Sega Megadrive repair - doesn't boot

[atariry]

So I recently got interested in sound ICs, as a consequence of checking some parts bought via Ebay were genuine. I found about about VGM file format, which allows tunes to be played by various sound chips. So I bought a cheap non working Sega Megadrive for the purpose of harvesting the YM2612 sound chip. The intention was to connect it up to an Arduino (there are many projects on the web) in order to play Megadrive tunes from VGMs.

 

Before I removed the sound IC, I was interested to see if I could figure out what was wrong with the unit. The reset switch was faulty, but that was a simple fix. Using a scope to look at signals around the MC68000, HALT and RESET have activity when the reset button is pressed. There is good 5V power and a good clock. Looking at some of the other pins I see 4 low going pulses of LDS and UDS, then no more activity. My guess is that these pulses are for the initial stack pointer and instruction counter (both 32 bit) which is 4 16-bit reads. I am a little uncertain, but perhaps the CPU halts if these are invalid in some way?

 

I have attached some waveforms. it was done a few weeks ago, so please excuse my faulty memory! I think that notRESET is the yellow trace; notHALT is the cyan trace; and two of the top four traces are UDS and LDS. So the CPU does appear to halt.

 

Here I admit that there was no cartridge inserted. I may have been thinking that there was some boot ROM, as I read something about TMSS (sp?). Nevertheless when I inserted a cartridge nothing changed.

 

I would appreciate any advice as to what to try next or any pointers to sites which have detailed technical details about the boot process. For example where are the initial stack pointer and instruction counter read from: built in ROM; or the cartridge?

[zylon]

Most common "black screen" issue I've found on Sega Gen/MD's has been cart slot pin tension. While I only use a v3, I used to tinker with the others for a long defunct game shop. Even my "parts" spare G3 turned out to have the same issue. I use a tiny screwdriver, or pick, and push the exposed tips of the contacts to the outside and down, gently. There's 2 types of cart slots in these systems, but the open top version with exposed pin edges, is most common by far.

Not having the proper sequence of pins connecting, won't let cpu start up

Edited January 23, 2023 by zylon