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Something old & something new In this story, the old and the new were one and the same thing - an old model VCR that had been stored unused in its original carton for umteen years. But why did it seem to have so many separate faults? The device was a National video recorder, model NV300. This model is around 9 years old now and was, I believe, the last of the top loading models in this brand. They were very popular and are still worth servicing, in spite of their age. So the arrival of another one in the workshop was no surprise in itself. What was surprising was that the customer brought it in neatly packed in its original carton. What was more, even a cursory examination was sufficient to tell me that it had had very little use. Its history , as told by the customer, confirmed this. It had originally belonged to another member of his family but, for some obscure reason, had been packed away shortly after it had been acquired. He had only recently obtained it but had not been able to make it function. I'm not sure as to the precise details of that last statement. In view of subsequent events I suspect that, initially, there was some sign of life but that it failed to function properly. Subsequently, and possibly as a result of his attempts to correct this, it went completely dead (no channel indicator display lights, no clock function, etc }. Well, I've been caught myself with that condition on this machine and, even as we were unpacking it, I felt sure I knew the answer. One of the front panel controls is a pushbutton designated "Timer Ree Switch" . This is activated to put the machine into the timer controlled recording mode, after the start and stop times have been entered. And once it is activated regardless of whether the timer has been programmed or not - the set cannot be used in any other mode and appears virtually dead. This point is made in the instruction manual but is easily overlooked. While the customer was still there I plugged it in, connected it to a monitor and released the aforesaid button. The recorder immediately came to life. I set it up in the tuner mode and tuned in the SBS test card, which produced a first class picture. That piece of " magic " rocked the customer somewhat. Crook sound But even as I did this, I realised that all was not well. The picture was first class but the sound was so weak as to be almost non-existent. I made a quick check of the tuning adjustment but this achieved nothing. And the result was the same on several other channels. Finally, before the customer left, I tried a test tape. This loaded OK and tried to play but the picture refused to lock, suggesting a servo (RON\ta>.N ~ c . ~ r - , _ _ ~ CON~ S\JS'Pee.TED 11-\E- -"R~ CONve."R..,-E.R "-' & THE.SE C.OST OVE."R. '$ \00 I 60 SILICON CHIP BJI002-J BJI002- 2 BJI002 -J BJIOOl -4 BJl002- 5 BJ I002 - 6 BJI002- I BJl002- 8 Pl 002 AC l4V AC 14V • c1001 om _OY AC 22Y AC 36V AC 2.65V AC iii'/ ~ BJIOOJ - I BJI003 - 2 GN O BJIO0J - 3 --- -- BJI003-4 BJIOOJ - 5 BJI003-6 BJI003- I BJJ003 - 8 RIOOS RI003 3300 15K 8100! 56K RIOP2 5600 + CI004 - sov 0.33 P6008 - I P6008 - 1 P6008 - 3 P6008- 4 P6008 - 5 P6008 - 6 01002 2S0973 18.S OIOIS RIOOB 1100 MAl6~ I ZW o, ISS133 - C1001 OIOIG + ROl3EB2or3 CIOII l6V47 + 35V4 7 PIOOS [?ff-ffi -~ Fig.1: the power supply circuit for the NV300 VCR. An 18V regulated rail is provided by Q1002 while lesser regulated voltages are derived from IC1001. fault in either the drum or capstan systems. So we had at least two rather nasty faults and, on that basis, I had to advise the customer that repairs could be expensive. At the same time, I pointed out that, whatever the cost, he would be getting a virtually new machine for much less that its original value. It transpired that he felt the same way, so it was agreed that I should go ahead unless anything more serious showed up. RF converter Initially, I was most concerned about the sound fault. My prime suspect was the RF converter (modulator) and these cost over $100 to replace. On the other hand, the servo fault