Items Covered This Month HP Omnibook 5700CT laptop computer. Mitsubishi CT-29AC1 colour TV. Sony KV-XF25M30 colour TV. Panasonic TC-29R20A colour TV (C-150 chassis). Panasonic TC-28PS10 colour TV.

I was sure I had fixed her notebook computer a year or so before, so when she recently came through the door with an HP Omnibook 5700CT F1350A in one bag and all her software and accessories in another, I was afraid that I might have done something wrong. Perhaps it was her serious facial expression that made me come to this conclusion but she was pleasant enough.

Penelope Mosman runs a firm of accountants and sure enough, I had fixed her machine before. Her problem now was that the notebook was making extraordinary and alarming noises and she thought that it might be the hard drive I had replaced last year.

I told her unconvincingly not to worry and that I would get onto it as soon as possible.

When I had the chance, I connected it all up and switched it on. I confess I was expecting the worst but was amazed when it booted it up perfectly. The computer was loaded with Windows 95B and everything was there and working properly.

Anyway, I decided to run a few diagnostic utilities which needed to be installed by CD-ROM. This HP machine can be booted with either a CD-ROM or a floppy drive connected, so I closed it down, fitted the CD-ROM drive and rebooted.

As it booted up, I noticed how microphonic it was but everything continued to work properly. After loading the software it required a further reboot. But this time, as Windows was loading, it suddenly developed a really loud high-pitched noise that was deafening.

My first reaction was to try to turn it down but I couldn’t immediately discern where it was coming from. Finally, when I had consulted what little commonsense I have left, I found that the sound was coming from the tiny internal stereo loudspeakers. And it was quite surprising just how much noise they were making – others in the same room and beyond were shouting at me to turn that ruddy noise down.

The only volume control on the computer itself that I could find was a pair of function keys. I was madly pushing the volume down buttons as fast as I could but this made no difference. In the end, I stuffed some rags over the loudspeakers and shut the computer down.

This was rather bizarre and I began looking at the computer more closely. There was no other volume control –not even on the CD-ROM player. However, I did notice a bank of three sound sockets on the side, so I got a 3.5mm stereo jack plug that fitted and kept it handy, because I knew I was going to have to go back in. When it booted up again, the noise returned, but I could now turn it off by inserting the plug in the external speaker socket when it got too much.

Next, I tried the Windows 95 volume control and turned it right down and even muted it, but still the noise wouldn’t go away. However, it appeared that tapping the case could make it go away and I began to suspect an internal dry joint.

By now, I was beginning to recognise that the noise might also be due to acoustic feedback. I found the tiny hole which the microphone hides behind and covered it with a dense cloth. This reduced the noise considerably and so, to confirm this hypothesis, I pushed the 3.5mm jack plug into the external microphone input. The sound ceased immediately.

Taking stock

It was time to take stock of the situation and try to work out what was happening. This hideous noise was being caused by acoustic feedback from the speakers to the microphone. But why didn’t the software and hardware volume controls work? And why was it alright with the floppy drive inserted and not the CD-ROM? Something was obviously not quite right.

I retried with the floppy disk instead of the CD-ROM several times and soon found that I could also get the noise with that as well. By now, I was fairly convinced that there was a dry joint or a crack somewhere near the CD/floppy drive input connector.

At this point, the only thing I could do was to go in and find out. Now anyone who has had a go at repairing notebooks will, like me, have a fear of doing this. There are all sorts of concealed clips and screws which make it very difficult for anyone with no experience to disassemble (and later reassemble) this type of machine.

Anyway, I persevered and successfully removed the keyboard and escutcheon. I then unplugged the miniature ribbon connections and eventually had the whole thing in pieces in front of me. The only trouble was, I couldn’t find any dry joints or cracks. Perhaps it was the plugs and sockets not making good contact?

Anyway, I couldn’t find anything wrong and eventually reassembled it, making sure everything fitted together properly. When I switched it on, everything seemed to work initially but unfortunately it didn’t take long for the howling to return.

By now, this was turning out to be another economic disaster – especially as I already had a "fix" for it in the form of the 3.5mm plug. And being an accountant, it was extremely unlikely she would need either the microphone or speakers together.

As a last resort, I decided to have another look at the software. Double clicking the volume control in the System Tray on the righthand side of the taskbar brings up the Volume and Balance controls. Under "Options", "Properties" there is a radio button for "Recording" which produces a new mixer panel when the "OK" button is clicked.

