Mac IIsi Repairathon

[RetroViator]

Alpha is A-Okay​

After spending time on Bravo's audio problems, I decided to check out Alpha's speaker problem while the subject was still fresh in my mind. Alpha was working perfectly after I restored it back in 2021, but I was using it as a known-good system for testing, and along the way, my sloppy probe work resulted in the speaker dying.

To get things started, I verified the signal coming out of the Sony ICs and followed it to the bottom of the board. Everything was making sense up to the TL071B op amp at UK3. The signal was coming into pin 2, but the output from pin 6 was garbage. It was coming out a -8.8V and there was just the faintest hint of a signal. All the other pins looked normal, so it seemed something had gone amiss with the op amp.

I ordered replacements, so I installed a new TL071B--and the problem remained. Maybe I ordered the wrong part; so I scavenged one off the battery-bombed Foxtrot--and the problem remained. I was now scratching my head, because all the signals and voltages coming into the op amp were correct, but output was wrong.

I pitched the question to ChatGPT and it suggested the problem was coming downstream of pin 6, keeping it from producing good output. I followed its recommendation and removed R41 to isolate the downstream circuit, and sound came out of the speaker! The sound was a bit hissy, but it was something.

So, I reinstalled R41 (not easy given how small it is) and removed Q11 (an NPN). The probem was back. So, I reinstalled Q11 and pulled Q10 (a PNP), and the speaker was working again. So, apparently I had damaged that transistor with my probe. I had ordered new MMBT3906s from Mouser, but I remembered the issue I had with Bravo, and I supected the pin orientation did not patch the footprint on the board. Since this circuit was easier to follow, I was able to figure out that collector and base were reversed on the new MMBT3906 compared with how the board is oriented. Luckily, I was able to scavenge yet another part from Foxtrot, and with its Q10 installed, the speaker sounded great.



In the end, I replaced UK3 (twice), and I had to reinstall nearby C41 when it came off while replacing UK3. I also pulled and reinstalled R41 and Q11, then finally replaced the faulty Q10. I was distracted by the odd output coming out of UK3, but after working my way down the cirucit, I found the shorted transistor.

 

[RetroViator]

Bravo is Beautiful​

Bravo is the system I've worked on the longest (technically, I started working on it in 2021), and it is finally good to go.

It's last problem was bad serial communication. TechStep was reporting a Serial Communication Controller test failure. Most specifically, it said there was a problem with the handshake input. So, I followed the handshake input pin on the jack to UA6, which is a 75175 (a quadruple differential line receiver). This IC is right next to C9, C10, and C11, and the legs did not look great. After some continuity testing, it looked like several of the IC's pins were not making good contact.

I decided to remove it, and yep, it was pretty dirty. It took quite a while to get the corrosion off the pads and fresh solder in place.



Once I reattached the IC, I connected the TechStep, and for the first time, Bravo passed all its tests!



What work remains?

• Figure out why Charlie won't shut down properly with the Power-Up IIusi replacement PSU
• Cobble together one more working floppy from the remaining parts
• Build four more Power-Up IIusi PSUs

 

[RetroViator]

Charlie is Copacetic​

Charlie was working well after the recap, except it would not properly shutdown with Caymac's Power Up IIusi replacement PSU. I followed the power signal on the PSU's pin 9 (J15) back to the Egret chip, a 68HC05, at UB1. The connection seemed good, so the issue was either with the Egret or the PSU.

With an original Magnetek PSU, I was getting 12.2V, 5.11V, and -12.07V. With the replacement IIusi PSU, I got 12.2V, 4.9V, and -11.2V. One interesting thing is I got a flash on all the power rails when I plugged in the PSU.

When I scoped the signals coming from the new PSU, pin 9 had a jaggy sawtooth signal.



I had harvested the wiring harness off an old PSU, and the pins were a bit green. So I depinned all the wires coming from the PSU and cleaned them with a fiberglass pen and some IPA. I also cleaned up J15, which looked rough too.



After clearning, I tested again, and while the shutdown still didn't work, the power rails no longer flashed when I plugged the PSU into J15 and the sawtooth signal was gone. While cleaning helped, it did not solve the problem.

When triggering a shutdown from the Mac's menu, the signal on pin 9 dropped very briefly but then jumped back up. I confirmed that the computer would shut down immediately when pin 9 was grounded, but would immediately power back on when the ground was removed. After plugging this problem into ChatGPT, along with the pertient schematics, it recommended installing a 100k resistor between the base of Q1, a 2N4401, and ground, providing a better base pull-down. As with my eariler post on the IIusi replacement PSU, all the issues seem to flow back to Q1.



After putting that in place, the shutdown worked perfectly! I tested the new PSU on all five of the IIsi boards, and they all worked just fine.

 

[Byte Knight]

I'm really enjoying reading your updates from the BBS using the TinkerDifferent telnet server. I did a ton of BBS'ing with my IIsi, so it seems very appropriate. I can't comment from the BBS since the telnet server is read-only for now, but I'm following with great interest!