Mr Crown's six year old TV set, a 66cm GE 29GP480A (Thomson TX92), was a typical example and it came to me by accident. About four months ago the set had died and he dutifully took it to his local GE service agent. Three months later, this "specialist" told him it was a pile of junk he couldn't fix and didn't want to fix. He suggested he throw it out and buy a new one. It was an extraordinary thing for a service agent to say about the products he represents!

Items Covered This Month GE 29GP480A TV set. Toshiba 3429DXH TV set. Samsung TV-516 TV set Sanyo CPP 2930 TV set (A8-A29 chassis).

A lesser mortal might have left it there but Mr Crown was made of sterner stuff. He took it somewhere else and they at least made some progress - even though they weren't GE agents. The set was now no longer dead but still had a few intermittent faults that had to be fixed.

I know their principal technician personally and when I called in to have a cup of tea and a whinge about our lot, I cried about how quiet it had been since the GST came in - how business had dropped off incredibly now that new sets were 20% cheaper while repairs were up to 10% dearer. Apart from TVs, only top of the range VCRs are worth tackling - providing the faults aren't major.

Anyway, the point of all this was that, since I wasn't all that busy, he suggested that I might like to finish off the repair on the GE set - as long as I could do it within an agreed budget and time. Well, what could I say? I had to take it on.

I took it back with me to the workshop and switched it on. The faults were all intermittent and included poor video, green raster, cutting out and varying picture size.

Fortunately, my colleague had also supplied me with a circuit diagram, but that was all he had. Most of the problems seemed to be centred around the intermittent green raster and this was controlled by the screen control (G2 or A1, depending on your nationality) on the horizontal output transformer.

Sensitive control

The control was set fully anticlockwise and was very sensitive. If it was turned up, the picture's poor video would improve momentarily before going into a runaway bright green raster with the set cutting out.

I started by trying to check the main HT, supposed to be 130V, on the cathode of DP51. There are no voltages printed on the circuit but someone had pencilled this in. While looking for DP51, I noticed that resistor RP98 (270kΩ) had been unsoldered and half pulled out.

I resoldered the resistor only to find that the set was now completely dead. The reason for this was that surface-mounted transistor TP96 (BC858B) had been destroyed. A new one fixed that problem and also fixed a few other faults. The picture size no longer varied and the set did not cut out unless the beam current was ridiculous.

My colleague had already replaced ICs IF01, IF02 and IB01, the latter being a TEA5101B - the CRT video output amplifiers. I measured the voltages on the CRT base and found that the cathodes varied between 112V and 121V (green), while the screen voltage seemed to be low at 210V. I changed all the electrolytics on the board, especially on the 196V rail, but this made no difference. I then checked TB18 and the diodes. They were all OK except for DB72, a BAV21, which was leaky.

I didn't have a BAV21, so I fitted a 1N4148. This improved the picture dramatically, especially after I adjusted the screen control.

Occasionally, however, when I switched the set on to AV1, from cold, it would still go green. I thought there might be a chance of an intermittent heater control short in the picture tube, even though changing the channel would take it out of the green raster mode. Anyway, I reconfirmed that this was OK by patching it onto another tube.

Considering the high voltages in these circuits, BAV21 diodes (DB72, etc) are quite critical for the automatic greyscale (AUTO CUTOFF) control circuits. 1N4148 diodes are rated at 100V while BAV21s are rated at 250V. I ordered and fitted the correct ones - except they weren't surface mounted. This made the set a lot more stable but I was still doubtful about the screen voltage on CRT pin 7; it seemed low at 210V.

I entered the service mode by holding down the remote control blue TEXT (VT) button and switching the set on. This is a fairly useless MENU system, which isn't self-explanatory. The first thing I discovered was that the channel or AV cannot be changed when in this mode so, first, it is necessary to tune the default program 1 to the signal pattern generator.

Navigation through the service mode takes a little working out but modes can be changed with the yellow MENU button on the remote control and the values with the volume + and - buttons.

The first problem, after starting in the service mode, is finding out what various things are - eg, BRAND3 and BD1 and (later) what is PEAK?

There are five greyscale settings for the three colours but no instructions on how to set them up. I adjusted the green to improve the greyscale slightly and left it at that until I could get more information.

It was here that I ran into a brick wall. Despite having telephone, fax, and email facilities, trying to contact the main service agent was appallingly difficult.

No kidding; it took two weeks and half a dozen phone calls, faxes and emails to get a minimal response. Basically, technical support is only for dealers and has to be faxed, not phoned (which means, definitely, no questions answered). Phone calls are only to the receptionist who blocks any further calls higher up the ladder. Their website requires you to become a member. The only trouble is that its membership form submission doesn't work and no-one answers the email.

