| Issue: 181 | Published: 20 October, 2003 |
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Serviceman's Log TV servicing is getting complicated... |
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Now that I am getting to see more modern TVs and get acquainted with their hi-tech digital circuitry, I am beginning to appreciate the simpler TVs of 10 or more years ago. Even base models now have extensive digital circuitry with protection circuits everywhere.
Does this mean that the new sets are more reliable and cheaper to fix? No, I really don't think so, as it is the new high-tech circuits that are now giving the most trouble - and they are much more difficult to troubleshoot as well.
The digital circuits normally consist of surface-mount components on PC boards with very fine "spider-web" tracks. These boards, which are also often double-sided, are easily ruined in a corrosive (eg, salt-laden) atmosphere and replacement boards are sometimes expensive and difficult to obtain. And, of course, the new module often requires reprogramming after it has been installed.
Unfortunately, these boards are rarely repairable to component level and due to their complexity, can suffer from a range of unusual intermittent faults. Furthermore, access to diagnostic software also requires a service manual which often isn't available unless you are a service agent. Similarly, specialised diagnostic tools are often only available to selected agencies that can afford them.
Is this really a big step forward? Comparing a 34cm or 48cm TV today with that of the eighties, there really is very little difference in picture and sound quality. It is only in the large screen sets with multiple options that you can better understand the reason for their complexity.
Traditional bread and butter faults like flyback transformer failure now often lead to multiple chain reactions as they ripple back through small signal circuits, taking out surface mounted transistors and diodes - especially in protection circuits. By contrast, in the old days, if a flyback transformer failed, it would probably just take out the line output transistor and the beam limiting resistor.
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Items Covered This Month
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A few quick jobs
I recently had a Philips 21PT238A/75R PV4 chassis in for service. In this case, the EHT had arced out of its insulation and had taken out the vertical output IC (IC7960, TDA9302H). I replaced both parts, using a substitute HR8304 for the flyback transformer, but the set still wouldn't start, with just the front LED flashing.
I checked all the diodes (about 10) around the flyback transformer to find that D6444, D6492 & D6484 were all short circuit or leaky. Unfortunately, working out what value they were was a bit more difficult as I couldn't find a circuit that exactly matched this set.
In the end, I found a combination of different circuits that showed that D6444 was a 15V zener diode in series with D6447 (also 15V). In fact, 22V zeners were originally used in the set so I replaced D6444 only with another 22V zener. D6492 was a 1N4148, while D6484 was meant to be a 13V zener but the original was a 16V unit and so I fitted the same again.
After setting up the screen control and focus, the set was once again performing properly.
I also had a very old NEC N1426 (white plastic cabinet from the late 1980s). Interestingly, this was one of the last NEC-made sets (from Rank Arena) using the PWC-2188A chassis. Later production models of the N1426A were made by Daewoo (C38N chassis) in Korea and the only external difference was the addition of one extra control in the front.
This particular set had low distorted sound which I traced down to the bias resistor R310 (82kΩ) going high.
By contrast, I have another Philips with no sound which I still haven't managed to solve as there is a fault in the Digital Sound Enable circuitry and I haven't got as far as getting the exact circuit for the control logic! There is sound output from the IF circuitry and the audio output amplifier and speaker works - it's just the bit in between!
More often than not, these digital circuits are intermittent and the fault is random. Sometimes, the only clue to be had is the error code but even that will be lost if the microprocessor and/or EEPROM itself is faulty.
A faulty Akai
I had another Akai CT2867AT come in recently with no sync when cold. Akai is now pretty much an endangered species with little or no spare parts obtainable anywhere. This was a 1996 chassis built 66cm TV.
Because the fault only showed for the first 5-10 minutes or so, I decided to wait for it to work and then hit it with freezer. IC302 AN5601K seemed to be the most sensitive to this treatment, so I replaced it. But it wasn't to be - the fault was still there.
Feeling somewhat annoyed, I persevered and replaced the more obvious electrolytic capacitors in the sync circuit. I changed C322, C348, C338 & C312 without result, before moving on to the power supplies where I changed C409, C410, C341 & C340.
However, I still wasn't getting anywhere. A check with the CRO showed that the video signal and sync pulses were arriving at the IC and the voltages around it were as expected. And so I went back to the freezer idea.
This time, I carefully hit some of the components around the IC and quickly noticed that C345 was the sensitive one. Fitting a new 3.3μF 50V capacitor fixed the problem. Obviously, the first time I used the freezer, it had bounced off the IC and hit this capacitor, causing me to mistakenly think that the IC was the culprit.
