Huge IBM 1800 find (and need some help)

Jon Elson elson at
Sat Mar 21 17:21:59 CDT 2015

On 03/21/2015 02:58 PM, Lawrence Wilkinson wrote:
> On 21/03/15 19:39, Jon Elson wrote:
>> On 03/21/2015 01:47 PM, Johannes Thelen wrote:
>>> Thanks Jon for tips!
>>> No, that is not option, I mean 2311 is have to get back 
>>> to life ;)  I think use ultrasonic or carbon ice 
>>> cleaning for 2311 and disk packs. Both are quire gentle 
>>> cleaning methods.
>> You have to be very careful with anything that requires 
>> temperature extremes.  Can anybody verify the 2311 uses 
>> the hydraulic head positioner?  If the oxide coating of 
>> the packs has deteriorated, which I think is quite 
>> possible, then the disks will not be usable. Anybody else 
>> have experience with 14" disk packs from 40" years ago?
> Yes, hydraulic actuator. Assuming it hasn't run for a long 
> time then flushing and replacing the hydraulic fluid would 
> be necessary.
I think it is going to need more tender loving care that 
that!  It is now over 40 year old.  Seals and hoses will 
have dried out and become brittle.
> The plus side is that you can pretty well replace a module 
> with a few transistors/diodes/resistors, if you have the 
> manual that shows you what is inside each one. Also, the 
> same SLT card will be used in many places so it's easy to 
> swap them around. For the Model 30 there was a manual that 
> showed where each card was used, which facilitated this 
> swapping. Of course there were odd cards that only 
> appeared once or twice.
Yes, this is true, and I was pretty amazed at your ability 
to fix the 360/30 you had.  But, time has been continuously 
taking its toll.  On the other hand, I was amazed to see on 
Wikipedia that 4 1800's were still running in 2010.  There 
were no SLT parts available after sometime around 1980, and 
IBM was supposed to get as much stuff back so they could 
support the FAA's 9020 computers, which were slightly 
modified 360/50's and /65's.  I'm pretty amazed they could 
keep those 1800's running for an additional 30 years. Maybe 
they were a lot more reliable than I thought, from my 360 
>> So, the likelihood of the CPU actually working after 
>> FORTY years of storage is pretty small.  There are, of 
>> course, no replacements for SLT modules, as IBM stopped 
>> making them in the late 1960's! It was a form of 
>> diode-transistor logic, and not terribly dense. A single 
>> flip-flop would not fit in one SLT module, it took at 
>> least 2, and the usual scheme was to integrate some 
>> steering gates with it and ended up with about 4 modules 
>> to implement the typical register bit.  They didn't use 
>> what we think of today as a FF, they used D latches, as 
>> it was simpler to implement, but required several clock 
>> phases.  One clock phase to latch input data to the ALU, 
>> wait for the ALU to settle, then a pulse to latch the 
>> result out of the ALU.
> The power supplies would be the first thing to work on - 
> they'll be switchers and have the potential to make a very 
> large bang.
I don't know.  The power system in the 360's was different.  
They used a "converter-inverter" that changed mains power to
DC, than inverted it to single-phase 120 V, 2500 Hz sine 
wave.  This was fed to remarkably small linear 
transformer/rectifier/regulator modules.  The downfall of 
this was there was little energy storage for momentary line 
fluctuations, so any tiny blip on the mains caused a system 

The 1800 may have been designed differently to work in a 
known-bad power environment.  it seems to have some quite 
large bulk supplies of relatively low power output, each.  
There is a mention in the FE manual about making sure the 
machine has the right 50 Hz or 60 Hz transformers.  I 
believe the 360s only needed to have one card swapped in the 
converter-inverter to set up the SCR preregulator timing for 
the right frequency.  So, anyway, it is not clear the power 
supplies are of the switching type.  but, in any case, they 
need to be checked before power is applied, or the filter 
caps could pop.
>>>   I like to dump core and all disc packs for future uses 
>>> ( = make original configuration).

>> I wouldn't worry about what is left in core, it would be 
>> interesting, but you might have to do a lot of testing 
>> and repair before enough of the machine is working to 
>> read it out.
>> Be very careful with the disk drives.  If you even get a 
>> scratch on the heads, you will not be able to replace 
>> them.  Also, if anything at all happens to the heads, you 
>> will need an alignment pack to realign any replaced 
>> heads.  Do you have ANY experience with old-school 
>> mainframe equipment?  If you think you will just dust 
>> this thing off, mount the disk packs and fire it up, you 
>> are quite crazy!
> I got away with it, but the Model 30 was only 10 (or 
> fewer) years out of service. By defeating interlocks it's 
> probably possible to get a 2311 to spin up without a pack 
> and have it load the heads (with something soft between 
> them!) to check head loading and cylinder actuation. I'm 
> not sure what diagnostic facilities exist for the 1800, 
> but the 2841 controller had microcode routines to manually 
> test drives.
Yup, I think you were VERY lucky, too!  It was not hard to 
make a big mess with the old 14" disk drives, the heads were 
very easy to scratch.


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