๐ธ๐ฐ Tesla PMD 85: Part 4 (PMD 85-2)
20Some progress, but unfortunately no boot yet. For video I decided to use the simple composite (or even straight luma/B&W?) video signal ... it looks that RGB is possible but I don't know the pinout.
Powering-on initially had a problem - the -5VDC rail was sagging (or soaring?) to around -1VDC. The PSU was fine so most likely something I had done.
I checked the replacement tantalums I had installed and noticed that I'd installed the one for what I thought was the +5VDC rail between +12VDC and ground. Not a big issue, but still there was only about 11ฮฉ of impedance between -5VDC rail and ground ... duh ... I'd installed the +ve leg of the -5VDC tantalum to the -5VDC rail ... but really it should've gone to the ground rail as it was "more positive" than the -5VDC rail ... ?
So I have removed it and now the -5VDC rail was as expected. I have not reinstalled a capacitor there ... my guess was to smooth the input power from the power supply ... I'm hoping that my modern Mean Well PSU is more stable and less ripply that than a 40-year old Eastern Bloc one is ... ?
Anyway, I was expecting, no video output. I tried re-seating all the chips - one of the 1Kbyte EPROMs containing the firmware ... a gorgeous ะ573ะ ะค1 (K573RF1) ... had a pair of broken legs, but looked like there were long enough to make contact. An assumption.
Unfortunately I managed to break a leg on one of the 16Kbit (Mostek 4116) DRAM chips ... it bent going back in, and broke when I tried to straighten it. I don't think I have any spare.
I have a bit more confidence now so I'll have another look at the 2B model I got originally ... it's in pieces so I'll need to re-assemble it, and I'll also try it out with the PMD 10 power supply.