I have had a run of Panasonic C150 chassis TVs recently which have been good for me. These days, it is rare to get common faults and each one is its own proverbial pain in the backside when it comes to tracing it. So often, the time taken is too long to charge it all out to one job and one can only hope that similar faults in other sets will offset the cost of finding the original cause.

The C150 has a common problem due to the design of its vertical output stage around IC451 (LA7838). The solder pads for this 13-pin in-line IC are too small and are prone to dry joints and/or fractures in the solder, resulting in a variety of intermittent vertical scanning faults.

In Australia, there were only seven models employing the C150/C150A chassis released since its introduction in 1996. And more often than not, this is the first fault that the customer has had in five years. What I like is that most people want it fixed in the home and I can go and do this in about half an hour – a win-win situation for everyone.

The only fly in the ointment is the intermittent part, because invariably the fault won’t show when I’m there. I can only go on the client’s description of the problem and there are so many people who are not articulate or that accurate in their observations.

The worst scenario is when IC451 fails and results in the set being dead because of the load on the 12V rail (pin 1). Repeated failures of IC451 can be attributed to C455 (100μF 35VW).

Items Covered This Month Panasonic C150 chassis TC-25R70AB Panasonic M19 chassis TC68GS90 Panasonic MX8 chassis TC68PS10 Panasonic M16M TX-33V2X

Caught out

Unfortunately, I was really caught out last month when Mr Peters brought his set, a TC-25R70AB, in with the fault description: "intermittent loss of picture and a horizontal white line".

I immediately took this to mean intermittent loss of vertical deflection and stupidly quoted on this basis. Naturally, I switched the set on and the picture was fine even when I belted it – my standard shock test for dry joints.

Still, I was extremely confident it was just dry joints on IC451 and so after waiting 10 minutes, I decided to resolder them. When I had finished, I replaced the back and put it aside for soak testing.

My confidence didn’t last long. About 30 minutes later, I was walking past the set and was mortified to see that the display on the picture tube was now a pulsating bright raster with a white horizontal line in the centre.

I immediately returned the set to the operating theatre and removed the back. However, when I switched it on, it was back to working properly again – much to my frustration.

I persevered and noticed that after about 15-20 minutes, the picture would go through a series of very slight changes but mostly getting increasingly brighter until the screen was an uncontrollable white raster. It would then start to pulsate, with the picture geometry breaking up and an even brighter white horizontal line appearing in the centre.

Because it took so long to happen, I attributed this to heat causing a component to fail. But where? The other clue I thought I might have was the rust on the metal screen covers – no doubt due to the owner living close to the beach.

I started with a hairdryer and this did tend to support my theory that heat was causing a part to fail. Unfortunately, after emptying one and a half cans of extremely expensive freezer, I couldn’t get the fault to reverse itself.

Next, I examined the rust/salt corrosion problem. This was mostly confined to the metal screening cans and was not visible on any component. Nevertheless, I sprayed the entire set with a fine mist of CRC 2-26 and then used the air compressor to make sure every part of the chassis was covered in a fine coating of oil.

But still the problem persisted. All the voltage rails checked out OK but I did notice some variations in the voltages applied to the CRT. These, however, could be restored by unplugging E/Y32 – the drive to the output transistors. I spent some time examining Q350 and its associated components, especially the diodes, and I replaced C350 and C360. But to no avail.

By now, I also noticed that in the process of getting brighter, the hue would change too – somewhat randomly. I checked the screen voltages and the screen control on the flyback transformer, because by now I had worked out that as it got progressively brighter, the set was unable to cope with the beam current. As a result, the flyback secondary voltages were dropping with the demand for power by the tube.

I must confess I did have some doubts about the flyback transformer but for now I was more interested in the jungle IC – IC601 (TA8844N). In the end, I decided to take the plunge and replace this 64-pin high-density device.

You can imagine my frustration when this made absolutely no difference.

What was missing?

So what was I missing? Well, as Mr Sherlock Homes would have noted to Dr Watson, I hadn’t been concentrating on the details. As the picture began to go into its runaway failure, one of the first things it did was ever so slightly change its colours and produce Hanover blinds. Now, this was happening only very intermittently and very subtly but it turned out to be the main symptom when it came to determining the cause of the whole problem.

This symptom made it appear as though the set was being switched to NTSC and this is usually caused by problems with the line pulses. Line pulses are often used for clamping DC levels for the output stages, so it was well worth following this line (‘scuse the pun) of investigation.

