Cleaning the disk drive, a disgusting and labor intensive task, which is best done on the workbench

RODENT EXCRETIONS CAUSED SIGNIFICANT CORROSION IN THE AIR PLENUM

The compartment inside the drive that pulls outside air through the SLT card compartment and forces it through a HEPA level filter that directs the air into the disk cartridge. That air flows around the platter and heads. 

The plenum was apparently a very attractive home for the rodents, because they filled it with bedding and produced a substantial amount of urine over the course of their residence. Add in some nut shells and you will imagine what it looked like inside the compartment. 

SOAKING AND WIRE BRUSHING ONLY REMOVES SOME OF THE SLUDGE

I soaked the plates, which were the floor of the rodent residence,  in Simple Green for hours but the wire brush had little effect. After I got the first few layers off with 409 and paper towels, we got down to this brown cement that appears to be a mixture of rust and animal droppings. 

Once nothing more would come off, I decided to chemically treat it (Naval Jelly) to cover the rust part to a protective material. When that was done, I closed up the chamber for the time being. 

DID QUITE A BIT OF CLEANING SO I CAN GET THE DRIVE CLOSED UP FOR MOVEMENT

I worked for a few hours cleaning everything I could on the drive. The blower motor was detached but I have to unmount the blower blades from the motor shaft to get in and really clean it. The rust (from the acidic animal products) is making it a challenge to get these apart. I may need a gear puller or similar tool. 

I then put the drive back together, almost fully, to protect it from any damage while being moved but also to ensure that parts don't meander somewhere hard to find. I also closed up the 1130 for the same reasons, so that it is ready to be moved a short distance away. 

I have just about run out of time at the shop for a while. I have a long day volunteering at the Space Force Museum tomorrow, several groups touring the museum and hanger C but with the added complication of the last Delta Heavy launch bringing crowds and tightened security

Then on Wednesday I begin my travels up to New Jersey for the VCF East conference. My first time using the Autotrain, as I normally enjoy driving and like the control I have over my own schedule. 

After I get back mid next week, I have two sets of visitors which will command a bit of my time. Then there is the move to the new shop. 

Replaced two capacitors in the +3V regulator card due to high leakage

CHECKED ALL CAPACITORS FOR VALUE, SHORTS AND ESR

In order to check the capacitors, I had to lift one lead from the circuit to isolate them. I carefully unsoldered one side of each capacitor and made measurements. All of the capacitors had their rated value, a testament to the high quality parts that IBM used in their mainframes. No shorts on any of them. 

What I didn't like was the relatively low resistance of two of the 100uf capacitors on the card. While they were kind of marginal in general, I suspected that this circuit might be more sensitive to having a resistor effectively soldered in parallel to the capacitors. 

I replaced them with two new electrolytics of the same capacity and a higher voltage rating. I am not convinced that this alone was enough to repair the regulator, but it can't hurt. 

PULLED OUT IBM 028 TRANSISTOR AND CHECKED ON THE CURVE TRACER

The voltage regulator uses a cascade of transistors, each stage amplifying the current so that ultimately the regulator can drive 20A of current. The final stage consists of four 108 transistors in parallel. The stage which drives the last one has a single 108 transistor. All of those had been tested on the curve trace a few days ago and were good. 

I moved down the chain to the IBM 028 transistor which drives the single 108. That was removed from the board and the curve tracer showed that it was good as well. All the semiconductors left to check are three transistors, two IBM 026 which form a comparator between the actual voltage and the reference voltage, plus an IBM 086 that drives the 028. All the transistors are germanium; the 086 is NPN while all the others are PNP type. 

Cosmetic adjustments, door repair, some reassembly of 1130 system prior to shop move

REAR COVER FASTENER

One of the rear covers of the 1130 was missing the ball stud that holds it closed when that stud presses between two spring loaded rollers on the frame. I made a substitute with a bolt and two nuts. 

