Items Covered This Month Sanyo C29PK81B-00 68cm TV set (AA1-A29 chassis). JVC C-21T1AU 55cm TV set (KY chassis). Philips Matchline TV set (FL 1.2 AA chassis)

Storm damaged TV receivers are a classic example of this problem. They are invariably dodgy; the extent and path of the damage is entirely random, is impossible to follow logically and it’s difficult to assess the best approach.

So it was with dread that I responded to a call from Mr Philips about his 6-year old 68cm Sanyo stereo TV set, a C29PK81B-00 employing an AA1-A29 chassis, which was dead. There had been a severe storm the previous evening, with a lot of lightning but Mr Philips had been out during that time. When he returned and tried the set, it wasn’t working.

I removed the back and found that the 4A fuse, F501, had blown. Though I knew it was futile, I replaced it and switched on. There were no signs of life. I them measured R508A, 3.9 5W, and from there traced the circuit to the culprit, the chopper transistor, Q313 (2SC4429), which was short circuit. At this stage, it was time to take the set to the workshop and so I loaded the wagon and set off.

I had to order a replacement transistor but, in the mean­time, I did a few checks with the ohmmeter in the primary part of the circuit, checking Q311, Q312, R320 and R321. All were OK.

I felt fairly safe in replacing the chopper, Q313, when it arrived. I switched the set on and monitored the main B1 rail but was horrified to find it was at nearly 180V instead of 140V. I checked the optocoupler which was OK but Q353 wasn’t. Replac­ing it brought all six rails up to scratch.

But the set was still dead, and even the standby LED wasn’t on. This was no great mystery as there was no 5V and, in turn, Q521 was open circuit and there was a dead short on the 5V rail. Replacing microprocessor IC701 and memory EPROM IC790 fixed that and at last I had a raster on the screen.

But there was still no sound and no remote control. A new RC preamp module fixed the latter but a bizarre thing was now happening. On standby, I had 5V but with the set switched on this rose to nearly 10V. From where was it getting the extra 5V?

To begin with, I felt sure there was something wrong with the supply via Q521, which is on the front control board, because there was 16V on the emitter of the 2SC2568. This was further complicated because, when I unplugged the board, the set wouldn’t start and the 5V was rock solid. Perhaps it was breaking down under load? This was possible but unlikely, especially with a new transistor fitted.

Gut feeling

So I put that on the back burner. My gut feeling was there was another power source that was breaking through onto the 5V rail via a faulty component – but only when the set was on. The problem was where; the 5V goes everywhere to all sorts of devic­es.

Rather than trace and disconnect the 5V circuit everywhere it went – with the high probability of switching the set off in the process – I decided to disconnect each power rail until the 10V on the 5V rail changed.

I started with the horizontal output stage, by shorting base to emitter of the line output transistor (Q432). This switches off all the horizontal transformer derived power rails, including a highly suspicious 5V rail from pin 8 of the horizontal output transformer (T471). However, in the event, this turned out to be a furphy; it made no difference and subsequently, it became clear that this circuit was not fitted to this model. (It is really only for Teletext sets).

So back to the drawing board and the six rails off the chopper transformer, T311. Disconnecting one at a time, I found no less than three rails were able to affect this wretched 10V, which, I might add, was probably causing oodles of problems to the devices attached to it. After all, if a device is supposed to work at 5V, it is probably very unhappy at double that voltage.

My only hope was that they could all just hang in until I solved the source of this higher voltage. I was as brief as I could be on each measurement with the set on and mentally apolo­gised to each component for the stress it was enduring. In the end, it was rails B2, 24V; B5, 15V; and B6, 12V; which caused the 5V to rise.

The common denominator between these and the 5V rail seemed to be the protect rail – but this didn’t seem to be very logical; surely this circuit would at least have turned the set off. Another furphy; that wasn’t the answer.

To cut down this somewhat elaborate tale, I found diodes D393, D362 and D363 to be in various stages of breakdown. Q792 and Q793 seemed to be OK but I replaced them anyway. That fixed this problem and the 5V was now steady in the STANDBY and ON modes. I must admit I really can’t quite see why this would raise the 5V rail so high, especially as there is never less than 33k between them. I can only surmise that if the voltage is high on pin 31 (protect) of the microprocessor IC701, it will feed back out on the Vcc rail (pin 27).

There is not much information about the protect rail cir­cuit. If pin 31 is less that 2V for a few seconds, the set should switch to standby. This is a fault I had with another similar model (CP29ST2T-00 using an AC2-A chassis), where R485 180 went high. This held the cathode of D486 high in the video output 210V supply, causing the set to shut down.

That wasn’t the end

Unfortunately and predictably, that wasn’t the end of my troubles with this set. And that brings me to the conflict that faces all technicians in deciding where to draw the line between a write-off and a viable business proposition. It is very hard to be a good technician and a good businessman at the same time.

