LM311 with weird output when "idle" in a Commodore 1541 floppy

[trondl]

I'm repairing a Commodore 1541 floppy drive (Alps) with multiple toasted logic chips as a result of a possible catastrophic failure on the 5v line (dumpster dived).
Body count is 6 chips and some trace repair from previous "attempts" by previous owner(s) so far.
All essential logic is repaired and now working to the point where the computer section, communication over serial and head/motor movement is responsive.

The still not working part is getting the data from the read/write heads to arrive in a correct state to the computer section of the drive.
The drive refuses to read, write or format any floppy I've tried (Yes, they are DD, not HD)
Also, all C64 1541 diagnostic software I've tried cannot read a valid RPM detection from the drive.

I do get triangle-ish waves appearing on the outputs on the video amplifiers UF3 and UF4 NE592 (Top right in the schematics) from the r/w head when attempting to read a floppy with verified data on it (written on an identical drive), so I assume that the head and amplification of the signal is OK.

Next in line is the comparator LM311 UE4 that when idle (nothing on the inputs), continuously outputs something that looks like a square wave with a slow rise time, almost a rising sawtooth with a soft curve at varying length.
This goes away when data is read from a floppy, and the pulses then looks more like genuine data with shorter and longer square-ish pulses.

Should the LM311 continuously output something when there is no "data" on the inputs?
Could this be the reason why no valid data is decoded?
The 5V line is only between a 510ohm resistor to the output.
A hint?
The signal is also present when UD3 74LS86 XOR is removed.

It is worth mentioning that UD4 9602 (monostable multi vibrator) originally had a short on pin 7 to ground and was replaced with a "genuine" ebay chip.
UD3 74LS86 XOR is a Fairchild brand with almost the same vintage as UA1 74LS14 that was toast on top left and right gates.
UD3 was verified as functional with my Minipro TL866, but its output on both gates to the 9602 looks more like random spikes that actual pulses.
The other two gates on UD3 is used for serial communication which is functional.

Should I consider LM311 and potentially 74LS86 toast as well?

Edit: also worth noting is that the plug from the r/w head to the mainboard was inserted in reverse to the header when I first opened it.
Could this possibly have caused anything?

[Peabody]

Is there also nothing on the ouput of the LM311?  The circuit shows the pin 7 output is the open collector, so there's nothing in the chip that would bring the output high.  So maybe something else connected to  the output is going high and low.

I was recently working on a Commodore +4 project, and found that they now have the Pi-1541, which makes an SD card look and behave like a 1541.  Around $30 I think.  It's a Raspberry Pi Zero with a hat that has a display showing which drive image is mounted at the moment.  Pretty cool stuff.

[Bud]

Next in line is the comparator LM311 UE4 that when idle (nothing on the inputs), continuously outputs something that looks like a square wave with a slow rise time, almost a rising sawtooth with a soft curve at varying length.
This goes away when data is read from a floppy, and the pulses then looks more like genuine data with shorter and longer square-ish pulses.

Should the LM311 continuously output something when there is no "data" on the inputs?
Could this be the reason why no valid data is decoded?
The 5V line is only between a 510ohm resistor to the output.
A hint?
The signal is also present when UD3 74LS86 XOR is removed.

That appears to be the correct behaviour. LM311 is a differential comparator. In idle mode both inputs are at the same potential through the pullup resistors and the output is undefined. What you see is the comparator triggering on noise. To verify you can short on of its input to ground - the output should go to a steady level.

[trondl]

I was recently working on a Commodore +4 project, and found that they now have the Pi-1541, which makes an SD card look and behave like a 1541.  Around $30 I think.  It's a Raspberry Pi Zero with a hat that has a display showing which drive image is mounted at the moment.  Pretty cool stuff.

A pi-1541 wouldn’t pose a challenge.
I’m reviving this drive to gain knowledge, not the easy path.

That appears to be the correct behaviour. LM311 is a differential comparator. In idle mode both inputs are at the same potential through the pullup resistors and the output is undefined. What you see is the comparator triggering on noise. To verify you can short on of its input to ground - the output should go to a steady level.

Thanks for verifying its behavior!
One chip less from the suspect list.

Now I’m suspecting the 74LS86 XORs output.
Maybe my DSO isn’t fast enough, but the output from both gates relevant to this section is very far from clean square pulses. More like very short spikes.

[trondl]

Old news, but want to close this case.
The "new and very genuine" UD4 9602 MMV was the offender, making pulses not within spec.
Bought five of them and the other four worked as expected.
Solved!