Failure to try to load heads as drive comes to speed narrowed to defect in one card

FIRST TRY TO HAVE HEAD LOAD SOLENOID ACTIVATE WAS UNSUCCESSFUL

The 90 second time delay relay, which I am waiting to replace, opens a circuit that is at ground initially but when disconnected is seen as logic high. When that signal goes high, it powers the head load solenoid to force the heads onto the disk surface. I used a wire with alligator clips to substitute for the relay contacts, intending to unhook the wire to trigger the head loading.

However, when I did this, nothing happened. It didn't take a lot of probing around to fine that the '70 percent latch' was active. This is a circuit that detects when the disk rotation speed slows below 1050 RPM, forcing the heads to unload and requiring a power cycle of the drive to reset. Now, I could see from the sector and index pulses that the disk was right on speed, so this was a spurious error detection. 

SPEED DETECTOR CIRCUITS IN THE DRIVE

The machine has circuits that validate when the drive is up to speed before the heads can be loaded, and they also detect when the speed slows below 70% of nominal. These two interact with each other and drive the 70 percent latch I mentioned earlier. 

The bottom of the disk platter inside the cartridge has eight slots evenly spaced around the periphery which generate the sector pulses, plus there is a second slot just after one of the eight which is used to generate one sector pulse per rotation. 

The sector pulses are used to drive the speed detectors. One of the detectors produces pulses that will set the 70 percent latch on, the other produces a reset signal that keeps the 70 percent latch from turning on. Thus the timing of the reset signal turning off is important relative to the setting pulses. 

The yellow line is the output of the integrator that turns off the reset control on the 70 percent latch. Its voltage rises with sector pulses until the threshold is reached that turns off the reset line, thus allowing the latch to get set by the other circuit. The green line is the inverted reset control, so when it goes high the latch can be triggered. 

The purple line is from the rate detector that creates a pulse for each sector and eventually will stabilize as a single level, unfortunately long after the reset signal (green) has turned off. I didn't capture a picture of the purple signal after it reaches that level, but there is an image in the IBM manuals that I included just below.

The purple signals create pulses, on the blue line, that set the 70 percent latch. These pulses stop once the purple signal stabilizes. The key to the proper behavior of the two detectors plus the latch is that the integrator (yellow line) should not start counting until the purple line has stabilized. There is a circuit called a squelch that should block the integrator but it does not appear to be working, thus the early integration and release of the reset signal. 

The trace above shows the squelch signal in purple, the integrator in yellow, the reset signal in green and the setting of the 70 percent latch in blue. The latch signals keep setting the 70 percent latch but the reset signal will immediately turn it off again (until the integrator reaches the threshold). 

When the pulses (blue line) setting the latch become so close together they produce a steady on level in the blue signal, which is what releases the integrator to begin its work. The squelch output (purple line) should stay low until the blue line is essentially steadily high. What we see is that the integrator begins counting, only being squelched for the first 80 milliseconds of the trace then climbing steadily. 

annotated with the scope trace line colors

from ALD

All of the circuits above are on the undocumented (to me) 7235 card you see below, packed with components. I will have to reverse engineer the squelch and integrator portion so that I can determine why the squelching is not doing its job long enough. 

The ramp generator behavior based on the IBM supplied trace images implies that it charges a capacitor up to +5V and discharges it with the incoming sector pulse. The different rates of charge and discharge mean that the recovery back to +5V is slower. If the next pulse arrives before the recovery is complete, then the high voltage drops. The trace implies that it stabilizes around 3V. 

The detector that comes next is apparently a threshold detector, activating for the time when the voltage is no more than about 3V, producing the set pulses for the 70 percent latch. Once the ramp gets down steadily below 5V, the set pulse is steadily on. That leads to turning off the squelch and allowing the integrator to build up. 

In the second scope trace in this post, you can see that the ramp detector pulses have not become a solid on signal yet, they are still spaced apart (blue line) but the squelch circuit output was drifting up (purple line) which no longer resets the integrator. 

My assumption is that the squelch input into the integrator, which should initially be high and only drop to low when the blue line is steady, is the malfunction here. The high input level causes the integrator to discharge the capacitor that is building up voltage from each sector pulse, so that it never accumulates a high enough level. 

