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It wouldn't necessarily have to be a ROM patch, as you would have to find a point to insert the instruction. Ideally you'd want it to run when system resumes from sleep but not more than once, and I think patches only run when the OS is running. A regular extension of some sort might be easier.

There's a few different ways you could do it, simplest would be changing the command from "sound on, clear latch" to just "sound on". In ResEdit for example change 2B from 03 to 01:

The RAM speed issue is due to a DTACK register not being accessed when the system wakes from sleep. The last 4MB of RAM from 4MB-9MB is run differently due an unreleased expansion card system, and if that register is not accessed after the CPU GLU is reset then the DTACK delay will be much...

I think I see what's doing it in the Portable Control Panel 1.3. Included are two ROM patches, one replacing the BatWatch VBL task and one for the SndWatch VBL task I posted earlier. I believe the BatWatch patch is to add dimming of the backlight when the battery is low and a new message for...

The registers aren't really documented anywhere. I believe that bit switches between PWM and analog output on the ASC, but as you see the analog is not connected to anything at all. There was also a thought I had on the ripple from the 12V regulator. The design guide for the LT1070 recommends...

MicroBug is a much more basic version of MacsBug, I believe it is either built in to the ROM or the OS. It looks like this:

One thing you can experiment with as well with a debugger (you should at least have MicroBug included) is disabling the output on the Apple Sound Chip. Hit the programmer/interrupt button and type SM FB0802 1, then see if you notice a difference in the noise levels.

I tried booting from that same French MacTest Portable disk on mine and seem to have the exact same sound every 10 seconds. Yet if I boot from a System 6.0.8 "System Tools" disk that sound does not occur.

Well as to "normal" if my guess as of the cause is correct I would say "yes, but also no". Take a look at the SndWatch VBL task from the Macintosh Portable ROM: SndWatch: movea.l PowerMgrVars,A2 lea (SwVBLTask,A2),A0 move.w #600,(vblCount,A0)...

On the M5126, the source and drain pins of Q5 are directly connected, so +12V is always supplied, and Q5 would be pretty useless. My guess was they decided that being able to turn off the sound wasn't necessary, like many other questionable decisions they made when making the M5126. As to...

A simple program/interface could probably written, and then run on basically anything with serial hardware. I suppose you could even do it from a smartphone. You can actually do quite a lot from that interface. Besides running the various hardware tests, you can dump memory, execute code, and...

The test has marked the upper byte (bits 8-15) of RAM as bad, so it could be any of the chips in the lower half. While it could be a combination of bits on multiple chips (for example 3 bad bits on one chip and 5 bad bits on another), it's more likely one or more chips are bad or failing to...

Assuming the select lines are connected in order, that range would match 4 chips, but since the error is in the lower byte the two chips for that range should be U4B and U1C.

*0 loads the starting memory address and *1 the end. If you try testing RAM on two different ranges, for example 00000000-0003FFFF and 00040000-0007FFFF, and you get the same error in both, then either bit 4 is bad on multiple chips or there is an issue with bit 4 across all of the RAM.

Anything like MacTerminal or Red Ryder probably should work fine. Proper settings should be 9600 baud, 8 bits, 2 stop bits. Be warned that linked documentation is missing most of the commands and has some commands that aren't in the Portable ROM. For example to run a march test on the first...

If you're feeling adventurous and have the means to hook up a serial cable and ground pin 7 on J22, you can likely get into the test mode and possibly isolate the bad RAM there, assuming there isn't an issue with data bit 4 across all of the RAM.

I guess there's also the CudaLite with PS/2 support, and whatever Cuda version was in the early iMac.

Those solder bridges are actually a form of bodge. There's line numbers next to the pads on the edges, and there are corresponding line numbers on pads connected to the driver chips on the opposite end of the board. If for some reason there's a bad connection, this allows you to connect the same...

Sure, go ahead and mail it over if you'd like. I could probably draw up a basic schematic as well. As for the Portable logic boards, I can look at them if you want but I imagine you could probably get them running pretty easily as well. I think the only thing I have compatible with the Radius...

Well I can't tell for sure since some traces are either under ICs or on inner layers, but here's some guesses on the card and notes on RAM cards in general, if anyone cares: Since it's using 512K x 8 chips, you'll have a pair of them on the 16-bit data bus for 1MB. Because of this you only...