resurrecting/upgrading power mac 7500/100

[fjm_48]

Failed electrolytic capacitors can have numerous different failures, here are a few:

- Short Circuit
- Open Circuit
- Out of spec capacitance (too low or too high)
- High ESR
- Turning into a resistor
- Electrically leaky, passing DC instead of blocking it



You need to replace all of the capacitors, not just some. Replace the rest of the capacitors and try to boot the board again. You also should clean the board well with something like CRC Lectra Clean. IPA is junk and usually just smears things around and leaves white streaks.

I'm not familiar with the 7500, but it may need a ROM SIMM to boot. It also may need a good PRAM battery, there are several mac models that won't boot or have erratic behavior with no PRAM battery installed.

Thanks for the response.

Short Circuit, Turning into a resistor, Electrically leaky, passing DC instead of blocking are different degrees of the same thing. If they were significant enough to affect performance, a voltage droop at the capacitors' positive terminals would be measurable: no voltage droop was measured. Open Circuit, Out of spec capacitance (low), High ESR may vary in magnitude, but they all are equivalent to reduction in capacitance. I doubt that the design could not tolerate the complete removal of any one capacitor. Out of spec capacitance (high) would improve power rail filtering/bypassing. I did obtain the additional needed caps and all have now been changed. That did not fix it.

Changing all capacitors may make sense to prevent future failures, but changing only bad capacitors would be needed to return the board to operational. In Defense, space and commercial aerospace, IPA is specified for soldering rework cleanup.

No ROM SIMM was installed in the 7500/100 system, and it operated properly in that same configuration.

The PRAM battery is new, and it measures 3.66V both installed and out of circuit.

The 7500 should not need a PRAM battery. Only early 7500s would need a ROM SIMM; most of them don't.

Thanks for the response.

It's been a moment since I restored mine but I seem to remember holding a reset button for about a second every time I make any changes to the machine, before powering it on. Much more about it here though https://68kmla.org/bb/index.php?threads/powermac-7500-100-dead.19371/

Thanks for the response. I followed the guidance in the link you supplied. There was no change.

There is one more data point that someone here may recognize. When I have (a) memory stick(s) installed, I get 3 seconds of white noise 1 second after power up. When I remove the memory, I get 1 second of white noise 1 second after power up.

 

[Zippy Zapp]

It is so odd that all of that will not work. The 7500 board is pretty robust. Unless you have the wrong memory or bad memory or broken PSUs it seems odd that all those components provide the same result. I have an old 7500 in a self-destructing case that I pull off the shelf of my office filling cabinet and power it on from time to time and it usually always works. I assume you have an ADB keyboard that you are using? Does the power button on the keyboard work? Can you hit the Ctrl+Command+Power key on the keyboard to see if it does a reset? Does the reset produce a chime? I have a PowerMac that refused to boot when powered on until I reset it then it would be fine. I traced it down to the video card and it wasnt needed because most Macs have video built in.

How are you installing the Memory DIMMs? Have you tried different slots? IIRC you have to use specific slots first and while they don't have to be pairs you get a performance boost for using matched pairs in corresponding slots. As already stated you do not need a PRAM or a ROM SIMM.

From the Apple Service Guide:
Screen is black, no boot tone, drive does not operate, but fan is running and LED is lit:
1. Reset Cuda Chip
2. Reset logic board
3. Remove all DRAM DIMMs and try replacing them one at a time (Slot A1 first. A1 and B1 are the slots for interleaving)
4. Replace Processor Card
5. Replace Logic Board
6. Replace Power Supply

How many different DIMMs do you have?

 

[fjm_48]

It is so odd that all of that will not work. The 7500 board is pretty robust. Unless you have the wrong memory or bad memory or broken PSUs it seems odd that all those components provide the same result. I have an old 7500 in a self-destructing case that I pull off the shelf of my office filling cabinet and power it on from time to time and it usually always works. I assume you have an ADB keyboard that you are using? Does the power button on the keyboard work? Can you hit the Ctrl+Command+Power key on the keyboard to see if it does a reset? Does the reset produce a chime? I have a PowerMac that refused to boot when powered on until I reset it then it would be fine. I traced it down to the video card and it wasnt needed because most Macs have video built in.

I do not have an ADB keyboard. I will use a wombat adb-usb adapter if I can get this power mac running.
I have not cards installed other than the CPU.

How are you installing the Memory DIMMs? Have you tried different slots? IIRC you have to use specific slots first and while they don't have to be pairs you get a performance boost for using matched pairs in corresponding slots. As already stated you do not need a PRAM or a ROM SIMM.

I have installed individual DIMMs, pairs of DIMMS, and 3 pairs of DIMMS in all combinations using paired slots specified in the user's manual.

From the Apple Service Guide:
Screen is black, no boot tone, drive does not operate, but fan is running and LED is lit:
1. Reset Cuda Chip
2. Reset logic board

My logic board only has 2 buttons; one is the power switch. The other is located under the cpu card. I assume that is the reset AND cuda chip reset.

3. Remove all DRAM DIMMs and try replacing them one at a time (Slot A1 first. A1 and B1 are the slots for interleaving)

Yep, I did that.

4. Replace Processor Card

I bought a used card on ebay (820-0849-A). The seller represented it as good; surely he wouldn't be misleading me (sarcasm).

5. Replace Logic Board

I bought a used logic board. No change.

6. Replace Power Supply

I replaced the psu. No change.

How many different DIMMs do you have?

I have 3 pairs of DIMMs. Each pair is different from the other 2.

I suspect that I have purchased used components that don't work. I wish that Apple gave more detailed processes in identifying bad components.

 

[skate323k137]

Are you in the US? I have a fully working (last I checked, a couple years) parts 7500/100. The original owner painted it purple, hastily, but the actual board, memory, etc is all fine.

 

[fjm_48]

Are you in the US? I have a fully working (last I checked, a couple years) parts 7500/100. The original owner painted it purple, hastily, but the actual board, memory, etc is all fine.

Yes, I am in the US; in Arizona. If you are offering help or parts for loan or purchase, perhaps you will direct message me. Thanks

 

[fjm_48]

Can anyone tell me for sure whether a Model 8500/9500 cpu card, 820-0849-A, is compatible with the 7500 logic board?

 

[ClassicHasClass]

That should work fine.