The big Sharp
A large 33-inch (78cm) "telly" was delivered to my workshop by two strong oung men and my instructions were to fix it if it was under $300. Mrs Serviceman had fortunately steered them directly to my workbench, so no further lifting was required.
The set was a Sharp DV7988X using an 8PLP chassis circa 1992 and employed a 78cm picture tube. Fortunately, this was a basic TV without many sophisticated digital circuits and its fault was that it was dead.
However, strictly speaking, it wasn't completely dead, with the red standby LED coming on. And when it was first switched on, it would try to start up, with the Green LED coming on and then going off. Furthermore, relays could be heard clicking inside and there was even the rush of EHT but it was soon back to "standby".
This was the first time I had seen this model and, from the symptoms, it seemed like a protection circuit was preventing it from starting. Interestingly, there was no sound at all.
I ordered the service manual and while I was waiting for it, I removed the chassis and checked it carefully for dry joints and other obvious problems. The set is essentially built on two horizontal PC boards at the bottom and one vertical board for the power supply. As usual, the leads are too short to let you pull the assembly out so that you can turn the bottom boards over.
Eventually, the circuit arrived and, mainly because I didn't really know where to start, I began by measuring the voltages out of the switchmode power supply. These were all OK except that there was no -27V rail. This could either be due to a problem with the source or the load.
A quick check with a DMM showed that the rail measured a short circuit to ground. It turned out that one of the diodes in D702 (RH-DX0284CEZZ) - a double-diode TO-220 package - was short circuit. Replacing it completely fixed the set and the sound.
An examination of the circuit reveals that there is a protection circuit sensor on this rail. This trips one of the three relays and switches the set off when there is a fault, which is why the set was shutting down.
A piece of cake, really; I just wish that all jobs were like that!
Another big set
Another extremely heavy set was dropped in with weird intermittent symptoms. This was a 1992 Sony KV-S29MH1 using a G1 chassis (KIRARA BASSO Series).
This is an overseas (German) deluxe model with lots of complex circuitry. It arrived with a list of problems to address, ranging from drifting tuning to an excessively bright picture.
Fortunately, I had worked on this chassis before and was aware of a few of its stock problems. These were mostly due to a series of dry joints on the IC regulators on half a dozen or so modules.
I went over all these joints and when I had reassembled it, all the faults had been fixed except for the brightness problem and a jitter effect on the On Screen Display.
I decided to deal with the brightness fault first and soon discovered that the control voltage from Microprocessor Module M was not varying when the button on the remote was pressed. Instead, it was stuck on 4.8V.
I opened up the Service Mode (press DISPLAY, 5, VOLT, POWER on the remote control) and checked the sub-brightness control (31 SBRT = 0D) but it also made no difference. I then checked the screen control adjustments (579V) before concluding that Microprocessor Module M (A-1306-428-A) needed replacing. Unfortunately, it was no longer available.
My next option was to replace the microprocessor (IC005) itself (M37204M8-A10SP, Part No. 8-759-069-76). Fortunately, this was available but replacing it is another story.
First, the chassis has to be unplugged and removed. A metal screen then has to be unsoldered and removed before unsoldering the module itself. That done, the large-scale high-density microprocessor IC has to be removed from the double-sided board.
Before resoldering the module to the motherboard, I took the precaution of removing the two EEPROMS and fitting two IC sockets in their place. The EEPROMs were then installed in the sockets (this was done so that they could later be easily replaced, if necessary). Unfortunately, when I finally got everything back together, the fault remained as before.
Next, I tried swapping the two identical EEPROMS (ST24CO2ABI) and to my total surprise, the fault cleared! However, I now had a new series of problems - some of the tuning menus weren't working.
The EEPROM (ST24C02ABI) used is a very common device so I tried another one in the set. This too gave the same result as swapping them but it also let me establish which of the two ICs was the crook one. It was the top one - IC002. However, despite this being such a common device, it is no longer available from Sony for this set (Part No. 8-759-043-86 which, presumably, is programmed with this chassis' options).
I then decided that if I couldn't fix this problem with hardware, I would try software. In the Service Menu, I found I could stop the jittering with the OSD by changing item 46 OSDOE from a "1" to a "0" (CRT Screen Display odd/even inversion default settings).
I then went through the entire service menu of 50 items, checking each one against its default, checking whether it was working and writing down its value. The only one still not having any effect was item 31 (SBBRT) which, though set at OD, varied from OO to FF with no effect on the screen.
I then noticed in the service manual, under "Circuit Adjustments", that there were five extra service commands. Pressing 7 and 0 on the remote saves all the values and made no difference. Similarly, pressing 8 and 0 sets all user controls to the standard state but that still made no difference. Pressing 9 and 0 sets the horizontal frequency automatically, while 2 and 0 writes all the 50Hz data to the 60Hz data or vice versa (not applicable here).
