exxos's Atari, Amiga & retro forum

Hi All,

Was wondering whether anyone could point me in the right direction with troubleshooting the following.

Mega 1 original TOS 1.02 being used as a games machine only with original floppies. This has been working fine, as forever, only this week the following behaviour has been seen.

Switch on machine/reset button with any disk in drive (all working disks as have other ST's to test them on) nearly straight away 4 bombs then flash on screen then 4 bombs again - FREEZE

Switch on machine/reset button with no disk in drive machine eventually boots to desktop where everything is fine, I can run programs/games on same disks that would give 4 bombs if present on restart!!

My problem now is I can't play games that will only boot from disk (i.e. won't start via GEM desktop) so has anyone seen this weird issue with booting with disk in drive. Thanks in advance.

What drive is it exactly ? long or short button ? Have you cleaned the heads either way ?

Also likely the PSU is likely failing as they all do being 30+ years old now.

Short button. No haven't touched the heads (really should now you've mentioned it as it was the case of ain't broke don't fix it)

Yes I was thinking the PSU failing would cause some funny random faults as it's not like anything has actually broken or failed just a certain procedure is causing issues.

Strange thing is I put a executable bootsector disk (fcopy AV bootsector) and it ran fine. Now all disks work fine again.

Utterly random again like you said possibly failing PSU.....

thanks

If a disk fails to boot at turn on, (bombs or whatever) do you press reset button and see if it loads a second time ? If its literally only failing at power up, then most likely is the PSU.. Though I would still clean the floppy heads as likely they do need doing anyway.

exxos wrote: ↑Tue Jun 11, 2019 2:35 pm If a disk fails to boot at turn on, (bombs or whatever) do you press reset button and see if it loads a second time ?

yeh tried, same 4 bombs as switch on.

But as it's now magically working again I will defo go with PSU recap etc. before it properly goes. Before I open it up are your PSU repair kits generic or do I have to have a particular PSU type?

thanks again.

There are several PSU's... Its probably similar to a SR98.. you would have to post a image of here...

My Mega 4 has an ASPF34/2 fitted so I'd suggest you check before ordering any bits.
HTH.
Jez

Another quick suggestion, when my DMA died on my 4mb Mega 1, it would show 4 bombs with a disk in the drive at boot up. So it may be worth checking some of the soldered joints on the DMA and if its socketed, then clean the pins and socket with IPA.

Carrying out exxos's 1772 mod is worthwhile too as is recapping the PSU, but as JezC says, check which one first as I've had 3 different PSUs in the 3 Megas I own. The resistor kits, although similar, may not be interchangeable.

Many thanks philc i did see the following article :

https://www.exxosforum.co.uk/atari/last/DMAfix/index.htm

Which mentions MEGA ST's so somewhere to go if the PSU fixing doesn't work.

I opened my MEGA 1 and took some piccies if I can identify them correctly my DMA chip is not socketed.

exxos; I have taken some pics of the PSU too small caps are quite wobbly, PSU model Mitsumi SR138, if you could advise on which recapping pack would be best.

thanks.

jwd wrote: ↑Wed Jun 12, 2019 11:19 am exxos; I have taken some pics of the PSU too small caps are quite wobbly, PSU model Mitsumi SR138, if you could advise on which recapping pack would be best.

ah SR138, couldn't think of the number. The SR98 kit will likely have the parts you need, though I haven't upgraded a SR138 or even documented it yet, so can't say if the caps will physically fit. I will try and check when I get back home tomorrow.