[Video] Mandatory Fixes: bus resistors on an STFM

[exxos]

Replacing all the resistor packs is not a good idea as your esecially performing an experiment with unknown conciquenses.

[LarryL]

I only toned out on RP2. On the address bus pin A5, I only toned out on RP6.

What exactly do you mean with „toned out“?

Put your measurement device into the mode to measure resistance.
Then do your measurements.
When you measure just a few Ohms, you know you have the direct connection
Since all data and address pins go via 10k to 5V, you will measure 20k to not directly connected pins - your multimeter might beep then too, if you are in „continuity mode“

Just an idea :-)

Cheers
Michael

[MacTech05]

By toned out, I mean the tone heard on the multimeter when checking for continuity and the steady tone is heard indicating continuity.

Randy

[LarryL]

By toned out, I mean the tone heard on the multimeter when checking for continuity and the steady tone is heard indicating continuity.

Randy

Exactly.
That’s what I meant, why you should use the resistance measurement and not the continuity mode.
A connection via two resistors ist also continuity
But you are searching for a direct connection of the address and database pins to the resistor packs

[MacTech05]

LarryL,

Thank you. I will retest using the resistance measurement.

Randy

[MacTech05]

Ok, I have retested and here are the results:

NOTE: Pin 1, 5V pin read 9.98K on all resistor packs, which is what I expected. Basically, 10K.

RP1 Data Bus (9.98K on Pin 1, 10.98K on the other pins.
RP1 Address Bus (9.98K on Pin 1, 10.98K on the other pins.

RP2 Data Bus (9.98K on Pin 1, .4 ohms on one pin, 19.9K on the rest of the pins.
RP2 Address Bus (9.98K on Pin 1, 19.9K on the rest of the pins.

RP3 Data Bus (9.98K on Pin 1, 19.9K on the rest of the pins.
RP3 Address Bus (9.98K on Pin 1, 19.9K on the rest of the pins.

RP4 Data Bus (9.98K on Pin 1, 19.9K on the rest of the pins.
RP4 Address Bus (9.98K on Pin 1, 19.9K on the rest of the pins.

RP5 Data Bus (9.98K on Pin 1, 12.32K on one pin, 14.65K on the rest of the pins.
RP5 Address Bus (9.98K on Pin 1, 12.32K on one pin, 14.65K on the rest of the pins.

RP6 Data Bus (9.98K on Pin 1, 19.9K on the rest of the pins.
RP6 Address Bus (9.98K on Pin 1, .7 ohms on one pin, 19.9K on the rest of the pins.

RP7 Data Bus (9.98K on Pin 1, 14.65K on the rest of the pins.
RP7 Address Bus (9.98K on Pin 1, 14.65K on the rest of the pins.

RP8 Data Bus (9.98K on Pin 1, 19.9K on the rest of the pins.
RP8 Address Bus 99.98K on Pin 1, 19.9K on the rest of the pins.

So, based on the resistance measurements for each resistor pack, I would say the 5 resistor packs to replace would most likely be:
RP1, RP2, RP5, RP6, RP7

Does that sound logical?

Thank you.
Randy

[exxos]

According to your results, you only found any connection from resistors packs 2 & 6 ? So I'm not sure how you're coming to conclusions with the other resistor packs when you found found no connection to any address or data pins on the CPU ?

If I get chance this week I'll look though all my motherboards to see if I can find the same one as yours as your finding are not making any sense to me. It shouldn't be this hard to locate CPU pins which go to resistor packs.

[MacTech05]

Exxos,

You are correct, I only see the direct connection with RP2 (from data bus) and RP6 (from address bus) as measured using the continuity setting on my multimeter. Technically same results using the resistance mode, RP2 (from data bus .4 ohms, basically a short/continuity) and RP6 (from address buss, .7 ohms, basically a short/continuity)

I was attempting to identify the 5 resistor packs that seems to be the number anticipated.

As I indicated in a previous message, I was able to get continuity with all 8 resistor packs by repeating the continuity testing with different data bus pins and address bus pins which tells me that each resistor pack is directly connected to one or more of the data bus pins or address bus pins. Hence, my confusion and the reason I asked for advice. I switched to repeating the testing using the resistance setting instead of the continuity method. You assessment is correct, only RP2 and RP6 are definitely showing direct connection to data bus pin D4 (RP2) and address bus pin A5 (RP6). That is clearly the case when measured using the either the continuity method or the resistance method. Since I only definitely identified RP2 and RP6 as resistor packs to be replaced with 2.2K resistor packs.

Everyone here suggested there should 5 resistor packs that could be replaced. But you are correct, none of the resistor packs are definite candidates for replacement. Three others have low resistance readings, but not essentially short/continuity as was the case with RP2 and RP6.

I would appreciate it very much if you could find the time to go through your motherboard to see if you have one that is the same as the one I have. I have searched for a schematic for my specific motherboard, but I have not found one. If you will notice, and this is what I have found in my search for STFM motherboards like mine, all of the schematics and photos I have found show the 68000 processor at the bottom edge of the motherboard in a horizontal position. My motherboard has the 68000 processor to the right side of the board in a vertical position.

Thank you.
Randy

[LarryL]

Hi,
It is not sufficient to measure only one of the data pins or one of the address pins.
Each data/address bus pin is connected directly to ONE Single pin of the several resistor networks
You have found the direct connections from D4 and A5 - now you have to find the direct connections (low resistance of just a few Ohms) from the other Dx and Ay pins
You might have misinterpreted the schematics - although a „bus“ in the schematics looks like the pins are all connected to each other, they are of course not!

Regards
Michael

[exxos]

I looked through my motherboards and afraid I don't have that type any more.