was unlikely to involve any expensive mechanical problems because the machine had had so little use. My first move was to replace the RF converter. I carry one of these in stock, expensive though they are. In fact, I carry several other major components under this brand even though they, too, are relatively expensive. While my accountant argues that such stock is hard to justify, purely on a turnover basis, I regard it as more than stock; it is also test equipment and relatively cheap test equipment at that, considering the time it can save on sticky jobs. The changeover is simple. The device clips into place and there is one multipin connector and the main RF lead to be changed over. And presto! - that cured the sound fault. So it looked as though we had a faulty RF converter. But the changeover revealed more than that. Now that we had normal sound level, running a test tape with a standard tone indicated that the tape was running spot on normal speed. On this basis, the capstan system appeared to be running at the correct speed, suggesting that the failure to lock the picture was most likely a fault in the drum servo system. The circuitry for the drum servo system includes IC2001 (MPC1504C) and its associated components (sheet 3-10 for those with a manual). Several waveforms are given and I checked these with the CRO. Unfortunately, results were rather inconclusive; the circuit was generating all the waveforms but with what precision was not easily determined. However, I had already developed a gut feeling that the servo system was trying to lock but couldn't quite make it. And that reminded me of R2022, a 50k0 trimpot labelled CYL.F.R. (cylinder free running). This trimpot is a coarse adjustment and sets the free running oscillator within the locking range of the servo system. While I had no intention of effecting a bodgie repair on a "twistand-fiddle" basis, I was anxious to know whether I could make the system lock, at least on a temporary basis. So, noting the present pot setting very carefully, I gave it a tiny nudge in one direction. And bingo - the picture jumped into lock. But it was something of a hollow victory because two more faults immediately presented themselves. One was that the picture had no colour and the other was that it was displaying very pronounced hum bars. Teetering penny My immediate reaction was one of surprise and frustration that a virtually new machine should have so many separate faults. Then I began to think a little more rationally. The penny - if it had not already dropped - was at least teetering on the edge. It was the hum bars that provided the vital clue. This meant a fault somewhere in the power supply and this would have to be fixed before going any further. What was more, there was good chance that at least some of the other faults were manifestations of this one. So I went straight to the power supply (sheet 3-8). This delivers a number of voltages, some AC but most regulated DC. An 18V supply is derived from regulator transistor Q1002 but the other regulated supMAY 1990 61 believe it or not, it now delivered perfect sound. So that was it. I threw in a routine clean and lubrication and the customer now had a virtually new machine, with first class performance. And it had cost him less than the price of the RF converter which I had feared I might have to supply. SERVICEMAN'S LOG -CTD Post mortem ~I ~ J I · . \ .. CZ(~L 0. 0 •. MY IMME:"'DIPt"f'~ 'RE:AC.-,101'1 WP-.S Ol= SU'RPR\"SE & FR~TION 11-\F,r ~ V\RTUP--U...'