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Old 03 November 2020, 17:46   #21
solarmon
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And yes, I also think that information out there is being mixed with the onboard chip ram mod, which requires a change to JP7A to prevent any expansion ram conflicting with it.
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Old 03 November 2020, 18:01   #22
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Originally Posted by MiaM View Post
My guess is that it's either ESD related or related to grounding of the Amiga and the soldering iron.

You should electrically connect the soldering iron with ground on the PCB you are working on and have an ESD wrist strap connected to those grounds. All those grounds should preferable be connected to something that has some kind of true ground (preferable the ground on an electrical socket).

If your soldering iron has a grounded plug and it's connected to an ungrounded socket and it has a switching power supply and/or a mains filter it will likely leak a small current from mains to the tip.That can be enough to fry semiconductors.
.... THIS. I really suspect what's happening is what you're describing here and was my first thought when I zapped it the very first time. I guess if I'm touching pads on the motherboard that are supposed to be ground and I had the Amiga power cabled in (but off) in order to ground (correct?) then this could have been zapping everything connected to ground. This at least seems logical and moves this problem out of the realm of mystery which is what I was actually driving me nuts.

Instead of using the amiga power unit to ground the board which might not have been working as desired I should clip a wire on a ground section of the MB and have that run directly into the mains ground pin.

All elementary stuff but I guess I've just gotten slap dash with an 'ah it'll be alright' attitude. On a side note, my new populated motherboard came wrapped in just bubble wrap yesterday!!! It worked but you'd think that would have been a recipe of disaster.
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Old 03 November 2020, 18:06   #23
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I was in the same boat trying to understand this mod initially. I also didn't get why JP7A needed to be cut or changed. It was only when I looked at the schematics and tried out the various permutations did I understand what it is for. So this information is based on my own experience.

If the middle pad of JP7A is floating then it will be seen as HIGH, as there is a pullup resistor connected to this _EXTICK signal (which is 'active low').
Now that figures! So when I want to enable my mod I do indeed want it all floating to have it pull up, be high and thus disable the ram. Disabled ram means disabled in a fast ram sense but it is addressable by Agnus providing top two JP2 pins are bridged.

If I've understood this correctly then I think I'm done here! Even if I don't get this mod working, I was keenly seeking the knowledge over the result. The result would be DEAD handy and is what I want but now I'm spooked.

Thank you so much for you help with this and your replies. I know that I went VERY VERY long with my explanations but I really want to gain 100% knowlege of the Amiga one day both HW and SW and although my questions may seem tedious to the very much indoctrinated Amiga modder, this stuff really helps me on the path. I will be sure to properly read the Amiga Hardware manual along with the schematics and hopefully some day 100% know this system's every knook and cranny.

Thanks!
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Old 03 November 2020, 18:15   #24
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And yes, I also think that information out there is being mixed with the onboard chip ram mod, which requires a change to JP7A to prevent any expansion ram conflicting with it.
Any chip ram mod needs you to cut JP7 so that the other 512k doesn't get mapped to chip ram and slow ram at the same time. It makes no difference to whether the ram is on the bus or not, but if the exram/extick signal is low, then gary will make the trapdoor (or second 512k on a rev6 board) visible at c00000 via the agnus.

Yes, it has a pullup near the gary socket.
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Old 03 November 2020, 19:02   #25
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Any chip ram mod needs you to cut JP7 so that the other 512k doesn't get mapped to chip ram and slow ram at the same time. It makes no difference to whether the ram is on the bus or not, but if the exram/extick signal is low, then gary will make the trapdoor (or second 512k on a rev6 board) visible at c00000 via the agnus.

Yes, it has a pullup near the gary socket.
The OP is about making expansion ram seen as CHIP ram - so is JP7A still required to be modded for just this?
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Old 03 November 2020, 19:32   #26
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The OP is about making expansion ram seen as CHIP ram - so is JP7A still required to be modded for just this?
Answering my own question...

Apologies. I was was testing with a memory expansion that has an on/off switch, and it was on the off position, which effectively made the _EXTICK/_EXRAM signal floating, which means it is HIGH.

So JP7A doesn't need to be cut ONLY if the expansion ram does have an on/off switch (which needs to be OFF, to make _EXTICK/_EXRAM go HIGH). Otherwise, yes, it needs to be cut to make _EXTICK/_EXRAM float and go HIGH).
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Old 03 November 2020, 19:39   #27
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Answering my own question...

Apologies. I was was testing with a memory expansion that has an on/off switch, and it was on the off position, which effectively made the _EXTICK/_EXRAM signal floating, which means it is HIGH.

So JP7A doesn't need to be cut ONLY if the expansion ram does have an on/off switch (which needs to be OFF, to make _EXTICK/_EXRAM go HIGH). Otherwise, yes, it needs to be cut to make _EXTICK/_EXRAM float and go HIGH).
Ah yes, I read about this and I think that's what JP1 is on my A501 board (going from memory of what I read last week so don't quote me on that). I don't want to fiddle with my A501 as it works so to do the mod I need to have JP7A floating which as you gather by my inane ramblings is what I had from the very start pre bodge wires etc.

Funny this whole modding thing, I had planned that it was going to be a 30 min job max and it's cost me a week BUT it has meant that I have finally engaged in this forum despite being a lurker for what could easily be 15 years! (I would have definitely visited here in 2005 if not earlier). So in some strange way I'm glad I bollocksed this up and got to meet you all.

Once again, thank you ALL for engaging in this mod. I have learnt a lot even though the end result wasn't in my favour.
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Old 04 November 2020, 14:44   #28
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The latest update on this:

Measured voltage of my soldering iron tip to mains ground and got a fluctuating reading of 0.03V. That's perhaps enough to zap something. After connecting the ground pin of my iron to mains ground the voltage reading of the tip to mains ground was 0V.

So I undid the bodge wires on JP7A, jumpered the top two pins, repopulated the board (but missing even CIA), connected the trapdoor expansion.

* Boots up and flashes dark grey light grey repeatedly. I think that's what happens when Agnus cannot see the expansion...

Jumpered the lower two pins of JP2, removed expansion.

* Boots up to light grey and screen flickers slightly every second. Although JP7A is floating therefore in a expansion disabled state, without an expansion board I'd expect it to boot at least. Also I managed to get my unmodded REV6 board to boot with the same set of chips before repopulating the modded one so it's possible I've zapped something on the motherboard itself.

I've just received two good CIAs in the post and I put them in my 3rd Rev6 board along with the expansion and I'm seeing 512k / 512k with all systems working so I can confirm the expansion is fine.

I guess if I repopulate my 2nd Rev6 with everthing from my modded Rev6 and it still boots then assume something on MB is zapped and either attempt the mod on my 2nd Rev6 or just give up because I'm spooked.
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