Once squelch drops to low, the capacitor is free to increase its charge with each injection of energy from a sector pulse until it asymptotes to roughly 5.5V. The threshold detector is set to some voltage that corresponds to approximately 1500 RPM rotation. This turns off the reset line for the 70 percent latch so that if it is ever presented with another set pulse from the ramp detector, it will latch on and stay that way until the drive is powered down. 

If the pulse arrival rate slows enough, the voltage on the ramp generator begins to climb back above 3V towards 5V. The detector then produces the set pulses, which turn on the latch. The reset signal won't turn back on until the drive has slowed to about 30 RPM when the integrator result gets low enough. The drive is already coasting to a stop at this point. 

Importantly, until the drive motor is turned off, the latch stays active blocking disk activity. The drive is reported as not ready and the heads are unloaded while the latch is set. Thus, if for some reason the disk speed gets dangerously slow (< 70%) during operation, while it keeps spinning the drive inactivates itself and the operator must turn it off completely before restarting. 

The fine ballet of charging, discharging, thresholds and pulse production depends on the values of capacitors and resistors on the 7235 card. This is in addition to the usual possibility of failed transistors or diodes on the card. 

Time delay relay for disk drive is bad - bought generic replacement

TIME DELAY RELAY IN INTERNAL DISK DRIVE FOR IBM 1130

The platter has to spin for about 90 seconds in order for temperatures to stabilize and any loose dust to be blown out of the cartridge before the heads can safely be lowered onto the surface of the disk. This is controlled by a time delay relay that is powered in parallel with the spindle motor relay. 

A time delay circuit inside the relay should cause it to take roughly 90 seconds before the relay pulls in, switching contacts that are wired into the drive electronics. The contacts are wired so that the normally closed contact is hooked to ground and thus the delay signal is at logic low initially. When the relay energizes this signal is disconnected from ground, thus becoming logic high to the drive circuitry.

When it goes high, assuming the spindle is still spinning at its nominal 1500 rpm, the drive electronics will activate the Head Load solenoid. This forces the backs of the heads down to the disk surface. They fly on an air cushion due to the 25 rotation per second spinning. 

With the heads at a point two inches in from the outside of the 14" platter, we have a 10" diameter circle thus about 31.4" of travel distance per rotation. The heads are moving 785 inches per second or roughly 20 meters per second around the circular track. A speed of 72 Km per hour is going to experience a lot of air resistance. 

The air is dragged along with the disk surface at the very small height that the disk head flies, which is what imparts the force to keep the head off the disk surface. 

The head load solenoid has a tab that depresses a microswitch when it has activated. The signal is used to switch on File Ready, the signal that informs the 1130 controller logic that the disk is ready to accept commands to move, read and write. 

REMOVED AND TESTED HOOKED TO 48V BENCH SUPPLY

I disconnected the wires and unscrewed the relay from the AC box of the disk drive. Hooking it up to my bench power supply, I supplied 48V and started a stopwatch to time the activation. Immediately, I noticed that the relay was drawing quite a bit of current, consuming about 36W of power during the time it should be simply charging an RC network to turn on a transistor in 90 seconds. 

The relay never activated and the power draw remained steady long after the time interval was up. I could feel some current flowing in the coil - insufficient to pull the armature down but enough that if I assisted the armature it would hold in the activated position. 

DISASSEMBLED AND INSPECTING COMPONENTS

The relay has a rectangular box on one side which contains the circuitry. It consists of a few resistors, capacitors, diodes, one transistor and one potentiometer. It has five wires running to the relay coil and the terminals on the relay, which suggests that the coil of the solenoid is not a single monolithic winding. 

I removed key components to measure them without any interference. The capacitors measured fine. One resistor had drifted about 30% high and the other was on spec. Both diodes tested good on the VOM at least. The transistor voltage gaps looked odd, however. They were not regular silicon nor germanium junction gaps. 

Given the partial energization of the coil and the failure to change state after the time delay, the transistor may be the bad part causing everything, but I am not certain. The transistor and capacitors all had private label markings, making it impossible to correlate to a part number I can order. For example, the transistor is marked GE P-1901-28 and the bigger capacitor is marked 1068-6720. 

GENERIC TIME DELAY RELAY PURCHASED TO TAKE THE PLACE OF DEAD RELAY

Generic delay relays are available at really good prices, offering a variety of activation voltages and delay adjustment ranges. I found a part that will activate with 48V DC and can be set to 90 or 100 second delay. When it arrives I will connect it to the drive thus restoring its ability to load heads and become ready for use. 

