Items Covered This Month Philips 29PT5683/79R TV set (A8-0 chassis) National TC2086 TV set (PBA-M14 chassis) Philips 29PT8419/79R TV set (EM1A chassis) Panasonic TC-21Z88A TV set (MX-5Z chassis) Toshiba 289D8A TV set Philips 29PT2152/79 TV set (L01.1A chassis) Philips Flat TV 17PF9945T2 (LC03 chassis) Panasonic TC-68GS90A TV set (M19 chassis) Philips 32PW8807/79R TV set (EM3 chassis)

One of the few interesting jobs this month was a Philips 29PT5683/79R TV set, which uses the A8.0 chassis. The client was complaining that the set was dead and buzzing!

Well, to me, that’s a contradiction in terms – the set can be either dead or buzzing but not both. Or perhaps it was buzzing first and then died?

Anyway, it was the latter. The usual 2200pF 2kV ceramic capacitor (C2168) on the collector of the output transistor had burnt out and after negotiating the all-encompassing plastic framework, I replaced it with a 6kV version. However, the set was still dead, with the red LED flashing error code 1.

This meant that the protection mode was on and that there was a fault in the x-ray, east-west correction and/ or vertical deflection circuits.

The earthy end of C2618 went to the EW panel via pin 4 of M61/E61 and then via a collection of coils to FET7680 (STP16NE06FP). Access to this module wasn’t easy and it had to be removed completely before I could remove the FET which was indeed short circuit. Ordering and fitting a replacement FET was the easy part. Reassembling the plastic frame was the difficult bit and took most of the time.

And that fixed the fault. All that remained was to reset the fault codes via the EEPROM and the set was ready to go home.

The humming National

An ancient National TC2086 with a PBA-M14 chassis came into the workshop. It’s only sign of life was a humming noise, so I removed the back and began checking voltages.

After establishing that the B+ rail was OK, I soon discovered that the horizontal drive transistor collector voltage was all over the place. And this turned out to be due to dry joints on the horizontal drive transformer (T501).

This in itself is unremarkable but had this identical fault occurred in something like a Sony BG-1S or G3F, it would have destroyed the line output transistor. The fact that it survived is probably due to the fact that many older sets were "over-designed". For the same reason, I also wonder how many current sets will last as long as the older designs, such as the Rank Arenas, etc of 1975.

The SSB strikes again

I have written before about the Philips A10 chassis TV sets and the problems with their Small Signal Panel Board. Well, it seems that the problems with this board can also extend to the later EM1A chassis!

For example, I had an 18-month old 29PT8419/79R come in with no less than six faults: no colour off air (but OK with AV), no pincushion correction, no Teletext, no picture-in-picture or double window, intermittent no remote control, and intermittent no control functions on the set itself when it got hot.

Ironically, the set showed no error codes at all! Replacing the SSB fixed all the problems.

However, it should be said that the SSB for this set is much sturdier than its predecessors. It’s mounted inside a metal cage and the connector is the same type as used for SDRAM modules in modern PCs, only with additional plastic supports to suit its much larger size.

After fitting the exchange module, all the geometry and other controls, plus various options, have to be reset. This all takes time of course and is fiddly. The Service Alignment Mode, by the way, is still accessed by keying in 062596 plus the OSD Index i+ button on the remote.

A simple repair

We recently had a 2003 Indian-made Panasonic TC-21Z88A television using an MX-5Z chassis come in under warranty. This set had an east-west fault with excessive width and it was showing the CHk mode symbol in the top righthand corner.

In the adjust mode, the width could change its value on the On-Screen Display (OSD). However, the width itself wasn’t changing geometrically. I replaced the EEPROM (IC1103, C3EBFC000025) and then realigned the controls to complete a fairly simple repair.

The blue Toshiba

A customer dropped off a Toshiba 289D8A TV set, complaining of a blue screen but no picture.

This 1990 set was different from the more usual 289X8M models we used to see but it took some time for me to realise this. The reason why the raster was blue was because the greyscale was so far out. This was soon corrected to a black raster by making the appropriate adjustments.

However, there was still no picture – at least not initially. Vibrating or tapping the set seemed to make no difference but then the picture appeared as the set warmed up. In fact, once it had completely warmed up, it was very hard to get it to play up at all.

I examined the main chassis very carefully, changing any well-known troublesome capacitors such as C303 (1μF) and C317 (2.2μF) in the vertical timebase (hopefully affecting the blanking and muting circuits) and soldering any suspicious dry joints. Unfortunately, nothing I did was having any effect.

Anyway, to cut a long story short, I found that the difference between this set and the 289X8M models was that this one had a Teletext module. And, as I quickly discovered, the three large ICs and the connection socket were riddled with dry joints (and well hidden by a shielding plate).

