Repairing a Nineteen Sixties mainframe: Fixing the IBM 1401’s core memory and power provide

A few weeks ago, I wished to make articulate of 1 of the normal IBM 1401 mainframe computer methods on the Computer History Museum, nonetheless the computer wasn’t working.1
This article describes the multi-week restore course of to catch the computer working again.

The whisper started when the machine was powered up on the identical time somebody shut down the predominant power, it sounds as if causing some fabricate of detrimental power transient.
The computer’s core memory entirely stopped working, making the computer unusable.
To repair this we needed to delve into the depths of the computer’s core memory circuitry and the skill affords.

The IBM 1401 computer. The card reader/punch is in the foreground. The 12K memory expansion field is partly seen to the factual in the back of the 1401.

Debugging the core memory

The IBM 1401 was a typical enterprise computer of the early Nineteen Sixties. It had 4000 characters of inside core memory with further 12000 characters in an exterior expansion field.2
Core memory was a typical fabricate of storage in this technology because it was quite instant and low-charge.
Every piece is saved in a diminutive magnetized ferrite ring known as a core.
(Whenever you happen to hang got ever heard of a « core dump », that is what the timeframe on the foundation referred to.)
The photo below is a magnified stare of the cores, along with the purple wires extinct to judge, learn and write the cores.four
The cores are wired in an X-Y grid; to access a explicit take care of, one of many X traces is pulsed and one of many Y traces is pulsed, selecting the core the effect they intersect.Three

Ingredient of the core memory in the IBM 1401. Every toroidal ferrite core retail outlets one bit.

Within the 1401, there are 4000 cores in every grid, forming a core airplane that retail outlets 4000 bits.
Planes are then stacked up, one for every bit in the discover, to fabricate your entire core module, as shown below.

The 4000 character core memory module from an IBM 1401 computer. Miniature ferrite cores are strung on the purple wires.

To diagnose the memory whisper, the physique of workers started probing the 1401 with an oscilloscope.
They checked the indicators that desire the core module, the memory attach watch over indicators, the incoming addresses, the clock indicators and so forth, nonetheless all the pieces looked okay.

The next step was to observe if the X and Y desire indicators had been being generated properly.
These pulses are generated by two boards known as « matrix switches », one for the X pulse and one for the Y pulse.5
Some take care of traces are decoded and fed into the X matrix swap, whereas the replacement take care of traces are decoded and fed into the Y matrix swap. The matrix switches then produce pulses on the optimistic X and Y desire traces to access the desired take care of in the core planes.

The photo below exhibits the core memory module and its supporting circuitry contained in the 1401.
The core memory module itself is on the bottom, with the two matrix swap boards mounted on it.
Above it, three rows of circuit boards (every the dimension of a taking half in card) present the electronics.
The head row includes inhibit drivers (extinct for writing memory) and the most modern provide and most modern driver boards (offering most modern to the matrix switches).
The center row has 17 boards to decode the memory addresses.
On the bottom 19 sense amplifier boards learn the information indicators from the cores.
As you would possibly perhaps perhaps also gaze, core memory requires tons of supporting electronics and wiring.
Also suppose the warmth sinks on all these boards due to the the high currents required by core memory.

At some stage in the IBM 1401 computer, showing the predominant parts of the core memory system.

After some oscilloscope measurements, we found out that one of many matrix switches wasn’t generating pulses,
which defined why the memory wasn’t working.
We started checking the indicators going into the matrix swap and
found out one matrix swap input line confirmed some ringing, it sounds as if sufficient to connect the matrix swap from functioning.

For the reason that CHM has two 1401 computer methods, we decided to swap cards with the precise machine to trace down the fault.
First we tried swapping the thermal swap board (below).
One whisper with core memory is that the properties of ferrite cores switch with temperature.
Some computer methods keep some distance from this whisper by heating the core memory to a relentless temperature in air (as in the IBM 1620 computer) or an oil bath (as in the IBM 7090). The 1401 on the replacement hand
uses temperature-managed switches to regulate the most modern based entirely on the ambient temperature.
We swapped the « AKB » thermal swap board (below) and the related « AKC » resistor board, with no finish.

The core memory uses a thermal swap board to regulate the most modern through core memory as temperature changes. The switches initiating at 35°C, 29°C and 22°C. The fabricate of the board (AKB) is stamped into the decrease left of the board.

Next we tried swapping the « AQW »
most modern provide boards that attach watch over most modern throughout the matrix switches.6
We swapped these board and the 1401’s memory started working. Replacing the unusual boards one at a time, we found out the putrid board, shown below.

The IBM 1401 has four « AQW » cards that generate currents for the core memory switches. This card had a depraved inductor (the upper inexperienced cylinder), combating core memory from working.

I examined the putrid board and examined its parts with an multimeter.
There hang been two 1.2mH inductors on the board (the properly-organized inexperienced cylinders). I measured Three ohms all the way in which through one
and Three megaohms all the way in which throughout the replacement, indicating that the second inductor had failed.
With an initiating inductor, the board would handiest present half the most modern. This defined why the matrix swap wasn’t generating pulses, and thus why the core memory didn’t work.

I gave the putrid inductor to Robert Baruch of Project 5474 for diagnosis.
He found out that the connection between the lead and the inductor wire was intermittent.
He dissolved the inductor’s package in acid and took photos of the winding contained in the inductor.7

The depraved inductor from the IBM 1401 showing the failed connection.

We looked in the spare board cupboard for an AQW board to interchange the putrid one and positioned out just a few.
Nonetheless, the replace boards had been diverse from the unusual—that they had one power transistor rather then two.
(Look at the photo below with the photo of the failed card from the computer.)