Breadboarding the interface from Arduinos to the 1130's 2501 reader controller logic

TWO ARDUINOS PROGRAMMED TO EMULATE 2501 READER BEHAVIOR

I wrote a program for an Arduino Uno that will respond to the requests from the 1130 controller to start the motor and to feed cards, producing the timing CB (circuit breaker) pulses. It also models the timer in the 2501 card reader which would turn off the motor 15 seconds after the last access by the 1130 controller. 

IBM uses the term circuit breaker to mean a microswitch that is activated by a cam. In the 2501, it is actually magnetic pickups that sense a permanent magnet on a rotating wheel, but the terminology remains. In the 2501, pulses are emitted by CB1, CB2 and CB3 to indicate points during the feed cycle of card movement. 

I programmed an Arduino Mega 2560 to model the photocells and read emitter pulses of a 2501 reading cards. It watches the CB1 and CB3 timing pulses from the other Arduino and receives control signals from the 1130 controller logic. When the controller energizes the Feed Solenoid, to move a card through the machine one position, the Arduino program will produce the outputs that would occur. 

The 2501 has three positions where a card can sit - the hopper, the pre-read station, and the stacker. Each feed cycle moves cards from one to the next position. Initially there are cards in the hopper but not elsewhere. Pushing the Start button on the reader triggers the controller logic to issue a Feed Solenoid and move one card into the pre-read station. 

After this first card situation, each Feed Solenoid moves a card from pre-read through the read photocells to the stacker, simultaneously moving one card from the hopper into the pre-read station. This continues until the Hopper Empty microswitch tells the controller that there are no cards left in the hopper.

A photocell at the left side of the pre-read station, the right side of a punched card, will be blocked as a card is moved into the station. It stays dark until a next feed cycle when the card is moved through the read photocells to the stacker. 

As the card moves away from the left side of the pre-read station, the photocell sees light. This is the trigger to the controller logic to set up for a card read as this occurrence defines the point where the left edge of the card is entering the reading photocell area.

The 2501 records timing pulses on a read emitter wheel that will produce a pulse for each of the card columns as they pass under the photocell. These 55 microsecond pulses are the Read Emitter output of the card read. My Arduino sees the signal to record the timing pulses which starts my production of Read Emitter signals, emulating what the 2501 would have done. 

The first pulse is column 0, so it is ignored by the controller logic. The next 80 cause the controller logic to sample the state of twelve photocells, one for each row of the punched card. The logic samples the photocells twice - at the start of the read emitter pulse and at the end. If the sampled values differ, a read check error is flagged. 

My Arduino presents the signal that would come from the pre-read station photocell as cards physically move into it and out of it. I set them up in advance of the Read Emitter pulse and hold them steady until a safe time after the end of the Read Emitter pulse. 

I set up the values for the 80 columns, with the state of the twelve row photocells for each, using Hollerith encoding. It is a card that has // XEQ CARL at the left side and C0001209 in columns 73 to 80. The end characters allow me to be sure that the emulated 2501 doesn't get out of sync with the 1130's controller logic and misalign the results. 

INTERFACING ARDUINOS TO THE 1130 LOGIC

The 1130 logic uses different voltage levels than the TTL levels of the two Arduinos. In some cases, 12V and -3 are present on input signals. 

To protect the inputs of the Arduinos, I have these signals feed an open collector buffer gate with the Arduino's internal pullup resistor reading a logic HIGH if the gate is not on and a logic LOW when it is active. This is true for all of the inputs into the Arduinos.

To drive the 1130 logic from the Arduino, I also used open collector buffer gates, since the 1130 logic doesn't care about a logic HIGH voltage, only reacting to a LOW level on the input. This is true for many of the outputs of the Arduino, but not all. 

Some of the signals going into the 1130 see a voltage of -3V and expect to be pulled up to +12V for a logic high. Those signals use a transistor to connect the +12V to the input when activated by the Arduino output. These are -Motor Hold, +Card Lever 1 (pre-read photocell), +Row 12, +Row 11, +Row 0, +Row 1 . . . +Row9 photocells. 

SIGNALS NOT HANDLED BY THE ARDUINO DONE ON INTERFACE CIRCUITRY

There are microswitches in the 2501 that detect jams in the stacker, the hopper empty condition, and open covers on the machine. These lines into the 1130 see -3V and expect to be fed +12V when active, controlled by switches in the interface circuitry and not by an Arduino.  

