How to fix the motherboard bugs on the A1200 yourself

In this section I will detail how you can fix the timing problems of the A1200 motherboard yourself, with the aid of a soldering iron and a bit of perseverance.
I personally have applied the fix on an A1200 REV 2B motherboard that would not work with my Apollo 1240, until I did the fixes.

At the end of this document I present a simple guide to removing surface mount resistors.

What are the timing fixes?

As far as I can work out, there are problems with the 7 MHz system clock and the 7.09/7.15 MHz quadrature clock used by the custom chips. Removing E123C and E125C (detailed later) eases a timing margin, which should help an accelerator card.

Are there any other modifications?

Yes. Some rev 2B motherboards appear to have some additional wire links. As far as I can tell these wire links are to correct for PCB tracking problems as they follow existing tracking.

NOTE: My Rev 2B motherboard does not have the following 3 modifications and it works OK. AFAIK it came from a Commodore A1200.

Clockport to Paula wire link (REV 2B only)

This wire mod connects A30 of the address bus (clockport P9A pin 6) to pin 46 of Paula (A30)!

 (Click image for larger view 159KB JPEG)

U8 8520 CIA Wire link (REV 2B Only)

Another wire link to connect something that the PCB should!
This connects Pin 44 (C) to Pin 2 (PA0) which then connects to POUT of the parallel port.

(Click image for larger view 108KB JPEG)

Resistor added by ROMs (REV 2B Only)

This is actually part of the timing mods detailed below.
The connection to C6B (brown SMT capacitor) is to pick up +5V. The resistor that has been added is a 470 Ohm, 0.25W resistor that then connects to DRA0 (DRAM address bus bit 0) by a PCB via. The timing modifications below state you should connect a 470 Ohm resistor from +5V to pin 43 of Alice, this modification does the same thing.

(Click image for larger view, 145KB JPEG)

What components you need to do

The actual timing fixes! (REV 1D4 and 2B only)

Locating E123R/E123C and E125R/E125C

These components to remove are on the underside of the PCB.
On this picture I have circled the two components you need to remove.
Note: It does not matter if you remove either the resistor or the capacitor, removal of either part will fix the timing issue.

(Click image for a larger view 839K JPEG)

Locating R118 (Rev 2B only)

This is near the CPU expansion of the A1200.

(Click image for a larger view 49 KB JPEG)

Remove the existing resistor and add a 220R resistor as shown.

That's it!

What does this fix do?

Here is the relevant section of the A1200 schematic concerning E123 and E125

You can see that E123R/E123C do not connect to any devices to the right of the resistor, denoted by the NC text. The same is true for E125R/E125C. The 7MHz clock and CDAC only connect from Alice (U2) to Budgie. So removing either the resistor or the capacitor will break the AC terminator.

This circuit is an AC terminator. It has a time constant of 1.30ns and was designed to remove high frequency components in the signal rise/fall time. Unfortunately the 22pF capacitor adds loading to the 7MHZ and CDAC lines which coupled with the input capacitance of Budgie (10pF?) + PCB trace of maybe 5pF, it increases the signal rise/fall times. The rise or fall time of a signal T = 2.2RC, where T is the rise time in seconds, R = output driver resistance, C = capacitance.

Most Commodore ASIC have a 4mA drive capability, which from spending too much time around IBIS/Spice models is approximately a 150 ohm output drive impedance
Plugging in the numbers, T = 2.2 x 150 x 37E-12.
T= 12.2ns

If we remove the 22pF capacitor, either by removing it or the resistor, the capacitive load drops from 37pF to 15pF. The rise/fall time is thus:
T = 2.2 x 150 x 13E-12

T= 4.95ns

Accelerator cards with logic buffers have a 3-7ns propagation delay added on. This helps to explain why some accelerator cards work better than others. I would need to re-read the A1200 CPU slot design notes to explain this further.

All formulas and rationale of driver output impedance derivation came from High Speed Digital Design, A handbook of Black Magic by Howard Johnson, chapter 1.

In summary,

Removing either E123 R/C or E125 R/C will perform the timing fix and allow your A1200 to work.

How to remove surface mount capacitors/resistor easily!?

Get a soldering iron with a 1206 SMT rework bit, apply to the resistor and lift it off the PCB!

Don't have a 1206 SMT rework bit?

Do not panic it can still be done. This may not be the best way but I have used it many times with great success.

Required tools

A soldering iron, preferably with a 2-3 mm flat tip.
Some solder.
A sharp knife or scalpel.
Soldermop/desolder wick to clean up.
A fine pair of pliers of tweesers.

Procedure

1. At one of of the component apply the soldering iron and some additional solder.
2. Quickly apply the iron and solder to the other end of the component until the solder is molten.
3. Now get your knife under the middle of the component, ready to lever upwards but wait.
4. Go back to the end you started at and apply the heat again. When the solder melts lift the component up gently.
5. If all went well, one end of the component is in the air at an angle of 30-45 degrees.
6. Using your tweesers, hold the component by the end in the air, heat the solder on the PCB and the component will now come off the PCB. That is the component removed.
7. Use the desolder wick to clean up the PCB pad by applying the desolder wick to the pad and then the soldering iron and whilst the iron is on the wick, drag it off the PCB pad for a nice clean finish.
    

Need more help or advice on soldering?

Try these web guides:

Everyday Practical Electronics (EPE) Magazine Basic soldering guide (Highly recommended)
Picture gallery for the above article.

How to solder surface mount components
 

Document written by Ian Stedman, 26th November 2003.
Updated 24th December 2008

Updated 30 August 2010