Christian: They are available here: https://www.electronicparts-outlet.com/en/integrated-circuits/4688-mullard-fch131-dual-4-input-nand-gate-with-rc-dip14.html https://www.electronicparts-outlet.com/en/module/iqitsearch/searchiqit?s=FCH+111 FCH131 is a 4 input Nand Gate. Can you use a 7420? FCH111 is a 8 input Nand Gate. Can you use a 4068? Those are much cheaper if they work. I didn't check the pinouts and operating parameters.   Derek      

Hi Derek Thanks fore information. I already saw these offers in the www. In between I desoldered all IC's from the PCB  and tested them on a breakoutboard. Found one mor defective FCH 131 wich had an steady High level on one output. I decided to solder all back in and only leave 3 IC 5 open  because the there are only two and three inputs of the NAND's used. I will try to rewire an CD 4012 an look what will happen....fingers crossed. Still waiting for deliver the CD4012, when they arrive I will report. The failing of the  IC's was, in my opinion, caused by the Z Diode 2D4 6,8 Volt. This diode was loose in the solderjoints by some strange human activity. If it's loose or not connected anymore, from my knowlege, the voltage on Emitter 2TR6 is raising near to the input voltage  Collector 2TR6 (haven't measured what comes out from the bridge rectifier) and fries the IC's.   Kind regards Christian