exxos's Atari, Amiga & retro forum

I am about to do the Bus Resistor Upgrade outlined in the Mandatory Fixes Item 12 post:

"12) ST/F/M/E /MEGA ST- Bus resistors
Bus resistors can cause all sorts of random issues. Plugging in a cartridge can make the machine unstable, changing the brand of the CPU or ROM's can cause various malfunctions. The bus resistors need to be changed for 2.2K to resolve the issues. Most STF/M machines had 10K resistors on the address and databus, these should be replaced with 2.2K. Similar with STE where early machines had 10K and some mixed 4.7K. These need to be replaced with 2.2K.
viewt ... 235#p27504"

My question is regarding the proper orientation of the five 2.2K resistors and the one 1.2K resistor. Can I assume the "diamond" printed on the pull-up resistors marks the "common" pin? The "diamond" is above the 2nd pin, so does that mean the second pin is the common? Or does it mean the common is the first pin on the end with the "diamond" mark?

I looked on my STE motherboard and did not see a clear marking identifying the common end of the bus resistor holes in the board. Do I just orient the bus resistors in the same way as the currently installed (by Atari at the factory)? This I what I planned to do, I just wanted to ask to be sure before I proceeded with the bus resistors upgrade.

Thank you.
Randy

Either a diamond or a dot on one end indicates orientation for the resistor packs.

Usually there is a square silk screened on the boards which also indicates alignment, but you should note orientation of the original and fit the same way.

Here is a picture of an empty SIL slot:


The square on the silk screen is where the dot on the SIL should be placed in.

Just be careful when you de-solder your SILs...

HigashiJun wrote: 02 Oct 2023 10:14
Just be careful when you de-solder your SILs...

Just to re-iterate what Jun said, be very careful! The rule to follow is: If they do not come out freely, they are not ready to come out. If you jiggle/move them to 'break' them out, you will most likely break the pcb connection to the pads, they are very fragile. A trick I've used in the past when I have a SIL that won't freely fall out after de-soldering: liberally apply flux, run the heat gun over the pads, that should make it fall out very easily. Or if no heatgun, run your 'clean' soldering tip up/down the fluxed pins till it drops out.

Steve,

Just be sure, when you say liberally apply flux, run the heat gun over the pads, you mean the pads on the bottom of the motherboard, not the component side, correct?

I do have a heat gun. I was either going to use my desoldering station, or my heat gun. The heat gun is probably the least risky method.

Thank you!

Randy

Probably safer on the rear, then you don't need to tape over any components. Although it shouldn't take much at all if you've already desoldered it mostly.

I personally don't like use a heatgun on ST mainboards, they are very easy to 'popcorn' and scorch. Everybody has their own experiences, and everybody's equipment varies to a degree, but that's my preference.

I use either my desolder gun or wick on ST boards and find they work very well, and adding leaded solder beforehand to the part to desolder can help lower the melting point of the solder you want to remove. Flux is also a key component.

Obviously if you have removed the bulk of the solder then applying heat with a gun can help to get stubborn parts out, for example where there are large ground planes, but you need to be able to control both the temperature and airflow of your heatgun to ensure you don't damage the PCB.

Also, and I'm not suggesting that this applies to you, but I have seen examples of damage done by 'heatguns' and it turns out that the tool in question was not a flow/temp controlled desoldering station, but a DIY paint stripping heat gun with no control at all, so I always like to be clear on this sort of stuff.

Of course however you do it is down to your own skills, experience and tools, but I always feel that the more info you have helps to make a good decision.

The other option you have is to destroy the old part in situ and pull the pins one by one, it can work very well.

I also use solder wick on both motherboard sides and it does marvels.

IMHO, last STE boards revisions are prone to "accidents", while first revisions are more robust, and parts can be de-soldered much more easily.

Yeah in retrospect as @rubber_jonnie says it's probably best to not use the heatgun (although I was only invisaging a 3-5 second burst of it) just running a clean soldering tip up/down the run of pins with flux will make it drop out just as easily.

You can heat up the resistor pack.. It will likely burn and break, but it gets hot enough to remove it. This way your not really heating the MB at at all, at least not directly. So it greatly helps not burning a hole in the board. If you burn a resistor pack removing it.. who cares.