs of distress at a moderate input voltage. But there was obviously another fault lurking somewhere. A resistance check between the HT rail and chassis confirmed my suspicions - a dead short. But where? Now I really did need a circuit; the idea of trying to track down a short of this kind, on a rail which would undoubtedly wander all over the place, did not appeal one little bit. Then suddenly the memory bank came good; the chassis I was looking at was a Sharp. With spirits revived I began searching through my Sharp manuals, hoping that I might have the right one and that I . : i ~m • I l!U , 7. ~ l oo - . 0 . Q) --- .... Having made that deduction, checking the zener (ZD702} was largely a formality and explained the HT rail-to-chassis short. It also suggested a likely sequence of events. My bet is that the chopper transistor failed first and the zener made the supreme sacrifice as a result - but not before the horizontal output transistor had also succumbed. That much established I simply lifted the zener and started from scratch again with the Variac . This time I was rewarded with a rail voltage and some signs of life, in the form of a raster. Well, that was something but there was no sign of a picture and no sound. Also, the raster was very bright, with visible flyback lines and some evidence of hum. The brightness control also appeared to be inoperative. Other damage ... wrn-1-rwo t\lE-W FUS&S IN 'PL.AGe::, l'T' VJ~S -r1N\~ -ro ~A~ M"< SNe:AK\NG UP ?ROCEl>~~ would be able to recognise it if I did. It so happens that, some time ago, a colleague retired from his business and I was able to buy a stack of manuals from him at a very modest price. This now paid off handsomely. The first clue was the photograph on the cover, even though this was not necessarily conclusive. But what was conclusive was a notation above it, in my colleague's handwriting, "Pye 14G4". With the circuit in front of me, everything made a lot more sense. I had done everything right so far, including picking the HT rail, which I now learned operated at 115V. More importantly I was reminded of a design trick used on many Sharp models; that of fitting a sacrificial zener diode between the HT rail and chassis to protect the rest of the set · in the event of a voltage regulator failure in the power supply. (This is not the only protective circuit in this set}. The zener is normally rated about 10V above the rail voltage and, in the event of a minor overload, will simply load the rail and hold the voltage down. But there is very little protective resistance in its circuit and a major fault will cause it to break down permanently. All the evidence suggested that there had been a major fault. The circuit is such that a shorted chopper transistor will put the full bridge rectifier voltage, some 300V plus, on the HT rail; more than double its intended value. And that could do a lot of damage. All in all, the situation didn't look too good. With no picture or sound I began speculating on what other damage the excessive rail voltage may have caused, particularly to the front end components such as the tuner, the IF signal processing IC (!201}, and the video signal processing IC (!401}. This latter includes the brightness control circuit. I searched through my stock of !Cs and to my surprise found that I had both the 1201 unit (0062CE} and the !401 unit (0118CE} in stock. I had apparently bought them at some time for a now long-forgotten job but had never used them. Well, at least it would not be hard to provide replacements if further checking supported such a move. But I had no intention of taking such a drastic step at this stage. To begin with, I was more concerned with the excessive screen brightness. The neck board of this set carries an adjustment called a "screen" control. This consists of a pot, R878, which varies the G2 voltage on the picture tube. It is used as an auxiliary brightness control during setting up procedures. On an impulse I wound this control back as far as it would go. It had some effect but not much. Then I had another inspiration; I wound back the bias controls - R851 , FEBRUARY1989 59 2SAIO 15fYl/ 2SA 10291CI uf u, J.lc•12 R43t 22oP a 47K J 0 406 v.:.,8 TO·H2/ R422\ , R426 5K(8) ; · Jl.. LU I 911,c;,mus I ~ 50M (8) R42S 10K (A) s•101 MAINS ~ LU -~ POWER 240v QACCL30vl!CEZZ TO-HI TO Fig.I: power supply and horizontal output circuitry for the Pye 14G4. The chopper transistor (Q701) failed and took out several other components with it. The neck board is supplied from point H2 at the bottom of the circuit (ie, from the HT rail via ZD702). R856 and R863 - on the red, green and blue driver transistors (Q850, Q851 and Q852). This reduced the brightness much more effectively, even though I wasn't sure just what this was supposed to prove. In fact, it proved something very important because a closer look at the screen