I went through 3 replacement DRAM chips and none of them worked. With all of them I got distorted noise instead of a delay signal. Yesterday I sat down with a schematic and beeped out every pin of the DRAM to the DSP. The RAS line (pin 45) of the DSP wasn't quite soldered down, so the RAS pin at the memory was open. Some flux and a quick touch of the soldering iron and now it works with external memory and runs the DL4 firmware correctly.
|New RAM chip. The flux mess was cleaned off afterwards.|
The DRAM that I know worked was a Toshiba TC51V17805CFTS-60. I think any 16 Mbit DRAM with the same standard pinout will probably work here.