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coco model 1 memory upgrade failure

Lurch666

Experienced Member
Joined
Dec 14, 2010
Messages
190
Location
Stoke-on-trent. U.K.
I have tried to upgrade my 16k coco to 32k using 8 4164 chips.
I replaced the chips,changed the jumpers to 32k and tried turning my coco on and all I got was a screen of garbage.Then I cut the capacitors next to the memory chips but that made no difference.Then I noticed I had missed a jumper and shorted out the pins for the 4k setting instead of the 32k setting DOH! (Should not do this when tired!).

Fixed everything so it was correct and double checked and now when I turn on I get a double OK near the bottom and two flashing cursors below them.I can type stuff in but when I press enter the system locks.Oh and there's no copyright text on the screen either.

I presume I have fried part of the system.Anyone got any idea where to start looking.
Thanks.
 

Chromedome45

Veteran Member
Joined
Jul 6, 2009
Messages
3,828
Location
Central Florida
So you shorted the pins with the 64k chips installed? Then set jumper to the 4k position? Well you may have fried the 4164's. But just a guess. By setting the jumper to 4k you may have hit the 4164's with -5 and +12 volts. Those chips only use +5 volts. Did you try putting the original chips back in? I am guessing so seeing as though you mentioned you got the ok prompt. Check around the memory decode/addressing area for bad parts, chips etc.
 

TRS-Ian

Veteran Member
Joined
Sep 10, 2011
Messages
1,036
Location
Melbourne, Australia
Lurch, is your Coco a D, E or 285 board? Each is upgraded to 32K/64K differently. I have the details here on how to upgrade each one.

Ian.
 

Lurch666

Experienced Member
Joined
Dec 14, 2010
Messages
190
Location
Stoke-on-trent. U.K.
It was an E board.I had a spare set of 4164's that I tried after getting fixing what I did wrong and the result is still the same double OK prompt as before. I followed the rainbow-may1984 instructions.One thing is that it says if the ROM is the old (c)1980 then it needs replacing.I figured that this was just if I wanted to go to 64k but I just want 32k.When I get the chance I#m going to try putting the original ram chips back and undoing the changes to see if my coco works as it did before.
 

TRS-Ian

Veteran Member
Joined
Sep 10, 2011
Messages
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Location
Melbourne, Australia
Yeah put it back to standard, check your rom versions, shoot me your email address and I'll forward you copy of the 32K/64K upgrade instructions I use, which are from Hot Coco.

Ian.
 

Lurch666

Experienced Member
Joined
Dec 14, 2010
Messages
190
Location
Stoke-on-trent. U.K.
Back to 16k.Problem persists so I have fried something.Here's my motherboard:


2013-04-30144726_zps905649f5.jpg

Am I correct in saying it's an E

And here's the screen I get on turning my coco on-It;s the same if I have 16k ram or with the 32k:

2013-04-30144753_zpsa2434bfb.jpg

Like I said I can type and what I type appears in double but as soon as I press enter the system locks.
 

TRS-Ian

Veteran Member
Joined
Sep 10, 2011
Messages
1,036
Location
Melbourne, Australia
That board is the PAL encoder board so Lurch must be in a PAL television country like Australia or the UK...

Can you tell me what the serial number of the motheroard is? Serial number is over near the Program Pak connector, just below it.

If it has jumpers for HI and LOW as well as 4K/16K and 32K on it then its probably and E board, if it only has 4K/16K jumpers, then its probably a D board.

The the computer respond normally aside from the awkward screen and do Program Paks work OK?

Ian.
 

Eudimorphodon

Veteran Member
Joined
May 9, 2011
Messages
4,401
Location
Upper Triassic
Something tells me that some magic smoke escaped from the SAM chip during all this, but that's totally a guess based on the strange pattern of the display corruption.
 

Lurch666

Experienced Member
Joined
Dec 14, 2010
Messages
190
Location
Stoke-on-trent. U.K.
Serial is 8709222 A.It does have a Hi/Low jumper and I'm in the UK so it's a PAL machine.The computer lets me type and characters appear on the screen (Albeit double on two rows after the double cursors) but pressing enter locks the machine.I don't have a program pak to try but I do have one on it's way-will test when it arrives.

Which is the SAM chip?
I'll try replacing it to see if it make any difference.

Thanks dudes.

EDIT:Looked around the net and found out which chip is the SAM and also found this out:

There are two versions of the SAM. The early one is labeled MC6883 and/or SN74LS783; the later version is labeled SN74LS785. There are some minor timing differences, but the major difference is the support of an 8-bit refresh counter in the 785 version. This allowed for use of inexpensive 16K by 4-bit and certain 64K by 1-bit DRAMs.

Now mine was a SN74LS783.Could I be having problems cause I have the older version?Should I look for a SN74LS785?
 
Last edited:

Lurch666

Experienced Member
Joined
Dec 14, 2010
Messages
190
Location
Stoke-on-trent. U.K.
Got my replacement SAM.I've set the memory so it's 16K again but the coco still doesn't work.I now get this screen:

2013-05-31 11.02.00.jpg


Since they are easy to get I tried replacing the CPU to see if that made a difference but no change at all.
I also received a coco cart to try but all I get with that is a blank screen.
Any other chips I could try replacing?
 
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