revealed that I now had a faint picture. Perhaps "proved" was too strong a word but it at least suggested that my fears about damage to front end components were groundless. Thus encouraged, I began a more detailed examination of the neck board operating conditions and it didn't take long to establish that the collectors of the three driver transistors (red, green and blue) had no voltage on them. Small wonder that the screen was excessively bright. These collectors are fed from the 60 SILICON CHIP 115V HT rail via a 100 safety resistor, R725, which connects to a line marked "HZ" (shown at the bottom of the circuit). This resistor was open circuit and replacing it brought the picture tube operating conditions back to normal. Of course, I had to reset the screen and bias pots back to near their original settings because the screen was now much too dark. (I subsequently did a complete grey scale adjustment). This left the lack of sound as the only remaining problem although I had no inkling of what solving this would entail. The sound section is contained in IC 1301 (0096CE) and my first move was to check the supply voltage to this IC, which is supposed to be 12.BV applied to pin 10. In fact, it was zero. This voltage is derived from the secondary wind- ing of the power supply transformer, T701, via diode D706 and filter capacitor C716. I checked across the filter capacitor but there was no voltage there either. I checked the diode and it was OK. There is also a safety resistor, R719, in the circuit and this checked OK. By this time, there was little left to suspect other than a lack of voltage across the transformer winding and this proved to be the case. But why? The winding was intact so why wouldn't it produce voltage? (you'll never guess the answer). One thing was obvious - there was something very silly going on in the power supply. As well as the lack of secondary voltage, there were two other symptoms: a quite obvious hum pattern on the screen (which I mentioned earlier) and, I now mode at the correct voltage by courtesy of the Variac (which meant that there was no voltage regulation). Had I paid more attention to the setting of the Variac, I might have woken up sooner. It also explained why there was no voltage across the secondary winding; there was only DC flowing in the primary instead of the chopped current from the chopper transistor. I fitted another 2SD380 - they are not cheap, by the way - refitted Q702, Q703 and ZD702, tidied everything up and tried again. I paid more attention to both the input voltage from the Variac and the behaviour of the HT rail this time and was gratified to note that the latter pegged at 115V as the input approached 240V. What's more, it remained constant as I simulated a typical range of mains voltage variation. And, of course, the sound came good, thus bringing the set virtually back to normal. As I mentioned earlier, it needed a grey scale adjustment and I found that the volume control was horribly noisy and had to be replaced. Otherwise, that was it. The set has now been running for several weeks and hasn't missed a beat. So that was another happy ending but that's about the only part of it that was happy. I don't want too many more like that. I WAS N\~e. CONc£RI\JE:D WITI-1 TH£. From the Apple Isle E:XCE.SSIV~ SG~E.E.N ~RIGH1tJE.SSoo• To change the pace, realised, a total lack of regulation within the supply. It was sitting on 115V simply because I had adjusted the Variac to produce this voltage. Regulator checks I switched the set off and began a systematic check of all the components involved in the regulator section. I pulled out transistors Q702 and Q703 and these checked OK. While they were out I checked some half dozen diodes and a zener diode in this section, along with the various resistors, but found nothing. That seemed to leave only the capacitors, since the chopper transistor (Q701) had already been replaced. Nevertheless, on a hunch, I checked the transistor again. And would you believe it, it was a dead short. How or why I don't know. The only explanation I can offer is that it failed when I first attempted to energise the HT rail, before I discovered that zener ZD702 had broken down (which is surprising because I advanced the Variac only marginally before I realised that there was unlikely to be any HT voltage). Still, this discovery explained a lot of things. As I mentioned earlier, a shorted chopper transistor in this set puts the bridge rectifier DC voltage directly on the HT rail, so the set had been operating in this here are three shorter stories from my colleague, J. L., in the Apple Isle. They emphasise the variety of problems a serviceman is expected deal with. Here's how he tells it. A customer brought in a Teac stereo amplifier, complaining that it continually blew fuses. In more detail, it had occasionally blown one of