This is a basic troubleshooting guide for more advanced Genesis faults where the system turns on, but displays only a black screen. Often times this can be caused by just a dirty cartridge connector, but when that is not the case, it quickly becomes an advanced repair. This guide attempts to point users in the right direction with regards to finding faults, and best practice on repairing said faults.
This guide assumes you are familiar with the general terminology used in Sega's service manuals, use of basic and advanced diagnostic equipment, soldering, and understand general Electrical and Computer Engineering terms. This will NOT teach you how to solder, what specific signals do, or the pin naming schemes of the console.
Common Causes
- Dirty Cartridge Connector
- Broken Traces
- Broken Solder Joints
- Short Circuit between pins
- Failed Component
Dirty Cartridge Connector
See this guide on cleaning cartridges, which applies to the Sega Genesis, more specific information below.
Using a Sega CD To Test:
A Sega CD, or anything else that can boot off of CN2 (expansion connector) is an extremely helpful diagnostic tool, as this connector is often un-used and clean, eliminating much of the guesswork of if CN3 (the cart slot) was fully cleaned. It also eliminates reliance on VA18-VA23 /MRES, /VRES, HSYNC, VSYNC, /YS, EDCLK, VCLK, /AS, /DTACK, /CE_0, /TIME, /CART and /M3. If a Sega CD BIOS boots, but games do not and the cartridge slot is clean. You may wish to move to the more advanced sections of this guide.
 | Often times just cleaning the cart slot is enough to bring a system back, but it may not always be the case. Repairing a Genesis or Mega Drive with a black screen is not at all easy or for the amateur beyond this point, potentially requiring advanced micro soldering or an understanding of how the 68000 works. Beyond this point the guide becomes increasingly more advanced and geared towards someone experienced in electronics repair and engineering. |
Broken Traces
Recommended tools:
- Multimeter that can test continuity
- Some form of magnification (optional, but recommended as these traces and pins get small)
- Soldering Iron
- Small wire
- UV solder mask
- Genesis Schematic
- Grinding Pen or sharp Tweezers
Broken traces can be common on boards subject to lots of corrosion and external damage, cleaning corrosion up first is required to find broken traces.
Visual inspection is the main way to find broken traces, but if the motherboard under inspection has an available schematic (VA3-6.5 MD1, VA0-1.8 MD2, VA3 MD2) it is a vital tool in hunting down problematic traces, but BE WARNED that Sega's schematics are often error prone!!!
The Z80 and VRAM traces can be damaged, yet the console will still be able to boot or get beyond just a black screen, the most common areas of trace damage are near the cartridge slot due to its semi exposed nature, but any harmed address or data line could prevent booting the console.
If a broken trace is found, it is best practice to repair it with as short of a bodge wire as possible, and avoid soldering to vias. Instead, scrape away solder mask on either end of the break, and bodge the trace in line with its original run. Then, cover the line in UV solder mask for protection.
Broken Solder Joints + Shorts Between Pins
Recommended tools:
- Multimeter that can test continuity
- Some form of magnification (optional, but recommended as these traces and pins get small)
- Soldering Iron
- Good flux
- Tweezers
- Genesis Schematic
- Magnification
- PATIENCE
It is becoming increasingly prone that the large form factor QFP chips can have their solder joints break free, causing inconsistent and difficult to track down behavior. Sometimes this can be found by pressing on sides of the chips until something happens, but more likely it will need to be found from careful probing with tweezers. If a single pin is found to be broken free, reflow the entire chip as more will follow suit with time.
Most often this happens to affect larger 208 pin chips, such as the FC1004 ASIC (315-5487, 315-5660, 315-5700, 315-5708) (will be referred to as just the ASIC from now on) and MCE (Mega CD engine found on the Sega CD, 315-5477, 315-5548, 315-5632) as these are the most sensitive to board flex. But it can and does impact smaller QFP chips with the same very small leads.
To find lose pins, a tweezer can be used to poke all of the pins of the chip, it requires very little force to move a lose pin, and it will likely be bent the moment force is applied. On the other hand, well soldered pins will likely bend the tweezer used to poke them, but in order to avoid damaging the pins it's best to be extremely careful.
Reflowing the very small pins of an ASIC is advanced drag soldering, and IS NOT TO BE ATTEMPTED FOR THE FIRST TIME EXPECTING GOOD RESULTS. It is very likely that someone without experience doing this will bend pins when attempting a reflow, if the pins break, or are bent beyond fixing on-board, the chip may need to be removed or grinded into before it can be re-used. It is best practice to use a soldering iron with a higher heat than normal, larger iron tip with more thermal mass, a good flux such as Stirri or Kingbo, and leaded solder (Don't use ROHS solder as the Genesis pre-dates it and mixing solder chemistries is bad). Combine these tools with a dragging technique where the iron effectively floats over the pins, avoiding direct contact with the pin, and instead opting for direct contact with the pads they solder to in order to avoid bending pins.
When a reflow is completed, it is vital that all pins are checked for shorts (this is the most likely time for them to appear) and pins that didn't properly solder (same tweezer method). All shorts and non connected pads must be fixed before testing or else the system will most likely still have a black screen.
Failed Component
Recommended tools:
- All of the above recommendations
- oscilloscope
- logic analyzer with at least 50 channels
If none of the above were able to fix the console, it is likely some component has failed, and finding it will be challenging.
Since the Z80, ZRAM, along with the video and sound subsections are likely not to completely crash the bus when failed, it's most likely the 68k, work ram, or ASIC.
| ASICs with the following part number: 315-5700, 315-5487-01, and 315-5708-01 tend to have a higher failure rate than other parts in the console. Most commonly the ones made by Fujitsu will suffer a condition dubbed "Fujitsuitis" where they corrode and fail internally, similar to the S-CPU A. |
It is best to start by checking the 7805's voltage output, if either (or just one) are outputting too high of a voltage beyond 5VDC, SHUT THE CONSOLE OFF IMMEDIATELY if 9 or 12 volts was observed, it is likely too late, and the entire console cannot be repaired. If 5VDC is confirmed, measure the resistance of the voltage rails to ground, if it's observed that any are a very low resistance, such as 6 ohms as opposed to ~300 ohms, a chip or part has likely failed short. Thermal imaging or feeling for heat can find the failed part, as it will heat up significantly.
An oscilloscope can be used to check clock lines, and reset behavior, along with general bus activity. If something is wrong it may be pointing to the failed part. If SRES is failing to assert properly, look into the reset buffer setup, which is either a discrete opamp or a part of 315-5684. Audio or video mods may inadvertently cause this to fail due to harming or eliminating the vref line. If the 68k is HALTed, look into /CART, as it needs to be asserted or the system will HALT.
Logic analysis can be performed if the equipment can handle VCLK speed and the bus width of the 68k. The 24 bit address bus + 16 bit data bus can't be traced by most modern logic analyzers, as they lack the needed channels to get vital information. TMSS systems begin executing the TMSS ROM, which, among other things, wants to read address $000100 for the "SEGA" header, this same ASCII data must also be written to $A14000-$A14002 or else VDP access will crash the system. If it appears as though the console gets stuck in an infinite loop at early addresses, it's due to the TMSS ROM crashing or failing in some way. Bypassing TMSS may be a path forward to see game code crashes, which are easier to trace.
If no solution can be found, it may be best to part out the console, or, use a donor console to test each component, until a faulty one is found.