RESET circuit tweaks

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exxos
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Re: RESET circuit tweaks

Post by exxos »

Forgottenmyname wrote: Mon Oct 22, 2018 2:11 pm And it's a reasonable price too. Works fine on my Megas
I do have a odd reset fault on my machine like I think you guys had also.. I have to press the reset button to get it to turn on sometimes.. I think using the proper reset chip is a good thing to do anyway...
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Re: RESET circuit tweaks

Post by exxos »

Did a quick board to replace the NE555 the TL7705...

The two electrolytic's will need removing from the motherboard and this adapter board soldering's place..

I added in a jumper link because there is a inverted and non-inverted output on the TL7705... We need the inverted one to straight swap with the 555, but also there is a inverter driver (U1 74LS05) .... This will be replaced with a noninverting type such as 74LS05, as such the jump on the board needs to be changed for the noninverting output.

So hopefully this new reset circuit will be a lot better than the old one. And also it does away with a inverter chip and uses a straight buffer instead... I plan to use these buffers to replace as many chips as possible on the STF...

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rs2.jpg
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As before, If people want to try it, here is the file...
STF_RESET_MOD.zip
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Re: RESET circuit tweaks

Post by exxos »

Reset boards just arrived :)

I got a few of these made, so what I will do is just include them in the 1772 PCB bag...

Will add these "kits" into my store hopefully later today..

As said before, I will just do it cheapest first class postage...

rstb.jpg
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STORE LINK

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Re: RESET circuit tweaks

Post by Icky »

Been working on the reset board today. Not much luck.

IMG_3254.png
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The board sometimes has the STF sat at white screen or black screen and on the odd occasion does reset and boot.

The reset switch has no effect before or after the machine has booted.

I have been playing around with different values of resistor / capacitors as there is quite a difference between the values for the board and what the MEGA ST circuit has.

Will start to trace the outputs etc to see what is going on.

One other thing, have been doing a clean up on the original patch as the wire link is no longer needed with this board in place.
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Re: RESET circuit tweaks

Post by exxos »

Don't forget those jumper pads give a inverted and non inverted output..
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Re: RESET circuit tweaks

Post by PhilC »

Mine is working well now with the original 555, so not sure I will tweak it yet.

Just need to get my floppy working again.
If it ain't broke, test it to Destruction.
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Re: RESET circuit tweaks

Post by Icky »

exxos wrote: Sat Nov 10, 2018 4:01 pm Don't forget those jumper pads give a inverted and non inverted output..
I actually tried out both as it wasn't too clear which one to use.

I believe from reading the thread that the inverted one is the one to use (middle and bottom jumper pads). Correct me if I am wrong Exxos.
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Re: RESET circuit tweaks

Post by exxos »

Not sure offhand... Pin 3 on the 555 should switch when you press the reset switch ?
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Re: RESET circuit tweaks

Post by PaulJ »

Most likely will not put in the reset flat in as the 555 reset circuit runs flawlessly here. It always powers on with no issues and resets reliably. In this case I may follow the old rule of "If its not broken don't fix it".. :mrgreen: :mrgreen: :mrgreen:

Also it breaks the zoning law that says within one chip of the CPU stack heights of greater that one story are not allowed. They block the ocean view of the CPU!! :cheer: :cheer: :cheer:
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Re: RESET circuit tweaks

Post by exxos »

The new board will have the new reset circuit in place.. Its part of my master plan to do away with the TTL chips :)
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