its two fuses for no obvious reason. But recently it blew both and now would not work at all. A quick examination showed that one fuse was identified on the board as a 3.15A 2AG type and the other as a 4A 2AG type. The reason for the difference was not clear because both seemed to be associated with the DC supply. I guessed that they might protect two FEBRUARY1989 61 SERVICEMAN'S LOG separate rails - the output stage guitar amplifier. The guitar faults and the rest of the works. were obvious and easy to fix but the I put the job aside until I could amp problem took a little ingenuity get a circuit or service manual. This to sort out. proved to be a lengthy exercise but Both guitars were suffering from eventually I had both. And five the same fault. The 6.5mm phono minutes later I knew all about this output sockets had been forcibly particular amplifier and its history. torn from their bodies and one of This amplifier was a Teac A-515 them had taken the tone control pot 38W per channel model. Teac also with it. made a 25W model, the A-313, usI understand that this is a coming the same circuit board. The mon problem. Young musicians try larger unit requires two 4A fuses in to emulate the on-stage antics of the feed lines to the bridge rectifier. their rock-and-roll heroes and a The smaller one uses 3.15A fuses in 2-metre lead will not stretch to this position. And rather than over- · three metres without something givprint different boards for each ing way. Either the plug comes off model, Teac printed the one board the lead or the socket comes out of to suit both types. the guitar. It seems that, in this amplifier, a One of these guitars had been in fuse had failed and it just happened for the same repair some time ago to be the one in the holder marked and I had made a new mounting 3.15A. It was replaced, not with plate for the socket from a heavier another 4A type but with a 3.15A plastic sheet than used on the type as marked on the board. original. The students had still Of course, this didn't last all that managed to break it so this time I long and was soon replaced with made a plate from 1.5mm-thick yet another 3.15A fuse. This ac- aluminium sheet. They won't break counted for the history of fuse that in a hurry. failures. The amplifier was reported to be When I examined the board more "distorting", though I couldn't closely, I found that the fuse holder understand how anyone would hear marked for a 3.15A fuse also had a that in the complex signals it was tiny rectangle alongside with the being asked to process. number A-313 inside it. The other I connected one of the guitars fuse, the 4A one, had a similar symand strummed a few chords. The bol marked A-515. output at workshop level sounded It could be argued that these quite clean. But then, who ever markings are adequate but this pre- heard of a rock guitarist playing at supposes the knowledge that there workshop level? So I wound up the are two similar model amplifiers, wick to loud, then to very loud, then using the same circuit board with to glass-shattering level - and different fuses. I didn't know that, finally I heard it. nor did the owner or whoever I wasn't quite sure what it was replaced the fuse on those previous but it sounded very like something occasions. Not a really creditable loose inside the amplifier cabinet. arrangement, is it? What I needed was a steady, high The reason for the failure of both level tone that would keep the rattle fuses was simply that the power going while I searched for the amplifier IC, an STK465, had source. My audio oscillator providshorted internally. A replacement ed a suitable signal and I soon had IC put the amp back into first class lO0Hz bouncing around the shack condition. And I scratched the and rattling even the books on the misleading fuse rating from the cirshelves. But it didn't rattle the amp. cuit board. That mistake won't hapSo I ran the oscillator up towards pen again. lkHz. Then I found it. At about School work 250Hz the cabinet back began to vibrate so strongly that it disapThe local high school sent up a peared into a black blurr. When I couple of electric guitars and a 62 SILICON CHIP looked more carefully, I found that each of the 16 screws holding the back was undone by about half turn. That bit was easy to solve. I continued to crank up the frequency and at about 800Hz there was another strong, noisy vibration. This time it was one of the tone controls on the front panel. It had come loose and the lock nut was free on the shaft. This was a little harder to secure but it was eventually done and the high school can now start next term with a distortionless rock band. a Customer problems I wish customers could be more accurate in their description of TV faults but I suppose that's hoping for too much. I've just spent the best part of a week going backwards and forwards to an