-< NE:W MP...CH\l\lE: ~\-\OU\...D k~\JS SO MAN\.f Ff\UL-TSoo~ plies come from IC1001 (STR1096). The 18V rail was spot on so I went to the IC which is readily accessible. Pin 1 is shown as 15.1 V and was spot on. Pin 2 is shown as 11V but was actually 14.8V. Pin 3 is chassis. Pin 4 is shown as 9.31V but was 13.6V. Pin 5, shown as 6.06V, was OK. The answer, of course, was a crook IC. They are a common enough device and I had one in stock, so it was easy to replace it. The result was more far reaching than I expected. The colour was 62 SILICON CHIP restored and the hum bars had vanished, which I did expect. I wasn't sure about the locking problem but felt I should reset the CYL.F.R. pot. This I did and the system not only remained in lock but exhibited a wide locking range. That left a query over the RF converter. I wasn't very confident that the power supply fault was to blame here, seeing that the replacement converter had worked, but it was simple enough to try and there was a lot of money at stake. So, in went the original converter. And But there is rather more to the story than that. I have to admit that, in hindsight, I should have done better. Experience has taught me that whenever I encounter more than one fault in a video recorder - and particularly if they appear to be unrelated - the first thing to check is the common factor; the power supply. Had I done so, I would have saved some time. But why did the power supply fault have the effect it did, particularly in regard to the RF converter? Well, for starters, let's consider the voltage delivered by pin 4 of IC1002 - 9.3V. This supplies the RF converter, the servo circuits, and the luminance and chominance circuits. The faulty IC gave a pin 4 voltage of 13.6V and this accounts for all four symptoms, at least in general terms. But why did one RF converter accept this fault while the other one wouldn't? My best guess is that individual modules may vary in their sensitivity to such a fault; some may work with it, some may not. At least, that's my theory. Amateur gear And now, for a complete change of scene, here's a story with an amateur flavour. I usually don't have much to do with amateur equipment as such devices are not really within my field. But every now and again my amateur friend encounters a problem and seeks my assistance - which I'm happy enough to give within my limited experience. On this occasion, the problem concerned his 2-metre transceiver; an learn Model 22S. I understand that these were a very popular unit some years ago, being regarded as very much state-of-the-art when they were introduced. They were TETIA TV TIP Sanyo CTP 7605A Symptom: No go. No voltage between collector & emitter of chopper transistor but normal output can be measured across bridge diodes. Fuses and primary of chopper transformer are OK. Cure: Dry joint at mounting post of C310. The can of this large electro forms a link between the bridge negative and the emitter of the chopper transistor and the two mounting posts must be securely fixed to the copper track. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. also regarded as good value for money. One of their features was a phase locked loop (PLL) to replace the bank of crystals employed in the previous model, the Icom 22. Such models were normally supplied with two or three sets of crystals of the customer's choice. After that, the owner had to supply his own crystals for whatever channels he chose. But crystals are expensive and not always easy to get for a particular frequency. So the PLL was a significant improvement, permitting operation on virtually any channel across the 2-metre band. Various switching systems were employed to select the wanted channel. The Icom 22S used a novel approach. It was equipped with a matrix board to which the owner could fit inexpensive switching diodes in a wide range of configurations to produce a chosen frequency. Each diode combination was fed from a 9V rail and there were 22 such rails, selected by the channel switch. Each diode in any one combination fed one of eight cross rails. Typically, from one to seven diodes would be needed for a particular channel. Thus the user could program the set for up to 22 channels. However, it didn't take the mob long to wake MASSIVE TEST EQUIPMENT SALE Completion of Various Contracts Allows Us To Offer Equipment Surplus To Requirements Racal Laboratory DVM Model 5005 - $1 200 Racal Laboratory DVM Model 5004 - $1200 Racal Laboratory DFM 9000 Series - $1400 Heathkit Transistor Tester IM-36 - $30 Hewlett Packard HP606A Signal Generator 50kHz-65MHz - $500 Hewlett Packard HP41 QC Voltmeter with Probe (900MHz) - $350 Marconi TF 886A