exxos's Atari, Amiga & retro forum

Going down the bad DMA route with no diagnosis is not the best way to get started.

The FDD controller or something else could easily be at fault.

You might change it out and still have the same issue.

Just go slow and carefully.

Id certainly get another floppy chip (wd1772 most likely) as I've found them to fail. Don't worry toouch about the whole bad dma thing as the drive will have worked at one time or another, so they can't have been that bad.......

Generally I would agree with you jonnie...but since the chips I ordered were so inexpensive..and this is a known issue.
I can at least eliminate the DMA chip.
Afterall ..the diag cart does say there is a DMA failure.
Seems logical to me as I have no other avenue of attack atm.
rgds

PhilC wrote: 01 Feb 2026 21:38 Id certainly get another floppy chip (wd1772 most likely) as I've found them to fail. Don't worry toouch about the whole bad dma thing as the drive will have worked at one time or another, so they can't have been that bad.......

I was tempted to try the wd1772 until I saw the price of them ...and scarce in the UK.
both the floppy chip and the DMA chip are soldered directly onto the PCB so whichever route I chose bases would have to be fitted.
Either way I'd have some soldering to do..
lol

Vipersan wrote: 01 Feb 2026 21:41 Generally I would agree with you jonnie...but since the chips I ordered were so inexpensive..and this is a known issue.
I can at least eliminate the DMA chip.
Afterall ..the diag cart does say there is a DMA failure.
Seems logical to me as I have no other avenue of attack atm.
rgds

Inexpensive can also mean fake, and not that I'm trying to put you off because the DMA could be dead, I'm just urging caution because I have seen a lot of things go wrong with desoldering and I've repaired plenty of torn traces myself as without good equipment and with a dose of impatience you can easily damage traces and then you have even more on your plate.

Before you get to taking chips out there is more you can check, such as the FDD cable, FDD power, check the drives work on another machine, even a PC by setting them to ID1.

BTW if you do the DMA test without using the correct test plug on the external ACSI connector it will fail and it isn't related to the DMA chip.

I know as I have the full test harness.

Hi Jonnie Already made cheat sheets for CPU / Floppy controller/ DMA chip etc etc...
Using a meter in continuity mode I checked where possible pin to pin or pin to resistor ..
All beeped out..which is why I need to start swapping out chips.
clock signals and supply lines also checked.
..I have no other direction to go.
Even J303 (floppy drive header) to all relevant chips/pins were checked.
I'm clutching at straws now...sadly.
I do have mechanical and electric solder pumps..though braid and flux is often the best least damaging way to desolder.
...plus Chipquik for those pins that dont take heat properly.
Also recapped the psu though I still have electrolytics on the mainboard to replace.
At this point I have nothing to lose but my sanity..
lol

Vipersan wrote: 01 Feb 2026 23:02
At this point I have nothing to lose but my sanity..
lol

And maybe some traces..

Did you read what I said about the DMA test failing if you don't have the ASCI test plug? I'm just concerned you're making a decision based on that result and it's a red herring.

No ...I did read it buddy....and I took note.
No my reasoning was based on the fact that the modified Sony drive works fine in a PC when the DS1 jumper is set.
..and that I spent hours testing traces between various chips.
I could only conclude that either the floppy control chip or the DMA chip was suspect.
...and when I watched youtube and my specific chip was mentioned as being problematic and giving very similar issues to mine.
I decided to try and eliminate that possibility.
Time will tell I guess.

Vipersan wrote: 01 Feb 2026 23:59 No ...I did read it buddy....and I took note.
No my reasoning was based on the fact that the modified Sony drive works fine in a PC when the DS1 jumper is set.
..and that I spent hours testing traces between various chips.
I could only conclude that either the floppy control chip or the DMA chip was suspect.
...and when I watched youtube and my specific chip was mentioned as being problematic and giving very similar issues to mine.
I decided to try and eliminate that possibility.
Time will tell I guess.

OK, cool, I'm not trying to be awkward, but have seen enough people react to 'Bad DMA' when the reality is it's much more complicated than just a bad chip and there is no substitute for proper troubleshooting.

I've also seen enough pictures of torn pads on motherboards to know that often people dive in without having suitable skills to do so. ST mainboards are not robust and can fail easily even just through manual handling. I've had vias break just because the board flexed when I plugged something in.

So I always urge caution to new members, I've been at this for over 20 years and have hit a few pitfalls along the way.

So a couple of other things I'd advise.

First off the sound chip does have a hand in floppy operations and if that is bad you may have issues. It does a lot more than just sound, including the parallel port, have a look at the schematics.

Secondly, have a look at this: Mandatory fixes

There is a lot of stuff in there that may have relevance in your case.

@Vipersan Please don't split topics half way and continue in this thread..