old Precedent that was supposed to be suffering from erratic fine tuning. In fact, fine tuning was only incidental to the real fault. The customer rang one evening and said that her picture had gone all wriggly, then had bent in the middle and finally dissolved into masses of dots. I asked her if the other channel was also affected. She hadn't even looked at that before asking for help. She had to leave the phone to check. Back on the phone, she said the other channel was OK but her favourite program was still all dotty. Would I come and have a look at it? I suggested that it sounded like a fine tuning problem which she could probably fix herself. I told her what to do and to ring me back only if that would not fix the trouble. I heard no more that evening, and it was later the next day tbat she rang to say that "the fine tuning fixed the fault last night but now it's back again and this time the fine tuning won't work. What's more, both channels are the same now! " There was nothing I could do but put her on the list for a visit the next day. When I finally fronted the set, the owner rather shamefacedly appologised because the set had been perfect since the previous morning. It was displaying quite a good wpoD FOR CHIPS ... WOOD FOR CHIPS ... WOOD FOR CHIPS ... WOOD FOR CHIPS ... WOOD FOR CHIPS ... WOOD FOR CHIPS ... WOOD FOR C NEW VARIABLE TRANSFORMERS NEW - National Semiconductor Data Books ~ ~ O;: 0 ~ Cl 0 ~ The latest editions (1988) just received. Hurry because they never lastl Interface contains all you could possibly require on. 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Approximately 412mm Wide x 80mm High x 370mm Deep UPS-350, 350 Watts Square Wave $754.00 UPS-501, 500 Watts Square Wave $841.00 HSG-0052 2 x 36- 0.1 in wire wrap 2 x 30 • 0.1 in wire wrap 1 x 48 • 0.1in wire wrap 2 x 50 · 0.125in PCB 2 x 40 · 0.125in WIW 2 x 40 • 0.125in RIA PCB 1 x 8 • 0.156in Solder 1 x 1O• 0.156in Solder 1 x 15 • 0.156in Solder 2 x 43 • 0.156in WIW 0-250V 0-2.52A$118.00 ew o w ---..-- - -0° Heavy Duty Connector Wooay won't tell us how he does it but here's another super connector special. • 24 Pin polarised plugs and sockets. • Chassis Mounting. • Genuine gold plated contacts. • Full 5 amp rating. v_w_,_,_ ,. ,.;; _ .._ • .;;.11_111.,.. , ' ' . ~ ~ .... - Dimensions Panel Cutout 65mm x 20mm Mounting Holes M3 clearance on 73mm centres Plugs Sockets Height 81 .8mm 81.8mm Width 22.2mm 22.2mm Depth 24.6mm 22.7mm You could pay over $25.00 each for these little beauties, but Geoff said to clear them at a measly unit price of $9.50 But, as they say on the box, that's not all I C65.M 6.5mm Panel Mount Mono Socket were $1 .40 now $0.50 PCB Mount "D" Connector• D-15-PLG-RA Metal Shell were $4.00 now $2.00 D-25-PLG-STR Metal Shell were $4.50 now $2.00 only $17.50 Now $7.30 $6.35 $8.00 $4.00 $15.95 $15.95 $15.95 $2.40 $3.75 $4.65 $17.05 $8.00 $7.50 $7.50 $1.20 $1 .90 $2.40 $8.50 Others claim to be No 1 in semis(or is that no-one) but we a// know it's Wood for Chips, so we have taken the axe to this selection of semiconductor specials. If he keeps giving stuff away like this the accountant will take the axe to Woody, so better get in for your chop now. 2N2160 Unijunction was $1.10 now only $0.65 2N2904A PNP 60V 600mA was $1.45 now only $0.70 2N4403 PNP 40V 600mA was $0.30 now 10 for $1.50 2N5401 PNP GPA was $1.90 now only $0.50 2N5485 NFET 25V 10mA was $0.80 now only $0.40 2N5550 NPN GPA was $0.30 now only $0.15 BD646 PNP D/ton 60V was $1 .40 now only $1.00 LM311 H TO5 Comparator was $0.55 now 10 for $1.00 MRF208 RF 225MHz 10W was $3.50 now only $1.50 PN3565 NPN 30V 150mA was $0.25 now 10 for $1.00 PN3566 NPN 60V 150mA was $0.25 now 10 for $1.00 PN3567 NPN aov 150mA was $0.25 now 10 for $1.00 PN3569 NPN 60V 150mA was $0.25 now 10 for $1.00 PN3641 NPN 60V SOOmA was $0.25 now 10 for $1.00 PN3642 NPN 60V 500mA was $0.25 now 10 for $1 .00 TSB3055 NPN 60V 15A was $3.00 now only $1.00 Three Super Semiconductor S ecials 2 6706 NPN T092+ 45V was $1 .00 now 10 for $2.00 2N6710 NPN T092+ aov was $1.10 now 10 for $4.00 PN3643 NPN 60V SO0mA was $0 .25 now 100 for $6.00 GEOFF WOOD ELECTRONICS PTY LTD 229 Burns Bay Road , (Corner Beatrice St.) iNc IN Nsw Lane Cove West , N.S.W. P. 0. Box 671, Lane Cove N.S.W. 2066 Telephone: (02) 4271676, Fax: (02) 428 5198. 8.30am to 5.00pm Monday to Friday, 8.30am to 12 noon Saturday. Mail Orders add S5.00 to cover postal charges . Next day delivery in Sydney add $5.00. Were $14.60 $12.70 CHIPS, CHIPS, CHIPS u, 0 NE~ VARIABLF POW RES PRN-553 Series • 320W Continuous • 640W 15 minutes • 1280W 4 minutes • 800V DC 750V RMS 33 ohm $166.20 Other resistance values from . 