Custom ROM SIMMs and Boot issues

[Chopsticks]

hi, Im interested in gathering data regarding the issues people have had using custom ROM SIMMs in their Macs. I have been making pcbs and working with using customs roms but I have ran into some intermittent issues around using non apple simms. I would like to gather any information I can regarding things such as the model on Mac used, the Simms used that doesn't work, the ram chip part numbers and speed as well as the thickness of the Simm boards measured at the contact edge using callipers etc. I'm trying to narrow down the reason why some Macs such as the se/30 will work with stock roms and not with custom rom boards. the more details you can provide the better as it seems from my research that using customs boards can be a bit like the lotto in regards to what machines or whatnot they run on.
For example I have a se/30 that without fail will not boot a custom Simm regardless of is a shim is used or not and I have even replaced the socket with a NOS AMP part, yet sometimes if I wait a little bit and hit the reset switch it'll boot fine. however without fail as soon as I swap with a. stock standard apple rom Simm it boots immediately.
any information you can provide I would be most great full for as this has really got me completely stumped, while I don't have a second se/30 of a different revision to example/work with perhaps its just my machine though I have heard/read on the web that many people have had similar issues. unfortunately getting other logic boards to test with here in Australia is quite expensive to the point I can't afford so I'm hoping some of you here can help me out with working on this problem.

btw im an engineering student, and yes I've done the recapping etc etc

 

[Cinan]

Hi,  where about's are you ?  I am in Melbourne and have this problem which I am thinking is a ROM issue.  I have a Toshiba marked ROM SIMM.

If you are in Melbourne you could test on my machine.

 

[Cinan]

[No message]

 

[Chopsticks]

im based in geelong, i dont think its an issue with my SE/30, at least not from my testing so far. ive fully recapped my board and replaced the simm socket with a new old stock AMP part and ive removed almost all the smt IC's and soldered back on with fresh solder, ive also scoped out a lot of the different signals etc and so far it remains a mystery.
 

i had a read of you post you linked, your se/30 likely needs a good clean, recap and more then likely the floppy drive needs a service, the logic board looks ok from a quick glance at the couple pics you uploaded  but i'd still start with a recap, along with the a recap of the PSU and the analogue board too. btw you ROM SIMM is a standard 2 chip apple simm, so i doubt thats the issue. as for the hdd its likely dead or the rubber beneath the platters has gummed up making the heads get stuck, that is assuming its the typical quantum 40/80mb that they shipped with.

depending what your skills are with soldering there are lists online and even mouser shopping carts available with all the specific capacitor size/values needed, otherwise i'd look for someone local who can provide that service for you at a reasonable cost. i do repair work down here in geelong when im not to busy with uni but distance wise its probably not practical for you tbh with the current state of things in Victoria restriction wise.

so recap logic board, analogue board, PSU, then pull apart the floppy drive and clean/service it, then try booting from floppy drive with HDD completely disconnected would be the order i'd start with and assess symptoms from there

 

[Chopsticks]

update:

so I have checked resistance on all the A2-22 address lines and they sit around .9-1.0ohm and I get the same readings when checking D0-31 too, if I load a stock Mac rom image on any custom Simm I have it boots straight away yet as soon as I use a custom rom image based on the IIsi I get the garbled screen, I was thinking that the customs roms are using some different (higher) address ranges during boot hence why I checked the resistance on each line. again if I wait a few seconds and then click the reset button on the side though it will boot. At this point I might have to pull the scope out again and start some serious investigation unless anyone has some input on this....

 

[Chopsticks]

final update:

turns out the zoology 8530 have partially failed, I left the Mac running for awhile today while I was scoping out signals and noticed that the 8530 SCC was getting just slightly warmer then id expect it to get. now these chips do get warming anyways but on a hunch I removed it and fitted a spare I had in my parts bin, now everything is running 100% reliably, still not sure why it would boot a stock apple ROM and not a IIsi based custom rom but likely it had something to do with the upper data bus (D24-31) that the SCC communicates with the rest of the computer with or the A1/A2 lines, I think either of these might have been causing some intermittent bus errors or something..

regardless all is well with this SE/30... so far... more extensive testing needs to be done but for now it seems to be running well
 

 

[JonThysell]

Did you try the rubber band method? I found the only way to get the MacSimm to boot in my SE/30 without a simasimac was to use a rubber-band to pull the top toward the edge of the board. Shims would not work. Also whenever I have to take out the logic board, whenever I put it back in make sure to give the simm a little push in the right direction, just in case it moved.

 

[Chopsticks]

it ended up being a failing 8530, i replaced it and its worked 100% ever since. i use a 2-3 layers of caption tape to solve the need for a shim between the rom and rom socket. though i did replace the original rom socket with a New old stock AMP socket that is of much higher quality then the one apple used. not sure if it matters but i use 8mb rom simms that got pcbs made for

 

[aeberbach]

Wow, locals - Hi, I am in Melbourne too. I do not have a lot of experience with custom ROM but I have just received a BMOW for the SE/30. It did not work unless two layers of kapton tape were added on the back of the SIMM. the thickness of the PCB is the issue so the tape needs to go over the contacts but also up the sides of the SIMM so that the clips act with just a little more force. 

 

[Chopsticks]

Mac ROM Simms are about 1.3-1.4mm thick, the Simms blow has been selling for many years now are only about 1.27mm. I believe he is/has got a new batch of Simms made that are around 1.3mm in thickness but tbh I’m not a fan of bmow products. I have 2 model a floppyemu’s I purchased from his store a long long time ago and both have failing lcds, his response was to just buy new ones from eBay yet with 1 line of code change he could have allowed people who own the old models/b units to upgrade to the same lcds used in the model c, there’s no technical reason why it can’t be done with a simple change to code but telling a customer to go source a lcd on eBay is pretty poor tbh. 
 

I only mention that because he should have picked up on the simm pcb thickness years ago and got pcbs manufactured using mills not mm. Telling people to just use a shim or apply tape to get their product to work is piss poor for a businessman