After two weeks, I finally received an email saying only dealers can use the website and the only service information I can get is by purchasing a CD. This I did and I eventually received it another week later. It carried 16 files with .DOC .DOT and .WPS extensions. These print out to a fairly good circuit explanation and a little fault finding but no voltages, alignment or adjustments, which was all I was really after. So much for technical support when needed.

I persevered with a few more faxes. Five days after the last urgent fax, they did eventually phone back and apologise for the delay; they told me their technician would telephone in a few days - and, surprisingly, he did.

The correct HT is 132V and the screen voltage normally lies between 200V and 250V, so 210V was fine. The screen control is set by first selecting an empty AV input (black picture) with the brightness set for mid-range and turning the control up until flyback lines can be seen, then backing it off a little.

In the service menu, Brand 3 means this particular model as opposed to others in this range. He didn't know what BDI stood for. To set the greyscale in the service mode, one really needs to feed a pattern generator into one of the AV output sockets.

You can exit the service menu with the TV/OSD button on the remote control, select the AV channel and then re-enter the service mode with the VT/TEXT blue button. Now, with the colour control turned down to nearly minimum (but not completely), and brightness and contrast set to mid-range, you can adjust the five menu controls to set the greyscale visually.

It's not very scientific and probably difficult on an aging tube but this particular one wasn't too bad. On a normal TV set, one would adjust the cut-offs in low light areas and the drives on high bright/contrast areas. On this set, one can only assume that R-DC and G-DC are the cut-offs.

Anyway, I was fairly happy that I had indeed fixed the set but I was troubled that the screen control was set so low and was so critical. Mr Crown, however, was pleased with the result even if I was dubious.

Troublesome Toshiba

And talking about accidental jobs, how about this one! I never met the owner of this 1992 Toshiba 3429DXH that came in but I can imagine his frustration the set. This is an overseas model and judging by the 2-pin plug, was probably bought in the United States or Japan. And because it was a 28-system automatic multi-voltage set, all he had to do was to plug it into the power here in Australia, tune it in and it would have worked.

And it probably did for years without a hitch until, recently, it was found by the roadside awaiting a council cleanup. The person (or persons) who found it carried the 70kg 86cm TV set to the nearest secondhand TV shop and sold it for $50. Their technician, whom I know personally, switched it on to find that the picture and sound were initially excellent but, after 30 seconds or so, it would lose sync (particularly vertical) and the picture would "go funny" - almost negative.

A simple fault, one would think, and fairly easy to fix but you would be wrong. First, this set has all the bells and whistles and then some - it even has karaoke! Secondly, access to the chassis is appalling. It has three flat chassis along the bottom of the case with plug-in modules that are soldered in. To get to the solder side of the main boards, it is necessary to unplug a lot of connections before it can be turned over, which means there can be no measurements underneath while the set is on.

Without a circuit, it was going to be impossible to know where to start. There are two tuners, two IF panels and a 64-pin jungle IC, as well as a 64-pin microprocessor - not to mention numerous peripherals.

The agents for Toshiba are Castel Electronics in Victoria and my friend was extremely lucky to purchase a poorly photocopied manual for $45. Even armed with this, the task was daunting and eventually he offered the job to me.

Why me?

Why do I take on work like this? I suppose I just can't help myself. Anyway, it was delivered to my service bench and I tried to make sense of how it all worked.

First, I had to try to control the set without the proper remote control and so I used a generic one (Pilot or VESA make ones that work this model). It was still hard to juggle the controls, which aren't an exact match to the set, but I was eventually able to tune in the local stations.

Tracing the signal through the five plan-size circuit diagrams wasn't easy, partially due to the poor photocopy but also because of mistakes in the schematic, particularly in following the video and sync paths. Often one would follow the path from what appeared to be a source only to find that the destination was the source with the arrows reversed on another module! Not only that but quite often they would become a dead end on a module.

I started on the Video Out from the IF unit board (pin BO) and followed it to a small 3-transistor circuit which had an output marked TV SYNC. The only problem, when I finally managed to hook an oscilloscope to the component side of this circuit, was that there was no output. This was exactly the sort of symptom I was seeking and I was starting to feel hopeful.

Unfortunately, it took about three hours of checking all the voltages (which were correct) and changing almost all the components before I realised that this circuit was only a TV sync detector and delivered a voltage to the microprocessor which helped determine the auto cut-off (no sync) and, presumably, the frequency.

I moved on from the main board via plug PVO2A-228 where the arrows suddenly went in the opposite direction but nevertheless the signal arrived at pin 36 of ICV01 (TA8777N) on the Back Terminal Board V003 (PB3013-1). This IC controls all the inputs and outputs from the three AV inputs plus the TV set. Once again, it took a bit of fiddling but I eventually managed to get the AV inputs to work and I connected a signal generator. Even so, the picture was still rolling and negative.