Some inputs to the 2501 will drive indicator lamps on the front of the machine. These are Ready, Feed Check, Read Check and Combined Attention. These can be fed to LEDs in the interface circuitry. 

The Start, Stop and NPRO pushbuttons on the 2501 go the the 1130 logic where they see -3V, but present +12V when the button is pressed. Again I use switches in the interface circuits. 

BREADBOARDING INTERFACE CIRCUITRY

I needed ten open collector buffer gates for the signals passing between Arduinos and the 1130 logic, six switches, four LEDs, 14 transistors for the circuits that swing between +12 and -3, plus the various pullup and current limiting resistors. 

Found issue with the voltage regulator, repaired it and began more debugging

TEMPORARY 22 OHM RESISTOR OF SMALLER WATTAGE PUT ON CARD FOR TESTING

I didn't have a large 22 ohm resistor on hand, but could install a couple of 1/2 W 47 ohm resistors instead while I figure out what is causing the overload. Once that is resolved and I buy the proper resistor, the card can be restored to original condition.

BINARY SEARCH OF CARDS USING 35V TO SEE WHICH IS OVERLOADING REGULATOR

There are five cards that are supplied with the +35V produced by the voltage regulator card. I decided to test card by card to figure out which is overloading the regulator. The first card I inserted, D2, worked just fine. I would then insert E2, H2, J2 and K2 in turn, with the other cards out of the compartment.

FOUND A DEAD SHORT ON CARD E2

When I powered on the electronics with card E2 inserted, there was a big puff of smoke since my tiny substitute resistors blew faster. I checked the resistance of the 35V input pin B09 to ground and found it to be zero volts!

I was pretty sure that the tantalum filter capacitor for that voltage on the card had shorted. I pulled it off the card and confirmed that it had failed with a dead short, as these types of capacitors will do. I grabbed another capacitor from one of my spare/junk cards, after testing it for a short of course. With the capacitor replaced, the voltage regulator was happy with E2 installed as well as when D2 was installed.

Rather than iterate through puffs of smoke, I decided to test all the tantalum capacitors on the remaining cards - H2, J2 and K2. Another was shorted on H2 but the other two cards were okay. The same process was used to repair this card - grab a good part from a donor card and swap it into the card. 

To be extra cautious, I checked all the tantalums on all the cards. They filter the +3, -3, +6, +48 and +35 voltages when they enter a card. In addition one card had a slew of tantalums used for various timing circuits, so I checked all of those while I had the cards out. 

The components used by IBM were rated at 60V and I have never found one fail when used with the main logic voltages +3, -3 and +6, nor the +12V rail. However, the same parts are installed on 48V lines where they have failed and now I found two that were installed on a 35V rail but somehow failed in spite of a very large voltage margin. 

MACHINE COMES UP AND NO SMOKE NOW THAT THE CARDS ARE REPAIRED

I was now able to bring up the voltage rails and work with the drive without smoking resistors on the voltage regulator card. Much seemed to be working well so I moved on to test the emergency retraction to Home, the spin up to Ready status, and then some access movements. 

EMERGENCY RETRACTION TOGGLED THE DETENTS BUT NO MOVEMENT OF THE ARM

When I manually moved the arms out from the Home position (track 0) before turning on the 48V, the logic in the drive attempted to retract the arm back. It does this by setting the detent for 20 mil movement and pulling the voice coil in reverse, iterating this until the arm reaches track 0 and switches on the Home microswitch. 

I heard a buzzing from the detent solenoids but there was no movement of the voice coil at all. When I pushed it back myself to Home, the buzzing stopped (as it should). 

MAINTENANCE MANUAL SUGGESTS VOLTAGE CHECKS - WHICH FAILED

The maintenance manual suggests to try to emergency retract and to verify four voltages on the backplane that indicate it is trying to do the right thing. They were all out of spec compared to what was expected. 

ATTEMPTED TO SPIN THE CARTRIDGE FOR 90+ SECONDS TO SEE HEADS TRY TO LOAD

When the motor is turned on to rotate the platter inside the cartridge, a timer is started. After 90 seconds it should switch a signal which tells the drive to activate the Head Load solenoid. When that solenoid pulls in, it turns on a microswitch that indicates successful head loading. The drive should then turn on its File Ready signal indicating it is ready to accept commands. 

