Kaypro II DOA

[KC9UDX]

The seller very clearly displayed it not booting. That is, it was instructing the user to insert a boot disk.

When it arrives, it doesn't power on. What would you do? Do I raise a stink with the seller or take advantage of the situation and delve into repairs?

 

[KC9UDX]

The mainboard has a 5V short...

 

[ldkraemer]

My opinion would be to remove the 10 screws, remove the top, and pull a Floppy Power connector and have a look at the +5 & +12 Voltages.
It could just be a Power Supply problem or fuse for Power Supply. If the Power Supply Voltages are proper, then you need to check for a
Clock Pulse at the Z80 CPU along with the other Control Signals that would prevent the CPU from starting the boot process.

Pin 24 /WAIT
Pin 16 /INT
Pin 17 /NMI
Pin 26 /RESET
Pin 25 /BUSREQ

Pin 6 CLOCK
PIN 11 +5.00 VDC
Pin 29 GND (Power Supply Common)

That should tell you what is going on.

Larry

 

[ldkraemer]

+5.00 VDC short somewhere on the Motherboard traces. I'd start by pulling all the socketed IC's and see if any of those
are the culprit. It's likely a Capacitor or an IC somewhere in the Motherboard. Don't forget to unplug a Floppy drive (leaving
one as a load for the Power Supply), then if the Floppy isn't the problem, switch the other Drive in. That will prove that
both are likely not the problem.

That short isn't much of a problem if you have a Lab Power Supply that will output .3 VDC at 200 mA, and a Meter with a 200 mv scale.
Or you could make a Circuit Short Detector, or Continuity Tester that I've been researching.

Send me a PM with an email address.


Larry

 

[KC9UDX]

It may not be a short. I used a better ohmmeter. Now my kitchen table looks like my bench.

5V to GND on the mainboard is 14ohms, which may well be normal. I'm guessing the floppy drives have a separate 5v supply because they always have 5V. Whenever the mainboard is plugged in 5V at the mainboard turns to 200mV. So it is probably a resistive path between the mainboard and the power supply.

Likely a cold solder joint at the power supply. That's a reasonable thing to change in transit!

Larry, I'm good on QRZ and an ARRL member with that kind of email can't PM very well from here (phone)

 

[KC9UDX]

Ok the seller is honest and true, the sale was fair.

It is indeed a, or rather multiple cold solder joints at the power supply connector. A little wiggling and the machine does what I expected. I guess I get to pull the mainboard again and pull the power supply. Not the end of the world.

Now I get to figure out if I have a bad A drive or two bad disks.

Thanks guys for the suggestions! I apologise for jumping the gun.

 

[tingo]

Don't worry about "jumping the gun" - for me (and probably others) it was an interesting question and story.

 

[KC9UDX]

Thank you.

I have yet to reflow the bad solder joints, but after careful examintaion I can tell you they are visibly separated. With all the cold solder joints I've been finding lately, and the fact that all my rolls of 25year old solder are going bad, I'm thinking I need to buy some eutectic solder for the future's sake.

I'm patiently ( thus far ) waiting for a boot disk to arrive from an eBay seller.

 

[KC9UDX]

I finally got round to doing something with this, and have repaired the cold solder joints.

Now, I have an intermittent disk drive problem, or so it seems.

I could not get it to read a disk in the A drive, so I swapped drives. It will sometimes boot just fine from that drive. But, it seems like it only works if I hold the drive up sideways. Even then it's intermittent.

So either I have a bad cable (I think this is unlikely) or a misaligned drive.

 

[KC9UDX]

Both drives (mechanical) and both drive boards act intermittent in exactly the same way. I did have to adjust the RPM on one drive, and I probably knocked the other one out of alignment upon finding the clamp on the seek motor spindle loose.

Unfortunately, not having a known good drive or known good mainboard to compare with, I'm going to have to assume the drives are good and there's a problem on the mainboard. So, I guess it's off to the mainboard with the scope tomorrow now that I found a schematic tonight.

 

[KC9UDX]

I couldn't find anything electrically wrong, so, I put the scope to the one with the loose seek motor clamp, and moved the head around.

I was able to get it to work. But, with the boot disk that came with it, I have to misalign the head to get it to boot. That is, if I move the head toward the front of the drive to get the first two tracks to read, but to read the third track, the head needs to be bumped toward the back of the drive. I thought maybe the stepper motor was moving in a non-linear way. But,

I realigned the second drive and was able to duplicate the results. So, the disk was written with a drive with a non-linear seek.