The Teletext module had to be desoldered to get it out for access but once the job was done, the fault was fixed.

A weird picture

Another Philips 29PT2152/79 TV (L01.1A chassis) came in with the weirdest looking picture. As far as the client was concerned, the picture was "all broken up" but it really was hard to describe. However, it looked as though it had a line interlace problem – there were multiple irregular lines all over the two-thirds scanned screen, with vertical foldover.

A quick check with an oscilloscope showed that the vertical drive output from the jungle IC (IC7200) was distorted. Unfortunately, the service manual is hard to use, because you constantly have to look up "Diversity Tables" to find the values of the components for your model (if fitted).

Anyway, after replacing IC7200 (TDA9565H/N1/4), the waveforms from pins 16 & 17 (S3 & S4) were OK all the way to pins 1 & 3 of the vertical output stage (IC7471, TDA8359J). I replaced this IC as well but it made no difference. I then checked the two voltage rails to pins 3 & 6. The former (13V) was OK but the latter (Vlot Aux +50V) was low at 45V, with significant ripple.

Next, I checked C2487 (47μF) and swapped L5472 from another chassis. This made no difference, so I concentrated on the IC itself, especially around the vertical output circuitry. In particular, I checked the components around pins 7, 9 & 4, along with the waveforms, but it was hard to pinpoint the culprit.

In the end, it was the output wave-forms that provided the vital clue. They were quite distorted, with significant ringing, which led me to suspect resistors R3477 and R3478 (150Ω) in series across the deflection yoke. They both measured high and replacing them fixed the fault!

Philips LCD TV

One difficult problem we faced recently involved a Philips Flat TV – model 17PF9945T2 with an LC03 chassis.

According to the client, this beautiful 17-inch LCD TV was taking a very long time to come on and was taking even longer to give sound and picture - though it was quicker when connected to a digital set-top box (or digibox). The problem was getting it to play up for us. Heating, cooling and vibration tests produced no measurable effects.

However, being an authorised Philips Service Centre, we are privileged to receive Service Information Updates and we had no less than three such updates for similar symptoms. The first update says to check that the wires from CN2, CN3, CN4 and CN5 on the Inverter Board are well connected via the four connectors to the LCD panel. The second update says to check the flexible cable for pin damage. And the third update lists two modifications: (1) R49 (10kΩ) is changed to 3kΩ (bias to Q8); and (2) R29 (0.33Ω) is changed to a link (5V supply to IC U1).

We tested it for a another week and it is now back with the client.

A frustrating job

Of course, there’s always one job that goes wrong when everything else seems to be going so swimmingly. The set concerned was a 1999 Panasonic TC-68GS90A using an M19 chassis and it came in DOA (as in "dead on arrival"). Apparently the cause was due to a power surge during a storm.

It wasn’t hard to diagnose the basic problem – there was no voltage to the power standby relay (RL801) and
diode D871 had been blown apart, with black marks going to capacitor C803 which had also blown.

I hadn’t seen this chassis before and was grateful to have the service manual – that is, until I discovered that the guy who drew the circuit must have been half-asleep at the time because of all the glaring errors in it. For example, the bridge rectifier (D801) is not shown at all, while D809 is only partially shown and the circuit around it is incorrect.

Fortunately, this didn’t really concern me at this stage, as I was concerned only with a simple standby switchmode power supply that controlled the relay. What did concern me was what voltage to expect from this power supply. The circuit keeps referring to an "RMCN-8v", which I initially took to read as an 8V supply for the remote control receiver on the front panel (ie, the G Board and there is another switched 12V rail to this). However, relay RL801 is a 12V relay (or at least, this is what is stamped on the component casing), so it cannot be an 8V rail.

Delving further into the service manual, I found on page 12-70, under adjustment procedure, that TPD35 (the output from this standby supply) is normally 12V but can range from 10-14V. Things were beginning to get foggy already but not to worry, as my DMM read 0V, so all I had to do was improve on that!

This power supply is very simple, with a maximum of 35 components. It consists of a negative half-wave supply which is fed into transformer T801 and from there to a 3-pin IC (FET IC881, MIP0210SYITV). Pin 2 of this device goes to –ve, while pin 1 (control) is supplied by a rectified voltage from a separate winding on the transformer.

The secondary of the transformer feeds diode D890 which is turn supplies the voltage to relay RL801. It also feed an optocoupler which uses an 8.2V zener diode (D892) and diode D891 to give a 9V (approx.) reference. The feedback to the control pin is first via the separate power supply winding and then the optocoupler.