The horizontal (line) pulses are generated from the line output transistor via C509/C510 and fed around to pin 35 of IC601 via horizontal centring control R509. I put a CRO on the line and watched it. As the fault occurred, so the line pulses began to start changing shape and cause "ringing" immediately after the pulses. And when I saw it going through the "H. Center" control, I suddenly remembered reading a service bulletin about it.

I dug up the bulletin which was about "intermittent colour phasing/missing at top half of picture". Well, this was not really the problem we were faced with, though there were some similarities. I read on and it said these sets may have a problem with the colour content of the picture. It can either be a symptom where intermittently the top half of the picture may have missing colour, or the top half of the picture may have a colour phasing problem (green colour at top).

It went on but I was beginning to lose interest as these weren’t the main symptoms I was getting. However, out of curiosity, I decided to vary the horizontal centring control (R509) and see what happened.

To my surprise, varying this control had a real effect on the symptoms. Fully anticlockwise, it was very bad but turn it fully clockwise and the symptoms would slowly vanish – and stay that way.

The Technical Information Sheet advised that R508 should be changed from 15kΩ to 27kΩ and C507 from 150pF to 330pF. I made these changes, reset the pot to centre the picture and that, as they say, was that. The fault was fixed completely!

Panasonic month

It’s been mainly a Panasonic sort of month, with all sorts of models jumping out of the woodwork.

One 68cm 1997 TC68GS90 TV employing an M19 chassis once again found its way to the middle of my bench while my back was turned. I keep telling myself Mrs Serviceman that she mustn’t allow these sets onto my bench while I’m doing important things – like making coffee – but it doesn’t do any good.

This set was dead and apparently dead urgent too. Someone in the Ladies Bowing Club (which owned the set) has influence.

It didn’t take massive amounts of brain power to discover that line output transistor Q552 (2SD2539LB306) was short circuit, as was D720 (RH3F). The only problem was why?

I switched the set on and sound and picture appeared in that order. But just as I had managed to tune in a station, the set died again. The same two culprits also got extremely hot before dying.

I had had a chance to see the picture before the set died and it was too small (about 20cm wide). What’s more, there was a large east-west pincushion effect.

With only the above information to go on, I had to make an intelligent guess as to what the problem might be. As a result, I decided to order a new flyback transformer (T501), along with the other components that had failed again.

The boss wasn’t impressed with the delays – the heat must have been on and I speculated that perhaps she was up to being elected to the committee. I was quickly reminded that it wasn’t for me to speculate on anything other than the job at hand. I bet brain surgeons don’t get treated like that.

Anyway, when the parts arrived and were fitted, I decided to cover myself and measure the B+ rail and a few other rails as well. I also connected the oscilloscope to the collector of Q552 to check the line pulse.

I switched it on and, of course, with a new flyback transformer, you have to reset the screen and focus controls before you can see the picture.

Oops! – too late. The two parts had failed again and were red hot. However, I did manage to check the voltage rails in time and was able to see that no change had occurred in the picture. Unfortunately, I missed the oscilloscope readout as it was on the wrong range and the set died before I was able to adjust the controls.

Well, my feeling was that I had at least covered myself with the flyback transformer and the two components were dying from delivering too much power into the east-west correction circuits. So that was where I had to investigate next.

The cathode of D720 is connected to the collector of Q552, while its anode goes to its partner D721 and to the X Board via D9, pins 7 and 8 and then to the line linearity coil and correction transistor Q703 via L722 and R765.

I took out the X Board and examined it very closely. I checked it for dry joints (there were none) before using a multimeter to check the main components. Q702 and Q703 were fine, as were all the coils, diodes and capacitors. Everything was looking fine but as a last resort, I decided to check the resistors too. (I really didn’t have much more in the way of clues in the old kitbag). It was just as well because I found R765 (10Ω) to be open circuit. I put a new one in after checking everything else, fitted a new transistor and diode and gingerly switched it on again.

Now I wasn’t really expecting much. I had been surprised that R765 was open, which made me think about Q702 and Q703 again – but I really couldn’t see this as being responsible for blowing up the set.

But as usual, I was wrong. The set was now working perfectly and the offending parts were no longer getting hot. I soak tested the set for as long as I dared before informing the boss that it was ready.

A bizarre fault

The next story involves a bizarre fault. It too was a Panasonic, a 1999 model TC68PS10 with an MX8 chassis. In this case, the top 10cm of the picture was full of flickering big dots (not snow, though), stripes and lines and data – somewhat like data corruption. It was really weird and even happened when the video was muted out to a blue screen.

