OT: C= SR4190R needs repair



#2

Let me first apologize for this OT post. I've seen a few articles about Commodore calculators here in the past so probably someone in this forum can help with my problem. I have already bothered several people with this but it seems no one had an idea.

Ok, here it goes: I recently bought a C= SR4190R. I knew it was defect but it was in a very good used condition so I couldn't resist. It tuned out that the batteries (3xAA rechargables, 600mA NiCad) leaked and burned a trace on the main PCB (ground for the leftmost digits in the display). I replaced the batteries w/ NiMH's at 800mA and soldered a wire to get the LED's back working. A few other traces were concerned too, but I've checked their resistant and they seem to be still conductive. Now two strange things occured.

(1) The batteries don't charge. They got approx. 4.7V but only about 10mA. This doesn't seem right

(2) After several hours on the AC adaptor (only charging, the calc is switched off and the CPU stays cold), I switch the calc on and notice miscalculations with every function that uses more than just x/y registers. The longer the C= is connected to the adaptor, the worser the problem is. After 30min off the AC, the calc runs fine again. It does so even when the CPU gets warm.

There is no RAM on the PCB, all registers are in the CPU. There is a voltage regulator identifying itself as 7735, a working germanium diode and an unknown transistor (the writing could be C1213). Now I have a lot of questions: Does anyone know the 7735 and can identify the specs (to get a replacement, this model seems to be out of production). I suspect this VR to be responsible for the low charging power. Does anyone have an idea what could cause the unreliable operation (defect CPU?/unsufficient current?). Could it be related to the VR? Where does the CPU get its clock signal from? This is another idea on my side, the clock speed might vary over the time and cause malfunctions. How to find out more about this calc?

Again, sorry to bother you with this problem. I would be extremly grateful for any ideas, guesses and pointers.

Thanks for listening,

Thomas


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