Hello everybody !

I am hobbyist trying to resurrect something which is out of my league. But I want to try .....
I got hold of 3 ATEN kvm switches. They are a switching device to switch between multiple computers with only single Keyboard/Mouse/Monitor. All of them have a problem. Their mouse ports dont work. The PC starts but at the BIOS it says "Mouse not found". Other ports do switch keyboard and VGA.

After rebooting many times thinking it might reset the switch I gave up. I tried different mouse models but to no avail. I then checked the voltages at the PS/2 mouse output port, pin number 5 (clock) and 1 (data) were oscillating with 5v max. Pin 4 also has 5v but it is steady. I cross checked from a fully functional PC, whose mouse PS/2 port (pins 5,1,4) voltages were a steady 5v.


I recapped the entire device thinking power quality might be a problem but the problem of oscillating voltages remained. I also check a nearby transistor with a component tester, but it was found to be normal. There is an IC nearby. It has been sanded down so I dont know its identity. But Mr Google search showed me what it might be.I think the chip is PIC16F72. I might have got it wrong but I dont know. It is same 28 pins and the dimensions are the exact same. I checked continuity with a DMM, IC Pin 2 and 3 have a direct line to the problematic PS/2 mouse port (pin 5 and pin 1). Voltage oscillation can be seen here too.
The functional keyboard port doesnt connect to this IC so it isnt affected.

What do you think ? Can the mouse ps/2 ports be made working again ?
Do you think the IC has gone bad ? If its a PIC16F72 and I get a new one, even then without the code its no use.
Can the problem be elsewhere making the chip behave bad ? I couldnt understand what to check next.

Thanks
Prashanth

 

All of them have a problem. Their mouse ports dont work. The PC starts but at the BIOS it says "Mouse not found". Other ports do switch keyboard and VGA.

Did you have a mouse and keyboard connected to the KVM and the port selected to the computer you booted before you booted it?

 

Did you have a mouse and keyboard connected to the KVM and the port selected to the computer you booted before you booted it?

I had the peripherals connected before I booted the KVM and the target computer. I know PS/2 peripherals are not plug & play so I took care of that requirement.
Another point I had to make clear, I powered up the KVM separately with nothing connected and checked the voltages. Pin 5 and 1 of the mouse port, as shown in the attached images above, keep on oscillating. While a PC with ps/2 ports has steady voltages.

 

What do you mean by “oscillating”? Please describe that better.
Have you tried measuring the voltages with a mouse plugged in? Or, if that is the case, without one?
Have you tried measuring the voltages on the keyboard port?

Since you don‘t know what normal behavior is for the device it is a mistake to become fixated on the changing voltages. Instead, just note that fact and keep looking for other possible problems. For example, where do the ports get the 5V in the first place? Does that supply also vary in voltage?

It is easy to get sidetracked by something that seems to be a cause when it is only an effect. One thing I would try is verifying that the electrolytic caps are in good condition. Although they all look fine, depending on the age of the device (and it seems to be 2005 production) electrolytics can be a problem.

By the way, it is possible but very unlikely that an MCU would exhibit such problems. If the MCU was bad, it would probably just not work. If the pins were bad (say ESD damage) it would probably not supply anything.

You need to look further than the first apparent anomaly.

 

Hurray, Today morning I got a different set of ps/2 cables and viola 2 out of 3 switches are working as intended. I think the problem was software and not hardware, as during boot one cable was causing the mouse not to be recognized. @Ya’akov, what you said came true. Problems might lie elsewhere.
But the one I recapped isnt working, VGA/Keyboard ok but mouse port not working. I have some fresh batch of nichicon capacitors, I am going to recap again and try. Thanks to both of you for pitching in.

 

Hurray, Today morning I got a different set of ps/2 cables and viola 2 out of 3 switches are working as intended. I think the problem was software and not hardware. @Ya’akov, what you said came true. Problems might lie elsewhere.
But the one I recapped isnt working, VGA/Keyboard ok but mouse port not working. I have some fresh batch of nichicon capacitors, I am going to recap again and try. Thanks to both of you for pitching in.

The capacitors are low hanging fruit and sometimes are the magical repair færies are satisfied with a capacitor offering but unfortunately that’s not always enough.

 

The capacitors are low hanging fruit and sometimes are the magical repair færies are satisfied with a capacitor offering but unfortunately that’s not always enough.

Hurray again, the third switch is also working now. All switches now are switching VGA/Keyboard/Mouse. The fairies who had rejected my old KELTRON offering now accepted my NICHICONS I changed over to fresh nichicons, mouse started being recognised. I will now recap all the switches to extend their lifespan. The earlier OEM caps were "S.Y" brand and were showing 4% Vloss. They were about to die I suppose.

As a side note I had recapped my DLink network switches with the same KELTRON caps and are now working fine 24/7. Maybe some circuits are too sensitive.

The original problem seemed to be that both cables have to be good only then the mouse will be recognised. Otherwise with only one cable the keyboard (which is the more important device over mouse) will be recognised but with only one cable mouse will not be recognised. Keyboard is essential.

Thanks @Ya’akov you were right once again, You brought my switches back to life
Now back to the workbench recapping all the switches.

Thanks
Prashanth

 

Hurray again, the third switch is also working now. All switches now are switching VGA/Keyboard/Mouse. The fairies who had rejected my old KELTRON offering now accepted my NICHICONS I changed over to fresh nichicons, mouse started being recognised. I will now recap all the switches to extend their lifespan. The earlier OEM caps were "S.Y" brand and were showing 4% Vloss. They were about to die I suppose.

As a side note I had recapped my DLink network switches with the same KELTRON caps and are now working fine 24/7. Maybe some circuits are too sensitive.

The original problem seemed to be that both cables have to be good only then the mouse will be recognised. Otherwise with only one cable the keyboard (which is the more important device over mouse) will be recognised but with only one cable mouse will not be recognised. Keyboard is essential.

Thanks @Ya’akov you were right once again, You brought my switches back to life
Now back to the workbench recapping all the switches.

Thanks
Prashanth

I am glad to hear it worked out. Perseverance pays off.