The switch never activated. When I restored my own 1130, I found that the timer relay was defective, yielding the same symptom. I bought a substitute timer switch as a fix. I will need to do the same here, but will pull out the relay and verify its failure, just in case there is a wiring issue instead. 

NEED TO STUDY THE LOGIC AND CIRCUITS FOR VOICE COIL MOVEMENT

In order to troubleshoot this, I need to dig through all the material and ALD pages so that I understand exactly what should be happening. I can then find deviations and from that hunt down malfunctioning components. 

Wrote two Arduino sketches to jointly emulate a 2501 card reader in order to test out 1130 controller logic

ONE ARDUINO MODELS THE FEED CYCLE AND TIMING

This monitors the request to start the motor or read another card, setting a 15 second timer that keeps the 'motor' running. As long as the motor is running, this emits three timing pulses at specified times during a 60 millisecond cycle of the machine. 

SECOND ARDUINO MODELS READING A CARD

The second Arduino watches the timing pulses from the first as well as inputs from the 1130 controller. Initially the machine appears to have no cards in the pre-read station, but the hopper remains full. 

This produces the output of a photocell that detects the left edge of a card in the pre-read station. As a card sits in the station, the photocell output is low, but when a feed cycle is requested (Feed Solenoid line activated), synchronized with the timing pulses, the light shows as the card starts moving and goes out as the next card is fed in from the hopper.

When the controller activates the Record Emitter signal, we begin a read cycle. The twelve photocells in the read station that look for holes in the card are set up just before each card column would come under the photocells. As we reach each column we generate a 55 microsecond pulse on Read Emitter. 

I set up a sample card image so that the photocells will be lit for each card column where a hole would be punched. It is a single fixed value, but if this works I can add in support for sending card images over the serial USB link, as well as the ability to turn on Hopper Empty to test out the behavior of the controller in that case.

When Hopper Empty is raised after having read cards, the controller makes the reader Not Ready. Adding cards (so that the Hopper Empty goes away), then pushing the Start button will resume reading. If instead of turning off Hopper Empty, one simply pushes the Start button, it sets up the Last Card condition and makes the reader ready again. The next read will complete and the Last Card bit will be set in the Device Status when read by an XIO Sense Device. 

 

A long day working on the 1130 internal disk drive

FIRST STEP - CLEANING DIRT AND DEBRIS FROM THE DRIVE

I spent a good hour using compressed air cans to blow dust and other loose material out of the drive. Each area was cleaned using 409 and paper towels until the towels were no longer black with dirt. There was a thick coating on some of the plastic parts that was particularly troubling.

REASSEMBLING MECHANISMS TO RECIEVE DISK CARTRIDGES

I put together the drive receiver mechanism - this is a shroud that tilts up when the loading handle is open, into which the disk cartridge is inserted, then it tilts back to position the cartridge perfectly for the heads to load onto the surface. There are springs, pivot bolts and other parts that fit together. The mechanism is quite clever, as it has to open the cartridge as it slides into place.

A disk cartridge is a housing that covers the 14" disk platter with its brown magnetic oxide coating. There is an opening on the front that is pushed open by the receiver while the cartridge is being slid into place. There is a plate on the underside of the cartridge which keeps dirt from entering the cartridge, but that has to be pushed up inside the drive to allow clean air to be blown inside. It is impressive how it accomplishes all this with just one smooth insertion of the cartridge by the operator. 

The green arrow at the top left shows the latch that is opened by the receiver to let the arms and heads move inside the cartridge. The lower green arrow is the metal valve that is pressed open to let in air from the blower. 

How valve lets in clean airflow

SCRAPING SOUND LEADS TO SCARY DISCOVERY

I put a cartridge in the drive to assess how the heads are positioned relative to the disk platter surface. The lower head was very very close to the disk surface, much closer than it should be. I hand rotated the spindle and heard a scraping sound, which I at first feared was the lower head scaping the disk platter surface. 

However, after I removed the cartridge, I turned the platter in the cartridge and still heard the sound. When I looked underneath, I could see that the metal plate which should be spring loaded to close the cartridge, was instead collapsed inside sitting on the platter surface. That caused the scraping sound.

I quickly opened the cartridge and checked the cause. The spring loaded plate is held by plastic rivets but the degradation of old plastics (through evaporation of the plasticizers over time) makes them brittle. The rivets just gave way and let the plate fall loose. Fortunately the disk surface itself was undamaged. I can relocate the good platter into another cartridge whose platter is more scraped up.