With so much effort having been poured into fixing this set this far, all I had was a raster. Would I be only one cheap component away from finally cracking the problem or would there be umpteen other components that would need replacement?

I was already down the mine for a couple of hundred dollars for parts, not to mention labour – was I going to write this off? Who was going to pay for the work done so far?

In practice, I often tend to muddle along on a wing and a prayer – the job is put on the back-burner and only brought out during quiet times. It is either eventually repaired and returned to the customer or cannibalised for parts (for more profit­able repairs) – or even fixed and sold.

In this case, I decided to quit before things got complete­ly out of hand. Mr Philips and his insurance company were advised that his set was uneconomical to repair and he received a new one. I scored the wreck with a little cash to sweeten the deal. All things considered, it was the right decision and would seem to be the logical approach in all such cases where insurance is available.

Much later I went back into it and checked transistors Q700 (Tuning), Q708 (Reset), Q182 (Sound IF), Q1709, D1705 and C1705 (a kind of spot wobble circuit?), all of which turned out to be faulty. When these were replaced, the set produced a good picture in the A/V mode with a VCR connected and a poor picture off air. The sound output ICs were both faulty (and hideously expensive!).

The final hurdle to restoring full sound and a good picture off-air was the jungle IC (IC101). The circuit is extremely confusing as it is tied up with the multisystem switching and A2 stereo decoder.

The main sound IF goes through IC181 (pin 14), Q182 (SIF multisystem filtering circuit), the jungle IC (IC101, pins 5 & 1) and then to the multiple sound processor IC1103 (pin 3). It then goes via Q857/858, with a sub-sound going from IC181 pin 6 to IC1103 pin 2 via Q183/184. The audio management control separates mono, stereo, A/V inputs and outputs, sound carrier and different decoder circuits – all controlled by the Philips I²C bus data lines from the CPU.

With this set you can – if one really wants to – record a TV broadcast while playing back a tape from another VCR, with all the connections going through the set!

There were a number of other confusing situations that occurred during the course of the repair but they were eventually all sorted out with the aid of the expensive service manual and instruction book. These included the child lock and censored program sites (private position) and the stereo separation settings which have to be done after the EPROM is replaced.

All in all, an interesting but unprofitable venture into the Sanyo way of thinking. But at least I eventually scored a working set.

A miserly customer

Mrs Parker wanted to know how much it would cost to fine tune her TV receiver. I reasoned that if she had to ask, then she couldn’t afford it and so I was ready for the inevitable "that much!" I pointed out that it was the same charge as 10 years ago except for the GST. But, I suggested, if she felt that was too much, why not go back to the instruction book and do it herself?

Somewhat taken aback, she hung up and I thought that was that. But it wasn’t. Three weeks later, she phoned again and wanted me to call. I asked her if she had been able to fine-tune the set herself? Well, she said, her friend is an electrical engineer and he managed to tune all the stations except channel 7, where there was no sound.

"Well", I said, "surely he could have tuned channel 7 as well". "No", she said and she wanted me to come around that afternoon and though she thought it was daylight robbery, she was prepared to pay my fee.

Mrs Parker’s set was a 1991 55cm JVC C-21T1AU which employs a KY chassis and is a multisystem receiver. I connected the aerial directly to the TV set in case the VCR was affecting it and checked each channel. All were perfect except channel 7 VHF.

I negotiated the on-screen menu system and adjusted the fine tuning which was able to improve the sound at the cost of the quality of the picture. I then tried auto-tuning the sta­tions. The set searched and found every station with perfect sound and picture – except channel 7.

Next, I tried the colour system which was the same in Auto as in PAL. What I was really looking for was a switch to change the CCIR system, because the symptoms were very similar to some which can occur in the CCIR system, using 6.0MHz intercarri­er sound. I could see from the specification in the instruction booklet that this set could automatically detect 5.5MHz, 6.5MHz and 6.0MHz sound IF but couldn’t be overridden manually.

The suggestion, therefore, was that interference was acting in such a way as to make the receiver believe that it was handling a 6MHz inter­carrier sound signal, rather than 5.5MHz. And why channel 7? Presumably because it was receiving the stron­gest interfering signal.

It was just a theory, of course, but it was the best one I had.

I told Mrs Parker I would have to do some research on this but she was totally unimpressed with the idea and started bleating about money costs.

I was past caring by now. "Look", I said, "do you want this fixed properly or not?" She admitted that it was important to her. I then asked her when the problem first started. She didn’t know this exactly but guessed that it was several months ago. I also confirmed that the aerial hadn’t been altered and neither had any new build­ings been constructed nearby.

Everything was now pointing to one major suspect; interfer­ence from digital TV transmissions.

Australia is the only country using digital transmissions on VHF as well as UHF and there are many sets which were never designed to work alongside broadband, high-level, digital chan­nels in the VHF band. A common symptom of digital co-channel interference is a white dot pattern on channel 9.

