Items Covered This Month Sharp CX3451 TV set. Sharp VC-90ET VCR. Philips 14CF1014 portable TV set.

Let’s get straight into it. The first contribution comes from Mr R. H. of Carine, Western Australia and he’s called it "A Sad Repair Story." I’ll let him tell it in his own words:

It concerns a Sharp CX3451 colour TV set which an elderly relative had imposed on me to repair, after she could not get a response from two repair shops advertising in the local newspaper.

My main business involves the repair of switchmode power supplies and other industrial electronics equipment and generally I shy away from domestic appliance repairs. I find that, in most cases, the time spent is much greater than one can legitimately charge a customer and even if you do charge a modest amount, you are still accused of overcharging.

So to my story. The set was working but could be induced to lose the picture, with the screen collapsing to a horizontal line, by tapping on the outside of the case. My thoughts were that it was obviously a loose connection or a faulty joint problem and that it shouldn’t take too long to find. I discharged the tube before removing the EHT lead and it was a simple matter to remove the main board.

I thoroughly inspected the board with a magnifying lamp but for the life of me, I could not detect a connection or soldered joint which I could honestly say would cause the symptom – and I have had many years experience in locating this type of fault. There have not been more than a few cases where I could not pick a faulty joint by visual inspection but the symptoms indicated that there was almost certainly one here.

I decided to try the shotgun approach and resolder all the most likely suspects but as I later discovered, this only caused the real fault to temporarily disappear. Anyway, having completed the resoldering, I put everything back together and could not get the set to falter when I gave the case a solid tap, so I took it back to my relative. No sooner had I fired it up than there was the same fault back again.

I took it home and decided to get the board operating out of the case. However, the leads from the degaussing coil were too short to allow this, so I decided to remove the tube as well and connect everything up on the floor. I arranged the tube with some soft wedges to keep it upright and when all was ready I fired it up.

I went around the suspect area of the board, tapping it gently with a plastic toothbrush handle, and found that I could induce the fault reliably with the gentlest of touches in a fairly small area. I thoroughly re-soldered every joint in a diameter of 75mm and this time, I was unable to induce the fault when the board was tapped. I had fixed the fault for sure but this was not the end of the story.

During the time the whole box and dice was set up on the floor, I had forgotten to reconnect the lead which connects the aquadag of the tube to the 0V rail on the PC board and the tube was thus sitting several thousand volts above chassis. Unfortunately, due to the very short deguassing coil leads, the aquadag finished up within a couple of millimetres of the metal casing housing the infrared receiver module.

I remembered I had heard a spark jump while tapping the board and looking for the original fault but I had not actually seen it. Since it appeared to have had no effect on the picture or the manual operation of the set using the front panel controls, I concluded it was of no consequence. I didn’t have the remote control unit with me anyway so I couldn’t check it out.

At this stage, I decided to put it all together and deliver it once again to my relative who was pleased as punch when I demonstrated that the original fault was now fixed. However, shortly after arriving home, I received a phone call from my relative to tell me that the remote control was not working.

Uh-oh, I thought, maybe that spark was telling me something after all. I decided to leave it until the next day and work on it then. I suspected that the IR module was dead and hoped that that was all there was to it.

I removed the module (a Sharp GP1U5 IR receiver), connected it to a 5V supply and connected a CRO to the output. Sure enough, the module was self-triggering in an erratic manner without any signal being received. I checked the Sharp parts supplier and he had a unit in stock at around $16. I thought this was rather high, so I decided to try the Tandy GP1U5 copy (276-0137) at $3.90. This Tandy unit was a vertical mount model but I was able to remove the internal PC board, change the connection pins and then remount it in the horizontal casing from the original unit.

This unit worked perfectly when tested on the bench, the unit responding to the remote control with a no-load output signal of around 5V p-p. I then refitted the module to the main PC board, connected everything together and gave it a try. It did not respond to the remote control so out it came again.

Next, I decided to just try the board without connecting the tube but in order to do this safely it was first necessary to remove the horizontal output transistor to prevent the EHT zapping anything.

I connected the CRO to the output of the IR module and when the remote was operated, the signal amplitude was only around 400mV p-p. This was insufficient to drive anything in the 54-pin Sharp IX1703CEZZ IC. De-soldering pin 46 on this IC and checking with a diode tester showed that the internal protection zener on this input measured high leakage in the reverse direction, thus indicating it was indeed zapped.

A new IC from Sharp was going to cost $43.50 (ouch!) and would take several days to obtain from Sydney. It seems that my small oversight in not connecting that chassis lead had accidentally led to both the IR module and the function control IC taking a direct hit and thus resulting in my good deed for my relative costing more than anticipated.

Multi-standard VCR

The next contribution comes from Mr A. B. of Glen Waverley, Victoria. He calls it " The (more than) Multi-Standard VCR."

