- The LaST Upgrade -

PART 25 - Falcon Clock Buffer Patch

Last updated August 30, 2021


Installing is pretty simple. At least on machines which haven't already got a patch installed. The problem I found was super glue was used to glue the wires down, so removing the old patch caused damaged to the PCB tracks and pads. I was lucky and the PCB VIA's were still in place to solder to.

If you are doing this update to a virgin Falcon, then you will need to remove 3 SMT resistors. R221,R222,R216. I would suggest just blobbing a huge blob of solder of the SMT resistor then you can GENTLY push it off the pads once all the solder has melted. Once all 3 SMT resistors are off, GENTLY clean the area with a IPA swab and check for pad damage under a magnifier. If you have damaged pads then I would suggest you abort the install as "trying to fit it" will likely destroy your falcon incurring a hefty repair bill!

DISCLAIMER. Installing this patch along with removing the old patch is done entirely at your own risk. I do not accept any responsibility if you kill your falcon while doing these modifications.


The buffer board comes assembled so you don't need to worry about soldering the SMT parts.

I suggest you remove the U63 to make it easier to solder the patch board onto the IC. 0V marked on the PCB is pin 10 as shown in the above image. Then the diagonal pin 20 is the 5V pin marked on the PCB. Make double sure pin 1 of the IC is to the left as orientated in the above image. Getting this step wrong and soldering the buffer board on backwards is not a good idea, so check and check again! The PCB needs to sit flat ontop of the U68 IC. So solder as illustrated in the above image.

TIP: I would suggest placing the 2 header pins in the buffer PCB and soldering them on the bottom of the PCB to hold them in place while soldering to the IC. I would also suggest as you likely need 3 hands to solder this, that you put a little solder on the IC pin and header pin, then you can hold the IC and PCB together in one hand and just heat up the 2 solder joins and melt them into 1. Then once held in place, solder the other pin and add a little more solder if needed.

TIP2: Make sure the header pin is just long enough to overlap the wider top part of the IC legs.

WARNING: DO NOT get solder on the narrower part of the IC pins which push into the socket. Only solder to the wider top half of the IC pins. If you get solder on the part of the pin which pushes into the IC socket, then clean it off, otherwise you will likely damage the IC socket and the IC wont contact properly! (this would be very bad!) .

WARNING: Double make sure you have not shorted out the IC pins next to where you are soldering the PCB header pins.

NOTE: Its unlikely to be able to solder the PCB ontop of the IC perfectly straight.



Trim the header pins and solder them on the top side of the PCB. Do not get the pins too hot otherwise it may melt the solder on the bottom side of the header pins and you likely will have to re-solder the bottom half of the pins again. Trim the tops of the pins once done.

Place the U63 & buffer PCB back into the socket. The first wire is marked with 2 red circles. I supply a small length of wire so cut to size (as short as possible) and solder the wire as shown above.


2nd wire to solder on.

NOTE: I circled the PAD as this is where the wire is supposed to go, but due to pad damage I had to solder to the VIA, hence the red arrow showing where I actually soldered it to.


3rd wire.

NOTE: I circled the PAD as this is where the wire is supposed to go, but due to pad damage I had to solder to the VIA instead.


4th wire.

NOTE: I circled the PAD as this is where the wire is supposed to go, but due to pad damage I had to solder to the VIA instead.

Also a modification to the falcon needs to be done and shown HERE

Later builds of the board have a jumper which is shown HERE.

Double check all your connections under a magnifier. Turn on your falcon and it should power up as normal. I've noticed the Falcon seems to take 2-5 seconds to output a display on my monitor, so it can make you worried when it doesn't output a white screen as soon as your power on! Though if your falcon has not come on within 5 seconds then turn off your falcon as you likely have done something wrong or damaged something. In which case, good luck ;)





https://youtu.be/wSL-1w9aW4k by mzry (Steven Timms )

http://www.stcarchiv.de/stc1998/06/falcon-beschleunigen Interesting read about clock patches. (thanks Mikro)



UPDATE October 31, 2016

I found there was some bad ringing on the Falcon clocks. So I designed a small PCB which has some buffer resistors and also makes the upgrade a bit neater.

If there is enough interest I will make up a small batch of these PCBs.

I also found a interesting page here which has 3 different ways this clock patch has been done. http://members.optusnet.com.au/~startreks/falccct.html

Also official Atari info here METHOD1 & METHOD2

I did not see "method 3" when I created by design. Though it seems steinberg recommended using buffer resistors. I added those onto my PCB anyway as I saw there is bad ringing on the clock signals. So this mod is a lot better than previous methods.


November 17, 2016

Above is the test board in place. This version has a typo as R216B should have been R216T. Where T=TOP of the resistor and B=Bottom of the resistor on the falcons motherboard. As my falcon already had the patch done shown at the start of this page, I had to removed it. unfortunately it was death by superglue and the PCB pads came off :-( However its not a pad thing really as there as via's right next to the pads, which I think make a more secure connection to solder to anyway.