It was here that I found that the microphone balance control had been turned fully up. I reduced this to its normal level and this finally fixed the problem.

Why it was intermittent is hard to explain but it was probably due to the threshold of the feedback which varied according to how the unit was handled. I also suspect that the material used to make the CD-ROM drive was probably more resonant than in the floppy drive.

Anyway, that is my explanation and I’m sticking to it.

The trivial & mundane

Though it has been a busy time in our little family business, the jobs recently have all had faults that have been routine and mundane. Alternatively, they have had symptoms and causes I have written about before. Many times, I have started doing a repair, excited that the outcome might be of interest to everyone, only to find that the cause is trivial and boring – so much so that you kick yourself for not latching on to it sooner.

Then, of course, you meet and deal with the many different characters who can and do add twists to the service you are offering. In fact, this is sometimes more interesting than the work itself. From an income point of view, a boring month is a good one with a quick turnover. An interesting month is usually less profitable, hence the Chinese proverb/insult "May you live in interesting times!"

The Mitsubishi CT-29AC1

One of the more interesting jobs I’ve had is Mr Landhurst’s Mitsubishi CT-29AC1, which came in with intermittent no sound and no luminance. This is quite a nice TV although this particular unit had not been well looked after – the set was dirty and extremely dusty, as well as being slightly rusty from being not far from the sea.

I connected a colour bar generator into the AV sockets and followed the luminance signal through with the oscilloscope. I started at the AV switch board and went onto the H-RESO board and finally went all the way to the jungle IC.

Jungle ICs are very complex these days and can develop very weird faults. In fact, it’s got to the stage where they are becoming one of the most common causes of problems in TV sets. In this case, though it was highly suspect, I decided to measure the voltages around the luminance section. I soon found pin 57, the luminance clamp pin, to be low and subsequently that the voltages on Q201 were also wrong. I traced this further to transistor Q306 ("P-off, Mute"), near the tuner.

Although I had run the air compressor through the back of the set before I started, I hadn’t been particularly diligent around Q306. I blew it out again, then removed and measured the transistor with an ohmmeter. It read OK but the legs were slightly tarnished.

Anyway, I resoldered it back in the set and switched it on. The set burst into life with an excellent picture. I went back to the transistor and measured the voltages – all were correct now. I then hit Q306 with freezer and then the hairdryer but the set was working fine now. I can only assume that a combination of dust and salt had caused the fault but to be on the safe side, I replaced the transistor and soak tested the set.

I advised Mr Landhurst to cover his set when he wasn’t watching it and keep it away from open windows on the sea-side of his home.

A weird Sony

The next set I had to deal with was a Sony KV-XF25M30 which had a weird symptom. I can only describe it as being like the teeth of a woodsaw on the verticals, with jagged vertical lines all over the screen.

At first I thought it was something to do with the line output stage. However, when I put the scope on pins 7 & 9 of the jungle IC (IC301), the normal colour bar staircase was scrambled with what appeared to be another signal superimposed on top of it. Interestingly, though, the sync pulses were rock solid. I tried disconnecting these pins but both luminance and chrominance go through them and the signal was no longer available.

Next, I spent some time investigating the comb filter circuit but found nothing wrong. Finally, I ordered another jungle IC. The new one fixed the problem.

A crook Panasonic

I had to go and visit a Panasonic TC-29R20A (C-150 chassis) with the complaint that the colours were poor, especially peoples’ faces. When I arrived and had a good look, I could see there was very little red in the picture – but I wasn’t sure whether it was no red or no R-Y which would create the fault in the colour decoder as opposed to the RGB output stages. I didn’t have a colour bar signal generator with me but being reluctant to cart it off to the workshop, I decided to have a go.

After taking the back off, I got Mr Humber to watch the screen while I momentarily shorted the red cathode of the tube to ground. He reported that there was a flash of bright red on the screen, so that absolved the picture tube. I then checked the voltages of the cathodes to find the red pin to be high, as I had expected. I swapped the red and green transistors over and made sure that the plugs and sockets to the motherboard were all OK but this didn’t reveal where the fault lay.

That was about as much as I could do in the home – the rest would have to be done at the workshop. When I got it onto the bench with the generator, I could see that it was only the red that was missing – even with the on-screen displays.