ADJUSTED DOOR ALIGNMENT

The two rear covers sagged down at the point furthest from the hinges and the replacement ball stud I made was entering low on the spring loaded rollers. I adjusted the mounts and now the two doors fit well, with an even gap from top to bottom where they meet. The cover with the ball stud latches properly and the other door closes easily with its latch handle.

INPUT-OUTPUT DEVICE CABLE PANEL REINSTALLED

A panel that has the connectors for the cables that would run to the printer, card reader and other peripherals was put back into place. I had removed it to get access while I was replacing all the rodent chewed wiring and inspecting all the other wires. 

SLIDING GUIDE FOR TOP COVER BROKE WHILE ATTEMPTING TO UNBEND IT

The top power, which hinges at the right side of the machine, had a brass sliding guide that would keep the cover stable as it was raised and lowered. This guide had become stuck at some point in the past, at which time some ham-fisted person bent and partially snapped the guide. 

The brass slide was twisted, bent, broken at one point and quite deformed. I tried to gently unbend all the areas and restore it to a workable condition. I oiled the joint through which the guide would move in order to make it work properly.

The guide had a slot cut in its length, so that a screw on a mount would hold the slot in position as the slot slid up and down. One side of the slot had a break in it where the material had snapped, and the corresponding other edge was pretty mangled. While I was working on flattening the bends and twists at that weak point, it broke just like the other side had. 

Later in the restoration, I will have to manufacture or buy a metal part with a compatible slot and width. For the time being, the cover does open and close it is just missing the guide. 

Higher accuracy reading of zener diode; preparing system for movement to new shop

DID A SIDE BY SIDE COMPARISON OF NEW ZENER TO THE ONE IN THE REGULATOR

I grabbed a zener diode from my parts cabinets and did A-B comparisons on the curve tracer to more accurately determine the voltage for the part. While I had believed the diode was 4.5 volts, I found that a 4.7V part exactly matched the IBM part. It even had the same slightly slow turnon, not rising vertically but on a slight slant, something I had suspected was an issue with the IBM part. 

EXPERIMENTING WITH MY CIRCUIT SIMULATION TO GUESS AT FAILED COMPONENT

The circuit simulation is running well on CircuitLab, although the parameters of the transistors are not accurate enough to support operation with a full 20A load yet. Basically the default transistors, low current models like the 2N3906, wouldn't be able to drive enough current to maintain the voltage under heavy loads, whereas the actual transistors used in the supply certainly can do this. 

SHOP MOVE AT END OF MONTH

I am moving my workshop to a different address, not very far away from its current location, partly to support the Chiropractor next door who is desperate to expand into my storefront. The new place is being prepared with a new air conditioner and 240 service thus the exact move date is still a bit hazy. The Chiropractor will move my equipment and supplies and since he rents the storefront as of May 1, the precise date of movement is not that critical. 

TRAVELING TO SPEAK AT VINTAGE COMPUTER FESTIVAL EAST

However, between now and then, I am traveling to speak at the VCF East event and once I return it will be the final week before the move, so I have to suspend work on the 1130 and finalize all the packing beginning today. I put all the covers back on the 1130 and prepared it for safe movement, with only the disk drive being moved separately. 

Tested blower motors and power rail voltage regulators; the +3V regulator is bad

BLOWER MOTORS CHECKED WHEN POWERING ON

Now that T2 is wired up, when the contactor switches on, it powers T2 whose output is 115VAC wired directly to the blower motors. There are three motors in gate A, three motors in gate B and one motor under the voltage regulators and lighting power supply. 

These spun up just fine with relatively little dust emitted for a few seconds before they ran clean. Before the restoration is complete, I will need to measure the air filter openings on the bottom of the blowers, find a compatible size available today and cut it down as necessary to fit the 1130. 

DISABLING THE POWER CHECK AND LOCKOUT WHILE I TEST POWER SUPPLIES

If I pull the connector off the SMS card pin A, the signal to activate the shutdown and lockout won't activate the reed relay on the card and we will merrily continue to have power. 