The middle scope image shows the Falcons default clock waveform, which also is the same as the output of the F04 without resistors. The second scope image on the right shows with the resistors added. As you can see the bottom of the waveform has almost 1volt of "bounce" and this is likely being seen by parts of the falcon's motherboard as a HI when it is in fact a LOW. Similar issues were riddled with the ST line aswell. Its clear Atari has some thoughts to add resistors on the motherboard to start with, but didn't quite get there into having stable clocks.

I have noticed that the 3 clock outputs need different values of resistors, so I need to experiment a little to find the best values, but its very close with the values I guessed at anyway :)

March 21, 2019

The "Blue Edition" Version 2 fits the same as the previous V1 (purple) board.

Version 2 solves a poor logic low level on the SDMA clock line.

The solder pad turns on/off a 3 gate delay which should simulate the delays of the second chip on the phantom making this board a "all in one" solution. This has now been confirmed to work with the Phantom (thanks Steve!)

How does it work ?

Problem is as always noise and grounding problems. You actually end up with something like 2v DC offset between the clock source and destination. So if "logic low" is 2v, you're not being at a logic low at all, hence the whole clock issue epic. In fact the logic low only happens because of undershoot. So when its crowbarred with 47R it's literally forcing the DC offset down to more like 1V and "mostly" the voltage swing is high enough to still push a logic high over 2.5v. Only on my 2 Falcons it clamps the clock down to more like 0.5v and that's not enough by far. Hence then my falcon won't boot as its clock is basically low all the time.

I solve this differently. I come from an audio background so know all about DC offsets and logic levels with MOSFET digital amps. Anyway, I don't crowbar the voltage to get it lower, I just remove the DC offset instead. I then have something like 33R to help stop ringing. But as the clock is going right across the board, all the inductance and capacitance and noise pickup all adds up. It's still there at the SDMA end,BUT the ringing / under /overshoots all happen outside of TTL logic level thresholds. So it basically doesn't matter.

So for some reason the DC offset drifts on this falcon over time which it shouldn't really be doing. This voltage very slowly increases until the logic low level gets to the point it was before all this modding was done. Basically audio crackles etc. It further drifts out then causes video issues like in the image I posted. Then eventually it just crashes and half the time won't even reset again.

Now we get back to the 100K which just pulls the clock down to 0V enough to keep the proper clock biasing. Basically you end up with like 0v to 4v on the buffer board end for example, then without the buffer board to correct the voltages, you end up with more like 2-6v on the SDMA as there is bad grounding between the 2 areas of the motherboard. With my buffer board, it auto corrects to mirror the voltage on the buffer board. So you end up with a nice clean 0-4v clock on the SDMA.


I have had a report of DSP sound issues on one particular Falcon. The symptom was crackling audio before and after fitting the clock patch. There was also some intermittent lock-ups and crashes after fitting.

I had this particular Falcon sent to me to diagnose and came up with a solution above to simply solder a generic 100K resistor as shown in the image above. Falcon is now totally stable and no more crackling in the audio. It seems this particular falcon is more problematic than others and require a little more "work" to keep it stable. There are certainly no problems fitting this resistor as standard even if your Falcon does not specifically suffer from any problems. In fact I would recommend this modification be done to be on the safe side anyway. Please note, the 100K resistor is only valid for exxos clock buffer boards.

Please do not confuse this resistor with other mods which use a "termination resistor" of about 47R. This is 100K and is NOT a termination resistor! Adding this resistor alone will not do anything without the exxos board fitted. The 47R is basically a crowbar for the clock. Its pretty bad as if you take 5 / 47 = 106mA peak current draw where normally outputs can't drive anywhere near that. As the voltage rises the current will fall. But I really do fear people who do that 47R mod, it's a wonder it's not blowing the clock drivers up by now. My falcons won't boot with it as it clamps the clock to about half a volt! I think I've already written about all that before.

Anyway, it's not termination which "solved" the problem. It's the fact it's overloading the clock and all but killing it. What it actually does is short the clock out to the point its half the voltage and because it's crowbarred down to 0v, it alters its DC offset which forces it towards 0v more and it then has a lower logic low level which actually works, albeit pending blowing stuff up.

The image shows a simple 0805 100K resistor which is soldered on the "bootleg" track. The white wire connects to 0V as shown. I'm sure people with a more steady hand will be able to do a little bit neater job than I could :)




Images below show the Falcon clock patch as was fitted on my machine. This is needed on some machines which suffer from weak clock signals. The IC acts as a buffer for the clock signals. The IC used is generally a 74F04.

According to Atari documents, SCSI can suffer and can also cause DSP sound related issues such as crackling. The original Atari mod can be found HERE Though it seems steinberg revised the mod somewhat since then which is basically listed below.

Since writing these instructions I have since found a 3rd method which uses buffer resistors. My tests as to why this mod is better is listed below HERE.

I would also recommend a 100nF ceramic capacitor across the power rails of the F04 (not shown in images).