Next, I connected a monitor to the output monitor socket of the Toshiba. Surprisingly, the picture was perfect on the monitor but not on the set itself.

The output to the monitor was via pin 30 for ICV01, whereas the TV signal was now called Y OUT on pin 18 and I could see the sync pulse being crushed and the centre parts of the colour bars also being crushed. I felt I was on the right track and so I ordered a new IC. And then I discovered a strange thing - with the colour turned off, the picture on the TV set was perfect. In fact, it turned out that just turning off the burst was enough to make the difference.

The new IC finally arrived and I was disappointed to find it made no difference. All the inputs to this IC gave a perfect picture to the monitor output but not to the TV set. I replaced all the electros around the IC and checked all the voltages but they were OK. I then started replacing the transistors around the IC: QV11, QV12, QV10, QV7 and QV5 but they were all perfect.

I must admit I was beginning to feel defeated. I couldn't understand why when all the inputs were correct, the Y out wasn't.

I spent some considerable time investigating the colour circuits, especially the colour system circuit, on the chance that when the burst was present perhaps the wrong system was selected, causing the sync to change. I ordered and fitted a new jungle IC (IC501, TA8783N) but that made no difference; nor did indeed removing the Teletext Board U009 (PB3017) and linking FA and FE. I removed the karaoke board as well but again drew a blank.

By now I was beginning to follow another clue. When switching it on from cold the picture was perfect for nearly 30 seconds, which might mean that heat from something was the key factor - perhaps even the power supply. I tried heating and freezing components all over the set but still couldn't find anything sensitive enough to change the symptoms.

I went back to ICV01, and noticed a number of links drawn on the circuit that could reroute the signals. One, in particular, reroutes the monitor output to the TV set instead. These are marked #1 and #2, so I swapped the links. This was a small breakthrough as the picture was now stable on the screen, though the picture quality was poor.

Swapping another set of links marked JP from pin 26 "COL IN" from the COMB filter to the monitor output gave a perfect picture. This made me decide to investigate the COMB filter, which is connected via PZ01A.

Locating and separating this board (D-COMB/LT 1/P-TUNER U012A 3020-1) was a feat in itself. It was bundled tightly on the lefthand end of the chassis and was screwed in with the wiring harness on a plastic frame and surrounded by metal screening cans. The actual board was marked PB3305-1 and is part of the picture-in-picture circuit. I noticed that the output to the monitor from pin 30 of ICV01 also went via QV02 to pin 300, and then via Q211 and Q212 to a large special IC designated QZ01 and made by Sony.

This is a digital comb filter and actually consists of five surface-mounted ICs on a double-sided ceramic module inside a metal can. There are two analog-to-digital converters and two line memory ICs with a DCF microprocessor. The signal arrived at pin 1 of IC QZ01 and left via pin 13, then went through five more transistors before going back to pin 28 of ICV01.

The CRO showed that the signal was fine going into the digital comb filter but it wasn't coming out. I then found that freezing the IC would fix the picture for a short while. My previous attempts with heating and freezing obviously didn't work because of the metal screening and because the module was "buried".

I ordered a new IC (SBX1692-01; trade cost $72) and eventually received a substitute list with an SBX1765-01. This also specified that QZ10 should be changed from 10μF to 100μF on the IC input.

This was the final solution to this problem and completely fixed the fault.

The correct remote control is on back order so that I can finally access the P-I-P and Karaoke menus. Surprisingly, the picture on this 10-year old TV set is stunning.

In fact, I was so impressed with this set that I approached my friend and struck a deal to keep it. Basically, I would swap three 51cm secondhand remote control TV sets for it, which I worked out as being worth about $750. The Toshiba was worth about $5000 10 years ago and it performed like a new one.

We were both happy with this deal. But, privately, I couldn't help recalling the rhetorical question I asked myself when I first tackled the job: "Why do I take on work like this?"

Frustrating Samsung

I had a frustrating time recently with a 1996 Samsung TV516, which had been dropped. There was slight cabinet damage but the set was still working, apart from a purity problem. I had a go at degaussing and readjusting it but after trying external magnets, I reluctantly concluded that the shadow mask had moved inside the tube (A51KQJ63X).

Fortunately, I had a secondhand tube that was fairly close to the original and so I fitted that. The tube I fitted was a little older and used a different yoke, so the dynamic convergence wasn't as good as with the original one - but it was acceptable. All that was left was to adjust the greyscale and that meant getting into the service made.

It also meant getting a service manual and, after that, there was the frustration of trying to make sense of it. It might as well have been written in a foreign language, because most of it was hopelessly inaccurate.

