68kMLA Classic Interface

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Classic II analog board low voltages
Posted by: schrockwell on 2026-09-06 08:11:00 Hi folks, I am restoring a Macintosh Classic II. I have recapped both the analog and logic boards. It was working fine yesterday. Today, it won't chime or boot. I found that the voltages were low: +4.5V, +10.5V, and -9.9V. I tried adjusting PP1 but even fully clockwise, they're not high enough. Some recent changes to the analog board: found CF4 installed with reverse polarity (blown up, that was my bad when I recapped it) replaced with new 470µ 35V; it worked well after replaced stock fan with Noctua NF-A6x25 FLX stopped working after 🙁 I checked all the known voltages on the schematic (below) - the green ones are good, the red ones are not. The biggest red flag to me is +38V measuring at +33V, which is 15% too low. Scaling up 10.5V and 4.6V by 15% would yield 12V and 5.2V which is way closer to being correct. I think this points to a problem with the ZP1 transformer? I reflowed all its solder joints, no improvement. I really can't imagine that I broke anything by simply replacing the fan. Any help or tips would be greatly appreciated. Rockwell
Posted by: Builder68 on 2026-09-06 10:37:45 Hi folks, I am restoring a Macintosh Classic II. I have recapped both the analog and logic boards. It was working fine yesterday. Today, it won't chime or boot. I found that the voltages were low: +4.5V, +10.5V, and -9.9V. I tried adjusting PP1 but even fully clockwise, they're not high enough. Some recent changes to the analog board: found CF4 installed with reverse polarity (blown up, that was my bad when I recapped it) replaced with new 470µ 35V; it worked well after replaced stock fan with Noctua NF-A6x25 FLX stopped working after 🙁 I checked all the known voltages on the schematic (below) - the green ones are good, the red ones are not. The biggest red flag to me is +38V measuring at +33V, which is 15% too low. Scaling up 10.5V and 4.6V by 15% would yield 12V and 5.2V which is way closer to being correct. I think this points to a problem with the ZP1 transformer? I reflowed all its solder joints, no improvement. I really can't imagine that I broke anything by simply replacing the fan. Any help or tips would be greatly appreciated. Rockwell View attachment 103657 I would focus on looking upstream to determine why the supply voltage at pin 6 of the TDA4605 is so low (8.3V).
Posted by: bibilit on 2026-09-07 05:29:40 DP4 (1N4148) and probably DP3 (1N4148) leaky. Common issue on those boards, they look and test ok but are always bad.
Posted by: schrockwell on 2026-09-08 05:56:19 Thanks for the ideas. I replaced DP3 and DP4, no improvement. I also pulled the QP1 optoisolator and it has 2 MΩ across C-E so that doesn't seem leaky either. I started looking at the 5V line with a scope. It has some ringing from 4.4-4.9V and it doesn't appear periodic at all, totally random. Will post a pic later.
Posted by: Builder68 on 2026-09-09 06:12:25 Thanks for the ideas. I replaced DP3 and DP4, no improvement. If replacing the 1N4148 diodes at DP3 and DP4 hasn't restored the minimum 11.8V input on pin 6, you'll need to keep tracing further upstream on the analog board to find the root cause. Keep in mind the TDA4605 relies on an initial kickstart voltage trickling down from the main high-voltage rail to power on. Regardless of what other symptoms the Mac has, restoring that pin 6 voltage has to be your first step. Without it, the IC remains stuck in its under-voltage lockout state and won't generate the necessary output pulses to wake up the board. Check values of resistors RP1 and RP28 next. These dropper resistors take a constant beating and are notorious for drifting out of spec or failing open, which directly chokes off that startup voltage. You should also verify CP4, If it has dried out, it will bleed your startup voltage straight to ground faster than the resistors can supply it.
Posted by: schrockwell on 2026-09-13 11:06:26 If replacing the 1N4148 diodes at DP3 and DP4 hasn't restored the minimum 11.8V input on pin 6, you'll need to keep tracing further upstream on the analog board to find the root cause. Keep in mind the TDA4605 relies on an initial kickstart voltage trickling down from the main high-voltage rail to power on. Regardless of what other symptoms the Mac has, restoring that pin 6 voltage has to be your first step. Without it, the IC remains stuck in its under-voltage lockout state and won't generate the necessary output pulses to wake up the board. Check values of resistors RP1 and RP28 next. These dropper resistors take a constant beating and are notorious for drifting out of spec or failing open, which directly chokes off that startup voltage. You should also verify CP4, If it has dried out, it will bleed your startup voltage straight to ground faster than the resistors can supply it. Thanks so much for this advice. I have checked all these components and found no issues or improvement. RP1 measures 22K. I don’t have a 3W replacement to try. RP28 also tested okay at 47 ohms, but I replaced it with a 51 for good measure. Only a small voltage drop with that, as expected. Now around 8.0V. CP4 was already new and measures okay, but replaced with a 33u/100V, closest I had. Also replaced CP5 and RP5. Feeling pretty stumped right now.
Posted by: schrockwell on 2026-09-13 11:08:43 And just to confirm, the starred voltages on the schematic are referenced to the primary circuit common, which I am taking from the (-) pin of the rectifier DP9. And measuring in VDC, not VAC.
Posted by: schrockwell on 2026-09-13 12:08:52 I finally got some improvement by cleaning the bottom of the PCB with 99% IPA. Must have been some leakage going on there. Got it up to 10V! Did some more cleaning, tried again, and pop, RP2 blew. Not sure what I shorted or how, but time to source a replacement.
Posted by: 8bitbubsy on 2026-09-16 03:12:16 Did you also clean the PCB on top where QP1 is? It's very important this chip is not interrupted by the conductive cap juice, as it's part of the feedback for voltage regulation. There can be electrolyte leakage under and around this chip on the top side of the PCB as well.
Posted by: schrockwell on 2026-09-20 13:53:16 Did you also clean the PCB on top where QP1 is? Yes, thanks, I always take the chance to clean under any removed component. After blowing the fuse and RP2, I located a short between the primary circuit common and the +306V. I suspected QP2 because of this thread, and sure enough when I removed QP2 the short went away. So I've ordered some replacement IRFBC40. I'm still not sure how that happened, but hopefully this will let me continue troubleshooting.
Posted by: schrockwell on 2026-09-27 12:56:42 I replaced QP2 with a new IRFBC40, adjusted PP1, and voilà! We are back in business. No more short, no more magic smoke, and the voltages look good again: -11.1/+5.2/+11.8V. That MOSFET must have been on its last legs. Now it boots into the BlueSCSI just fine again. Whew. Now to figure out why the audio sounds like a falling bomb and this project will be complete.
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