Items Covered This Month
Loewe Planus 4781ZW Q110 chassis
Panasonic TH-42PA40A 106cm plasma TV set
Resurrecting a Windows 3.11 computer
The scams people try
A Loewe Planus 4781ZW TV was brought into the workshop. Oh goody, I thought, a heavy CRT set! At least there was a chance I might be able to fix it to component level.
This set employs a Q110 chassis and was reportedly DOA (dead on arrival). However, when I got it onto the workbench, it turned out that this rumour was somewhat exaggerated. As I quickly discovered, the set would click on and then click off again as a protection circuit cut in.
I began by checking the B+ rail and this was correct at +150V. However, there was no EHT and there were no short circuits on this rail. From there, it didn’t take long to confirm that there was no horizontal drive coming from the signal board.
Before investigating this large assembly, I checked the remaining voltage rails. This was a good move because the 12V, 9V and 8V rails were all missing in action. I then checked FET Q663 BUZ71A (and changed it for good measure as these do give trouble) but it was F661, a 3.15A IC fuse that had failed.
I soldered in a new fuse and switched on. The set clicked, gave out an acrid smell and then immediately died. I should have known it wasn’t going to be that easy.
Now before I had replaced the fuse, I had checked for shorts on the 12V rail (by measuring across C668) and everything had been OK. Now, however, it was measuring just 4Ω between this rail and ground, so there was a short somewhere in the works.
I unplugged the main signal board and found that the short immediately disappeared. I then removed and carefully examined this board but there were no shorts to be found so I replaced the fuse again. The supply rails now came up OK when the set was switched on with the main signal board still disconnected.
Next, I checked the main signal board for solder dags and bent pins but could find nothing that might be causing the intermittent short circuit. As a result, I reinstalled the board and checked it out again before switching on. Damn! – the short was back.
This time, I left the meter connected while I disassembled it yet again. It wasn’t until I got to W1201, a 3-pin lead to the rotation coil on the CRT, that the short disappeared. Now I really had something to work on.
I closely inspected the coil and noticed that it had a small, sealed assembly in the middle. However, I could find no reference to this on the circuit diagram and eventually, feeling somewhat exasperated, I phoned Loewe technical support. It turned out that there are two versions of this set and guess what – my circuit was the wrong one for this set.
The good news was that this is a well-known fault and whatever it is inside the concealed lump, it does go short circuit, causing the fuse to blow. Fortunately, the set works quite well without this assembly connected (it is designed to straighten the picture to compensate for geomagnetic forces).
No milk or cookies
I’m writing this back in mid-December and the holiday season is almost upon us. And as usual, some customers are flying into a panic as to whether “it” will be fixed before Santa arrives.
Not only are we expected to have done our duty at half the usual price, no matter how late the call, but we don’t even get milk and cookies, let alone a red coat and a team of reindeer.