Q Magnification Meter 1 5-180MHz - $1 50 Hewlett Packard HP614A UHF Signal Generator (800MHz-2.2GHz) - $250 Marconi TF2092 Noise Receiver (75!l) - $50 Hewlett Packard HP6188 Signal Generator (3.8-7 .6GHz) - $200 Marconi MF Monitored Attenuator DC-550 kHz, 0-11 0dB , 0.1 dB Steps - $100 Wayne Kerr Universal Bridge 8224 - $350 Hewlett Packard HP 86548 Signal Generator 1 0-520 Mrlz - $2500 Gilmac RF Bridge, 20-180 MHz, Digital Frequency Readout, Direct SWR Readout $2500 Advance Power Supply PP15 0-15 Volts 0-3 Amps - $100 Gilmac DC Power Supply 13.8 Volt 25 Amp Continuous, Rack Mount, Fan Cooled , New - $400 Systron Donner Pulse Generator Model 101 - $250 General Radio 1564A Sound & Vibration Analyser, 1/3-1 / 1 0 Octave Level Meter $150 Gilmac Lab Power Supply 0-15V 0-10 Amps Metered, New - $350 Singer Microwave Spectrum Analyser, IF Section Only, 1 kHz Resolution - $200 General Radio 1156A Decade Scaler, 100MHz in 10MHz Out - $50 Siemens D21 55 Level Meter With W3155 Tracking Oscillator 200Hz-61 0kHz Digital Readout Current Model - $3500 Tektronix 454A 150MHz Oscilloscope - $1350 Tektronix 453 50MHz Oscilloscope - $950 Bruel & Kjoer Electronic Voltmeter Model 2416, 2Hz-20kHz, Displays DB, DBM , Volts (RMS, AV, PEAK) - $60 Systron Donner Spectrum Analyser 762-2A 1 0MHz-40GHz CW, All Waveguide Mixers 1 kHz IF Resolution - $4000 Green Screen Video Monitor P31 Phosphor Composite Video Input - $50 Hewlett Packard HP200 CDR Audio Oscillator 5Hz-60kHz , 20V RMS 600V output $80 Micro Power Model 221 Sweep Generator 8-12 GHz, AM - Pulse - $250 Tektronix CRO Tubes 5-lnch New: 1 x TO541-31, 1 x TO501-7 P/No. 154/0082 $100 Each Racal Laboratory Pulse Generator Delay, 1500 Series, As New - $1200 Racal Logic State Analyser 205 With All Options, As New - $10,000 Racal Universal Switch Controller, 1200 Series, As New - $1200 Racal DFM 1 0Hz-3GHz, 9921, as New - $1000 Racal DFM 1 0Hz-200MHz, 9913, As New - $250 Racal DFM 1 0Hz-200MHz, 9914, As New - $450 HP Spectrum Analyser 0-1200MHz 141T - $2500 Avo Bridge G2457/6, As New - $300 Avo Super Megohmmeter RM 170, As New - $400 Racal-Milco Transmission Test Set, Model 220, As New - $350 Swan-Astro 102BX HF Transceiver, 150 WRF, As New - $750 CCD Subminiature Camera, Auto Iris Lens, 0.5 Lux Sensitivity With CCTS, As New $400 Racal HF Transceiver, 7948, 100 Watt, New, 24V with CCTS - $750 Bruel & Kjoer Artificial Voice Model 4219 - $150 Drake SSR1 Shortwave Receiver - $100 Call, Fax or Write: AV-Comm Pty Ltd, PO Box 386, Northbridge, NSW 2063. Tel: (02) 949 7417; Fax: (02) 949 7095. All Reasonable Offers Accepted. MAY 1990 63 View inside the Icom 22S, with the matrix board hinged forwards. The fault was discovered on the sub-hoard, beneath the matrix hoard. to the fact that there was a 23rd positio'n on the switch and the matrix board, and that these could be wired to an external combination of diodes and switches which could select any channel at will, over and above the permanent 22. All in all, it is a very versatile arrangement. In common with all 2-metre transceivers using a PLL, there is also an offset function. For use with repeaters, it separates the transmit and receive frequencies by 600kHz (the standard repeater separation). The dark side Unfortunately, there was a dark side to this model. It is now generally acknowledged that some batches suffered from a crop of dry joints, particularly on a sub-board situated beneath the main PC board. This sub-board carries the PLL, voltage controlled oscillator, offset circuit and a plug for the matrix board. In most cases, these dry joints could be cured by blanket resoldering. So what was my friend's problem'? The set normally serves as a base station but can also be used as a mobile unit in his car. And it all started when he fitted the set in his car and travelled north for the Gosford Field Day - an annual event normally attended by every amateur and his dog within a 200km radius. The set performed perfectly during the day but was used only in the simplex mode. It was not until he returned home, set the unit up for base operation, and tried to activate his favourite repeater, that things went wrong. The set has two indicator lights .1.m.,,;. ;r.