It is certainly difficult to debug this kind of problem.
Try getting output from hid_listen when it happens, it may be helpful.
Also OS system logs would be very helpful but I don't know how to get them on Windows(I assume seeing you said 'chime'). If the problem also happens on Linux check logs under /var/log/ like kern.log or syslog, you may find USB errors or something.
The converter firmware can do something perhaps but I don't have definite idea.
I'd think of aspect of hardware before tweeking firmware.
1. ProMicro bootloader starts first and then runs converter firmware at power up. Meanwhile Teensy and TMK converter runs firmware immediately without bootloader intervention. This may cause the problem.
If you have Teensy or TMK converter try it and see difference.
2. Some keyboards draw more rush current on plugin, this can make microcontroller unstable and causes Brownout-reset in the result. This doesn't matter in most cases because it is recovered with brownout-reset.
This can happen also on TMK converter and Teensy when plug Model F with converter already powered from USB port. To prevent/mitigate this connect keyboard with converter first, then plugin them USB port. Use decent USB cable too.
Try other keyboards and see difference. Soldering big enough capacitor like 100uF or more on ProMicro may prevent this.
3. Check USB cable, keyboard cable, wires and soldering points. Unstable connection may cause unexpected result.
Hasu,
I just converted a Northgate Omnikey 102 from PS2 (scan code set 2) to USB, and I'm having an issue. Specifically, When plugging the keyboard in, I get the "device connected" chime, immediately followed by the "device disconnected chime," and the two back LEDs on the pro-micro go red and nothing works.
TMK PS/2 firmware doesn't control any LED on Pro Micro. What those LEDs show when working normally?
I guess bootloader controls the LEDs.
Pro Micro seems to have various version/configurations from various vendors/manufacturers and this makes really difficult to support people with them in trouble.
Note that this doesn't happen every single time, sometimes I'll plug it in and everything will work normally, or I can also get it working by resetting the pro micro a bunch of times.
How often does the issue happen?
Actually, testing this some more just now, it seems like the only way to reliably get it working is reset the pro micro (at least once, sometimes more) by bridging the GND and RST pins.
I believe I'm having the same issue on another ps2-usb conversion I did on an RT101+, I think the behavior is the same.
Bridging is not reliable from my experience. I also bridge the pins with tweezers and know it doesn't work sometimes. You should solder push button perhaps.
Post detailed pics of both keyboards and Pro Micro so that we can know how your keyboard is converted. That would be useful to help you.
EDIT: And try Soarer's converter and see difference.