Atari Lynx Power Circuit

[@rt]

Hi Guys
I could have sworn the forum had a specific vintage computing section. Was it removed?

I have an Atari Lynx with momentary pushbutton on, and a separate momentary button off.
most of this circuit is a complicated latch, and some diode protection, with a suicide diode.
At the end of the day though, it's just a cheap Zener regulator at the far right of the schematic.
It's prone to failure, overvolting the unobtanium silicon with the ~9V battery supply.

I want to replace this with one of the Tracopower switchers that are designed to replace a 7805.
I'm just unsure if what I'm doing will interfere with whatever is going on for latch feedback at TP19.
Should I keep the 120R, remove the Zener, and connect the reg input across where the Zener was?

[@rt]

ps. it looks to me now that TP19 controls oscillation like in some large linear power supplies.
That makes things more complicated. It would be fairly easy to do a FET button switch and reg on a small board,
but not as easy to integrate a reg here.

[NiHaoMike]

The simplest solution would probably be to add a TVS or zener diode for overvoltage protection and a fuse on the input. Then only minimal changes would be needed.
If you're going to completely rework the power circuit, consider using 1S lithium with a boost converter.

[@rt]

I have indeed considered a crowbar. It would prevent the worst of the potential damage,
but not a complete solution. It appears it's a very common problem.
I might actually breadboard this circuit and experiment on the right hand side.

[Kim Christensen]

Also, be aware that the switching regulator pass transistor (Q11) is in the negative lead of the supply.
If you put a 7805ish Tracopower switcher in the positive rail that may have consequences.
ie: Where does the emitter of Q4 connect to? What else might be referenced to the positive rail?

Should I keep the 120R, remove the Zener, and connect the reg input across where the Zener was?

No, it not that simple... The zener is part of the feedback circuit for the crude switching regulator made up of U6 (pins 1 thru 6, 8&9), Q11, L6, D14, etc. When the 5 volt output gets too high, the zener conducts, turning Q13 on, which via the feedback path, reduces the on time of Q11 so that the output voltage stays stable. Opposite happens if the 5V output drops too low.

[Bud]

Hi Guys
I could have sworn the forum had a specific vintage computing section. Was it removed?

It is a child board in "Computers" section.