Tuesday, September 2, 2014

Stuck in neutral gear on core memory but made good progress on disk drive restoration


I will have to dive in with an oscilloscope and track down exactly where the failures are occurring, in order to find the failing component. This requires the use of a current probe on the scope, something I don't have. I tried to borrow one from the CHM, but there were only two broken partial probes (e.g. one missing connector,the other missing the mandatory amplifier). Bought a current probe online and will receive it sometime Thursday, so further diagnosis is delayed until this arrives.

To use the probe, I have to remove a jumper block on the core stack, replace it with discrete wire jumpers, place the current probe around the wire to be monitored, and then set the machine into a continuous read/write cycle with the CE Storage Load switch.

Buncha-jumpers - $60 at Frys - to replace molded jumper block to permit current measurement
I did remove all the SLT cards that I had moved in the A1 compartment and reseated them well. I checked all the cables between the compartments and reseated the cable connections over in B gate compartment C1 where the memory is interfaced to the rest of the processor. No change at all.

This set of stuck on/stuck off bit position errors is highly frustrating because I struggle to find any common point that could be causing everything I am experiencing. To explain this by failed cards would mean that seven cards have failed spread across all four sections of 4K words and the two compartments.

A cable failure between the memory (D) gate and the rest of the processor would affect all sections indentically, which is not what is happening. The last things I touched were in the D-A1 compartment but failures showed up in the D-B1 compartment as well as D-A1.


I wiped the heads with 99% IPA and lint free wipes, then look for any signs of damage. As far as I can see with a simple mirror inspection, the heads are in good shape. They are metallic in appearance, thus highly reflective which makes lighting and focusing on them hard.

Disk heads appear to be usable
The brown at the top of the head is a reflection of the nearby structure
I need to clean the drive with IPA and oil as recommended. The main issue remaining is all the dirt from the crumbled sound deadening material and 'protective' foam that IBM used. I will blow it all out, replace the filters, and wipe it down before oiling to ensure I get most of the dust out of the drive.

The main filter is inside a plenum under the drive, and it was filthy, absolutely filthy. As I removed it, chunks of clay like residue fell to the ground, I could see more of it on the plate covering the plenum, and then . . . I saw the filter itself. I will clean and blow dust off this, but I really need a new filter in its place. The IBM parts room is not available and I have not seen this on the market for many years, so a bit of a sourcing challenge.

Crud falling off bottom plate of filter plenum - clay-like material

Dirty enough? Yikes

Plenum bottom cover
I will then let it operate with the blower for a hour total time in order to dislodge and capture any remaining dust. I will need to do a similar cleanup of my chosen disk cartridge - blow out any dust, wipe it down carefully with 99% IPA, then let it sit in the drive with the blower active for about 15 minutes before letting the heads load onto the platter.

I picked up some clean air canisters, as I didn't want to use my air compressor because of the oil that is added in small amounts to lubricate nail guns and other connected devices. Oil sprayed at high velocity is not something I want to introduce into the drive or cartridge. Using those, I blew out everything as much as I could.

Next, I wiped rails, racks, detects, tachometers, spindles and many other objects, including the heads once again. I used some wooden stirring sticks to back the lint free cloth and IPA, ensuring I removed as much oxide and gunk as possible. With the drive as clean as I can get it, it was time to put the light oil and grease where it was recommended.

Mirror to inspect heads from outside the drive, since my head won't fit inside
Here are some more pictures of the disk heads. The mirrored surface just confuses the heck out of the iPhone automatic focusing. For each picture here, there are probably a dozen that were discarded because they were even blurrier. May be hard to believe, but these are the sharpest I could achieve.

Bottom head - the reflection from upper mechanism creates the curved line you see

Upper head shot from below. You can see how close the two heads are, tough to shoot in between

Bottom head showing a reflection of the shape of the upper head

Bottom head, rail behind reflected by upper head to lower head then to you

You get a sense for how mirrored the surfaces are from this picture

While I look for a replacement air filter, the best I could do was to blow out the dust on the filter (blowing from the clean side (inside) outward. It would be better to put in a new filter, but this appears to be good enough to clean the air and still pass enough air volume to meet the needs of the cartridge and heads.

I created the official IBM test tool for checking the volume of air through the filter, doing exactly what they specify in the maintenance manual. I took a blank punched card, folded it in half at column 40 and folding it in half another time at the column 20 and 60 line. Then, I tore off one of the folds, used scotch tape sparingly to hold the sections of the card from unfolding. This is the official test tool.

Official tool to test disk drive filtered air volume
The official tool is placed across half of the air outlet on the top, where the clean air will come out from the filter plenum. If the folded card can sit in place without be moved or blown off, airflow is inadequate.

The 'tool' was put in place, the machine turned on and the result was the card hopping right off. I also took the opportunity to manually move the head far out towards a high cylinder number, to verify that the drive will retract the head to the home position as part of power up. It did exactly that, which tested many functions - the detent stops, the voice call, the tachometer, the movement circuitry, and the power up logic.

The automatic homing of the arm gave me that wonderful buzz that the 1130 disk drives are famous for producing during seek operations. I was almost tempted to stick in a cartridge and watch the heads load and fly, but that would be rash at this point. First, I need to blow out dust and clean the platter on one of the cartridges, then have it sit inside the drive with the 'file switch' off for at least 15 minutes to have pure air blowing out any remaining particles.

I haven't yet decided how to blow out the dust, since the compressed air cans are really cans with a fluorocarbon propellent that evaporates and is claimed to leave no residue, but I don't want to have any coating or interaction with any materials in the 1130 or its cartridges. I need pure air untainted by oil or propellent - might channel air coming through a room fan, with some filter material on the inlet side.

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