Over the years, I have seen many Panasonic M16 TV chassis with a wide variety of faults. This doesn’t mean that it is an unreliable series – on the contrary, it is just that it is such a popular set and with good reason. It is very well built, reliable and a good performer, although the remote control is a bit over-complex.

The first models were released in 1991 and then a second series called "The One Up" was released in 1994. The latter had a lot of small changes to almost every part of the set which regretfully meant that the modules and their PC boards were not interchangeable between the two models. Even the cabinet detail is different.

Recently, I had an 80cm TX33V30X (M16MV30 chassis) come in which was dead. This set had already been looked at by other technicians and I wasn’t too happy at being the bunny at the end of the food chain. In this situation, you not only have to fix the original faults but also any introduced along the way. And of course, the client expects to pay less than he’s already been charged.

Anyway, I quickly established that the main switchmode power supply board (D PCB – value $593.26) was severely damaged, with 4A fuse F801 and resistor R809 (3.3Ω 15W) having also failed on two other boards. So was this due to a power surge and if so, was it a possible insurance claim? I really wasn’t sure.

Repairing these power supplies is always rather tricky, as there are about 25 components that need to be checked, tested and replaced where necessary before switching on and doing voltage checks. If you miss any or make any mistakes, you can be in for an expensive repeat failure of everything.

From the soldering, I could see someone had already been down this path and a lot of parts had already been replaced. Nevertheless, I found three transistors that had failed – Q801 (2SC4706F137) which is the main 130W power chopper transistor and drivers Q802 & Q803. I also found the 6.2V zener diode that’s in parallel with the two emitter resistors (1Ω and 0.82Ω) to be short circuit.

I ordered original parts and replaced these, along with R809 and capacitor C580 (220μF 200V), which was looking dodgy. I then connected a Variac to the supply input, disconnected R560 on the "X" board and clipped a 100W dummy load globe across the 140V rail (TPD1). I also disconnected protection diode D560.

When I wound the Variac up, the power supply started working at about 90V and at 110V I was getting a steady 140V output with no stress. I then checked the 5V output from IC803 and the 12V rail to the primary of the remote control power supply. The output from regulator IC802 was high at 15V (with no load) but after swapping the device, I realised that there was a parallel resistor (R817, 5.6Ω) across it, which accounted for the discrepancy.

By now, I was feeling pretty happy that all was going well and that the power supply was working. So, feeling confident, I wound my ancient Variac up to firstly 200V and then up to 240V. Well, it reached the first part OK but at 240V there was a spark and a smell from the Variac – and then all was silent. B@#$%&r!!

It took a while to work out was happening. The Variac fortunately wasn’t damaged but because the wiper contact was old, worn and tarnished, it couldn’t handle the increased current. As a result, I stripped it down and cleaned and polished the copper windings. The 3.3Ω 15W resistor (R809) had failed in the set too and this convinced me that the power supply had blown again.

I checked everything in the power supply again but surprise, surprise – nothing else had failed and after replacing R809, it was still giving a +140V output from 90V AC input.

Being a little too dumb, I tried turning up the Variac again very carefully. It got to 220V without problems but as I increased it past this, the 4A mains fuse suddenly blew.

Well, something was seriously wrong to blow a 4A fuse and damage a 3.3Ω 15W resistor but not damage the switchmode supply. Obviously, there was something amiss with the input from the N board. This board receives 240V AC from the mains filter S Board and includes a full-wave rectifier which provides around 300V for the chopper transistor.

But hello, hello, what about all the extra circuitry on this board? I suddenly realised that this set is designed to work from 110VAC to 240VAC 50/60Hz and the N Board has a Triac (Q812) which switches power to C809 (330μF). Its gate is controlled by transistors Q811 and Q814 which are in turn switched by a voltage detector circuit. This latter circuit ensures that Q814 is only on when the mains voltage exceeds 120V.

Items Covered This Month Panasonic TX33V30X TV set, M16MV30 chassis Panasonic TX80P300A TV set, EURO 7A chassis Panasonic TC-68P22A TV set, MX8 chassis Panasonic TC-15PM50A TV set, (MX-7Z chassis) Creek Audio CD43Mk2 CD Player Sony TCD-D7 DAT Walkman