I began by using an oscilloscope to confirm that there was no output from the jungle IC (IC601). I then measured the voltages and checked a number of pins around this IC but eventually came to the conclusion that the IC itself was faulty. I ordered a new TA8844N (a 64-pin high-density IC) and this fixed Mr Humber’s problem completely.

The frustrating thing is not being able to tell precisely what, where, why and how the fault was caused other than a large block called the jungle IC.

One can draw an analogy to a person’s car stopping and being told that it is the engine – just change it. One would really like to know what part of it had failed. However, judging by the thousands of different sets out there, it is unlikely the same symptom will show itself again.

Off-colour Panasonic

Mrs O’Brien invited me to her home because her Panasonic TC28PS10 was showing no colour when playing her video. I felt sure it was her video that was the problem, probably because it had been mistuned or switched to the wrong standard.

When I called, it didn’t take long to establish that there was indeed no colour on playback or EE (Tuner). I was still suspecting the video until, while trying the TV tuning (in case it was off), I tuned in the video in full colour in all modes. So, there had to be something wrong with the AV input.

Only a few days earlier, I had dealt with a Philips Matchline digital TV which had no sync (or colour) on one of its AV inputs. I found a menu on the TV remote which could change the colour system for each input, so I tried it with the Panasonic. Well, I found it was on the correct system and standard but I did find a menu for the AV colour which had been turned right down for the AV input she was using for the video. Resetting the colour to the correct level (about 32) and storing it fixed the problem.

So it turned out the be a simple problem but tricky to find.

Another Panasonic

I did have one other TV set, a Panasonic TC-2969S (M150 chassis), that came in with weak contrast. This set was really past its use-by-date and the lack of contrast could easily have been due to a low-emission picture tube.

However, apart from all the dry joints this set had, the usual solution to this is the well-known R525 resistor connecting the +113V rail to the beam limiting circuit. This resistor has a habit of going high – not surprising really, considering it is such a small size (1/8W or less).

The main difficulty is identifying the value of R525, because it varies from model to model (due to the size of the picture tube). This particular set had (I think) orange, orange, yellow, gold, making 330kΩ. The service manual said it should be 91kΩ for 29-inch sets.

In the end, I compromised by using 100kΩ, which seemed to improve the picture enough considering the age of the picture tube.

Monitor repairs

I am still repairing computer monitors but the time I am going to spend on them from now on is going to be limited. Frankly, I am fed up with trying to support manufacturers who are determined not to support us or their clients. There are far too many companies importing computer monitors and then dumping them in Australia without any service back-up at all – including no service manuals and no spare parts.

How they are allowed to do this is beyond me, with even many supposed reputable brands doing it too. My shop is littered with the corpses of sophisticated digital monitors (many large screen sizes) and in some cases I can’t even find out who the importer was let alone obtain a manual.

One in particular stands out in my mind – a Shamrock SRC2102L, which is a 1994 21-inch digital monitor. The owner bought it secondhand and I believe it might be a Hitachi but can get no information whatsoever from anywhere on it.

The set was dead with a flashing green LED on the front. I found the line output transistors Q609 (two 2SC5048 in parallel!) short circuit – well, actually, it was only one of them but it might just as well as been both. In fact, I’m not even sure whether there should be two of them because someone else may have made a modification – or was it designed that way?

That didn’t fix the problem, so I concentrated on the power supply board. This is a dual-FET switchmode design and one of the FETs (Q102, 2SK1723) was short circuit, in turn taking out R105, Q108 and D107. I replaced these and all the small electros and the power supply and the green LED would now come on. However, there was no +85V to the line output stage (I say +85V but I can’t be sure what voltage it is because I can’t get the circuit).

Anyway, it appears that half the power supply (the one that blew up) still wouldn’t come on – probably because of the Green Energy circuit tied up with the sync from the computer’s VGA card.

Now the monitor is probably a multisync type and has five BNC sockets on the rear, as well as a 9-pin D connector. The cable and instruction book were not supplied with the monitor, so I had to guess again what type of cable was required. I tried a standard multisync 9-15 pin cable but that didn’t want to work, so I can only assume that it has a non-standard pinout.

I then tried the BNC to 15-pin D connector and had a better luck with this. The connectors are Red, Green + Sync, Blue, Horizontal and Vertical pulses, Vertical pulses – the significant one being the Green lead. With that connected, you can hear a relay click in and out – but still no +85V or EHT. I gave up at that stage – life is too short and this job was just too hard and I can’t see the money in it for all my efforts.