The green path is the power that flows to keep the contactor (K1) energized and power applied. It does so as long as relay RR1 is not activated, because of the circled contacts. When the time delay relay TD1 fires, the yellow path flows through the coil of RR1 and activates the relay, breaking the contacts that keep K1 energized thus dropping power to the machine. Pulling pin A blocks the route to activate RR1. 

DISCONNECT REGULATORS FROM CPU LOGIC BEFORE TESTING THEM

We don't want any problems with the power rails to cause damage to the SLT cards or other circuitry, so we will disconnect the output wire from each of the three regulators. This ensures that anything we do with the regulators is not reaching any further into the machine. That is terminal 2 of the +6 and +3 regulators and terminal 5 of the -3V regulator. 

CONNECT RAW POWER SUPPLY TO REGULATORS BUT LEAVE THEIR BREAKERS OFF

The three regulators have their own circuit breakers, which we will initially flip off. Then the wires from the raw DC power supply are connected so its output will power the regulators. Raw DC flows to the inputs and the outputs are disconnected until we carefully check the power quality and correctness. 

TURN ON EACH REGULATOR AND CHECK OUTPUT VOLTAGE FOR SANITY

One by one, we flip on the circuit breaker and look at the voltage level produced by the regulator. These have a narrow range of acceptable values centering on their nominal 3, -3 or 6 volt target. If the values are slightly off, we won't adjust them here because the correct procedure is to measure the voltage as delivered to the terminal strip at the bottom of the logic gates and adjust the regulator potentiometer to get the voltage correct at that point, not at the output terminal of the regulator itself. This compensates for voltage drop in the distribution wiring. 

The results were excellent for the +6V regulator as well as the -3V regulator. The +3V looked a bit suspect, as it was at 3.26V which is outside of the maximum range of 2.88 to 3.12V. 

HOOK UP LOAD RESISTOR NETWORK TO FULLY LOAD EACH REGULATOR

I had a resistor network which I use to present full or nearly full load on the regulators. They must hold their voltage not only when no load is present, but maintain voltage regulation up to their capacities of 25A for 6V and 20A for the others. 

I wired together 10W ceramic 1 ohm resistors in series and parallel combinations to achieve the net low resistance to load down a regulator and to have each resistor at or below its 10W dissipation limit. I could demand the full 25A capacity from the 6V regulator but for the two 3V supplies, the resistors I had on hand could only get low enough to pull a bit under 17A. 

While this is not the full 20A the regulator can support, it is close enough that all power transistors must be good to handle this and therefore it would scale up to 20A with no problem. The four germanium power transistors are in parallel across the load with each able to handle about 5A. If only three were working we would have begun to sag already. 

As expected, the +6V held its voltage right on the nose at the full 25A of load. The -3V regulator held its voltage steady up to almost 17A. The +3V regulator did not do so well.

With no load, it had operated at about 3.26V. With the load applied, the voltage began ratcheting upwards from that already unacceptable level. When I saw it going north of 3.67V I flipped off the circuit breaker before the overvoltage (crowbar) card would fire and trip the breaker off. The regulator is not doing its job. 

REGULATOR REMOVED AND PUT ON THE BENCH

I disconnected the regulator and pulled it out the 1130. It was put on the bench where I could figure out the defect and repair it. The power transistors are on heat sinks that fold out from the regulator to give good access to all four. Each heat sink wing holds two transistors and an aluminum bus bar holds the two wings together electrically when folded up. 

REMOVED AND TESTED ALL 108 POWER TRANSISTORS BUT ALL ARE GOOD

Every other time I have had a bad power supply, whether on SLT systems or 1401 systems, the fault has been one or more 108 transistors that shorted or went open or are otherwise nonfunctional. I desoldered and removed the four 108 transistors from the wings, as well as the 108 on a heat sink on the SMS regulator card. 