The machine in question was an elderly Sharp VC-90ET VCR, a top-of-the-line, multi-standard unit that does everything. It has stereo hifi, will record/playback NTSC, SECAM & PAL (all at a bewildering variety of chroma frequencies), and features standard play and long play. All-in-all, a very fine machine.

It was, alas, somewhat sick, hence its arrival at my workshop. The owner brought it in saying "you were recommended by my neighbour, you fixed up her 78 RPM gramophone." This set me back somewhat; I know I’ve been fixing things for a while but I am not that ancient! Actually, it was an old Garrard turntable that I’d repaired, so that some vintage jazz 78s could be copied onto tape but that’s another story.

The Sharp VCR was not well – the front panel display was very dim, the mechanism was making horrible grinding noises, it was chewing tapes and the audio was shocking. As well, there was some patterning on the received off-air picture.

This latter problem was tackled first. This VCR, like most modern machines, has an RF output in the UHF band and this needed to be checked. To do this, the test switch on the back of the set was turned on and the workshop TV set put into search mode until the two white tuning bars were found. The test switch was then set to "Off" and the poor picture revealed.

When the VCR operate switch was turned off, the monitor screen produced a faint Ch31 picture – ie, classic co-channel interference. By using a small screwdriver, I was able to adjust the RF output clear of the occupied channel and a good picture came up.

Next, a dummy tape was inserted and the machine put through its paces. Fast forward and rewind (FF/REW) were horrible – the capstan motor obviously had dry bearings (a common problem) and a squirt of CRC 226 gave a temporary cure, pending a proper motor strip down and clean/lubricate.

The PLAY mode was then selected and this started the head drum and moved the tape guides into position, as expected. I then pressed the EJECT button. The cassette ejected OK but the guides hadn’t retracted, which meant that the tape hadn’t unlaced. Little wonder it was chewing up tapes.

This sequence was repeated a few times and there were variations – sometimes it would work perfectly, sometimes the head drum wouldn’t start and sometimes it displayed the original behaviour. This indicated an intermittent mode switch so it was removed and cleaned, which cured this fault. As well, during all these repairs, the relevant drive and loading belts were progressively replaced.

The sub-loading arm was also very stiff (this manifests itself as no – or erratic – tape counter) and so this was also sprayed with CRC 226.

The final mechanical problem was poor rewind, especially the last few minutes of a tape (or the first few minutes if playing from the beginning of the tape). The cure for this is to disconnect power, remove the cassette cradle, and manually rotate the loading motor until all arms, etc are clear of the take-up reel. This is then removed, the dust, etc cleaned out from underneath, the soft brake pads deglazed from the two brake arms and the rotation sensor cleaned. It sounds complicated but it’s easy after you’ve done it a few hundred times!

As a final step, the worn (glazed) pinch roller was replaced and the heads (ie, audio/video/control/erase) cleaned. We now had a mechanically functioning VCR.

The next step was to repair the front panel display. This problem was traced to multiple faulty and out of tolerance capacitors in the power supply module – again, so far so good and I now had a nice bright readable display.

The last (and most perplexing) fault involved a number of audio problems. The VCR was connected to the workshop stereo amplifier and speakers, and a stereo test tape run. This resulted in the classic "chuff chuff " sound of an FM carrier dropping in and out. My first thought was worn audio heads – very rare on Sharp machines but not unknown.

The pins from the head drum were desoldered and the Q of the heads measured – they were still excellent and more to the point, both heads were very close to the same reading. This ruled out worn/damaged heads. A further test at this stage is to select PLAY and listen – with the stereo heads physically disconnected, the machine should default to mono audio and any FM noise should disappear. As well, the front panel level meter should show a mono signal.

This wasn’t happening; there was still heaps of noise and the level meters were showing stereo L and R – not possible! As well, it was now observed that there was FM white noise coming through with the mono signal on the monitor. I resoldered the heads in disgust and wandered off for a cup of tea and a good think!

The machine had been left running while the cup of tea was organised and, while idly sitting with the brain in neutral, I suddenly realised that there was modulation coming through the stereo speakers but that it wasn’t from the tape – it was from a local AM broadcast station! I traced out the audio lead from the head amplifier pack and noticed that just wrapping my fingers around the connector where it entered the audio processing board was enough to remove the interference. This was obviously not a long-term solution unless I was prepared to move in with the customers!

The connecting lead was removed and the continuity of signal conductors and earth screening was checked – all OK. What now? – the thing looked like it was functioning as designed and the manufacturer had provided proper screening of the low level signal leads. And then I suddenly thought of the junk pile at the back of the workshop.

I had scrapped a whole pile of monochrome IBM monitors a few years back and remembered they all had toroid formers on the signal input leads, obviously put there to suppress any noise from the computer radiating via the monitor leads. One of these was chopped off the IBM, and as luck would have it, there was a big enough hole to pass the connectors on the end of the Sharp audio lead. There was also enough cable to be able to wind two turns around the toroid. This was lashed into place with nylon cable ties and, much to my relief, the problem was cured!