I told Mrs Parker that I would be back and, as good as my word, I returned two days later armed with a few gizmos. Number one in my armoury was a very expensive Polytron adjustable filter, model TFV-3K. Connecting this in the antenna system and adjusting the two trimmers restored the sound completely. The only trouble was that it was well out of Mrs Parker’s budget. Next, I tried some attenuators but they just added snow.

Finally, I reconnected the VCR into the set, via RF and A/V leads, and made sure that it was able to receive all channels satisfactorily. The point here was that the VCR tuner is not bogged down with an AUTO function to select the sound IF; it is fixed permanently on 5.5MHz.

I then explained and demonstrated the options available and suggested that using the VCR tuner was the cheapest and most convenient solution. Had I not demonstrated the filter to her, she would never have believed me. As it was, she begrudged paying me for my time and advice.

Subsequently, I was talking over this problem with a col­league and he solved it in a different way altogether. He had a similar model JVC, which was also multisystem. He very cleverly modified the automatic system circuit so that when the set switched to system I (6.0MHz sound), the 5.5MHz ceramic filters were switched in instead of the 6.0MHz filters. This meant that the set was now permanently aligned for CCIR system B/G Australia (5.5MHz) and the sound was perfect.

A crook Philips

Mr Stephens wanted a service call on his Philips TV which had just gone "crack" and then the picture disappeared. However, he still had a picture and sound from his video, which was connected via the AV sockets.

I enquired whether this had happened during a storm but apparently it hadn’t.

He told me that it was a 1991 Philips Matchline Collection (36ML8906/00B), employing an FL 1.2 AA chassis, and cost about $8000 new 10 years ago! This set is very large, employing an 86cm screen tube and is extremely heavy and difficult to move.

Despite this, I informed him that the difference between the cost of my taking the workshop to his set, which I had never seen before, or of him bringing it to me could be somewhat significant. That clinched it – his set was on my bench the very next morning.

This digital set has all the bells and whistles and surprised me how technically advanced it was for 10 years old. I soon found that the second tuner in the set gave a good picture and stereo sound in the PIP mode (Picture in Picture) and could be swapped around so that the main screen gave the off air/terrestrial broadcast picture. But the main tuner was not working properly. Intermittently, it would flash, drift off and give a snowy picture. Occasionally it even came good.

The tuner IF module (1160) is a long metal can and cannot be removed without first using a very hot soldering iron to remove the earth lugs. Once out, I took the covers off and examined it. I was about to replace all the electros inside it and touch up a few dry joints when I noticed black soot markings around IC7507 (TDA3856). It was fairly clear that something drastic had happened to this IC. In fact, considering the black markings adjacent to its legs, it looked as though this IC had exploded – and yet it was still almost working.

This IC is not available as a spare part and neither is the tuner. It is only available as an exchange repair. So it was duly packed up and sent off to Philips. My big mistake was not to have carefully written down all the details that were marked on the tuner (I think it was an FQ816MS KR11 21122).

When the replacement tuner arrived (it looked like new), I again didn’t pay any attention to the part numbers, but was somewhat aggrieved that it was more than the quoted price.

I soldered it in and switched the set on. Well, the new tuner didn’t seem to be any better than the old one so I tried to tune the stations in. Although I could do this, there was absolutely no sound at all!

I read the manual and found that there is an Option Code procedure for tuners and perhaps this needed setting up. To do this, the service mode is engaged by shorting pins S23 and S24 together and selecting the Option Code 1 and 2 menus and assigning a number from the list according to the hardware options fitted in the set. You then add up all these options and punch in the numbers with the remote control and store it with the "pp" button on the remote control

The original option code 1 was 154 for this set but I couldn’t deduce how to get this number. I tried a variety of numbers consistent with what I thought we had.

Finally, I got extremely technical and worked out why no sound was coming out of the tuner – it was because there were three missing pins on the replacement tuner (pins 16, 17 & 25), pin 25 being the L + R/A signal to the stereo decoder (IC7200, TDA­8417).

I sent the tuner back and received an FQ816ME/1 which had the right number of pins and the correct quoted price.

This time, when fitted, the stations tuned in correctly and the sound was terrific. Option Code 1 was indeed 154, being the sum of 2 + 8 + 16 + 128:

2 = Front End FE816/ME

8 = PIP module fitted

16 = NTSC-M reception possible with FE816/ME

128 = Second front end for PIP fitted

I assume FE means Front End but I have no idea what FQ means. Option code 2 remained at 4 for 100Hz high-end box fitted (modules L and M).

There are no less than six variations of tuner for this set. The correct part number for this particular model is 4822 210 10507 (Repair).

Mr Stephens was happy to get his set back, but we are still puzzled as to what caused the problem in the first instance.