~1;i1:.1;: RCS Radio Pty Ltd is the only company which manufactures and sells every PCB E, front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. 64 SILICON CHIP on the panel: a green one to indicate an incoming signal and a red one to indicate that the transmitter is on. When he pushed the PTT button, the red light came on all right but so did the green one; a clear indication that something was wrong. More to the point, the repeater did not respond. After a couple of more tries, the green light ceased to show and the indications were that all was now well. But there was still no response from the repeater, despite the fact that the set's own power meter indicated normal RF. Several other repeaters were also tried with the same result. At this point, my friend assumed that vibration during the car journey had unearthed one of the aforesaid dry joints but he was puzzled as to why the set appeared to be delivering normal RF but could not access any repeaters. To confirm the RF output, he connected a dummy load and made a power output measurement. It was spot on for this set. So, if it was delivering power but not reaching the repeater, it was most likely that it was off frequency. And that's where I came into the story. The first I knew of his problem was when he approached me, and requested the loan of a frequency meter. I don't use the meter much so I was happy enough to agree, wishing him luck as I handed it over. Later, he filled me in on subsequent events. He connected the frequency meter to the dummy load tapping which normally feeds the power meter, switched the set to low power, and pressed the button. The meter should have read 147.875 MHz but actually read 147.675 MHz; ie, 200kHz low. Several other repeater frequencies were tried, all with exactly the same result. But something else emerged from these tests; the problem was confined to the offset function. When the transmitter was switched to the simplex mode, all frequencies were correct. At this point my friend decided to do a little judicious prodding around the suspect boards. If that continued on page 99 Ultrasonic Proximity Detector for Cars ; Simple Computer Sound Repeater; 120W PA Amplifier (With Balanced Inputs) Pt. 1 ; How to Service Car Cassette Players; Massive Diesel Electrics In the USA. February 1989: Transistor Beta Tester; Minstrel 2-30 Loudspeaker System; LED Flasher for Model Railways (uses LM3909); Build a Simple VHF FM Monitor (uses MC3362). Pt.1 ; Lightning & Electronic Applian ces; Using Comparators to Detect & Measure. March 1989: LED Message Board , Pt.1 ; 32-Band Graphic Equaliser, Pt.1 ; Stereo Compressor for CD Players; Map Reader for Trip Calculations ; Amateur VHF FM Monitor, Pt.2; Signetics NE572 Compandor IC Data; Electronics for Everyone - Resistors. April 1989: Auxiliary Brake Light Flasher; Electronics for Everyone : What You Need to Know About Capacitors ; Telephone Bell Monitor/ Transmitter; 32-Band Graphic Equaliser, Pt.2 ; LED Message Board, Pt.2. May 1989: Electronic Pools/Lotto Selector; Synthesised Tom -Tom ; Biofeedback Monitor For Your PC; Simple Stub Filter For Suppressing TV Interference; LED Message Board , Pt.3 ; Electronics for Everyone - All About Electrolytic Capacitors. June 1989: Touch-Lamp Dimmer (uses Siemens SLB0586); Passive Loop Antenna For AM Radios; Universal Temperature Controller; Understanding CRO Probes; LED Message Board, Pt.3 ; Coherent CW - A New Low Power Transmission Technique. July 1989: Exhaust Gas Monitor (uses TGS812 Gas Sensor) ; Extension For the Touch-Lamp Dimmer; Experimental Mains Hum Sniffers; Compact Ultrasonic Car Alarm ; NSW 86 Class Electrics; Facts On the PhaxSwitch - Sharing Your Phone Line With A Fax Machine. August 1989: Build A Baby Tower AT Computer; Studio Series 20-Band Stereo Equaliser, Pt.1; Garbage Reminder - A 7-day Programmable Timer; Introduction to Stepper Motors; GaAsFet Preamplifier For the 2-Metre Band; Modern 3-Phase Electric Locomotives. September 1989: 2-Chip Portable