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Offline pikuTopic starter

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Amiga 500 - Can't figure it out
« on: November 22, 2023, 03:21:36 AM »
Hello all, I was hoping someone more experienced might be able to help this Atari guy.
I have an Amiga 500 that had a Vampire V2 on it.  It worked fine until recently and then stopped booting up.  The screen on the RGB was just yellow.
I also have an Amiga 2000.  Using that machine I've checked to be sure the gary, paula, denise, cias, cpu and fat agnus were all good.  And they all are.
I ordered diagrom and couldn't get it working.  Nothing at all.  I figured out eventually that I need an adapter for a rev 5 board which I ordered.  Still no dice.  I tested diagrom on my Amiga 2000 and it works perfectly and gives output out the serial which proves the diagrom works as well as my usb serial setup.
After getting the diagrom the bus activity in the 500 looks very much exactly like the 2000.  You can see lots of activity.  All the ram chips have activity on the outputs.  The lows have a sawtooth pattern to them which I was alarmed by but it looks like my 2000 does the same thing.  I've replaced all the data path IC's on the 500.   Still even with diagrom in the screen just stays yellow and the serial port is completely silent. 
PSU was replaced with a meanwell and I've checked that all voltages are (and were) correct.
I'm just not sure where to look next.  What else fails?  Resistor packs?  Capacitors?  I checked all data and address lines for shorts and they all read the same resistance.  I checked the rom for connectivity to the cpu and all pins seemed good.   I'm at a total loss.  This thing just doesn't want to work.  Meanwhile my 2000 just chugs along.
 

Offline Boing-ball

Re: Amiga 500 - Can't figure it out
« Reply #1 on: November 23, 2023, 10:06:03 PM »
Hello all, I was hoping someone more experienced might be able to help this Atari guy.
I have an Amiga 500 that had a Vampire V2 on it.  It worked fine until recently and then stopped booting up.  The screen on the RGB was just yellow.
I also have an Amiga 2000.  Using that machine I've checked to be sure the gary, paula, denise, cias, cpu and fat agnus were all good.  And they all are.
I ordered diagrom and couldn't get it working.  Nothing at all.  I figured out eventually that I need an adapter for a rev 5 board which I ordered.  Still no dice.  I tested diagrom on my Amiga 2000 and it works perfectly and gives output out the serial which proves the diagrom works as well as my usb serial setup.
After getting the diagrom the bus activity in the 500 looks very much exactly like the 2000.  You can see lots of activity.  All the ram chips have activity on the outputs.  The lows have a sawtooth pattern to them which I was alarmed by but it looks like my 2000 does the same thing.  I've replaced all the data path IC's on the 500.   Still even with diagrom in the screen just stays yellow and the serial port is completely silent. 
PSU was replaced with a meanwell and I've checked that all voltages are (and were) correct.
I'm just not sure where to look next.  What else fails?  Resistor packs?  Capacitors?  I checked all data and address lines for shorts and they all read the same resistance.  I checked the rom for connectivity to the cpu and all pins seemed good.   I'm at a total loss.  This thing just doesn't want to work.  Meanwhile my 2000 just chugs along.

A yellow screen usually denotes a RAM error. Either Chip or Slow RAM (Fast RAM on other Amiga systems). Sometimes this could be a red herring and something else could be causing it such as an add-on card not playing nice.

Do you have any extra RAM expansion connected to the under belly connector? If so, try removing that first.
 

Offline pikuTopic starter

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Re: Amiga 500 - Can't figure it out
« Reply #2 on: November 24, 2023, 02:30:53 AM »
I'm somewhat suspicious of the color meaning anything.  If I stick the other Denise in it's black.  And the color doesn't change at all.  It just starts yellow and stays.  Also if I jiggle the video port it sometimes changes to purple or light blue.  I think I'll have to start tracing activity line by line or hook it up to a logic analyzer.
 

Offline F0LLETT

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Re: Amiga 500 - Can't figure it out
« Reply #3 on: November 24, 2023, 09:08:46 AM »
Doesn't yellow mean CPU exception error?

I'm guessing you have tried reseating all chips? and that it also works as a standard A500?
Else, I would proberly go with and issue with Vampire. I have not read anything about rev 5 issues.
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Offline pikuTopic starter

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Re: Amiga 500 - Can't figure it out
« Reply #4 on: November 24, 2023, 03:04:45 PM »
No no, vampire is long gone and now installed in the 2000.  I am just trying to get it to work as a base 500.  No trapdoor or anything weird.  And yeah, all chips reseated of course as first step.
« Last Edit: November 24, 2023, 03:05:22 PM by piku »
 

Offline F0LLETT

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Re: Amiga 500 - Can't figure it out
« Reply #5 on: November 24, 2023, 04:39:20 PM »
Sure none have bent pins?
I have done this in the past and spend a while trying to workout issue, only to find a bent pin.

Also, when you removed agnus, did you do it properly, as snapping the corner can cause the plcc not to make contact correctly.
Quote from: Hungry Horace
Resolute and Industrious Grand ruler of the yellow people and the Ultimate Amiga Empire
Ultimate Amiga Network (Home of SONY PSP Amiga Emulator and AMOS Factory)

Quote from:  He who shall not be named
"Chris is that you!!!"
My all time favorite quote.
 

Offline pikuTopic starter

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Re: Amiga 500 - Can't figure it out
« Reply #6 on: November 24, 2023, 08:01:29 PM »
So exciting, I finally found the issue after 6 months of off and on trying.  REGEN line which is pin 23 from fat agnus to pin 18 of gary was disconnected.  You could see the tiniest amount of green death on one of the vias in the path.  Connecting a bodge wire fixes it!  So happy!  When I put the scope on pin 23 of fat agnus the screen freaked out which is how I found it.  A more careful look at the physical board could have also found it.
« Last Edit: November 24, 2023, 08:02:24 PM by piku »