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> 5vdc In 9vac Line On Commodore 64, My c64 works now, but I'm still curious
dustfilledhobo
Posted: March 28, 2012 05:10 am
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Hello,

Recently I have been having trouble with my breadbin C64. First I was having problems with it shutting off and re-starting, so I checked the solder joins of the power supply connector inside of the C64 and it fixed the problem for while. Now I have an issue where the internal fuse keeps blowing. I re-checked all of my solder joints, tested continuity (as well as checking that points that should not be connected were not connected). I have tried two different power supplies, and I have tested those power supplies with a different C64 (which worked great).

When I plug the power supply into the computer and test the voltages on the back of the motherboard I get an odd reading. The 9Vac line reads 9Vac like normal, but the same line reads 5Vdc as well. Also, the line that should read 5Vdc is only reading about 0.3Vdc.

Has anyone encountered this, or do you have any ideas as to how I could fix it?

Thanks.
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dustfilledhobo
Posted: March 28, 2012 05:22 am
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I decided to try one more fuse (my 5th), and for some reason this one isn't blowing and the C64 is functioning properly. I am still curious why leaving out the internal fuse would cause 5VDC to show up in the 9VAC line. Thanks for reading and any information you can provide me with.
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CWB
Posted: March 28, 2012 12:21 pm
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schematic ?


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dustfilledhobo
Posted: March 28, 2012 02:53 pm
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Here's a schematic: http://www.zimmers.net/anonftp/pub/cbm/sch...251138-2of2.gif

It is the fuse labeled F1 on the 9VAC line rated at 1A. Thanks.

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CWB
Posted: March 30, 2012 02:28 pm
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using the quick SWAG method after a quick peek at the schematic ...
i would hazard that due to the high impedance of your DMM , you were reading the "hummmm" induced in the circuit .
odd things happen in "non-complete" circuits .

going through several fuses before the unit worked seems suspicious to me .
if all those fuses were bad out of the box ... you better go play the lottery .
(this is assuming that they were of the correct type)
i would guess that a filter cap or something similar was drawing excessive current until it "healed up" a bit .
keep an eye on those electrolytics .
it may not hurt to seriously consider re-capping the most likely culprits .


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dustfilledhobo
Posted: March 30, 2012 07:04 pm
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Thanks for your explanation. Since I use this C64 for music I was considering re-capping the entire computer to help reduce some of the noise in the audio line. Do you think this would help reduce audio noise, or should I only focus on the caps near the power supply line?
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CWB
Posted: March 31, 2012 02:21 am
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if most of your noise is hissss instead of hummm (the fundamental plus harmonics thereof) , there is not much you can do except roll off the high end .
those '64s and many others of that era/type were noisy to start with .
man , even the old jupiter keyboards were notoriously noisy for a musical instrument !

re-capping the power supply sections is a good place to start .
it is cheap enough and high on the list of priorities .


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