AM Stereo Radio (uses MC13024 and TX7376P) Pt.1 ; Alarm-Triggered Telephone Dialler; High Or Low Fluid Level Detector (uses LM1830N) ; Simple DTMF Encoder (uses Texas TMC5089); Studio Series 20-Band Stereo Equaliser, Pt.2 ; Auto-Zero Module for Audio Amplifiers (uses LMC669) ; A Guide to Hard Disc Drives . October 1989: Introducing Remote Control ; FM Radio Intercom For Motorbikes (uses BA 1404 and TDA?000) Pt.1 ; GaAsFet Preamplifier for Amateur TV; 1 Mb Printer Buffer; 2-Chip Portable AM Stereo Radio, Pt.2; Installing A Hard Disc in the PC; A Look at Australian Monorails. November 1989: Radfax Decoder For Your PC (Displays Fax, RTTY and Morse); Super Sensitive FM Bug ; Build A Low Cost Analog Multimeter; FM Radio Intercom For Motorbikes, Pt.2 ; 2-Chip Portable AM Stereo Radio , Pt .3; Floppy Disc Drive Formats & Options ; The Pilbara Iron Ore Railways . December 1989: Digital Voice Board (Records Up To Four Separate Messages , Uses Texas TMS3477NL and 256K RAMs); UHF Remote Switch ; Balanced Input & Output Stages ; National Semiconductor LM831 Low Voltage Amplifier IC Data ; Install a Clock Card In Your PC ; Index to Volume 2. January 1990: Service Tips For Your VCR ; Speeding Up Your PC; Phone Patch For Radio Amateurs; High Quality Sine/Square Oscillator; Active Antenna Kit; The Latest On High Definition TV; Speed Controller For Ceiling Fans. February 1990: 16-Channel Mixing Desk; High Quality Audio Oscillator, Pt.2 ; The Incredible Hot Canaries; Random Wire Antenna Tuner For 6 Metres; Phone Patch For Radio Amateurs , Pt.2 ; PC Program Calculates Great Circle Bearings. March 1990: 6/ 1 2V Charger For Sealed Lead-Acid Batteries ; Delay Unit For Automatic Antennas ; Workout Timer For Aerobics Classes ; 1 6-Channel Mixing Desk, Pt.2; Using The UC3906 SLA Battery Charger IC; Digital Waveform GeMration Using a PC; The Controls On A Model Aircraft. April 1990: Dual Tracking ± 50V Power Supply; VOX With Delayed Audio; Relative Field Strength Meter; 16-Channel Mixing Desk, Pt .3 ; Simple Service Tips For Your Microwave Oven ; Model Aircraft Aerodynamics ; Active CW Filter For Weak Signal Reception . Note: November 1 98 7 , December 198 7 & January 1988 are now sold out. Serviceman's Log ctd from page 64 didn't reveal anything, at least he had some background which might assist some of his more experienced colleagues in pinpointing the fault. So, with the bottom cover removed and the PTT button permanently activated - by simply clamping the microphone gently in the bench vice - he went over the boards with an insulated prodder, gently at first, then more vigorously when there was no response. Nothing happened until he touched one corner of the matrix board, ever so gently. The readout flickered and up came the correct frequency. That produced mixed feelings. All the joints on that board were of his own making, so if one was dry it was his fault. But careful scrutiny with a high power glass revealed nothing. What about the 10-pin plug and socket feeding the matrix board? It was a long shot but a spray of contact cleaner wouldn't . do any harm. That done, he was replacing the matrix board when something caught his eye. On the sub-board, beneath the matrix board, there are several components; four ICs, three transistors, and several resistors and capacitors. And it was one of the resistors. These are mounted vertically, with the upper pigtail turned over to return to the board. And each pigtail was protected with a plastic sleeve, in case it made contact with the underside of the matrix board above it. But the sleeve on one resistor had worn through, revealing bare pigtail. And it didn't take a genius to work out that it had been rubbing against the underside of the board, eventually making contact with it. Of course, that was it. The cure was to simply push the resistor over at a slight angle, thus reducing its effective height by a couple of millimetres. That was several weeks ago and there has been no trouble since. But I can't help wondering how many Icom 22S sets may be gathering dust due to a simple fault like this. ~ MAY 1990 99