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Old 23 August 2011, 18:23   #1
gunni
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Unexpected surprise inside A500

Since my girlfriend managed to acquire a box of various commodore goodies a while back I haven't had much time to really investigate other than getting one A500 up and running and doing a bit of gaming and testing out the 3 C64s (2 dead one working but I'll get back to that later).
Today I decided to get the (hideously yellowed) other A500 out of its box to see if it was working and came across a curious switch next to the joystick 1 port. I popped in a workbench disk and switched on, all went well so I decided I would try to boot up after flicking the switch to see what happened. Nothing happened... the insert disk screen was there and the floppy drive was doing nothing.
Even more curious I opened it up and even through the shielding I could see there were "extra bits".
Anyway... can anyone give me a clue to what is going on with this frankenstein A500?



Sorry about the pic quality by the way... any suggestions to the wiring on this memory expansion? The yellow wire has no wire exposed so I don't think its been connected anywhere and the other 2 have been connected together...

Also, with regard to the C64's - the 2 non functioning ones seem to have chips missing although I can't tell if it would be the same chip or not as one is a 64C. Can anyone point me to someplace that might help me identify the missing chips?
Thanks!
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Old 23 August 2011, 19:29   #2
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there's some service manuals here:

http://www.bombjack.org/commodore/commodore/

Part of the great depository indexed here: http://www.bombjack.org/commodore/

If you can repeat the IC numbers, then it would be easier to let you know, as well.

Cheers,

c
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Old 23 August 2011, 19:31   #3
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Looks like the daughter bord for a 2mb memory expansion that goes onto the Gary (I think its been a while)

scrap that idea, just looked at the pic again and i am talking out of my arse
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Old 23 August 2011, 19:44   #4
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It's a Roctec RM2MC 2Mbyte Expansion board (I can just about make out the writing on the 2nd pic)

http://hardware.amiga.hu/exp/rm2mc
http://www.amiga4ever.nl/expansions.php?id=157

An additional 512K Chip RAM + 1Mbyte Fast RAM = 2Mbytes total

Last edited by alexh; 23 August 2011 at 19:49.
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Old 23 August 2011, 19:46   #5
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Quote:
Originally Posted by alexh View Post
It's a Roctec RM2MC 2Mbyte Expansion board

http://hardware.amiga.hu/exp/rm2mc
maybe I am not a thick as i thought then
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Old 23 August 2011, 19:50   #6
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Nope, you were right
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Old 23 August 2011, 20:07   #7
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Thanks guys! It was only showing 1/2 meg RAM when workbench booted so I assume the loose wires from the expansion board have something to do with it.

@amiga1 thanks for the link, I'll have a look see if I can identify what I'm missing later
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Old 23 August 2011, 20:07   #8
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Quote:
Originally Posted by deicidal View Post
Looks like the daughter bord for a 2mb memory expansion that goes onto the Gary (I think its been a while)

scrap that idea, just looked at the pic again and i am talking out of my arse
oh yeah... i had one for these for my A500 back in the day
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Old 23 August 2011, 23:51   #9
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everyone is right on the mark about the rest of the pics.nice one.

plus the wires i think you are talking about (by the trapdoor)are for a switch.thats missing.

hope this helps

if you post pictures of the inside of the 64's someone will identify the missing parts.

Last edited by roy bates; 24 August 2011 at 00:03.
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Old 24 August 2011, 00:10   #10
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I have now established the missing chips from the C64's are the SID chips and it seems I'm probably just as well picking up a cheap complete 64 as they are so pricey (cant believe someone on ebay is selling one for around 100 quid!).

