Items Covered This Month Panasonic TC29R20 TV set (C150A chassis) Teac Televideo MV4822 (CP-420 chassis) LG PF60A3Q rear-projection TV set (MP87A chassis) Philips 46PP8621/79 rear projection TV set (DPTV325AA chassis). Sony KD32DX40AX FD Trinitron TV set (FE-2, SCC-U78A – A chassis)

Between 1992 and 1997, Panasonic released a range of TV sets covering the MX1, MX2, MX3, M16 and C150 chasses. The C150 later became the C150A when the B+ was increased from +115V to +125B. It was during this period that Panasonic also began using more IC regulators and optocouplers (the C150 series has four IC regulators and three optocouplers).

Recently I had a Panasonic TC29R20 come in with Mr Moss, who complained that it kept "cutting out".

Well, when I switched it on, I couldn’t even get it to "cut in".

Most of the faults that occur in this popular 9-year old set are well-known by now and this set was no exception. There is a series of modifications that are designed specifically to fix these particular symptoms (intermittent shutdown), which were released on 11th July, 1997 by Panasonic Technical Information. In particular, two resistors and two capacitors have been changed to improve the threshold level of the overcurrent protection circuit.

These parts are R835 (now 0.33ω), R848 (33kΩ), C831 (3.3μF) and C838 (0.1$mu;F).

These parts hadn’t been replaced in this set, so this was a pretty good place to start. Unfortunately, it made no difference – the set was now completely dead.

Even though the above modification didn’t change anything, I thought it wise to also replace all the common components that cause the same symptoms. There are nine all told and I had already changed four. The remaining five are electrolytic capacitors C817, C820, C824, C455 & C1121 in the switchmode power supply. Even in the event that this didn’t fix the fault, I couldn’t send this set back to Mr Moss with a warranty when these components are known to fail.

Besides, they don’t take very long to replace.

Anyway, this didn’t fix the fault but it did change the symptoms slightly. The set would now occasionally come on for a few minutes (with perfect picture and sound) and then cut out.

When the set is on, the red LED is on, being fed by the 5V rail from C1106, which is in turn fed by 16V from D812 and Q802. This rail drops to 9V in the Standby mode. The three optocouplers between the primary and the secondary are for voltage control feedback and standby switching (D803) and to provide protection (D811 and D836).

In this case, I was beginning to suspect that it was the protection circuits that were closing the set down. When the set came on, I managed to check that the main power rail at TPE1 was +125V but couldn’t confirm that the set subsequently switched off because it might have intermittently gone high. However, I did confirm that there was +325V coming out of the main bridge rectifier (D806) – ie, across C809.

It was time to disable the protection circuitry step-by-step until the set stayed on. This can be a bit dodgy if there is a real fault on that line, because it will now have the opportunity to really destroy other parts without hindrance.

The entire protection circuit can be switched off by disconnecting D823 on the primary hot side which feeds Q827. This told me that the set itself was OK, because the picture and sound were rock steady and there was no sign of any strain. This meant that it was highly likely that the protection circuits themselves were faulty.

Next, I resoldered D823, then moved to the cold side of the supply and shorted Q805’s base to ground (emitter). This too allowed the set to switch on and function reliably.

One of the most common faults in this set is dry joints on the vertical output IC (IC451, LA7838). If this fault is allowed to fester, it will eventually destroy the IC and cause too much current to be drawn from the 24V rail. This is detected by Q454 which in turn switches on Q805. However, this wasn’t the case with Mr Moss’s set.

Gradually, I disabled each of the protection feeder circuits until at last I found that disconnecting D832 allowed the set to function. This was surprising, as I had already replaced C831 and R835 in this main B+ current protection circuit, making it less sensitive. Basically, this circuit includes Q831 which is switched on if the voltage across R835, the main HT feed resistor, rises too far, due to excess current. This in turn switches transistor Q805 on via D832.

In all, a total of seven components are involved in this circuit and they all tested OK in-circuit. Replacing Q831 (2SA1018) finally fixed the fault, although the original tested perfectly after removal (I_C = 0mA, I_B = 4.56mA, h_fe = 91, I_r = 2.5mA, V_BE = 0.73 and no collector emitter leakage).

I wasn’t that surprised because leakage can often develop between the leads of a transistor due to dirt and corrosion – especially if the set lives near the beaches. Just by moving the leads slightly (eg, when removing the transistor) can cause this corrosion to "flake off", restoring it to working order. That said, it’s quicker, cheaper and more reliable just to replace it.

Teac Televideo

Mrs Austin brought in a smart silver little 1999 Teac Televideo MV4822 (CP-420 chassis). The problem was a tape jammed inside and a relay that was chattering on and off continuously.

Unfortunately, I had no circuit for this model but being adventurous, decided to go in blind. I started with a careful examination of the whole set, beginning at the power supply, looking for "hot spots", marks or stains on the PC board and on components that might have been under stress.

