- The LaST Upgrade -


Last updated December 3, 2023


The next generation of 16MHz booster is the V2.2 version. Speed is marginally faster than the V1.5 booster as the ROM decoding is done on board. The V2 also holds TOS206 in addition to TOS104. As with the V1.5 booster you can switch between 8MHz speeds and 16MHz speeds.

NOTE: GB6 tests have been updated since the image above. For those who are BETA testing GB6 (or have a copy) Results will likely show slower speeds than above. Also note GB6 results cannot be compared to previous GB versions as the tests are totally different and show different benchmark results than GB4 for example.

NOTE there are some modifications & changes to the PCB.

1) A wire from pin 31 of the 68K header to the right hand side GAL where a pin in lifted and the wire connected to pin 7.
2) Pin 11 of the 68K header is intentionally missing.
3) Pin 23 of the right-hand side GAL is deliberately bent over to the previous pad. This leaves a free hole in the PCB and GAL pin 22 is cut.
4) R3,R6 are deliberately missing not required.
5) JP3,JP5 are for debug purposes only and should not be fitted or used.



Fitting is fairly simple. Like my other boosters you unsolder the old CPU and solder in the new kit with the addition of a couple of wires.

While the booster can be fitted directly into the motherboard, the 2 SIL resistor arrays do make space limited above the board so you should double check you do not have a clearance issue with the top metal shielding before fitting. It might be possible/advisable to unsolder the 2 SIL arrays near the CPU socket, and re-fit them flat to the PCB so the booster PCB may sit a fraction lower down.

The next wire is soldered onto pin 23 of the left side GAL. This wire solders onto the motherboard (BG). Now if you do not have the exact revision of board as listed below, then you should check with a meter from pin 11 of the motherboard CPU socket and locate a VIA where you get a solid reading.

Below is a example of a later motherboard revision.

Above shows the VIA with a red circle, which connects to pin11 of the motherboard CPU socket.


For 8/16mhz switching, pin 13 of the right side GAL connects to anywhere which is GND/OV on the motherboard or metalwork.


The top left corner of the board (PAD1) is the usual wire to solder to pin39 of the snifter. This supplies the board with its 16mhz signal.

That is basically it!

I supply the TOS select as a jumper link on the board, though you can run a toggle switch to it if required to switch TOS versions.


The 16mhz line (top left PAD on booster board) goes to pin 39 of the shifter. However, if the MEGA is not stable then the MEGA BUFFER BOARD as listed further down this page will need to be fitted. The 16mhz line from the V2 booster goes to CN pin 4 which is a buffered 16mhz line from the GAL.

The BG WIRE solders on pin 23 of the left side GAL) and solders to the blitter pad as shown in the above image. Note if you do not have a blitter (go buy one from my store!!) then there will be a blob of solder across the 2 pads where you solder the wire to. If you have a blitter (yay!) then you must solder to the bottom of the 2 pads and make sure you do not short out the pads as you solder. Double check with a meter to make sure you haven't accidentally bridged the 2 pads! Otherwise smoke, death, destruction etc.



The metal shielding angle may touch the front row of pins on the booster board depending on how low down you fitted it. I strongly recommend placing some strong card on the underside of the metal shielding as a extra precaution.



MEGA BUFFER board is supplied as a kit. While I have fitted sockets, they are not supplied (and likely cause height problems with the PSU if the shifter is fitted in socket). I suggested constructing the buffer board and trying it in your MEGA before fitting the V2 booster. This way you can verify the board is stable in your machine before fitting the V2 booster.

Please note these modifications were done to a MEGA ST C103277 REV2.1 board. While it is possible to fit the V2 booster and buffer board to any MEGA ST, I do not own those boards to test so likely some debugging and investigation will be needed.

I recommend soldering the header pins in first, then solder in the shifter second. Otherwise the shifter will be in the way of soldering the header strips :) Once the strips are soldered, trim the pins ontop of the PCB as much as possible otherwise the shifter may not lay flat on the PCB like it should.