Thanks for your help everyone!
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Old 24 August 2011, 07:55   #11
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Amibay is a place to go when you need replacement parts or even a complete system. Ebay is evil.
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Old 24 August 2011, 19:19   #12
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ok, had another look at the innards of the A500 today, the switch I mentioned is connected into the "EVEN CIA" chip.
with the switch in one position the A500 boots up and appears to only have the standard 512K, in the other position the floppy drive refuses to operate.
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Old 25 August 2011, 06:20   #13
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It sounds like a "turbo" 68000 CPU: some accelerators (of sorts) just double the 68000 clock, but the main clock to the motherboard is generated by the 68000 itself (E-clock).

When doubling the CPU (hence the E-clock too) the CIA chips goes nuts and the floppy will refuse to work.
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Old 25 August 2011, 06:30   #14
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Quote:
Originally Posted by rkauer View Post
It sounds like a "turbo" 68000 CPU: some accelerators (of sorts) just double the 68000 clock, but the main clock to the motherboard is generated by the 68000 itself (E-clock).

When doubling the CPU (hence the E-clock too) the CIA chips goes nuts and the floppy will refuse to work.
im glad you mentioned that.in the third picture theres a switch missing for configuring the fastram.
maybe its worth a look.
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Old 25 August 2011, 14:46   #15
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Quote:
Originally Posted by rkauer View Post
It sounds like a "turbo" 68000 CPU: some accelerators (of sorts) just double the 68000 clock, but the main clock to the motherboard is generated by the 68000 itself (E-clock).

When doubling the CPU (hence the E-clock too) the CIA chips goes nuts and the floppy will refuse to work.
Ah ok, thanks for clearing up that little mystery!

Quote:
Originally Posted by roy bates View Post
in the third picture theres a switch missing for configuring the fastram.
maybe its worth a look.
Yeah, as I recall 2 wires connected to each other are setting it to "ON" with the "OFF" wire being cut. Might try wiring in a switch to see if it does anything. I've been searching online to try find a manual to no avail to see if the jumpers on the expansion are set correctly
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Old 25 August 2011, 16:15   #16
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Quote:
Originally Posted by gunni View Post
the switch I mentioned is connected into the "EVEN CIA" chip.
Some expansions which have on/off capability connect to the CIA chips so that they can be switched on/off by holding down a button at reset.

Holding down LEFT mouse on re-boot = CIA (U7) pin 8
Holding down JOYSTICK FIRE = CIA (U7) pin 9

Without quality (i.e. macro mode and in focus) photographs at a reasonable resolution we cannot really comment.
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Old 25 August 2011, 17:02   #17
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yep i agree with you alexh a hi-res picture from above would help alot.the switch thats missing is only a single pole rocker/toggle switch anyway.you should have three wires coming from the pcb.the centre wire should correspond to the centre pin on the switch.

although my memory is a bit blurry on this as to wether it was activated via key press on startup i would try what alexh suggested.as i did have one that did function that way.
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Old 25 August 2011, 18:12   #18
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ok, had another look at the innards of the A500 today, the switch I mentioned is connected into the "EVEN CIA" chip.
with the switch in one position the A500 boots up and appears to only have the standard 512K, in the other position the floppy drive refuses to operate.
It seems to be a simple boot selector. It swaps the internal and external floppy drive numbers so it's possible to boot from an external drive with KS1.3 (especially useful if you have a 5.25" drive) .
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Old 25 August 2011, 18:25   #19
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Interesting stuff... never knew that

http://eab.abime.net/showpost.php?p=191053&postcount=24
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Old 25 August 2011, 21:41   #20
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FIXED!!!

for future reference looking at the card in place, front to back of the miggy -
writing on pcb - "on" orange wire, yellow wire, green wire, "off"
the orange and green wires were soldered together for god only knows what reason, looking at it and thinking of a throw switch directly on top I figured it should be orange-yellow or yellow-green.
stripped yellow connected to orange, boot up: 2197944 free memory as opposed to 362792 previously

thanks again guys for your help, might come begging again soon when I feel brave enough to tackle the drive-less A590 boards I've got in the box here!

Last edited by gunni; 25 August 2011 at 21:47.
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