Regulator card

Overvoltage/Crowbar card

USED MY CURVE TRACER TO IDENTIFY VOLTAGES OF ZENER DIODES

Having just used the curve tracer to verify the health of the 108 power transistors, I decided to leverage it to identify the voltages of the two zener diodes on the regulator card. It would be hard to reverse engineer the regulator circuit without specific component details like this.  They are 10V and 4.5V by the way. 

The 10V diode had a very nice almost vertical turn-on at the 10V point, but the 4.5 had a bit of slope rather than the right angle that is characteristic of a good zener diode. I guess I could substitute a new 4.5V zener and see if that fixes things, but that is a bit of a blind shot. 

STUDYING CIRCUIT SO I CAN COMPARE WHAT IT IS DOING WITH WHAT IS CORRECT

I set up the circuit with all the parts values in Circuitlab, an online tool that lets me simulate various circuits I have designed. The transistor parameters were not accurate, however as I don't know the specs for 026, 086 and 108 transistors. I tried to get the 108 set up to look like a modern PNP Germanium power transistor. 

The simulation did let me regulate by adjusting the potentiometer setting until it produced exactly 3V as an output. I put in various load resistances with zero change in the output voltage until I got down to .45 ohms. This corresponds to a design that could handle 6.5A or a bit more, not the 20A in the IBM supply. However, I attribute this to the inaccurate transistor specifications, since the drive current to keep up with higher current depends on the abilities of the transistors in the circuit. 

This allows me to record the expected voltages at different points on the regulator, which I can compare to the failing regulator on the bench. Hopefully this will point me directly at the failed component(s). The alternative is to unsolder all the transistors on the regulator card and put them on the curve tracer to look for a bad part. 

I keep seeing references to an IBM publication named IBM FE Theory of Operation : Power Supplies SLT, SLD, ASLT which would have been very helpful, rather than having to reverse engineer the supply to understand how it must operate. Back in the day, the solution to a problem like this was for the local FE to install a spare part regulator card and if that didn't fix it, replace the entire power supply. The factory would deal with diagnosis and repair. 

Repaired contactor and finished testing out sequencer box and some power supplies

PICTURES OF REPAIRED POWER SWITCH AND PROG LOAD BUTTON

I included pictures of the dummy light positions whose socket I swapped for the broken button. In addition, here are the Prog Load button and Power switch in their repaired state. 

CHECKED ALL CONTACT RESISTANCE FOR CONSOLE BUTTONS AND SWITCHES

While I had the console open to make the repairs above, I took the time to test every button for a good contact in both pushed and released states. The Prog Start, Prog Stop, Load IAR, IMM Stop, Check Reset and Prog Load were all near zero ohms for all contacts. The Console/Keyboard toggle switch was excellent as was the previous cleaned Power toggle switch.

REMOVED, CLEANED AND REINSTALLED THE CONTACTOR

I found clumps of rodent nest material between the contacts, between the parts of the contactor that should touch when activated, and in other parts of the component. I picked the all out and cleaned it up. After reinstallation, it is totally silent other than the actual clunk of activation, no buzzing like before.

JUMPED AT VERY LOUD TIME DELAY RELAY ACTIVATION

When the sequencer activates the contactor, it starts the time delay relay TD1 which provides the initial reset signal to all CPU logic until it kicks in after the delay. It also plays a role in protecting the machine if one of the three main power rails is missing - +6, +3 or -3. 

It is very loud with the sequencer box open, about as loud as the contactor itself. Thus, when I switched on the machine and was checking voltages produced by the raw DC power supply, and heard a loud unexpected clank, I thought something went wrong. Soon I realized what this noise was and was no longer startled by it. 

POWER PROTECTION CIRCUIT WORKING - LOCKOUT IF RAILS NOT GOOD

The sequencer logic card has three reed relays whose contacts are in series, with each relay energized by one of the main power rails. As long as all three come up to voltage and turn on the reed relays before the time delay relay fires, all is good. However, if one or more is not present, the machine shuts off the contactor. 

