Combating SE / SE30 voltage drop

[zigzagjoe]

So: Much to my surprise, the SE logic board harness (from the analog board) uses 22AWG for all wires, even the two +5v leads (orange).

In an accelerated SE / SE30, you could be pushing 6 amps over these two wires, which leads to nasty voltage drop! With the original PSUs, you can at least adjust the voltage to compensate, though your 12v line may be 13v when you're done.

However, on a seasonic PSU where you can't adjust the voltage? It's a real problem. In my SE/30, I was seeing around 4.72v at the LB floppy port with a Carrera clone, Asante MacCon, grandvimage card, HDD, and seasonic PSU. Somehow this was stable, despite visual noise on the GrandVimage's output.

Solution is to replace the 22ga +5v wires with 16ga wires. Happily, molex makes a near-exact replacement, gold plated, too! Just a hair longer than the original. After doing so, that same machine is now sitting at 4.85v - a 0.13v improvement! While it's still not ideal, it's a heck of a lot better, and importantly drastically reduced the noise on the external video output.



Molex P/N : 2153402223 , 225mm 16G Red cable w/ gold mini fit jr socket terminators.
Digikey stocks it for $1.39 per: https://www.digikey.com/en/products/detail/molex/2153402223/13144614

You can remove the wires from the original connector with a tactically modified paperclip (pictured above), or the original Molex 11-03-0044 tool.

 

[obsolete]

Ooh, that's a slick mod. I didn't know these pre-crimped jumpers were available. Will be throwing some into the next Digi-Key cart for sure.

 

[ymk]

Nice work. I couldn't get my SE above 4.65V with the pot pegged, so I added a resistor to push the adjustment range upward.

It's now at 5.0V / 12.6V with no card installed.

For those with power hungry expansion cards, a buck converter at the LB or card starts to make sense.

If you want to increase the voltage further, is there a reason you can't do the same with the ground wires?

 

[cheesestraws]

If you want to increase the voltage further, is there a reason you can't do the same with the ground wires?

When I've had to replace the interconnects I always replace them with rather chunkier wire both on power and ground wires - it definitely helps with the voltage, but I can't quantify how much changing all of them makes over just changing the +ve ones,

 

[zigzagjoe]

Nice work. I couldn't get my SE above 4.65V with the pot pegged, so I added a resistor to push the adjustment range upward.

It's now at 5.0V / 12.6V with no card installed.

For those with power hungry expansion cards, a buck converter at the LB or card starts to make sense.

If you want to increase the voltage further, is there a reason you can't do the same with the ground wires?

That's a crazy amount of voltage drop - I think you might have something else going on beyond just voltage drop over the wires. What voltage do you see at the AB to PSU connector?

While there's no reason you couldn't do so with the grounds, it has 6 of them on the LB connector, so I'd expect much less voltage drop there even at 22awg. I haven't measured it though.

The other idea I was entertaining - for retrofit PSUs only - was running a couple more +5v leads directly from the ATX unit to double-crimped wires at the LB connector, which would mitigate voltage drop from the 2 minifit connectors on the analog board and 2 18 gauge +5v lines on the original PSU out.

 

[ymk]

That's a crazy amount of voltage drop - I think you might have something else going on beyond just voltage drop over the wires. What voltage do you see at the AB to PSU connector?

I remember it was low right at the PSU, so the drop over the wires wasn't outrageous. If the source measures <4.75V, no amount of wire is going to help.

The PSU is fully recapped and after the resistor mod, has been working very well.

My theory is, these PSUs expect a certain load on 12V and without it, the 5V line sags.

 

[zigzagjoe]

I remember it was low right at the PSU, so the drop over the wires wasn't outrageous. If the source measures <4.75V, no amount of wire is going to help.

The PSU is fully recapped and after the resistor mod, has been working very well.

My theory is, these PSUs expect a certain load on 12V and without it, the 5V line sags.

Agreed, the problem is definitely low output at the PSU. Sounds like you've got it sorted though.

Modern group regulated PSUs do have that issue: 12v is the primary regulated rail on group regulated PSUs; the other rails track 12v and go out of tolerance if there's not enough load on 12v. But, at least the Sony and Apple Singapore SE PSUs don't have that issue (of 5V sags with no 12v load) - it's the opposite, 12v tends to go out of range especially if +5v requires adjustment for voltage drop and/or you don't have a HDD installed.