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Temperature Issue (Possibly) Explained

A project log for Ferrite Core Memory Module

A sub-project of the ED-64 Computer to document the development of a 64-byte ferrite core memory module

andrew-starrAndrew Starr 04/25/2017 at 01:552 Comments

I have some more boards for the ED-64 being fabricated at the board house, so while I'm waiting for them to turn up, I have turned my attention back to the temperature issue. As the computer warms up, the data flip-flops have a tendency to stick 'on'. This can occur whether or not memory accesses are occurring, and I believe I know why.

The current sense amplifier design is a cheap and cheerful affair, the core of which is an NPN and PNP transistor connected as a Sziklai pair. This configuration has a huge amount of gain (the gains of the individual transistors are multiplied together), so allows easy detection of the core switch signal. I was originally going to go with a common base amplifier, but they are tricky beasts to design, and when I found that the Sziklai pair worked ok, I went with it.

There are 2 amplifiers per channel, to catch both positive- and negative-going core switch signals. The centre of the pulse transformer secondary is has a forward-biased diode to bias the NPN transistors almost to the point of switching. The PNP transistors are wire-ORed together, and go to the active-low SET input of the flip-flop. When a core switch pulse comes in, one or the other of the PNP transistors turn on, pulling down the SET input, setting the flip-flop. The sensitivity of the amplifier is tweakable with the 20k pot.

I think the temperature issue is caused by heat increasing the gains of the transistors (exacerbated by the fact they are multiplied together), to the point where the existing diode bias is enough to pull down the SET input enough to hold the flip-flop 'set'. The pot then has to be adjusted to reduce the bias and fix this condition.

I believe the solution is to replace this design with a high-gain AC-coupled amplifier. I have a design in mind, more on this later.

Discussions

Andrew Starr wrote 04/25/2017 at 10:06 point

Yeah, I have a scope, but it's hard to probe where I need to, and the sense amps are quite fragile (kludged kynar wire and mid-air solder joints). I was just happy to be able to load code and data into the damn thing :)

Thanks for the link! I'll check it out. The new sense amp design has a common base stage as a signal input, followed by an AC coupled complementary transistor pair with some negative feedback for stability...I just need to figure out how to sensibly bolt on a current sink output to the flip-flop...

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Yann Guidon / YGDES wrote 04/25/2017 at 09:53 point

Nice log !

I think you'll like reading  http://sound.whsites.net/articles/followers.html

And an oscilloscope might help you unravel the mystery too :-D

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