The replace AQW card had one transistor rather then two, nonetheless was supposedly treasure minded with the primitive board.

Despite misgivings from some physique of workers people, the putrid AQW card was replaced with a one-transistor AQW card and we tried to power the system back up.
Relays clicked and followers spun, nonetheless the computer refused to power up.
We attach the primitive card back (after replacing the inductor), and the computer tranquil wouldn’t initiating up.
So now we had a bigger whisper. Apparently one thing had gone injurious with the computer’s power affords so
the debugging effort switched focal level.

Diagnosing the skill provide whisper

The skill provide system for the IBM 1401 is more complex than you would possibly perhaps perhaps quiz.
Curiously, the predominant power affords for the system are contained in the card reader;
a 1250W ferro-resonant transformer in the card reader regulates the road input AC to 130V AC, which is fed to the 1401 computer itself through a thick cable below the bottom.
Smaller power affords contained in the 1401 then way the fundamental voltages.

Because it was built ahead of switching power affords grew to alter into popular, the IBM 1401 uses fat linear power affords.
The photo below exhibits (left to factual) the +30V, -6V, +6V and -12V affords.8
Within the decrease left, below the +30V provide, you would possibly perhaps perhaps also gaze eight relays for power sequencing.
The circuit board to the factual of the relays is without doubt one of many « sense cards » that checks for appropriate voltages.
Below the +6V provide is a tiny « +18V differential » provide for the core memory. Foreshadowing: these parts will all be crucial later.9

Vitality affords in the IBM 1401.

After measuring voltages on the a pair of power affords,
the physique of workers concluded that the -6V power provide wasn’t working factual.
This was quite puzzling since the AQW card (the one we replaced) handiest uses +12 and +30 volts. Because it does no longer articulate -6 volts in any appreciate, I didn’t gaze how it would possibly in reality most likely perhaps perhaps mess up the -6 volt provide.

At some stage in the IBM 1401’s -6V power provide.

The physique of workers eliminated the -6V provide and took it to the lab.
Within the photo above, you would possibly perhaps perhaps also gaze the heavy AC transformer and properly-organized electrolytic capacitors contained in the skill provide.
Measuring the output transistors, they found out one putrid transistor and a few worn transistors and decided to interchange all six transistors. Within the photo below, you would possibly perhaps perhaps also gaze the unusual transistors, mounted on the skill provide’s properly-organized heat sink. These are germanium power transistors; your entire computer is pre-silicon.

The -6V power provide from the IBM 1401 uses six power transistors on a properly-organized heat sink.

The -6V power provide examined okay in the lab with the unusual transistors, so it was installed back in the 1401. We hit the « Vitality On » button on the console and… it tranquil didn’t work.
We tranquil weren’t getting -6V and the computer wouldn’t power up.

Within the next restore session, we tried to make a selection why the computer wasn’t powering up.
Purchase the eight relays talked about earlier; these relays present AC power to the skill affords in sequence to way clear that that the affords initiating up up in the factual characterize.
If there is a project with a voltage, the next relay in the sequence would possibly perhaps perhaps no longer shut and the skill-up course of will be blocked.
We looked at which relays had been closing and which weren’t, and measured the voltages from the quite tons of power affords.
At final we obvious that about halfway throughout the skill-up course of, relay #1 was no longer closing when it’ll, stopping the skill-up sequence.

Relay #1 was driven by the +30V provide and was activated by a « sense card » that checked the +6V provide.
However the +30V and +6V affords had been powering up suited and the sense card was switching on properly.
Thus, the problem perceived to be a failure with the relay itself.
Correct ahead of we pulled out the relay for making an strive out, somebody found out an updated schematic showing the relay didn’t articulate the frequent +30V provide nonetheless instead obtained its 30 volts throughout the « 18V differential provide ».eleven
And the schematic for the 18V differential provide had a pencilled-in fuse.10

Could the skill whisper be as simple as a burnt-out fuse?
We opened up the 18V differential provide, and clear sufficient, there was a fuse and it was burnt out.
After replacing the fuse, the system powered up suited and we had been back in enterprise.

The 18V differential power provide in the IBM 1401 affords 12 volts to the core memory. The fuse is below the properly-organized electrolytic filter capacitors.

With the computer operational, I would possibly perhaps perhaps at final scurry my program. After about a malicious program fixes, my program extinct the computer methods’s reader/punch to punch a card with a explicit gap sample:

A punch card with « Merry Yuletide » and a tree punched into it.

Blissful holidays all people!12

Conclusion

Regardless of all the pieces this debugging, what was the foundation trigger of the issues?
As some distance as we can characterize, the unusual whisper was the inductor failure and or no longer it is precise a twist of fate that the problem befell after the skill loss during system startup.
The unusual AQW card will must hang triggered the fuse to blow, even though we attach no longer hang a smoking gun.13
The cause the -6V power provide wasn’t showing any voltage is which means that of it was sequenced by relay #1, which didn’t shut thanks to the fuse.
The putrid transistors in the -6V power provide whisper had been it sounds as if a pre-existing and non-extreme whisper;
the precise transistors handled sufficient load to connect the skill provide working.
The precise from all that is that conserving an primitive computer running is appealing and takes a gifted physique of workers.

Thanks to Robert Baruch for the inductor photos.
Thanks to Carl Claunch for offering diagnosis.
The Computer History Museum in Mountain Inspect runs demonstrations of the IBM 1401 on Wednesdays and Saturdays so try it out whilst you happen to’re in the dwelling; the demo schedule is right here.

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Notes and references