Sorry for not posting, folks. I had some very nice hours with my miss Piggy. We even came to the point where the hole Display was working.
As I mentioned before I suspected some bad solderjoints to be the problem. I then resoldered everything that is somehow connected to any element in the display: The driver ICs, every resistor, the transistors, every wire and the 7-segment elements themselves.
Pic 67: What the display should look likeSure enough, it all worked after that. Look at Picture 67: The only thing photoshopped is the size. Every Element worked, trust me!
I then built everything back into its right place, only to find the same elements as before to be not working. WTF?! I grabbed my DMM and started measuring. Every voltage was where it should be, but three of the four related transistors hat a somehow lower BE-Voltage. I ripped them out and replaced them with BC550's I had at hand. All the elements lit up. A bit dim, but they are there. Built in - elements dead. AAAAARGH
Ok, take a breath. Measure again. Everything is fine, these suckers just do not work as they are intended to do. Checking again, one element came on, another one on another 7-seg died. Came on again a little later. WHY?!
Again a soldering-party. Retouched every joint again, resolderd even every single track on the display PCB (No soldermask on there). Turn it on, some random elements dead.
I then decided to rebuild the whole wire harness connecting PCB 201 with the Display board. I had seen it was built with solid wires, maybe there were some broken ones? Percussive maintenance would sometimes help to turn on or off a random element. Seemed to be a possible problem. This took me the whole afternoon yesterday. Yes, I tried to bind my wires in the same way it was originally done. Does not even look too bad (pictures will follow later). Good luck I beeped every wire before I ripped them out - the display module is not original! It is not connected in the same way the manual said. The 7-seg-displays are not the ones listed in the part-list. So maybe this is the reason why so many solderjoints seemed to be retouched? Maybe someone had this issue before? But I am sure this was done by someone who knew what he was doing. Well, 40 Wires later - it worked!
At least for about a hour. Now its the same as before.
I then went a bit crazy and again, with some percussion
some more elements went dead. But this time I pressed directly onto a 7-seg and this element went completely dead after that. But these were resoldered several times by now. I can only suspect the elements being bad themselves or possibly their leads to be corroded and not taking the solder as they should.
At the moment I think of rebuilding the hole display-PCB on stripboard. Maybe I should get me a desoldering gun first.
That's where I am right now. I cannot simply bodge in some modern 7-segs as the original ones have a very uncommon footprint. They are not only wider than normal while not being very high but also have an unusual pin-arangement. Also, I want to keep the device as original as possible on one hand. On the other, if I start to go crazy with this repair, I am going to replace every electrolytic as well. Hmmmm.
For the temperature switch: As far as I could find out it works like a usual merury thermometer. Inside is the mercury on one side, some other compressible stuff on the other. The hotter it gets the more the mercury will expand, taking more room in the tube and eventually reach the switching contact which than gets closed. Please refer to Picture 66:
The small red wire goes to the lower contact ring on the glass tube, the yellow one to the upper. Both contacts can be seen on the outside of the tube, the lower as a ring, the other in form of an eight. These Contacts also have a Pin that is embedded in the glass and reaches to the inside of that tube to allow contact with the mercury. On my photo these inner contacts cannot be seen, sorry for that. On the right side one can see a larger cave inside the tube. This is a kind of blast protection: It simply gives more room for the materials inside the tube to compress. This information on general function is taken from an
Wikipedia article (german only!) In the G-1001.500 this tube is part of a two-point-temperature regulator. The mercury tube is obviously the temperature sensor. It is on one end embedded in a metal block. This has holes for the transistor cans (they can be seen on the left). Also there is a coil on the round part of the metal, connected to the big red and clear wires. All the other wires are for the FETs of course. If the mercury does not reach the contact (that means it is open), the coil will heat up the construction. The large piece of metal gives some thermal mass for speeding down the effects of heating up and cooling down a bit (giving some kind of hysteresis). Then, when the mercury closes the contact, the coil is shut down. The device starts to cool down, eventually open the contact again to restart the cycle. This Switch is built to hold the construction at about 55°C.
I especially like the fact that this construction cannot wear out. Very reliably as long as you don't breake the tube. Which is nearly impossible. Just see how it is held in place by the flexible wiring, a drop of glue and completely surrounded with nice soft foam. Even if the PCB brakes it sits safe in its own can with the leads mechanical uncoupled.
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