Thursday, April 25, 2024

Burned again with transistor substitution chart for IBM house numbered transistors

SUBSTITUTE TRANSISTOR ARRIVED FOR IBM 086

Per the substitution chart, the replacement should be a 2N214 which I had purchased on eBay. When it arrived, I put it on the transistor curve tracer and found that it has a very different (lower) beta than the IBM part. The original 086, which seems to go bad when driven to the voltage and currents designed into the cards, shows a beta of about 20. That is, a 5ma input current to the base would generate a 100ma current through the collector. 

The 'replacement' transistor has a beta of just over 5.5, so that the same 5ma input current only produces about 27.5 ma of output current. Since this transistor has to drive two emitter follower stages to increase the current to the level that will drive the four parallel IBM 108 power transistors to control a 20A load, being relatively weak is a problem. 

Even with the weaker transistor, I should be able to regulate a light load, 1A or less, but the same runaway takes place. I turned my suspicions to the remaining two transistors, a pair of 026 that act as a comparator between the reference voltage and the actual delivered voltage. Turning the potentiometer does move the voltage up and down, at least in the early seconds when the regulator is behaving, so the likely culprit is the other 026. 

When I can get to it, I will remove that 026 and test it on the curve tracer. Since I move my shop at the end of the month, and further since my wife and I will visit our daughter in Providence for a few days next week, I have to put everything into shape to be moved the tenth of a mile distance from the current site to the new location. 

I am not able to find any IBM 086 or 026 transistors on eBay, only the purported substitutes from the chart. Based on my bad experience with the 2N214 as a replacement for an 086, I am loathe to buy the 2N270 that is listed as the 026 substitute. 

I did try removing the base connection to the 026 I suspect in my circuit simulation, and indeed the regulator fails to hold a voltage in that case with the output running high. Since the circuit parameters of the transistors in my simulation are not patterned on the real IBM parts, all I can do is take this as a qualitative indication that the 026 in question has failed. 

I did find an SMS regulator card for a related IBM 3V power supply. It does have two 026 transistors on it and a similar structure to the card for the 1130 power supply. I bought it as it is a good source for parts and might even serve as a swap in replacement card for the VCF machine's supply. It cost $50 including shipping, a bit steep but has the correct transistors. 

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