What could be easier than that? A few quick checks showed that anti-surge resistor R881 (1.2Ω, 2W) was open circuit, while C803 and the IC FET were both short circuit. I replaced all three, expecting an easy result but not so. The circuit just wouldn’t do its thing and although I did a thorough check of the 35-odd components using a multimeter, I couldn’t fault it at all.

By now, I suspected that transformer T881 was the culprit. I mentioned this to two other colleagues and they both checked the circuit out too. In the process, all the electrolytics were changed, along with the optocoupler, but we were getting nowhere.

In the end, a new transformer was ordered in and duly fitted. This was a good move, as the power supply now began to pulsate slowly, but it didn’t have enough "herbs" to trip the relay. We tried a variety of other loads and almost every part was either retested out of circuit or replaced but it had us beaten.

Between us, we have about 80 years worth of experience on TV repairs, so this was particularly galling. I’m beginning to think I’m getting "past it".

Anyway, the set was temporarily put aside while we got on with jobs until, by accident, I discovered another D Board (TNPH0165) half-stripped of components in a corner of the workshop. All I needed was another identical board to make comparisons – this was the answer to a maiden’s prayers (well, to my prayers, anyway)!

I reinstalled the missing parts on the board just to make this circuit work, which fortunately it did. Now I could measure the voltages I should expect at various points and hopefully I would be home and hosed.

Well, it was not to be. Comparing the voltages between the pulsating circuit and this one really only highlighted one difference – one was pulsating and the other wasn’t. The voltages were almost the same except, of course, that they were varying in the faulty one.

Disgusted with myself, I tried to make sense of the figures. The only slight clue I had was the voltage supply for the optocoupler on the primary (hot) side. Before the transformer was changed, there was no DC voltage on the cathode of D883. Afterwards, it was pulsating at about 6V but on the good supply, it was rock steady 7.38V.

This diode (D883) has an 820pF capacitor (C877) in parallel with it, along with a 0.1μF smoothing capacitor (C887). This arrangement then directly feeds the transistor inside the optocoupler (D884).

I decided to replace the lot with new components and see if anything happened. If it did, I could then analyse the parts afterwards to find out which one was faulty.

Of course, nothing changed – the parts were all OK. And then it hit me!

In mitigation, I have to say that the circuit board labelling (the layout isn’t shown in the service manual) is not the best. It’s done in shellac white paint and it’s all too easy to confuse the numbers C877 and C887 at the best of times.

Yes, that was it – I was comparing the two boards together, side by side, when it suddenly dawned on me that the 0.1μF and the 820pF capacitors had been interchanged when they were replaced earlier on. Switching them back made all the difference and the set burst into life, with the relay operating correctly.

Now I don’t want to point the finger but I’m sure it wasn’t me who was responsible!

Let the sound be with you

We had another late model Philips TV come in under warranty, with what looked like a simple fault. It was a 2001 32PW8807/79R employing an EM3E chassis and the fault was no sound.

I started by measuring the righthand loudspeaker and it was OK at 8Ω. I then checked the sound signals into the Small Signal Board and found it was there all the way to pins 10 & 18 of the audio output IC (IC7700, TDA7490).

Before replacing the now suspect audio output IC, I checked all the voltages on all its pins against those shown on the circuit diagram. Everything, including the supply rails, was spot on except for the "Standby-Mute" pin (pin 6), which measured 0V.

This pin is controlled by six transistors, which are in turn controlled by "Sound-Enable" (from the SSB), "POR" Power-On-Reset (from the 11V line deflection power supply), "Protection 1" (from IC7700’s output) and "Standby-Mute" (from pin 6 of IC7702).

I started checking this circuit by desoldering pin 6 of IC7700 and measuring the voltage that was coming in – it was still 0). Similarly, I tried desoldering pin 6 of IC7702. I then checked all six surface-mounted transistors with an ohmmeter in circuit and these measured OK.

Next, I turned my attention to transistor 7707 (BC847B). This was switched on by transistor 7706 (BC-857B) via R3714 (47kΩ). However, there was no voltage on the collector of this latter transistor – so where was the 0.6V on transistor 7707’s base coming from?

I tried shorting out the base and emitter of 7707 and the sound was restored. I then removed transistor 7707 and sound was still there. A quick check showed that this transistor was leaky and replacing it fixed the sound – well, nearly!

Sound was now only coming out of one speaker – the righthand one. However, it was distorted and substituting another speaker showed that the distortion was coming from the IC7700. I also checked the lefthand loudspeaker and it was open circuit.

Replacing the lefthand loudspeaker restored sound to that channel, while replacing the IC, which I originally suspected, fixed the distortion as well. I guess I should have gone with "the force" earlier on!