Osborne 1B Drive Issue

[Westinghouse]

Hi all,

I recently acquired an Osborne 1b.
It's got both the ScreenPac board and the Double Density board.

I've addressed a number of issues with it already.
Within 15 minutes of my first use, it blew the paper caps and the fuse in the PSU; these were replaced.
I adjusted the display size on the monitor, which was a little overly large for the bezel.

The last issue, which continues to frustrate me, is that the B> drive is not functioning properly.

I've done quite a few things to try to correct this; bear with me as I lay this out.
The spindle motor on B> was audibly slow, and would throw BOOT ERRORs if trying to boot from it, and BDOS errors when accessing B> after sucessfuly booting off of A>.
The motor would actually slow right down to stopping if I let the BOOT ERRORs add up.

I opened the Osborne and swapped around some components.
Let's call the drive PC boards 'A Board' and 'B Board', and the other drive hardware components 'A Chassis' and 'B Chassis'.

A Board + A Chasiss -- works in both A> and B> positions.
B Board + B Chassis -- fails in both A> and B> positions.
A Board + B Chassis -- works in both A> and B> positions.
B Board + A Chassis -- fails in both A> and B> positions.

I don't have an oscilloscope, but I took voltage readings from several sites.
Voltage is generally as described in Sam's Computerfacts, with the following exceptions:
- Supply to R24 is constant and at the expected voltage, but on the other side of R24 the voltage slowly drops.
- Supply to Pins 6 + 7 on U10 (the 555 timer) slowly drops - this is downcircuit of R24.
- Voltage on Pin 3 of U10 slowly drops.
- Voltage on Pin 9 of U18 slowly drops.
- Voltage on the anode side of D7 slowly drops; cathode side is correct voltage and constant.

The voltage drop eventually, over a series of minutes, gets to the point where the spindle motor stops.
It seems to have a cool-down period; leaving it alone for a few minutes gets the voltages back up somewhat; a longer period will get it back to initial conditions -- but under load, the drop always happens.

I've replaced a number of the components in these areas, to no effect.
In the attached image, the components that I have replaced are shaded red.


If I reduce the resistance at R23, the spindle will start off too fast, but after slowing down a bit, the drive will stop throwing BOOT ERRORs and actually boot to a prompt; this usually only lasts long enough to execute a DIR command before the spindle motor becomes too slow. This leads me to believe that the stepper motor part of the circuit is fine.

The gradual voltage dropoff and associated cool-down period have me thinking it might be a component heating up, but the only element which seems to show any appreciable temperature increase is the TIP110 riveted to the heatsink and PCB.

I'd be really appreciative of any advice, as I'm kind of stumped.

 

[Westinghouse]

Over 100 views with no responses makes me wonder whether I did a poor job of explaining the problem, or it's just such an odd problem that no-one has any insights.

If anyone has a follow-up question, or wants more information, please -- let me know!

 

[modem7]

Welcome to these forums.

The voltage drop eventually, over a series of minutes, gets to the point where the spindle motor stops.
It seems to have a cool-down period; leaving it alone for a few minutes gets the voltages back up somewhat; a longer period will get it back to initial conditions -- but under load, the drop always happens.

Definitely sounds like there is a temperature sensitive (beyond the norm) component somewhere.
Have you thought about using freeze spray (for electronics) to quickly identify the problem component.
My money is on C21.

 

[ldkraemer]

A good logical approach would be first verify all the DC Supply Voltages are within the specifications. If all the voltages are proper, then start looking for AC ripple
on the DC Supplies. That should give you some clue if the filter caps are working properly. It would be even better if you had a good computer to compare
with your problem child.

It's likely some bad caps in the Power Supply, but no amount of guessing or Easter Egg'in will likely find the problem. It could also be a cold solder joint on or around
the Power Supply. It's going to take a logical troubleshooting process.

Larry

 

[modem7]

A good logical approach would be first verify all the DC Supply Voltages are within the specifications. If all the voltages are proper, then start looking for AC ripple
on the DC Supplies. That should give you some clue if the filter caps are working properly. It would be even better if you had a good computer to compare
with your problem child.

It's likely some bad caps in the Power Supply, but no amount of guessing or Easter Egg'in will likely find the problem. It could also be a cold solder joint on or around
the Power Supply. It's going to take a logical troubleshooting process.

Note that the OP has already addressed the initial issues, and has logically deduced that the final remaining problem, a faulty floppy drive (of the two present), is caused by the logic board on that drive.

 

[Westinghouse]

Oops; I replaced C21 in my first round of substitutions, but I see that I didn't colour it in my markup.

Thanks for the suggestion regarding freeze spray; I'll give that a shot. Won't be for a while, though - busy time of year.

 

[Westinghouse]

Hate to leave a loose thread, so I thought I'd close this out by saying that I never solved the issue.

I tried applying freeze spray, but never got any positive results.

I ended up buying another cheap beat-up Osborne with two good drives (but no keyboard or DD board), and replacing my bad B> drive with one of those.
The replacement known-good drive worked fine, so it was certainly some issue with the board on the faulty drive.
I ended up passing the donor Osborne (and the faulty drive) along to someone else, so the problem with that drive is not something that I will likely ever know.