Well, where do you start with such a phenomenon? I could only think data, computers, micros – so I went for the microprocessor first (IC1101, MN1873284TF1), then the EEPROM (IC1102, 24LC08B1PA22) and finally the jungle IC (IC601, TB1237AN). To my frustration, changing them had no effect at all.

Surprisingly, it turned out in the end to be the tuner/IF module (TNR1, ENG39602G) that was the culprit. But having replaced all the other ICs, there really wasn’t much left to change – except the cabinet!

Mr Edwards’ 92 telly

Mr Edwards is a nice enough guy. He is semi-retired and lives with his wife in a modest house, with a private income from a block of luxury units. To look at him, you wouldn’t think him to be a wealthy man – until you saw the TV. It was a 1992 Panasonic TX-33V2X using an M16M chassis.

So what, you say? Well, this was a pretty expensive telly back in 1992, even by today’s standards. Back then, it cost about $5000 for this set which came with an 84cm picture tube and all the bells and whistles.

Despite its complexity, the M16 is a pretty reliable chassis. It was something of a watershed in Panasonic design and used plug-in modules, rather like the early M4 chassis of the seventies. That is probably where the similarity ends but you get the drift – it’s big, heavy (60kg) and complex.

So why was I doing a housecall on it? I can only put it down to the set being big and heavy and me being both optimistic and stupid. In mitigation, from Mr Edward’s fault description, it could easily have been something simple – after all, he said it intermittently wouldn’t start with the remote control.

Anyway, I thought it worth a look, so off I trundled to the Edwards’ house.

I started by looking at the remote control and found that it is actually two remotes in one. One side has a simple, basic remote control, while the other side has a much more complex layout. It was obvious that this wasn’t going to be easy to drive – especially as there were also a couple of tiny slide switches just inside the battery cover. And the 100-page(!) instruction booklet was like a small telephone directory.

I started by removing the batteries from the remote, thus disabling it, and tried switching on the set. The master on/off switch brought it onto standby, with the red power indicator coming on in the middle of the control door. One can never tell whether or not this is a fault condition, because most sets have an arrangement that when the power is switched off by the master switch, the set defaults to its last status – ie, if it was on stand-by, it will come on in standby and if it was on, it will come straight on.

The next trick was to find the standby/on switch on the front control panel. On this model, it is the program up/down buttons.

When I tried it, the set wouldn’t come on. I then put the batteries back in the remote and the set still wouldn’t come on. I wasn’t one hundred percent sure how the set was meant to be switched on from standby, so I tried the volume 1 controls plus lots of other things as well but got no result. However, after many repeated attempts, the set suddenly came on. I tried the remote control and that now worked too – it even switched the set on and off very positively.

This was the worst scenario I could imagine. There was probably an intermittent fault within the monster and the only place to tackle that was my workshop bench – definitely not here.

However, muggins me decided to poke about inside and proceeded to remove the back. Inside, the only accessible modules are the S-Board (AC Filter), N-Board (Power In) and X-Board (Deflection Correction).

The all-important D-Board Power/Deflection is one of the mother boards and is almost totally inaccessible. I resoldered some potential dry joints on the S, N and X-Boards and reassembled it. Despite bashing the set, I couldn’t make it go off so I then spent some time checking the tuning and set up.

There were two other problems. First, the sound was only coming out of the lefthand speaker. Locating the menu for adjusting the balance control wasn’t easy but I eventually found it under the "Auto/Function" menu on the set and the "Sound Menu" on the remote control. Readjusting it brought sound in the righthand speaker up correctly but I couldn’t find a menu that would switch the annoying "MONO" indicator to "STEREO" every time I changed channel.

This set has two separate stereo decoders – "A2 (German)" Zweiton and Nicam Digital – which it can automatically select. The only control is choosing between "Stereo" and "Bilingual". Everything, I tried left it in the MONO mode so there was obviously a second fault here.

I discussed this with Mr and Mrs Edwards who agreed to try it as it was – they weren’t too fussed about not getting stereo. Unfortunately, four days later, this all unravelled when Mr Edwards phoned to tell me the set wouldn’t start again. I still thought that this might be due to a dry joint on the D-Board and so I called in and picked up the chassis so that I could rework it at the workshop. However, apart from a few dodgy connections, I couldn’t find anything wrong.

It was then that I had an amazing bit of good luck. I was reading a technical magazine on repairing this very set and there, in black and white, were the two faults I had observed and the solution for both of them.

It all concerned the 5V rail from IC803 and, in particular, C885 and C889 (both 330μF 16VW). When these two electros dry out, the 5V rail drops to 4V, affecting the digital circuits. I replaced both parts on spec and took the reworked chassis back to the Edwards. I am happy to report that we had a good result.