The plate should be pressed down over the opening to keep dirt out. The drive has a plastic point that raises the plate to let air inside. 

LOWER HEAD HAS BEEN BENT IN THE PAST

My inspection of the head positioning uncovered the lower head failing to rest firmly against the solid holder. The head is on springy metal that has been pre-bent so that when it is screwed onto the holder, the head is pressing down on the holder. This positions the head with a gap from the disk surface.

How the springy metal should fit on solid holder

The mechanism that loads heads onto the spinning disk platter presses the backside of the head, forcing that spring loaded head to move off the solid holder and up against the surface. However, when the head is not loaded, it should be pulled back against the holder firmly. The lower head was not. 

Looking very closely, I can see a bend in the springy metal, not the result of manufacturing. I suspect some ham-handed prior repair. I was planning on removing the head and getting the springy part returned to its intended condition. 

LACK OF CONTINUITY IN LOWER HEAD WINDINGS, A FAILED CONNECTOR

One of the tests I did was to verify continuity through the erase and read/write coils of each head. The upper head was perfect but I didn't get good readings from the lower one. The worst case would be an internal break inside the heads; it is essentially impossible to find replacement heads. 

I separated the connectors to isolate the failure to either the side with the cable down to the head or in the side with cable back to the disk electronics cage where I tested. Immediately I could see the problem - two of the three pins of the male connector from the disk head side were broken off in the female drive side connector. I carefully extracted them, so that I can figure out how to repair the connector and restore continuity. 

REMOVING HEADS IN ORDER TO TEST OUT REST OF DRIVE

I am not yet ready to risk the heads or a cartridge by lowering the heads down onto the surface. I could block the mechanism that loads the heads, so they stay above the disk surface. The arm could move in and out, letting me check a lot of the drive out before loading heads.

However, because of the bent lower head, which is already too close to the surface, it wouldn't be prudent to move the arm in and out as there is too much risk of an impact with the lower head. I therefore chose to remove the heads from the drive, putting back only the solid holders. That way, the arm could still move back and forth but there will be no risk of damage to heads or platter surface. 

VERIFYING TWO KEY MICROSWITCHES

The function of the drive depends on the correct operation of two microswitches. One is turned on when the disk arm is fully back at track zero location - called Home position. The other is turned on when the head loading solenoid pulls in; it indicates that the heads are loaded at  the same time that the mechanical bits are pushing the backs of the heads onto the platter. 

I used the VOM to verify a good connection and correct operation. The Home switch only activated when the carriage was pulled back to the home position. The head load switch worked properly when I moved the pivot that is pulled by the load solenoid. 

WIRING UP POWER SUPPLIES TO BENCH TEST THE DRIVE

To test the drive outside of an 1130, I can use the CE switches to command movements when the drive believes that the heads are loaded on a spinning platter. This requires that I provide all the DC and AC voltages with my own power supplies. 

I set up four supplies, which will provide the +3V, -3V, +6V and +48V that the drive requires. I crimped ring terminals on wires from the supplies and screwed them onto the terminal block where DC power enters the drive. I have an AC cord attached to the AC box which powers the blower motor as well as the spindle motor that spins the disk cartridge. Finally, I wired a switch with ring terminals to attach where the start switch inside the 1130 would be connected. 

SPINNING UP DRIVE BUT BEGAN TO SMELL SMOKE FROM CARD CAGE

When the start switch is turned on, the drive will spin up to speed (1500 RPM) and wait for 90 seconds. This allows dust to be blown out of the cartridge and for the platter temperature to stabilize. At the end of the 90 seconds, the drive will activate the head loading solenoid, which pushes the backs of the heads down onto the spinning surface of the platter. 

The drive should then switch into Ready status and accept commands issued to it. My intent is to let the head loading appear to succeed, but of course the heads were removed so they won't actually be flying. The CE switches will allow me to move the arm back and forth in single and double track steps. 

The drive did spin up but after only about fifteen seconds I began to smell smoke so I shut everything down. 

VOLTAGE REGULATOR CARD SEVERELY OVERHEATING

I found that the voltage regulator card in the electronics cage was the source of the smell and a resistor on it was quite toasty. This card is said to reduce the +48V down to +35V for use in read/write and access circuits, plus a highly regulated +6V for disk head bias. 

I can see that the load resistor is damaged and cracked, the source of the smoke. Whether it is just a bad component or their is a short elsewhere is hard to tell without the schematics. 