Luckily, the voltages were actually marked on the printed circuit board (bless the Teac designers; if only other manufacturers would do this more often). The main power supply itself seemed OK but although the EVER 6V rail was correct, the EVER 8V rail was down to only 5.5V. The 6V rail is usually for the microprocessor and digital circuits, while the 8V rail fed the remote control relay solenoid.

This 8V rail is supplied by a separate switchmode standby power supply and ICI807 (TOP210) was running hot, its legs turning brown. Before ordering a new one, I noticed a small 47mF 25V electrolytic (C840) in its control circuitry and decided to replace it – just in case.

Well, Mrs Austin and I were both lucky as this was the cause of the problem. After replacing it, the set came on and ejected the tape correctly. And the relay stopped chattering.

The rear-projection LG

Mr and Mrs Paterson have a massive 60-inch ((150cm) LG rear projection TV (model PF60A3Q, MP87A chassis). Its problem was that the picture was flashing black and white and the sound was giving a knocking noise intermittently.

Well, the set was so big that it wouldn’t fit into our Toyota Hi-Ace for transport back to the workshop. As a result, I removed the top screen assembly so that at least we could take out the base assembly.

Back on the bench, I fixed up a temporary screen just below the ceiling using a large sheet of cardboard. At least, this would allow me to monitor what was a very intermittent problem.

I started by checking the supply rails and then the deflection outputs but everything was rock steady. Gradually, my attention moved to the small signal board which also has the twin tuners, IF stage and Picture-in-Picture circuitry, plus the AV interface boards. I tried heating, freezing and vibrating these circuits but derived no new clues.

LG were helpful and suggested replacing crystal XT75 but this too made no difference. Finally, despite its cost of $500, in the end an "executive decision" was made to replace the whole panel.

Well, the new one was fitted and the picture was now consistently perfect but there was no sound at all!

After a lot of crawling around on the floor on my hands and knees, an audio amplifier probe finally revealed a problem with the relay RL601. It had poor contacts, so I replaced it with one from an old board.

Finally, the set was loaded back into the van and reassembled back at the Patersons. After reassembly, it all worked properly. Thank goodness.

Fixing Philips

One of the most common faults in projection TVs is the failure of the convergence output ICs.

Recently, we had a Philips 2002 46PP8621/79 using a DPTV325AA chassis. Both the STK392-120 ICs (ie, 7044 and 7045) on the C4 small-signal module were short circuit and the reason wasn’t obvious until you examined the IC heatsink. Insufficient silicone heat transfer compound had been applied in the factory and on both ICs, there was a large air bubble, resulting in overheating due to poor heat transference.

Applying an even layer of thermal compound over the entire area of the new IC fixed the problem completely.

The mystery fault

I encountered a mystery this week, for which I have no answer.

I was called out to Mrs Downie’s Sony 1997 KV-G2152 (BG-2S chassis), which had no sound. Normally, these are pretty straightforward to fix and the cause is fairly obvious, like the earphone socket has become dry jointed or the headphone lead is frayed.

In this case, I found that capacitor C253 (100mF 16V) which hangs off the 11V rail on the cathode of D251 and the emitter of Q202, had exploded – literally.

Mrs Downie said she was watching the TV when the sound suddenly died. She claimed she heard no bang or any other loud noise.

IC203 (TA8248K), the sound output IC, was also dead and replacing both these components fixed the problem completely.

But why, or rather how, did this fault come about? There were no other problems I could find with the set and you would expect something like a power surge to cause this violent fault to occur but apparently there was none – it happened on a normal sunny day for no reason. Explain that.

The big Sony

Mr Mosely insisted we collect his heavy 2002 Sony KD32DX40AX FD Trinitron TV (FE-2 SCC-U78A – A chassis) when I said we would be unable to fix it in his home.

When we picked it up, he said that it had gone off while he was watching it and now only the standby light flashed. When I got it on the bench, I found that it gave one green flash and then two red flashes, indicating an Over Current Protection error (OCP).

A quick check soon revealed that the horizontal output transistor Q533 BU2515DX had gone short circuit. And that was because flyback transformer T511 (Part No.1-453-308-41) – which was the real culprit – was arcing.

Replacing these parts quickly restored the sound and picture but I wasn’t quite out of the woods yet – a 4:3 picture with a bright object on it produced noticeable "blooming" of the raster.

It didn’t take a genius to figure out there was problem in the ABL (Automatic Brightness Limiter) circuit. This comes off pin 11 of the flyback transformer and is held down by two resistors to ground – R518 6.8kW and R510 470kW. The former had lost a little chunk out of its body where the grey band should have been and was open circuit.

Replacing it finished the repair properly. Now the difficult bit – getting it back to Mr Mosely.