Documentation and Parts

The single most important document you will need to repair this machine is the PDP-8 Maintenance Manual, 6th edition printed in January 1971.   This may be downloaded from the resources page.

I spent many years working as a Field Engineer for both CDC and Data General in the 1970’s and 80’s and this was my first encounter with DEC’s systems and documentation.  Consequently, I’m sure there will be a place in heaven for the unnamed author(s) of the PDP-8 Maintenance manual.  They clearly understood the working of the machine down to individual gate level. It is clear, concise, comprehensive, and indispensable.  Print it off, store it in a ring binder, and write your notes on the pages.

I also suggest you print off the Engineering Drawings separately and clip them together as you will reference these regularly, along with the signal source pages.  It is always helpful to know where signals originate! 

Depending on the state of your machine, you may need to use the Factory Memory Tuning Guide to calibrating the core memory.  The document can be downloaded from the resources link.

There is one very important change to this document which I will highlight during the memory tuning section. I only discovered this change because a Field Engineer 50 years ago wrote a revised setting on the circuit diagram showing the slice voltage of 7.7v as opposed to the 7.2v shown in the manual and the guide.  The former brings the machine to life, whereas the latter does not – at least that was my experience.

A full list of Circuit Diagrams is available and can be downloaded from the Resources link. 

Note, the board revisions may not match those in your machine.

Another very helpful find was the DEC Part Number Cross Reference Guide. This is in the form of an Excel spreadsheet and can also be downloaded from the Resources link:

Sourcing the exact components, particularly transistors can be a problem as most are no longer in production and are available only from collectors or electronics component retailers in the USA who specialise in selling old stock.

Ultimately, I resorted to searching on Alibaba.com and found that you can source just about any transistor regardless of how old, brand new from China. I typically brought them in lots of 100 and usually for less than $1.0 each. The retailers are often one person ‘companies’ with a website, who have a relationship with various manufacturers.  I used two or three different suppliers and had no problems either with communication, shipping or quality. The PDP-8 would not be working today without these products.

It is easier finding local replacements for the common diodes used on the boards. The most common diode is the DEC664 which is found on most cards, with a couple of hundred of them on each of the twelve Accumulator and Memory Buffer cards.  I used a 1N4149 which is a low cost small signal diode but anything similar should be fine.  I replaced more than 2,000 of these diodes, but more on this later.

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