VOLTAGE DIFFERENCES among SE & SE/30 Analog Boards

I have 1 Analog Board that came with the Mac SE @Kai Robinson very graciously sent to me in the past (from the UK), and I also have 3 other Analog Boards that I use with my SE/30 machines. All Analog Boards have been recapped, trace-repaired, and have the same Noctua 60mm fan installed. The SE has a recapped SONY PSU inside, with POT set to max by me. Motherboard is the SE Reloaded board I built myself, with 4MB RAM and WarpSE installed — working perfectly.

I tested all 4 Analog Boards inside my Mac SE (from Kai) last week. All 4 produced a perfect picture on the CRT. But the stock "SE" edition Analog board (Euro edition) shows the highest voltage at the external floppy drive connector, while the other 3 boards showed the exact same "lower" voltage.

Mac SE with Euro edition SE Analog Board (5.16V)



Mac SE with 3 different US edition Analog Boards (5.04V)


I am located in Japan, so AC power into the SONY PSU was 100VAC 60Hz.

Note that the main wiring harness cable connecting the Analog Board to the Motherboard was beefed up by me with 16AWG wire to ensure the smallest voltage drop.

5.16V - 5.04V = 120mV

120mV is a large difference, especially so when you consider that all three Analog Boards from my SE/30 machines showed the same exact 5.04V.
Have any of you have seen such a difference among your own Analog Boards before when tested on the same machine?

GreenBar0n said:

For clarification, are you using the same recapped Sony PSU, with your (4) different analog boards?

Click to expand...

Yes.

Everything is exactly the same in each of the four tests except the Analog Board.

You are asking about the 74LS38 VCC Pin 14 voltage, but I don't have the EU edition Analog Board currently installed right now to test. Even so, before I do any board swap to perform such a test, note that the voltage levels at the Analog Board are higher than what my Mac-O-Meter shows at the external floppy drive connector. The reason is because there is resistance in the analog board traces, more resistance in my upgraded 16AWG main wiring harness cable, and more resistance in the motherboard traces. So if I remove the current A.B. and swap in the EU edition A.B. and then measure at 74LS38 VCC Pin 14, it very well could be 5.26V or slightly higher.

For everyone else reading this thread, you may be wondering...

Why does this even matter?​

ANWSER: Because, especially on the SE/30, "low voltage on the 5v rail" is a real world issue for many, especially if you have upgrades, and absolutely if you have the untouched stock main wiring harness.

In the past I was getting on order of 4.65V at times on my tricked-out SE/30 machines. Maxing out the voltage POT inside the SONY PSU and beefing up the main wiring harness wires to 16AWG largely eliminated that.

But now that I have discovered a key difference between the EU Analog Board and my 3 US Analog Boards, it would be good to nail down the cause. Why? Because some of you out there use the Baby Face or similar modern PSU which uses Meanwell MODULES that have fixed voltages that cannot be cranked up like you can on the stock SONY PSUs. No need to do that for anything but the 5V rail, but still. It is what it is. And so, if you use a MEANWELL MODULE type modern PSU replacement, you'll get a rock solid 5.0V when measuring at the Analog board, but when you measure at the motherboard like I am in this thread (using a Mac-O-Meter for convenience), you will get lower than 5V, and it could be quite a bit lower if you have a lot of upgrades, and shockingly lower if you've not upgraded your main wiring harness.

So if we can determine what on the US boards are drawing more power than the EU board (which would explain the voltage difference of 5.16V versus 5.04V), maybe something on the Analog Board could be changed to eliminate that voltage drop, thereby giving the user a higher voltage level on the 5V rail, which again is what you want if you have a lot of power hungry upgrades.

GreenBar0n said:

Curious what the 74LS38 VCC Pin 14 voltage is of the EU board, is it 5.16V there and also the collector of Q2?

Click to expand...

Sorry for my delayed response.

On one of my SE/30 Analog Boards (all 3 of which report 5.04V at the FDD connector via Mac-O-Meter), I measured the following tonight:

• Collector of Q2 is 19.68V.
• Pin 14 of U1 (74LS38) is 4.878V

Sorry, but I didn't have time to do a board swap to test the SE/30 A.B. (which shows 5.16V on Mac-O-Meter).

Wow, that’s pretty neat.

I must assume that in all of your machines you are using the stock wiring harness, whereas my harness has been upgraded to 16 gauge wire. Wire thickness has a rather dramatic effect on the voltages you will read at the external floppy drive port.

Another important consideration is that my recent voltage measurements presented in this thread have been made with the small POT inside the Sony PSU set to its maximum setting.

I normally leave the white vinyl cover on the solder side of the analog board off so that I can more easily measure voltages at the very back of the main connector where the main harness will connect on the flipside of the board. And almost always, that voltage level is going to be higher than what you’re going to measure at the external floppy drive port on the motherboard due to resistivity losses in the main wiring harness and in the motherboard itself. And if you have a lot of upgrades installed, that voltage measured at the external floppy drive connector will be lower still. All of that makes logical sense electrically speaking because it’s basically Ohm’s law, where you’re running all of that power through the main wiring harness and motherboard traces that have a small amount of resistance. But that will affect all four of my analog boards and not simply just one of them.

Even when using 16 gauge wire, there are still resistivity losses because the resistance even in that thicker wire is not absolute zero. But I always find there’s a slight voltage increase, when measuring at the motherboard’s external floppy drive connector, when I use the 16 gauge wire upgraded harnesses.

Due to the thickness of the insulation on the main wiring harness, I always thought the actual copper wire thickness to be 18 gauge, but in fact it’s only 22 gauge. That’s why there’s a pretty good difference when you upgrade your main wiring harness with true 16 gauge copper wire.

I am using straight out of the wall 100 VAC 60 Hz power here in Japan, and the exact voltage is going to vary a bit so it may be 103 V or even 105 V or even as low as maybe 98 V at times.

But despite that lower AC voltage from the wall socket, the fact remains that the voltage measured at the external floppy drive connector on the motherboard is different when I compare my stock SE analog board to my three analog boards that came from my SE/30 machines. And the reason I am currently testing with the Sony power supply’s POT set to Max is because if I don’t set it to Max and then test my 3 SE/30 analog boards, the voltage I measure at the motherboard’s external floppy drive connector will be less than 5 V. I was actually measuring 4.90 V originally, with my SE/30 analog boards installed, and that’s why I decided to boost the voltage inside the Sony power supply to bring those up to 5.04V, but that results in my stock SE analog board measuring 5.16V at the external floppy drive connector using Mac-O-meter.