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Member Since 9 Feb 2016
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Super Mario Bros for the c64

Thu Apr 18, 2019 7:55 PM


Replacing a 7805 linear regulator with a DC-DC step-down module

Sat Feb 2, 2019 12:12 AM

Just sharing some info that might help someone here.


I'm a little particular when it comes to powering my precious 8 bit machines with old power supplies. The Atari power supply pictured below was one of the first types distributed with Atari machines in Australia during the early days. There is also an identical looking one that goes with the c64. The only good thing about these is that they're rebuildable. Besides there being no mains fuse anywhere they utilise a very simple circuit and a 7805 linear regulator which runs very hot ( even with a heatsink ) and is very inefficient.



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Here is the schematic which almost matches this circuit for the original power supply I have. 


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I want to replace this with the more efficient switching regulator so I chose this DC-DC buck converter. I begin by doing some prep work and cut two wires to equal lengths.


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All soldered up to the input and outputs.


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I remove the cover of the power supply and the heatsink.


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Two things have to go. The  voltage divider ( two resistors ) and the voltage regulator. I also remove the large cap temporarily to test it before putting it back in. At the time of writing I realise that I don't need the smaller cap on the output so I'll remove this later once I get my fuses.


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I power up the supply to locate 12v with my meter. This will be the left most pin ( input ) of where the 7805 used to be.

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Diagram of the device from the front.


Attached File  Voltage-regulator-pinout.png   7.58KB   17 downloads


I solder the red wire to where the input used to be and the black wire ( ground ) to the ground pad of PCB then power it up. The module has a blue LED that lights up when it's powered on.


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I connect my DMM to the output of the module to check the voltage which I'll need to turn down.


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There is a little trimpot on the module. You'll need to turn it anti-clockwise quite a bit before the voltage starts to change. I adjust the voltage to just over 5v because I know it's going to drop under load slightly.


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I now connect the other ground wire to the ground pad and the red wire ( positive ) to output of where the 7805 used to be.


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With the Atari 65XE powered up the voltage only dropped by a hair.


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Final adjustment to 5v


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Optional step on the scope. Checking for a nice clean DC signal


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Just making  sure she pasts the hardware test.


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I've been running this for a few hours now and the voltage and temperature is really stable. Even without a heatsink this barely gets warm. Not bad for a few bucks



How do folks here verify 128kb RAM ?

Tue Nov 20, 2018 4:16 AM

I'm currently porting a game from the Apple 2 to the A8


Is there a more efficient way to verify 128kb of ram instead of writing/reading back arbitrary values in the other 4 ram banks ?


For example.



STA portb


LDA #$55 ; store arbitrary value in normal ram
STA $4000
LDA portb
EOR #$1C
STA portb
LDA #$AA ; store arbitrary value in bank 0 - extended memory
STA $4000
LDA portb
ORA #$1C ; switch back to normal ram
STA portb
LDA  $4000
CMP #$55 ; $4000 should still contain initial value of #$55

Altirra debugger - log execution to a file.

Wed Oct 17, 2018 3:46 AM

Perhaps I didn't read the documentation well enough but I couldn't find anything that could trace the given CPU to a file, as in what MAME can do.


trace {<filename>|OFF}[,<cpu>[,[noloop|logerror][,<action>]]]


Anyone know if there is similar feature available ?


Shoestring's Atari 8bit RAM tester

Sat Aug 18, 2018 7:58 PM

Hi all.
I've been fairly busy working on a DRAM & Basic ROM tester for 64kb Atari systems. This is really my first attempt at writing any 6502 for the Atari but I wanted to start off with something simple before I started porting some c64 games to the Atari. It gives me a chance to learn more about the hardware. I've finished testing in MAME and Altirra and I am currently testing this on real hardware to catch any last minute bugs or make improvements before I release it...  for free of course.
This program installs in the OS ROM socket and is used as a means for testing the DRAM and Basic ROM. If you have some spare 27c128s lying around then this can easily be installed. 
I've currently purchased a few different systems to test my program in. But I might need some help from someone to test in an A800XLF system if I can't get my hands on one.
What it does.
 Performs data line integrity checks ( checkerboard and inverse, walking 1s and walking 0s, 1s and 0s and exhaustive )
 Exhaustive takes the longest and is probably overkill.
Tests 0x0-0x7ff first to test integrity of the stack and lower mem before using it. 
Tests 0x800-0xbfff 
Copies the program rom into DRAM once the above memory verified good.
Turns the OS ROM off then tests the DRAM 0xc000 - 0xFFFF.
Displays bit pattern / test being performed written to DRAM  ( also useful for verifying video hardware / PMG ).
Plays sound to indicate CPU is functioning ( for systems playing blind ).
To do.
1. Code for address line integrity ( walking 1s/0s ) checks are in but are currently not being used, this will be added in a later version of its feasible and necessary.
2. Write a version for stock 600xls or adapt the current image to handle those systems.
3. Look at writing a version for the Atari 800 for stock 48k systems. 
4. Add another indicator / arrow to point out which memory region is bad.
5. Any other suggestions ?
Here is a video of an example of bad ram and bad basic rom.