Strange 1541&RAM expansion problem

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Kakemoms
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Strange 1541&RAM expansion problem

Post by Kakemoms »

I have this strange problem which gives black screen if both 16K RAM expansion and a 1541 drive is plugged in. It boots if I unplug the RAM expansion or if I unplug the power cord to the 1541. Switching the 1541 off does not fix the problem - I have to unplug the cord. Very strange.

I am thinking that it may be noise from the drive power supply, but it sounds non-trivial since the machine boots fine without the RAM expansion in place.

Ideas?
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eslapion
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Re: Strange 1541&RAM expansion problem

Post by eslapion »

I suspect your 16k RAM expansion is not from Commodore. What type is it ?
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mrr19121970
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Re: Strange 1541&RAM expansion problem

Post by mrr19121970 »

If the 6502 is socketed you can try it in another machine ? Similarly a different 6502 with this machine.

I have a 6510 that also black screens with certain components in a C64. Replacing cures it. Worth a try.
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MCes
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Re: Strange 1541&RAM expansion problem

Post by MCes »

switch off the 1541 drive or unplug its power cable have to be the same, if it isn't then may be that you have a current dispersion between two DIFFERENT and FLOATING electrical potential: the first is the metal of the internal structure (the ground of 220V cable) and the second is the ground of internal low voltages (5V, 12V).
check if the RAM modification form a bridge between this two DIFFERENT potential, for example from the ground of 5V and a screw that fix it on metal structure.... some pics could help....
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Kakemoms
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Re: Strange 1541&RAM expansion problem

Post by Kakemoms »

Its not commodore and I have no idea who the manufacturer was. The casing says 16K, but it gives me 24K+. Actually I get the same behaviour from my own 37K expansion and there is no shortening between 5V and GND there.

The 6502 inside is actually an Atari chip, e.g. the 6502C "Sally". I modified it so it would work in the Vic-20 and it has worked in there for some time without problems. I will try to swap it just to be sure.

That being said and I still don't understand why unplugging the 1541 it should make a difference. But the tip with GND noise coming from it may be a good hint... I could have tried to measure that tonight, but my picoscope died a few days ago so I have to get some BNC plugs & probe for an older oscilloscope to measure noise levels.
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MCes
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Re: Strange 1541&RAM expansion problem

Post by MCes »

I not indicate a short circuit between 5V and its ground, but I ask to check isolation between the negative of big capacitor (ground of low voltage) and the screws that fix the board to metal chassis (ground of 220V and filter of EMI suppressor integrated into 220V connector).

some pics could help....

Image
Last edited by MCes on Thu Aug 18, 2016 1:48 pm, edited 2 times in total.
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eslapion
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Re: Strange 1541&RAM expansion problem

Post by eslapion »

Kakemoms wrote:Its not commodore and I have no idea who the manufacturer was. The casing says 16K, but it gives me 24K+. Actually I get the same behaviour from my own 37K expansion and there is no shortening between 5V and GND there.
I suspected you might have a memory expansion that uses DRAM and this might have made it more sensitive to certain types of electrical noise but I guess that's not the issue if you have the same problems with different memory expansions.
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