1 ohm to 300 ohm available on short delivery. PRN-800 Serles • 640W Continuous • 1280W 15 minutes e aoov DC 750V RMS 106 ohm $174.90 Other resistance values from 0.5 ohm to 500 ohm available on short delivery. EDGE CONNECTORS We have previously advertised the closed version of these transformers, but we now stock the panel mount variety in two sizes. Other sizes up to 26A are available to special order on short lead times. HSG-0022 0-250V 0-1.2A $94.00 "'"ROSE All prices INCLUDE sales tax. Tax exemption certificates accepted if line value exceeds $10.00. BANKCARD, MASTERCARD, VISA, CHEQUES OR CASH CHEERFULLY ACCEPTED ~ a .,, C, a :l) 2 j Don't miss a single issue r ~ ! Electronlc house number :J· Convert a CB radio IF YOU'RE THE KIND OF READER that can't wait to get the next copy of SILICON CHIP, then why not have the magazine delivered direct to your door? 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Just ring us on (02) 982 3935 and quote your Bankcard or Visa Card number and the expiry date. I I I I I I I I I I I I Detach and mail to: FREEPOST 25 SILICON CHIP PUBLICATIONS PO BOX 139 COLLAROY BEACH NSW 2097 • No postage stamp required in Australia. I I I I I X I ~-------------------- - - ----- - --- ---- - -~ 64 SILICON CHIP -- e •- • ~~-- 0 I CONN~CT'E-t> ON& OF 11-1~ 6U\~'R.~ 8t St'RUN\H\E:'0 ~ ~ CHORDS oo• picture, except for some worminess in the yellows. This is usually seen in sets that are tuned almost into sound bars. This seemed to confirm my feeling that it was a fine tuning problem, though I had yet to see the full-blown symptoms as described by the owner. I tried fine tuning the set but nothing happened. Then I noticed that the AFC switch was on so it was no wonder that the fine tuning wouldn't work. With the AFC switch off, I tried again and this time it worked. As luck would have it, I tuned deeper into sound and as the picture broke up the owner cried, "That's it! Thafs what it did the other night!" Adjusting the fine tuning "the other night" had apparently cured the fault. I asked her if the AFC switch had been on or off when she tried to tune the set. She couldn't remember but thought it had been on. If that had been the case, how was she was able to tune the set? In the circumstances all that I could do was to carefully retune the set, then advise her to call me if the trouble recurred. She was back on the phone that evening. The next day and the day after that were repeats of the first one more or less perfect pictures, with only slight maladjustment of the fine tuning. In the end, I decided that the only way to come to grips with this fault was to run the set in my workshop. It could sit there for days if necessary and I would be on hand when the symptoms reappeared. And so a few days later I delivered a loan set and collected the villain. The loan set turned in a first class picture which removed any doubt that the customer's antenna system might have been contributing to the fault. Next morning I positioned the set on the bench and set it to channel 2. This was the customer's usual channel so it was the one most likely to show up the symptoms. Again, the fine tuning seemed to be slightly out but it responded to adjustment. I couldn't work out why it was just a little different every day and always in the same direction. The way things were going, we would eventually run out of adjustment! Still, the set had been running for only about two hours when the real cause of the trouble was revealed. Although I was working on another job at the time, I just happened to look in the right direction when the Precedent decided to play up. First, the colour disappeared. Then vertical lines in the picture started to weave from side to side. Finally, the whole picture collapsed into diagonal lines as the horizontal hold lost its grip. So that was the real symptom loss of horizontal sync - and not tuning drift . Of course, the customer couldn't tell the difference between the sound bars I had created while testing and loss of horizontal sync. Once I had seen the fault, I found that I could easily provoke it by tapping the chassis in the vicinity of one of the auxiliary boards, the video IF module. These Precedent sets use a Toshiba model C8.10 chassis which is well known for its dry joints on both the mother board and the plug-in modules. They use double sided boards with numerous pin-throughs which are notorious for bad solder joints. In this case, the dry joints were on the video IF module (FM-106) in the vicinity of the video detector (IC-102). The result was a crushing of the video output which reduced the amplitude of the horizontal sync pulses. Also, the video detector provides the control voltage for the AFC module, so the fault had to have some affect on the fine tuning. Fixing the fault was easy, once I had seen it. A few minutes work with a hot iron on the IF module cleared up the trouble. But like most intermittent faults, it wouldn't show up during the brief periods I could spend in the customer's home. And of course, the customer's description didn't help a bit! ~ FEB RUA RY1989 65