To get into the service mode, one has to press the "HIDDEN" service key - which didn't exist. Nor was there a "STATUS" key and, in the menu, there isn't any "AGING" mode. The service mode is actually accessed by pressing, in sequence, the STAND-BY, P.STD, HELP, SLEEP and POWER ON buttons on the remote control.

All I had to do was adjust the five greyscale menus, which I did. However, I made a mistake on one entry and wanted to cancel it, which I am supposed to be able to do by pressing the nonexistent HIDDEN or STATUS key. In the end, I phoned technical support who were very helpful. They suggested I press the reset menu option and correct the error.

So I did, not realising that this wipes out the entire EEPROM memory, including the options. When I tried to retune the TV set, it would only tune on UHF.

I had to go back into the service/factory mode (is there a difference?) and set up the "Option Bytes". The only trouble was I didn't have the figures for this Australian model. I went back to technical support and they didn't have them either but later faxed BYTE 0 = 40, BYTE 1 = 08, BYTE 2 = 23. The only problem was, this model had only two BYTEs, not three!

So I had to guess what the value was for each bit and calculate the byte 0 and byte 1 additions. I made BYTE 0 = 0 + 0 + 0 + 0 + 8 + 4 + 0 + 1 = 15, and BYTE 1 = 0 + 32 + 0 + 0 + 0 + 4 + 0 + 1 = 37 from the ambiguous list of options in the service manual.

I also had a look at a Samsung TV486 which had BYTE 0 = FC and BYTE 1 = 01. Unfortunately, I don't quite know what criteria they worked those out on.

There is no model number mentioned on the TV516 but I assume it is either CX, CK or CW. It is marked PIB03 on the main PC board under the horizontal output transformer and I assume this is an SCT 11B chassis variation.

It was all far too confusing for an old codger like me. I managed in the end, more by luck than judgement, to get the thing working properly. But my advice to anyone is that, before making adjustments to any menu, you should write down the values first - especially the OPTION BYTES. Whatever you do, do not Reset it!

A real mystery

Following this exercise in frustration, a reader brought in his Sanyo TV set and Sony VCR with a baffling problem. When playing certain parts of certain tapes, the TV set would switch on and off!

The TV set was a 1992 Sanyo CPP 2930 A8-A29 chassis, while the Sony VCR was a hifi SLV-X822AS. It was normally connected to the TV set via AV leads but the fault occurred just as readily via an RF lead on channel 36.

Well, where does one start with such a mystery?

Fortunately, the customer brought in a tape that would trigger the problem regularly and I was able to observe the problem first hand. However, if the tape was tested in another VCR and TV set combination, it had no effect. Similarly, I tried a variety of my own tapes in the Sony VCR but none would affect it.

So how could parts of a particular tape turn a TV set off and on? It took me a little while to think of how this could happen.

First, what could be different between various tape recordings? There is no clue just before the problem occurs, such as picture or sound degradation. However, many prerecorded tapes have Macrovision protection, a trick involving sync pulse distortion to prevent illegal copying.

Second, what ways are there to turn this TV set off other than via its remote control and the power switch? Well, there is an OFF timer and there is an auto shut-off feature which turns the set off after a broadcast has ended. And the latter is triggered when the sync pulses are no longer transmitted.

The common denominator for both of the above is the sync pulses, or rather, the lack of them. Having created a hypothesis that might meet the facts, testing it and devising a way to work around it is a lot harder.

I managed to obtain a service manual but imagine my reaction when I discovered that the only circuit missing was for the sync and reset module(s). Some sets have two separate modules and some have one combined. Nether are drawn in the service manual other than as a box in a block diagram.

I had to ask myself, is there an actual fault here with the VCR or the TV set, or is this a design problem? I tried connecting the VCRs and putting the tape in other players - but in the end, I felt this wasn't a fault; the TV set just wasn't designed for Macrovision. That being the case and because the set was nine years old, perhaps Sanyo knew about it.

Service bulletin

The next problem was that Sanyo no longer services its own products but fortunately they do have service agents, one of whom I know well. I asked my friend whether there were any technical service bulletins that might be similar to this problem. He had a good look before telling me the good news. There was one dated 27-4-94 (no. 24) for the CPP2930 and 2530 models, the fault being: "The TV set switches off and on (CPU resets momentarily) on VCR search mode".

The cause is put down to the fact that "the horizontal frequency detection circuit range is too narrow". Ironically, this didn't happen in search mode on this particular set but the rest was close enough for me.

It was then just a matter of purchasing a kit of parts and fitting them on the SYNC and RESET PC board (VE2689). Identifying their location was time consuming without the circuit or board layout but everything was otherwise straightforward and the modifications completely fixed the problem.