?OM errors

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bleachin
Posts: 6
Joined: Wed Jun 10, 2020 9:50 pm

?OM errors

Post by bleachin »

Hi all,

Have just purchased a dragon 32 to relive my early computing days. Machine turns on and sort of starts, cutting off after the D fo data on the first line but still gives me an ok prompt, which then fails to do anything. Reset will then normally give me the full basic dialog.

At this point the entry of anything, when hitting return I just get a ? OM error. The introduction manual index tells me to look at page 76 where its not mentioned at all. I do believe this was the reason for me getting rid of my original machine many years ago.

I am assuming that the initial power on issue and the ability to not do anything are one in the same issue but any pointers on where to start would be greatly appreciated.
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robcfg
Posts: 1301
Joined: Sat Apr 04, 2009 10:16 pm
Location: Stockholm, Sweden

Re: ?OM errors

Post by robcfg »

Welcome!

If I remember correctly, OM stands for out of memory error, which may indicate a ram, rom or sam chip errors.

It could be that the chips are ok but there are broken track or corrosion on the motherboard.

Can you post pictures of both sides of the motherboard?

Cheers,
Rob
bleachin
Posts: 6
Joined: Wed Jun 10, 2020 9:50 pm

Re: ?OM errors

Post by bleachin »

Thanks Rob,

Got both sides of the main board, thrown in the power as well, just in case. Looking at the top there appears to be some additional resistors added, not sure if that a good or bad thing or makes zero difference at all!
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bleachin
Posts: 6
Joined: Wed Jun 10, 2020 9:50 pm

Re: ?OM errors

Post by bleachin »

Having just found the page of mother boards on the site certainly looks like this has had some changes made in the past!
bleachin
Posts: 6
Joined: Wed Jun 10, 2020 9:50 pm

Re: ?OM errors

Post by bleachin »

Anyone any ideas?
sorchard
Posts: 425
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: ?OM errors

Post by sorchard »

The extra resistors are fairly normal and are tweaks to the display. I've seen these on quite a few boards.

As for the fault, there are many possibilities. It could be a RAM IC, ROM, CPU or SAM as previously mentioned. It could also be a bad power supply.

Try lifting each socketed IC a little and pushing it back in again to see if that improves things.

Applying light pressure to the board in various places while switching on can sometimes provoke a change in behavior and therefore reveal if there is a bad connection somewhere. I've had to resolder CPU and ROM sockets a few times before.

I don't know if this helps, but the most common faults I've encountered are bad soldered joints, bad IC sockets, shorted tantalum capacitors, and bad memory ICs.
Stew
sixxie
Posts: 1126
Joined: Fri Jul 18, 2008 8:36 am
Location: Hertfordshire
Contact:

Re: ?OM errors

Post by sixxie »

A Dragon 32 board I thought was dead magically came back to life for as long as I held a probe to the RESET line on the CPU - turned out that pin was just not making a good connection in the socket!
prime
Posts: 245
Joined: Fri Apr 10, 2009 1:40 am

Re: ?OM errors

Post by prime »

sixxie wrote: Tue Jun 16, 2020 3:59 pm A Dragon 32 board I thought was dead magically came back to life for as long as I held a probe to the RESET line on the CPU - turned out that pin was just not making a good connection in the socket!
I had a problem almost identical to that on one of my Jupiter Ace boards except it was IIRC A8 of the Z80. It was a dual wipe socket, which have contacts made out of stamped , folded metal. The metal had become flattened and didn't make proper contact with the IC leg. Replaced it with a turned pin socket and it worked perfectly after that.

Cheers.

Phill.
bleachin
Posts: 6
Joined: Wed Jun 10, 2020 9:50 pm

Re: ?OM errors

Post by bleachin »

I'm guessing this might be a difficult one to traceout then.

A press, wiggle and push of most things does appear to have at least made it more reliable when turning on, it does also now type better but I am still seeing the om error.

A couple of questions

Is this something I should consider returning to the sender as turning on, is very different to the "working" as they described it? or

Is this something that I could work though and find the dryjoint/cracked path if I spend time on it and if so where is best to start this process?
sorchard
Posts: 425
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: ?OM errors

Post by sorchard »

It's worth sounding out the seller to see if they are willing to do anything. It may well have been working when last used, but has deteriorated with time, and being rattled around in the back of a van was the last straw. It's impossible to know for sure.

I should have said initially that the first thing to check is always the power supply, namely the +5V, +12V and -5V rails. The heatsink is 0V, and you can find the supplies either on the power connector, or three of the four corners of each the RAM chips.

After a bit of searching it would appear that 4116 DRAMs are prone to failure, as they are sensitive to problems on the -5V supply. This does seem to be a common theme with Dragons. (And Spectrums too)

It's worth checking if any of the memory chips are unusually hot compared to the others as this is a known failure mode.

I'm not yet convinced there is a problem with connectivity. In the case of a bad soldered joint or dodgy socket I would expect either the machine to boot up fine and then crash when light pressure is applied to the board in certain places, or it will only boot up when pressure is applied. That said, a relatively easy thing to try is resoldering the SAM, CPU and ROM sockets, followed by the RAM chips. You never know, that might do it.

Tracing connections with a DMM is a bit on the tedious side. I would suggest concentrating on the CPU, SAM, ROMs and RAM and making sure that each leg is connected to something else. (If it's supposed to go somewhere that is: You may need a schematic to guide you) This is a job that's much easier with an oscilloscope and an eye for unusual signals.

If you search for 'Dragon 32 faulty memory' on youtube there are a couple of videos that show memory repairs so you can see what's involved. This also throws up some coverage of power board problems. Well worth a look.

Don't lose heart if you don't have any luck. I'm not sure when the next Dragon event will be but I'm sure there would be someone there who would be happy to have a look at it for you. Failing that, even broken/untested machines are sought after, as it is often a low-cost fix for someone with a bit of experience.

Keep us posted!
Stew
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