exxos's Atari, Amiga & retro forum

nick_p wrote: 07 Apr 2021 16:52 DMA chip removed.... still got just 43 Ohm resistance between 5V and GND on the board which I'm guessing isn't good.

345 Ohm between pin 20 and 40 on the unsoldered DMA chip so not a dead short or really high....

I think exxos might be right... move on to known working purchase.

Not a dead short, and I can't check, but it seems like the resistance is a bit high.

nick_p wrote: 07 Apr 2021 16:52 345 Ohm between pin 20 and 40 on the unsoldered DMA chip so not a dead short or really high....

I just checked a working DMA chip here and I read 2.2 MΩ between 20 and 40.

Thanks all.

I've ordered a replacement chip anyway so will chuck it in once it arrives if I can work out why I have next to no resistance between the 5V pins on the board and GND first.

Cheers

Nick

derkom wrote: 07 Apr 2021 17:07

nick_p wrote: 07 Apr 2021 16:52 345 Ohm between pin 20 and 40 on the unsoldered DMA chip so not a dead short or really high....

I just checked a working DMA chip here and I read 2.2 MΩ between 20 and 40.

Much higher than I thought for a working one. Trouble is all my stuff is in storage ATM.

nick_p wrote: 07 Apr 2021 17:42 Thanks all.

I've ordered a replacement chip anyway so will chuck it in once it arrives if I can work out why I have next to no resistance between the 5V pins on the board and GND first.

Cheers

Nick

I wouldn't worry too much about that, but before you fit the new DMA check that 5v is good at the socket.

rubber_jonnie wrote: 07 Apr 2021 18:30

nick_p wrote: 07 Apr 2021 17:42 Thanks all.

I've ordered a replacement chip anyway so will chuck it in once it arrives if I can work out why I have next to no resistance between the 5V pins on the board and GND first.

Cheers

Nick

I wouldn't worry too much about that, but before you fit the new DMA check that 5v is good at the socket.

Ah okay, great stuff. Thanks :D

Hi all,

Quick update just to finish the thread..

The new chip turned up. I installed a new socket and tested voltages before installing the chip and they seemed good.

I put the chip in and powered on. Good news is the chip is no longer getting hot, and I now have an occasional flicker on the activity LED, and it sometimes spins up the FDD but unfortunately that's as far as it gets. (I've replaced the belt and cleaned the heads and can hear it is spinning the discs)

I've decided to cut my losses on this machine for now and have sourced a working STFM motherboard which I can transplant into my cases. Once I know that is working, I will spend an afternoon swapping chips between this board and the new one to see if I can work out what is good and isn't which may lead me to the issue, or at the worst, give me a set of working spares :D

Cheers

Nick

nick_p wrote: 09 Apr 2021 12:44 I've decided to cut my losses on this machine for now and have sourced a working STFM motherboard which I can transplant into my cases. Once I know that is working, I will spend an afternoon swapping chips between this board and the new one to see if I can work out what is good and isn't which may lead me to the issue, or at the worst, give me a set of working spares :D

It does often go that way. I feel quite lucky that my experience and help from people on this forum over the years has really given me the confidence to dig deeper, but sometimes you're just stuck and the damn thing won't work.

Or like my Mega ST4, just keeps dying time after time.....

Cheers rubber_jonnie and all.

I'm definitely not giving up on this one, but a working one gives me more to compare and test against without bothering you experts, but I can guarantee I'll be back shortly with more questions :D

Cheers

Nick