This situation also turns on another reed relay which stays energized to lock out the system. Flipping the power switch off and on will not reengage the contactor. The machine either has to have the CE switch flipped off or the plug pulled out of the wall to reset that lockout relay. 

Since I don't have the regulators wired up yet and thus have no power rails present, the card is doing its job to shut down and protect the precious circuitry. During the time delay from turning on the power switch until TD1 energizes, I could check all the voltages on the power supplies and fuses. 

TESTING PRESENCE OF 230VAC AT ALL RELEVANT FUSE HOLDERS

Everything is looking good with the power sequencer box and the wiring. I put in fuses to each circuit to check out their operation, except for two that I can't test yet. The convenience outlet fuses won't get power because of the frozen, defective relay R3. The fuse for the SMS connected peripheral AC power requires that transformer T2 be wired in to drop 230V down to 115VAC for the peripherals such as the typewriter motor.  I don't have T2 connected yet because it also will spin the cooling blowers. 

TESTED RAW DC POWER SUPPLY

I put in the fuses for the raw supply and verified it is putting out the raw voltage, around 8V, that will be regulated down to +3V and -3V. These are two different outputs, one per regulator. A third circuit produces around 12V that will be regulated down to 6V for the third rail. The other side of the raw supply produced +48V and +12V, actually slightly higher which is normal. 

TESTED LIGHTING DISPLAY POWER SUPPLY

The many lamps in the display pedestal are powered by 7.25VAC from a beefy lighting supply that has its own fuse on the sequencer box. I validated that the proper voltages were present in the display box. 

FINISHED LACING THE FUSE WIRING AND CLOSED UP THE SEQUENCER BOX

With the fuse holds in place, I manipulated the wires which include a few I had replaced due to rodent damage. When I had them formed in the correct bundle shape, I laced them up to return them to a state similar to how IBM built the machine. 

The box can be closed since all fuses and the circuit breaker are on the outside and don't need access. I will open it again when I do a couple of deferred repair tasks later. I am deferring expenses until we determine that the machine is in reasonable health and likely to be fully restored. 

CLEANED BLOWER AREA IN PREPARATION FOR SPINNING THEM TOMORROW

I hooked up transformer T2, which drops the building 230V supply down to 115VAC for use by the blower motors and various peripherals which use 115VAC motors. Before turning on the machine with the blowers in the circuit, I wanted to make sure I wasn't blasting dirt, nut shells and who knows what else into the logic cards and power supplies. 

When removing the nesting material from the contactor earlier, I found that some of the gnawed strands of wire were mixed into the bedding, making it very capable of shorting out pins and components. This had to be cleared from the machine before first blowing air through the machine. 

COUPLE ITEMS TO ADDRESS LATER IN THE SEQUENCER BOX

I found that relay R1, circuit 1, had a resistance of 93 ohms when the relay activated to connect the common to the normally open contact. This adds relay R2 coil in parallel with TD1's coil. I suspect it will be good enough to fire R2 which is what delivers +12 and +48 to the machine when the power rails are verified to all be present. If not I can go back in earlier and burnish the contact to lower the resistance but since it probably will work okay for now I will wait until I am working on the relays later (see below) to handle this. 

Relay R3, which should be activated whenever the main circuit breaker is turned on and T1 is producing +24VAC, is frozen. It's only purpose is to connect the 230V from the circuit breaker output to a pair of fuses (F3 and F4) and through them to the primary of transformer T3. T3 drops the voltage to 115VAC for the convenience outlets in the 1130 and its larger peripherals. 

The convenience outlet is unnecessary for operation and served as an alternative to wall outlets which don't exist in computer rooms. The museum where it will be exhibited will have wall outlets, thus this is not essential. As long as I can repair R3 or replace it at a modest cost, I will do this just to get the 1130 as close to factory condition as possible.

