WHY MODEL AND HOW
LTspice is a free version of the SPICE circuit modeling software. Entering a schematic allows the operation to be simulated and graphs to be produced of the voltage and current at selected nodes. Very valuable to understand the operation of a circuit in detail. I also find it useful to modify components to see how they would behave if they are defective, in order to confirm a hypothesis about which part has failed.
The libraries provided by Analog Devices when you download LTspice include many component, but not every possible part. Usually the Spice model for a component can be found online and added to your schematic. However, there are almost no models online for Germanium transistors and diodes.
IBM USED GERMANIUM SEMICONDUCTORS IN THE SMS AND SLT PRODUCT FAMILIES
Standard Modular System (SMS) was the basis for IBM's transistorized computers in the 1950s and early 1960s. Solid Logic Technology (SLT) as the basis for the 1960s and 1970s computers such as S/360 and 1130. The diodes and transistors used in both families were Germanium based.
While the industry was transitioning to Silicon semiconductors which are much better for most purposes, IBM needed massive quantities of transistors and diodes, including manufacturing their own. Staying with Germanium assured them of adequate supply and avoided the need to re-engineer many circuits. Similarly the industry was beginning to use integrated circuits (ICs) in the 1960s but IBM's volume needs and other factors led them to stay with discrete transistors and diodes in the SLT generation.
IBM used many different transistor types in their products in SMS. In SLT, the transistors and diodes used in the SLT modules were much more standardized but SLT cards often included separate transistors in a myriad of types.
SPICE MODELS FOR GERMANIUM SEMICONDUCTOR COMPONENTS
There are only a handful of models to be found on the Internet for Germanium transistors. Music effect pedals make use of Germanium because of the way that signals are distorted by the device's characterists - pedals such as a Fuzz box - which is by far the largest use of Germanium today. Thus, the few transistors that are used in effect pedals do have some models to be found.
However, there were many hundreds of different Germanium transistors sold, the vast majority of which have no models online. IBM made use of almost 200 transistor types for SMS and SLT. This is where my problem lies.
GERMANIUM VERSUS SILICON GROSS BEHAVIOR DIFFERENCES
Silicon semiconductors tend to have a turn-on voltage across the base-emitter junction of around 0.6V while Germanium semiconductors turn on at much lower levels. typically 0.1 to 0.3V. The operation of a transistor with a given voltage at the base can be very different between Germanium and Silicon, as a consequence.
Germanium transistors operated in reverse mode (switching the emitter and collector) work better than Silicon transistors in the same mode. The amplification factor (beta) is less in reverse mode, but at a reasonable level with Germanium. Other characteristics worse for both types in reverse mode, but there are some circuits (especially effects pedals) that chose to use the transistor in reverse mode.
Germanium devices have higher leakage currents and are less stable under temperature variations than Silicon. They also are more susceptible to failure due to air leaking into the package than Silicon. Silicon forms an insulating oxide when exposed to air, which limits the impact on the device, while Germanium does not. Finally, the way that the component leads enter the typical Germanium package were susceptible to breaking or corrosion forming at the entry point.
IBM labeled their transistors and diodes with their own designator, even if the part was procured from an industry source who sold the same part with an industry recognized number. Thus even if a model had existed for a particular transistor under its commonly known number, to model an IBM circuit required knowledge of the IBM to industry standard number translation.
GRADUALLY EXPERIMENTING WITH SPICE MODELS FOR SOME IBM CIRCUITS
I have been hacking at some models that exist for Germanium devices, trying to get the IBM circuit to operate as it should. I have found a table that circulated among IBM repair people (Field Engineers - FEs) during the SMS era that purports to list industry standard transistors that could be substituted for an IBM numbered transistor in a pinch. These may list five or six different transistors, thus making clear that the substitute is NOT an exact match.
I have located some spec sheets for the industry standard transistors in that substitution table and from that I have worked up a potential set of specifications for the IBM part, such as hFE and Vceo, that would be the basis for a spice model of that IBM part. This is challenging because a spice model does not have entries like hFE, but instead has a myriad of parameters as you can see below:
My challenge is to turn the potential specifications I derived such as hFE into the parameters above. This is a work in progress. It will be quite valuable in the long run to be able to model IBM circuits accurately.