Finally, there was 5 and 0 Service Data initialisation which had the following warning: "Be sure not to use usually". Apart from the poor English, I had no idea how dangerous this measurement was but I had scraped the bottom of the barrel and had no more ideas.
I tried it and that, as they say, was that. Suddenly, everything was perfect and the brightness control worked perfectly. I guess somehow - perhaps because of a power surge - that the data had become corrupted. Resetting it fixed the problem.
The whistling Panasonic
I had another beast in recently which I thought was eminently fixable. It was a Panasonic TC21S10A (MX3 chassis) which was dead and making a whistling sound.
I began by checking the line output transistor and the main B+ rail for shorts. There were none but there was no voltage on the B+ rail at switch on.
From the noise emanating from the set, it was obvious that it was under stress - probably from a short circuit on the secondary. However, it was also possible that the switchmode power supply was not oscillating properly, so I replaced two small 47μF electros - C805 and C825. This had the effect of altering the pitch of the whistle slightly, so perhaps I was half right.
After spending some time checking the main B+ rail, I switched my attention to the other voltage rails and soon found what I was looking for - D835 (MA2560) was short circuit. This was a 56V zener diode, which meant that the output from the power supply must have exceeded 56V - probably because the two electros I had already replaced were faulty.
Anyway, a new one fixed the problem.
An embarrassing situation
I had an embarrassing situation last week when a well-known elderly client of mine brought in his 1990 Philips 21GR6756/74R (G110S chassis) with the complaint that the set was dead.
Though an oldie (the set I mean), most faults are well known and relatively easy to fix. I soon found that the 125mA "micro" fuse (1963) in the supply line to the east-west correction circuit had "blown". This fuse often goes open circuit for no apparent reason but I did find a possible cause in the form of dry joints on the east-west coil.
I also upgraded the fuse to 1A (some models actually have a link instead of this) and then put the set aside to soak test. Both the picture and sound were excellent.
Two days later, the customer waltzed in and picked his set up. Another straightforward repair - or so I thought.
Then, barely two hours later, a somewhat frazzled client reappeared with his "telly", saying that it had " lasted less than two minutes"! Apparently, he had just plugged it in and changed channels when he smelt burning, after which the set quickly died.
"Oops", I thought - "perhaps I shouldn't have uprated that fuse"!
Anyway, back on the operating table, I found that the fuse was still intact but the horizontal output transistor (7927, 2SD1577) was now short circuit. This indicated that the flyback transformer had gone - was the set normally kept in a damp environment, I wondered.
I replaced the transistor and switched on carefully. The picture took what felt like hours to come on and the set was making noises I didn't like. When the picture did eventually come on, the raster was intermittently distorted in all directions (trapezoidal) and though the picture was good, this was looking serious.
I switched off and removed the deflection yoke to reveal what I suspected - classic shorted turns had begun to cook and melt. This is unusual in Philips TVs except, of course, for the old 2BS chassis employing an A51EBS60X picture tube. Hang on - that was exactly the tube inside this model too!
Well, regretfully, that is the death knell on this set. Not only are these yokes not sold separately but everyone wants them for their old 2BS sets, so there's not even a chance of obtaining a secondhand one.
I had to refund my client who was not impressed but that's life!
The red Panasonic
I was nearly caught out with a Panasonic TC33AV1 M16M chassis, which came in with a red-only picture. When the service switch was engaged, there was a white line and I immediately jumped to the (wrong) conclusion that this was an electronic drive problem from the jungle IC (IC601, TA8719AN).
However, shorting the cathodes of the CRT to ground momentarily while watching the screen told an entirely different story - the red was intensely bright but blue and green were both pathetically dull.
A check with my CRT analyser showed that though the cut-offs were good, the drive was as low as 0.1mA (as opposed to 0.6mA for red). An 80cm TV set can cost over $1000 to replace, so I decided to give it a "tickle" with the rejuvenator as there was nothing to lose. But even with 9V applied to the heaters, I couldn't get any response from the blue and green guns.
At this stage, my colleague persuaded me to continue the rejuvenation process while the set was switched on. This was pretty scary as the EHT is typically 32kV and I was expecting a flashover through the CRT to my machine. However, because the tube was so flat, this didn't happen and with my colleague rapidly switching colours, we finally managed to "kick-start" the guns into conduction and start the rejuvenation process.
In the end, we managed to get both the blue and green guns up to 0.6mA emission but ironically the greyscale tracking was appalling. The set ended up with pink highlights, green mid-contrast and blue "low-lights". Still, it was worth a try as the CRT had little or no life left in it anyway.
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