Repaired Prog Load button, cleaned contacts of Power switch and tested up to energizing the contactor

PROG LOAD SWITCH HAD A BROKEN SOCKET AND WAS WEDGED WITH PAPER

As I looked closer at the console when I was testing the Power on/off switch, I noticed some paper wedged into the side of the Prog Load button. The socket had been broken thus the button would pop up out of the socket when released. These sockets are a plastic piece that IBM used for both buttons and lights on all the SLT systems, with a single part that works for pushbuttons, toggle switches and lights. 

The left side of the console has two dummy light positions that have a blank frosted white part inserted. Since they would not be used and are held in place by the metal console plate, I swapped a good socket from one of the dummy positions for the broken socket from the Prog Load button. The button is now as good as new. 

POWER SWITCH CLEANED TO GET LOW RESISTANCE ON CONTACTS

I was not happy with the contact resistance of the Power On/Off toggle switch on the console. I had to remove it to clean it and get deoxidizing solution into the contacts. The switch is locked onto the plastic socket by a 9/16" hex nut but the gap between the switch body and the plastic socket didn't leave enough room for any tool to turn the nut. I even have some narrow IBM wrenches, but they were much too thick for the small space they needed to enter.

I went to the local Ace Hardware and while they don't carry any specialty very thin wrenches, they have cheap thin parts that come with grills, used by the staff when assembling grills for display. It was close enough to 9/16" and thin enough because of how cheaply it was made. It was perfect for loosening the hex nut and I soon had the switch out.

The switch was filthy inside the stem but I cleaned and deoxidized it, resulting in very good low resistance contacts in both positions. I reinstalled it into the console. 

EPO (RED EMERGENCY SWITCH) HAD BEEN PULLED, RESET

In my debugging this morning I realized that the EPO switch had been pulled out by somebody years ago, in a childish desire to pull on the forbidden switch. The EPO switch locks into position when this happens, but it can be reset with a screwdriver from inside the display pedestal if you know how. It is now in its normal position and conducting electricity just fine. Somehow I had thought that I tested the EPO yesterday but obviously I did not. 

TURNING ON THE POWER SWITCH ENERGIZED THE CONTACTOR

With the system circuit breaker turned on and a fuse in F5 to power the 24VAC transformer, the system was just waiting for the Power On/Off toggle to be moved to the On position. I heard a clunk and then a noisy contactor which delivered 230V to the majority of the fuses and other circuits inside the sequencer box. It also energized transformer T2 which will drop the 230V down to 115V to drive all the cooling blowers, but I disconnected T2 as I wasn't certain of the state of the blowers or what debris might be flung around if they came on. 

VERY NOISY CONTACTOR, NEEDS RODENT DIRT REMOVED FROM CONTACTS

Loud buzzing in a contactor is a sign that the armatures are not pulled fully against the 'on' stop. This can be due to inadequate voltage on the solenoid but also by debris blocking the armatures from fully sealing. The latter is the much more likely scenario given the rodent nesting that took place inside the sequencer box. I will remove the contactor and clean it up so that it will work quietly. 

RELAY R3 FOR CONVENIENCE OUTLET POWER FROZE UP AGAIN

The rust solidified again and is keeping the armature from moving on R3. This blocks application of 230V to the fuseholders F3 and F4 which feed the primary of transformer T3. That transformer drops the 230 to 115 and feeds power to the convenience outlets in the 1130 system and its peripherals. 

The relay switches 230V to the transformer primary with the fuses specified at 3 2/10 amperes each to support loads of up to about 735 watts in total on the convenience outlets. If I get a replacement relay, its coil should work with 24VAC and its contacts have to handle the high voltage and 3.2A currents, plus tolerate reverse EMF when the power to the machine is turned off by the circuit breaker or pulling the plug from the building. It should also mount in the footprint and area where the existing R3 fits. 

I won't spend the money on this relay until I am further along in the restoration and comfortable that it will be restored to full working order. At that point I can purchase a substitute relay and install it.