exxos's Atari, Amiga & retro forum

When I checked it was 12.05V.

It says that R26 (2.7K) is voltage adjust resistor. Guess if you slapped like 100K Across the should shift the voltage divider down and I assume raise the output voltage a little. The side-effect is the 12V may well increase slightly as well.

According to my workings out.. With a ideal 5V and 12V output, the output of the resistor network would be about 2.5V.

Now when running a 4.77V rail, that voltage then drops down to about 2.3V. So in order to satisfy the original condition, it would need a 47K across R23.

Does not seem like R26 as much effect on output voltage adjustment :shrug:

But I would just try 100K across on the 5V feedback resistors, R23, R24, See how it affects the 5V and 12V. Just play around with it, will probably be quicker than trying to mathematically work it all out :lol:

I'll see if I feel brave enough. :-)

stephen_usher wrote: 10 Dec 2021 21:50 I'll see if I feel brave enough. :-)

Well you know we all want to see it happen, good or bad, video it ;) :lol: :fire: :fire:

Hehe.

Anyway, back to the RAM problem: I've been pawing through the schematics and the only chip which could possibly be causing "bit flipping" is actually the TMS4500 memory controller. It's the only chip which uses the address bus and which could possibly generate memory addressing errors rather than bit flipping, which would be impossible as the bits are on different chips. The RAM controller could be activating the wrong outputs when specific addresses are input to it.

Seeing as the errors decrease as the machine warms up I just hit the chip with freeze spray... and the errors increased, so I think we may have the culprit. I've ordered a replacement from *shudder* eBay for a fiver. I'll have to desolder the current chip and put a 40 pin socket on the board.

Rechecked again this morning. This time making sure that it wasn't something else I was cooling changing things. So I first sprayed the underneath of the board (to check that it just wasn't the soldering) and then the chips around the memory controller, all no change. Freeze the memory controller and suddenly errors in segment 0x4000 and lots more in 0x6000

stephen_usher wrote: 11 Dec 2021 09:59 Rechecked again this morning. This time making sure that it wasn't something else I was cooling changing things. So I first sprayed the underneath of the board (to check that it just wasn't the soldering) and then the chips around the memory controller, all no change. Freeze the memory controller and suddenly errors in segment 0x4000 and lots more in 0x6000

Nice, sounds like you have a culprit :)

TMS4500 pulled from the board and a socket installed ready for the replacement.

Well, the system's not any worse than it was, so I've not broken anything further. Always a good thing.

Unexpected item in recycling area.