[Video] Mandatory Fixes: bus resistors on an STFM

[exxos]

viewtopic.php?f=31&t=149

[MacTech05]

Exxos,

Thank you very much!

Randy

[MacTech05]

I retested the resistor packs from both the D4 databus pin and the A5 address bus pin on the Motorola 68000 processor chip.

From the D4 databus pin, only one resistor pack, (RP 2 - Black), gave the long continuity tone. That does not make sense.

From the A5 address bus pin, only one resistor pack, (RP6 - Black), gave the long continuity tone. That also does not make sense.

I retested from other databus and address bus pins, with the same result.

At this point I do not know which resistor packs to replace. Any ideas.

Randy

[exxos]

Sounds normal to me. I don't understand what doesn't make sense ? If D4 has connection to RP2, then you found 1 resistor pack already.

[MacTech05]

exxos,

You are correct. I found 1 resistor pack on my 1040 STFM. I just expected at least 4 or 5 relevant resistor packs, motherboards shown in the Bus Resistor Upgrade in the Mandatory Fixes section. If one resistor pack I all I need to replace with a 2.2K resistor pack, then cool. There was also one resistor pack found when testing them from the address bus. I assume I should replace that one as well. I will replace the two resistor packs I found with 2.2K resistor packs and move on to recapping the power supply.

My concern was I thought I metered out the resistors incorrectly. But, I did the tests multiple times with the same result.

My 1040 STE had 6 resistor packs to replace with five 2.2K resistor packs and one 1.2K resistor pack.

David a BW Techoshed posted a video of him performing the bus resistor upgrade on his STFM (different motherboard though).

Thank you.

Randy

[exxos]

There will likely be 5 resistor packs to locate. You may want to look at a STFM schematic to get an idea on how the resistor packs connect to the CPU. The numbering will likely be different but it should still be enough to show you how it's wired up.

[MacTech05]

I do not have schematic for my specific 1040 STFM version. So, not sure if other STFM versions schematics will help. However, if the motherboards are basically the same and some of the components are just in different locations on the board, but labeled the same, such as RP2 being same on my board as it is on other STFM boards, then I can probably work with that. However, I am not sure that is the case.

There are a total of 8 resistor packs on my board. Can I just replace them all with 2.2K resistor packs?

I am sure there is a reason for you recommending only specific resistor packs be replaced in the Bus Resistor upgrade section, but is it possible to just replace them all, without negative effects to my STFM's function and performance?

My experience in tracing schematics is old and was on solid state and analog (tubes) communications equipment, not computer systems. However, if I had a motherboard and matching schematic, I could probably increase my skill and proficiency.

Thank you Exxos!

Randy

[Badwolf]

There are 23 address pins and 16 data pins.

Each pin will go to one of the fives resistor packs.

Each resistor pack will therefore service up to 8 pins (one will only service 7 pins as there 3x8=24 not 23).

You've identified two of the five packs so far.

Try again now with different data and address pins. I'd suggest something in the D8-D15 range (one pack) and in the A8-14 (one pack) and A17-A23 (one pack) ranges.

This should yield five packs total. Almost certainly the two on the left over by the ROMs are the data packs and the three others will probably be over to the right of the board.

BW

[DoG]

Here you have some schematics. RP1, RP2, RP5 is resistorpack for address bus. RP3 and RP6 is on data bus. But the RP-number might not correspond to your motherboard. So don't trust them. Anyway, I hope it will help.

Bus resistors.png

[MacTech05]

Now, I am confused. I metered the resistor packs again. This is the result.

As before, from databus pin D4, I only toned out on RP2. On the address bus pin A5, I only toned out on RP6.

Next I cycled the databus pins and address bus pins, testing each of the resistor packs RP1-RP8.

This is where I am confused. As I metered each resistor from different databus and address bus pins, I ended up with at least 1 databus pin toning out 1 or more of the resistor packs (RP1-RP8) eventually toning out all of the resistor packs between the different databus pins I used to test the resistor packs. Same with the address bus pins, I ended up with at least 1 address bus pin toning out one or more of the resistor packs (RP1-RP8), eventually toning out all of the resistor packs between the different address bus pins I used to test the resistor packs.

Now this does not sound right. There should be 5 resistor packs that will tone out that I need to replace with 2.2K resistor pack. My tests tell me I should replace all 8 resistor packs. But that can not be right. Correct?

What may I have done wrong? I mean, I can replace all 8 resistor packs with 2.2K resistor packs, but I am hesitant to do so. On my 1040 STE, one of the resistor packs had to be replaced with a 1.2K resistor pack, but the other 5 were replaced with 2,2K resistor packs.

I am unsure about the resistor pack values. Below is what I have, with information printed on the resistor packs:

5 each resistor packs labeled: LC91 1002J USI 8908 (BLACK)
1 each resistor pack labeled: LC91 1001J USI 8907 (BLACK)
2 each resistor packs labeled: BI M9*1-472J-903 (RED)

The 2 red resistor packs are RP5 and RP7 beside the 68000 processor. The other side of the processor has a black resistor pack (LC91 1002J USI 8908).

If I should replace all 8 resistor packs, should one of them be replaced with a 1.2K resistor pack, with the other 7 replaced with the 2.2K resistor packs?

I very much appreciate all of the help and advice everyone here has provided.

Thank you so much!
Randy