MEGA DRIVE PAL VA6 WHO DON'T LIKE OVERCLOCK

Hi, I overclocked several model 1 mega drives and never had a problem like this.

I have two pal va6 who seem don't like be overclocked at 10 mhz, after a while game freeze or get garbled graphics. The problem lies in the boards as I transfer all the cables, switches and oscillator to a md jap va1 and everything is fine there.

anyone have experienced something similar? I was sure that va6 models work with overclock and I'm pretty sure I did some va6 overclocked some months ago as with tmss I never found a pal va5 so they had to be va6.

 

According to this page the Signetics CPUs doesn't work with overclocking. I have only tried with Motorola CPUs, so I can't confirm this. Might be the reason why it doesn't work for you :/

 

These two va6 got motorolas.

I always had success overclockin va4 with signetics cpu at 10mhz, i think is a wrong belief spread around the web or simply they can't manage more than 12 mhz (only tried 10 mhz myself)

Anyway I never read about va6 which are not overclockable and now I have two of them at home!

 

I assume you're doing it with a switch, and long wires switching the signal?

What happens if you wire the crystal oscillator directly to the clock pin, without long wires?

 

I did it with a switch, not tried the direct method, anyway I think is not a cable lenght-related problem as I put same cables, switch and oscillator in an another mega drive and work flawlessy there.
anyway I use 8-10 cm good quality wires which never gave me problems and never got a problem overclockin a unit till these 2 motherfuckers va6 in a row, one after another one, so I changed wires, switches, oscillators and errthing before considerin' that's a pcb revision problem

 

Thanks, that's good to know. I have a Signetics in my Jap MD1 so I might try to OC it to 10Mhz soon

 

you will not have any problem, the one I modded yesterday with success is a jap va1 with signetics cpu and never got a problem with a jap unit at all

 

The VDP in them might be the FH1001 chip instead of the older YM7101 found on older Mega Drive consoles.

 

i am doing a mod soon on a va4 i will add mini ram heat sink on the vdp to help the cooling.

 

That FH1001 VDP has "FH1001" written on it. If the console is using a Motorola 68000 and has a FH1001 VDP it *WILL* have a 220R resistor on 68000 pin 6.

 

Two issues:

-When you overclock by inserting a new oscillator into a system you no longer have a synchronized system, there will be guaranteed timing violations if the VDP assumes a fixed phase relationship between components, which it probably does (asynchronous interfaces increases latency and cost so there's no reason for them to include it, especially considering that the MD was designed with a single oscillator).
-When you switch clock signals manually with a passive switch (rather than a proper clock switch circuit) you're also inclined to create runt pulses or worse during the switch which also cause timing violations.

Timing violations lead to glitches and crashes. The Signetics 68K probably is implemented in slow, dynamic logic, which is extra sensitive to both issues above.