Mouse port 1 drops voltage

[lyzanxia]

I recently bought a a500+ motherboard from someone on the internet.
Ofcourse there were some repairs done to it because of the battery leakage (not by me - by tmb says on the motherboard).

Everything seems to work fine, except in port 1, my usb mouse with adapter does not seem to work. A "normal" amiga mouse works fine.
The usb mouse works fine on my other amiga 500 and 1200.

If I connect a "normal" mouse in port 1 and the usb mouse on port2 , they both work in diagnostics. If I connect the "normal" mouse in port1 , start the diagnostics and afterwards plug in the usb mouse in port1 , it also works.

Seems as if the initial 5v drops to 4.8v when booting up with the usb mouse connected. When measured on my other a500, it stays at 5v.

Any idea where this voltage drop occurs, and how to fix it?

Thanks for all suggestions!

[xArtx]

There’s a current limiting resistor in series with the 5V supply at each of those ports. Can’t find them now because the Amiga schematics site is down... One of those might be duff. As a connected device draws more current front he port, the voltage should drop.
If both of those resistors are fine, both ports derive power from the same rail after that, so it could only be PCB trace from the offending port that makes any difference between the two ports.

[Shadowfire]

Back in the day when this machine was made, the spec for 5V rails was usually +/-10%, and on some of the later stuff, +/-5%. Seeing 4.8V with a load means there is nothing at all wrong with the Amiga itself.

Your symptoms are more suggestive of a problem with your USB mouse/USB adapter choking on computer powerup. Basically, when you first turn the machine on, the voltage rises from 0V to 5V over (typically) several tens or even hundred milliseconds. When you plug the mouse in after the machine is on, the rampup to 5V is on the order of microseconds. Many microcontrollers have the poweron rise times specified in the datasheet, most of the time they have a maximum poweron (Vdd/Vcc) rampup time specified, sometimes a minimum is spec'd too. (Modern micros have watchdog timers and brownout reset circuits to deal with these kinds of faults, but on the older stuff this will cause powerup failures when they are exposed to too-slow rampup conditions.)

The only thing I think you can really do in this situation that *might* help is to recap your power supply's output filtering capacitors.

The current limiting resistor on the 5V pin seems to be 4.7ohms per my schematic. A 0.2V drop suggests that the unit is drawing around 40mA of current. In the old days, the +5V power line was meant to only power your paddle controllers, the 4.7ohm resistor was insignificant compared to the 1meg pot, but limited the current (if you had a short circuit, as could happen if you accidentally severed the cable) to an amp, which would probably collapse the 5V rail powering the machine (causing it to stop working), but prevent any kind of fire as the resistor would overheat and open up if the short was left in place.

(The USB spec permits a current draw of up to 1/2 amp. If you were to connect such a device to the Amiga, this would drop the apparent voltage on the port to 2.65V, and I seriously doubt that anything would work at that voltage. The Amiga mouse adapters rely on the USB mice to have a suitably low power draw so that they can still operate at whatever voltage it drops down to.)

[xArtx]

Originally Posted by Shadowfire

Back in the day

Not disagreeing with anything you said, but the OP said the mouse consistently works with one port, and not the other.
The only difference between the supply between those two ports IIRC are the two individual resistors, and of course the length of each port's PCB trace to the 5V supply (also forming another resistor).

I suspect a straight wire (perhaps suitably fused) between 5V rail and the port would fix it, or a slightly lower value resistor.

[lyzanxia]

Thank you guys for thinking with me on this problem!

I did some more tinkering..some more info..

Forgot to mention: this is a recent power supply from Individual Computers (ca-psu)

Measured the e401 emi, resistor r401 same values as on my other 500+ board, swapped gary+denise+paula, socketed u15+swapped.
Am testing with diagrom from Chucky. Only difference I see between boards is the one has a 8375 agnus and this one a 8375r0.

If I start diagrom with the usb mouse plugged in in port1 , it does not work. But if I pull the mouse out and hot replug it again, it works fine ??!! Tried with 2 different usb mouses same model usb adapter, and 2 ps2 mouse adapters+mouses.

Also swapped emi401 and resistor r401 from a known working board, no fix.
I bridged r401, I now measure a stable 5.0v from off, to power on on the 5v pin with the mouse attached, but even this does not fix it. As suggested I tried 5v straight to mouse voltage pin also does not fix it.

Don't know what I can try anymore now? I have a recap set on the way, will try this anyway.

Hot replugging mouse does fix it...mouse + adapter work on another rev8,rev6,rev5 board

[lyzanxia]

Fixed: it was a bad socket connection from Paula. When pushed down, it worked. New socket.