Macintosh Plus - Trouble after doing SCSI Term Diode Mod

[Paulo Gago Camara]

Hi!

Continuing my Macintosh Plus 4Mb total restore, I bough a BlueSCSI2 external module and installed a number of disk images.

It was working like a charm.

Then, I went to the CR1 diode mod, so that I could have TERM power on the SCSI port to avoid the need to power the BlueSCSI2 from a usb cable.

The shop didn't have a 50V 1 Amp 1N4001, so I bought the 1000v 1 Amp 1N4007 diode that they had in stock. (more diode voltage tolerance shouldn't be a problem)

Installed the diode and assembled everything.

The first 5 or 6 times the Mac worked just fine, booting from BlueSCSI2 normally and powered by the moded external scsi port.

Then, while using the computer I noted that the internal speaker sound started to get really low.

I went to the control panel and although I set the volume higher, it still was sounding really low.

Turned off the machine and when I turned it on again I got garbled screen, no bong, and after the normal while, it appears to have an unreadable sad mac icon and error message.

After a few seconds on this screen it started to give a low "moaning" sound with an increasing lower pitch that doesn't come from the internal speaker.

It appears to come from the circuitry (don't know if its from the logic board or the analog). When I heard this moaning sound I disconnected the computer immediately. It seems something is going to give,,,

The picture, although garbled, is CRT perfect, no interferences no hissing nor flup flup sound.

I removed the CR1 diode mod and the behaviour continues the same as described above.

I check the "Dead Mac Scrolls" and no case matches mine.

I've checked the voltages at the external FDD port and they’re 12,10V, 5,03V and -12,14 V

The "Macintosh Repair & Uograde Secrets" does have a low sound situation described, and it points to the -12 +12 rail, but the voltages appear to be fine....

Any help will be much appreciated .

Thanks!
--
PGC

 

[MacOfAllTrades]

Maybe powersupply portions of the analog board is going on you by coincidence / unrelated to the diode mod?

 

[Paulo Gago Camara]

I've previously done a total recap on the analog board.

The CRT image is totally sharp and stable with no strange noises or ozone smell.

Being the analog board it would be on the switching power supply part of it.

The voltages at the FDD port are correct as before.

But I agree that I can't rule out being a coincidence of something going bad on the switching power supply....

 

[Paulo Gago Camara]

I've been testing the machine with half the memory modules (2 mb) and I get the same screens each time.

It doesn't bong.

When I turn on I get this screen while testing the ram:

then I get this screen with a sad mac but cannot read the whole error code:

 

[reallyrandy]

I was gonna do this mod on my two Pluses so I could leave my SillyTinySCSI mounted on the back with no external power, but now I think I'm gonna wait until you can post what the problem was. I just recapped the analog boards and serviced the floppy drives and they're working beautifully. I don't wanna screw that up.

Do you have another analog board you can test with?

I do have one question about the mod though; how does that affect plugging in a standard powered external drive, like my Jasmine 20MB or my Ehman 60MB?

 

[Paulo Gago Camara]

I was gonna do this mod on my two Pluses so I could leave my SillyTinySCSI mounted on the back with no external power, but now I think I'm gonna wait until you can post what the problem was. I just recapped the analog boards and serviced the floppy drives and they're working beautifully. I don't wanna screw that up.

Do you have another analog board you can test with?

I do have one question about the mod though; how does that affect plugging in a standard powered external drive, like my Jasmine 20MB or my Ehman 60MB?

Unfortunately I don't have another good analog board. But the one I have was totally recaped with the Console5.com kit and was functioning perfectly before the mod.

The problem did not arise right after the mod, it only started to malfunction after a few boots and when I was testing a number of images on the BlueSCSI2.

I noted the sound becoming really weak. And then it no longer booted.

The voltages are OK at the FDD port, but I still haven't tested them for ripple with the oscilloscope.

I not 100% sure that the mod was the origin of the problem. I've ordered new RAM SIMMS to test if the ones I have are malfunctioning.

After the RAM and the ripple test, I'm going to recap the logic board and test the MC7805CT voltage regulator out of the board before I try anything analog board related.

I'll update on the progress. If anyone has more ideas or experience, please share.

Thanks!

 

[Paulo Gago Camara]

I made a video with the behaviour of my Mac Plus.

It starts with a garbled screen, then goes to the desktop pattern and beeps really low, then at the end you can ear the strange sound that I described above. I quickly turned it off because it didn't sound like it was going to end very well...

Any ideas?

 

[reallyrandy]

The reason I ask is because maybe the overpowered diode skowly burned something out. I don't know enough about electronic components to know if the 1000v vs the 50v could have harmed something but when you mod something and then shortly after that something goes wrong, it's rarely a coincidence.

 

[Paulo Gago Camara]

The reason I ask is because maybe the overpowered diode skowly burned something out. I don't know enough about electronic components to know if the 1000v vs the 50v could have harmed something but when you mod something and then shortly after that something goes wrong, it's rarely a coincidence.

I agree that this kind of occurrence is rarely a coincidence.

Theoretically the 1N4007 that I used, versus the 1N4001 that I should have used, only differ on the amount of reverse voltage each can handle without breaking (1000v for the 1N4007 and 50V for the 1N4001).

According with the datasheet the rest of the parameters are equal, including forward voltage drop. ~1V

1N4007 pdf, 1N4007 Description, 1N4007 Datasheet, 1N4007 view ::: ALLDATASHEET :::