NEED TO SEARCH FOR SCHEMATICS - MY ALD DOES NOT MATCH THE DRIVE

However, I ran into a snag trying to move forward. On the ALDs that came with my machine the internal disk drive has a single card in slot L2 that does the voltage regulation. On the actual drive I am working with there is a double card 01352 in slots L2 and L3 instead. 

I will be looking at any other drive ALDs I can find to see if any of those have the double card alternative. I wasted hours debugging the 1132 printer controller because of a difference between the VCF machine's design and the ALDs from my system, until I realized that the logic was moved to a different card slot and type of card in the VCF system. This disk drive difference, occurring at the same spot as a failure, could be equally confusing. 

This is why it is so essential for those buying or keeping vintage computers to keep the ALDs and other service documentation with the machine. While I can get close using the few sets I have found online or my own, there are always situations like this. 

MAINTENANCE MANUAL SUGGESTS TESTS ON POWER TRANSISTORS

The maintenance manual for the disk drive suggests that the power transistors often fail and should be checked quickly with a VOM. Because some of these are Germanium transistors, seemingly bad readings can be acceptable transistor condition. for example, a few hundred ohms resistance across the transistor can be acceptable for the Germanium type 042 transistors. However, this makes it harder to check for shorted junctions. 

I even removed them from the circuit to be certain that I was measuring them without interference from other components. I used the diode testing function which showed good junction voltage drops for the silicon power transistors but only .1 volt for the Germanium 042 components. This is indeed the drop for low currents as used in a VOM. Based on that, these are likely okay. 

SUGGESTED QUICK TEST WITHOUT A CARTRIDGE

The design of the drive has it attempt to retract the heads all the way to the home position (track 0) if it is turned on but not yet ready. To test this, I would remove the cartridge, manually position the arm out about halfway, then apply the DC power. It should move back to home. 

I tried this but the smoke from the regulator increased and I chickened out of waiting to hear the retraction. 

Card now passes tests on bench. Final tests to be done, then ready for installation in my machine

CARD MODIFIED AND RETESTED ON THE BENCH

With all the bodge wiring the card was ready for the full set of tests. I verified every aspect of the card, from the output to the 1627 through all the signal timings. The sense device bits are correctly generated, operation response, busy and plotter not ready. Injecting -24V turns on the plotter ready status.

The drum and carriage movement commands pull the output line, normally sitting at +12V while idle, down to ground for 1.9ms. The busy state continues for an additional 1.9ms, a total of 3.8ms. The pen up and pen down commands pull the normally 12V output lines down to ground for 50ms and keep busy status for an additional 50ms. The additional time covers mechanical movements in the plotter. 

Issuing any of the six movement commands turns on the outputs for the target duration and triggers an interrupt on level 3. When a Sense DSW is issued, if an interrupt is requested then it returns operation response (bit 0). If the reset bit 15 is set during the Sense DSW, the op response status is reset and the interrupt request is dropped. 

The timing of the pen movements was a bit long, at about 63ms instead of the ideal 50ms duration. I adjusted the resistor value for the two 122 chips that handle pen movement, making the duration match the specification. 

I carefully checked every one of the six movement commands and all of the DSW bits generated for a Sense Device. This card now produces the same results as the IBM 586223 card we are replacing. 

As a result, I transmitted the updated PCB design to JLCPCB. I also updated the bill of materials for the components to match the adjusted values and to drop the unnecessary 74LS04 chip then ordered another set of components.

HOOKING UP ELECTRONICS OF A 1627 TO VERIFY IT TRIGGERS FROM MY CARD

I have a disassembled 1627 plotter, because the drum is mangled. I can apply power and attach the new card to the device to see how the electronics behave. I should see movement of the servo motors based on setting up a value on the associated B register bit and raising XIO Write, Area 5 and T6 to trigger the card. I also should see the -24V coming from the plotter causing the card to report that the plotter is ready for use. Conversely, when -24V is detached it should report the plotter as not ready.

TO INSTALL THE CARD IN AN 1130, CABLING AND CONNECTORS WILL BE NEEED

A cable would plug into a socket in an 1130 card compartment and run over to the SMS card connector area of the machine. A female SMS socket would be needed in that area to allow the 1627 to plug in, via a male SMS card. I will work on these once the 1627 is restored to operation. Mainly it is waiting on a replacement for the drum, which I will need to have made.