A follow-up phone call a few weeks later to the customer revealed all was well and they were recommending me to all and sundry .... an ideal way to finish a job!

Faulty memory

Contribution number three comes from Mr J. R. of Deer Park, Victoria. There may be a temptation to describe it as a memory fault – but it has nothing to do with computers. Anyway, this is his story.

I’m a TV/video tech by trade but I’m now on a disability pension. I did a mature-age apprenticeship which took me through a few employers who fixed all kinds of gear, ranging from TV sets to video and audio equipment. My last job involved computer monitor repairs.

These days, I fix electronic equipment for friends. I maintain a small workshop in my 3m x 3m shed and carry a reasonable stock of passive and active spare parts, mainly for electronic projects from SILICON CHIP which I build for myself or mates.

Recently, one of my mates landed me with a Philips portable TV set Model 14CF1014 (CF-1 chassis), explaining that it was his daughter’s set and that she had been complaining of funny colours. She uses this set mainly for her beloved Playstation. He also complained that, on some of the games she played, the picture would roll.

The second fault I already knew about (of which more later) and I didn’t think the second would be hard to fix, so I told him it should be ready in a day or two. He urged me to fix it ASAP as, in the meantime, she would want to use his main TV set for her game playing and he’d miss out on all his favourite shows. I promised him top priority.

When I switched the set on, I found that all the colours were indeed wrong. I was watching soccer highlights and found that the grass was blue and the player’s faces were green. I then switched my workshop portable on to compare the colour differences and found that a billboard which should have been green was actually red.

I’d seen this fault before many years ago in an old Princess TV set and I attributed that fault to a faulty colour processor chip. Feeling that this would also be the problem here, I attacked this section first. First, I measured all the pins on the chrominance/luminance chip, which was a TDA3560 (designated as 7192). These all measured OK so I next checked transistor 7148 (BC548), the Y signal buffer, for correct voltages. These were also spot on and capacitors 2218 (2.2μF), 2191 (1μF) and 2192 (22μF) were all within tolerance.

I next checked the 185V rail filter capacitor (3.3μF 250V) and found it to be down to only 1μF. Bingo! I replaced this with a sturdy 350V job, turned the set on and found I was back to square one; still the same mismatch. Damn – I thought I had it. I was now beginning to suspect the aforementioned TDA3560 chip. I didn’t have one in stock so I ordered one from my supplier.

When the chip arrived the following day. I proceeded to fit it. But prior to that, I’d fitted an IC socket to make the job easy. Isn’t it strange how quickly one likes to know whether you were right with your diagnosis or not? After a quick fit and a turn on, I stared at the screen in disbelief. I was still seeing mixed colours. So much for blaming the chip. Where to now?

I plonked the set face down on the bench and began making some more voltage checks in the hope that I had missed something. As I was prodding, around the chroma chip, I decided to look at the picture by tilting the set up and bending down to look at the screen. To my surprise, all the colours were back to normal!

What did I do to fix it? I well remember a computer monitor that had a no-power fault but fired up when I touched my multimeter probe on a resistor in the start-up circuit. But what could I have prodded to get this set to behave normally? The answer soon came.

I switched the set off and stood it up into its correct position and fired it up. The mixed colours were back. Ah-ha! Now I’ve got you! With the set still on, I then turned it face down. The colours went back to normal.

I now knew I had a purity problem, not a colour-processing fault as I originally had thought. I turned the set off, turned it back into its correct position, and went straight for the degaussing thermistor. When I pulled it out, I gave it a bit of a shake, and I could hear something rattling inside. I’d finally found the culprit.

Luckily I had one in stock and fitted it. I switched on – the picture was still a bit poor but a whole lot better than before. I brought out my magic wand and degaussed the set and that was it – a perfect picture. I did a few geometry and white balance adjustments and the set came up like new.

Now I had to tackle the vertical hold problem. I removed the vertical hold adjustment – a 1MΩ trimpot (3413) – and replaced it with three wires to which I joined a large 1MΩ pot. Then I drilled a hole in the front panel (bottom right) and fitted the pot in there. They now have an external vertical hold control.

A cheaper way of doing this is to adjust the original trimpot to a centre frequency, between 50Hz and 60Hz. To do this, you need a chipped Playstation and a PAL and NTSC game. You alternate, changing between the PAL and NTSC games, while slightly adjusting the trimmer until you get no picture roll on either formats. And that’s it – instant fix but I like to give friends an external control.

My friend didn’t waste time picking up the set. His daughter had complained that the large 68cm TV set she had purloined for her Playstation were making all the characters look too big and she wanted her little set back.

When I think back to my apprenticeship days, I remember being told to degauss a set with gross colour problems first before condemning the electronics. Looks like I need more experience . . . or a better memory.