There are 2 resistors and 2 capacitors and a GAL to solder as shown in the image above. Use a resistor leg to bridge over the "L1" pads.

The tricky part is that there are so many revisions of the MEGA ST (I only have a MEGA 1) That The following mod might not be possible, or the resistor might be located in a different place..


R150 is a resistor in the 16mhz line from the shifter. As there is a resistor on the buffer board then R150 needs to be removed and a wire link placed there (or bridged over with a wire link). Some MEGA's do not have that resistor, others may have it in a different location.

16mhz line can be taken from CN pin 4 (4th pad from the left). I have also powered the V2 from the R150 link and that worked also. Though using pin 4 on CN header will spread the load across 2 GAL outputs instead of 1, so likely that will be the preferred method.

NOTE - if you do not remove R150 then the voltage drop across both resistors may be to low and the MEGA will not boot. Unfortunately with so many MEGA revisions , some experimentation may be required. Please let me know how people get on as I can't debug machines I do not have :)

Troubleshooting - If machine will not boot or is unstable. Then likely you need to do the modifications in the section below. With so many revisions of MEGA, the R150 may be missing totally, or might need bypassing. If this is not done, the 16mhz line will be unstable. Some MEGA's may require larger than 33R on the buffer board (47R or 100R typically) depending on the amount of noise on the particular revision of the board. I would also suggest trying the buffer board without the V2 booster connected. If your machine is unstable then likely you have a noise issue on the 16mhz line somewhere. MEGA machines are unfortunately unstable and the amount of instability depends on the revision. A scope is must in diagnosing such issues.



Certain models of STF/M, including the MEGA ST, exhibit an interesting phenomenon related to the use of 10K SIL (Single In-Line) arrays on the address and databus lines within certain motherboards. The concern stems from the apparent inadequacy of the 10K resistors to effectively pull up the bus, particularly when factoring in the additional load introduced by the GAL (Generic Array Logic) logic. While it might be expected that the CMOS-type CPU would impose a lesser load on the bus, the reality is more complex, potentially involving stability issues related to higher junction capacitance or other contributing factors.

This observation hints at a counterintuitive situation where the 10K resistors may not consistently provide optimal performance. While some individuals may report success with this configuration, the presence of variations results in a subset of boards functioning properly, while another set encounters operational challenges. The discrepancy in outcomes underscores the intricate nature of the interactions among components and highlights the need for careful consideration of resistor values in the context of the specific motherboard design and components involved.

To address this issue, it is recommended to replace the 5 SIL arrays on the ST/F/M boards with 2.2K pull-ups. In the case of the MEGA 1, 4.7K resistors are used on the address lines, and 10K on the databus lines. However, for increased reliability, changing the 10K resistors on the databus lines to a lower 2.2K value is advised. Noteworthy issues on STFM's include random crashes during boot, especially before or during loading from floppy, and the appearance of a row of bombs across the screen on power-up. Testing with gembench, particularly in 16mhz mode, can help identify potential problems. If crashes occur in 16mhz mode, changing all resistors to 2.2K values is recommended.

Due to the multitude of motherboard revisions, it's challenging to provide a comprehensive list of which boards require modifications. Generally, these SIL arrays are located near the CPU, ROM, and blitter, totaling about 5 arrays. It's worth noting that even with two seemingly identical motherboards, one may function in 16mhz mode while the other does not. This variability is attributed to tolerances across all chips on the motherboard.

To assess the likelihood of issues, a resistance check between PIN 32 of the 68000 CPU (DIP type, bottom corner pin) and the PSU 5V line (typically the red wire) is recommended (with the power off). A resistance reading of 10K may indicate potential issues, while around 4.7K could be a bit more stable. For maximum assurance, replacing all SIL arrays on the address and databus with 2.2K resistors is suggested.

See this forum topic for more information https://www.exxoshost.co.uk/forum/viewtopic.php?f=17&t=2235