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1200XL Power Fail

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I have three 1200XL's, including one that I picked up about six years ago from kjmann that was modified to use a 5V DC adapter.

 

Long story short, I was testing the keyboards on all three units and accidentally plugged the OEM 9V AC power supply into the modified 1200XL. I heard a little hum from the monitor speaker and then the red power LED went off just as I realized the mistake. When trying the correct adapter, the power comes on but nothing but a black screen.

 

I'm assuming I've fried this particular motherboard but just wondering if there might be an potential fix or something salvageable.

 

 

 

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It's probably fixable and it may be a simple fix or may be complicated. It kind of depends on how the unit was modifed when it was converted to 5V DC. In the stock system, the 9V AC power is run through a rectifier that produces 12V DC which then feeds two 7805 voltage regulators, and those in turn output the 5V DC the system needs. If the mod just feeds 5V DC straight to the main power plane, 9 VAC might have blown a capactor or worse, an IC chip. I think you're gonna have to do what I've been doing the last two weekends with my 1200XLs - checking components and swapping chips into a working system to narrow down potential sources of the problem. :(

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Thanks DrVenkman. Yes, the rectifier was removed for the mod (as discussed in this post), so I imagine the 9VAC did some damage. Not sure if I want to take the risk of swapping chips and ruining another good 1200XL. I'm pretty sad about my dumb move and losing that one, since it did have a nice s-video and audio out mod as well.

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Thanks DrVenkman. Yes, the rectifier was removed for the mod (as discussed in this post), so I imagine the 9VAC did some damage. Not sure if I want to take the risk of swapping chips and ruining another good 1200XL. I'm pretty sad about my dumb move and losing that one, since it did have a nice s-video and audio out mod as well.

 

Have you opened it up to look inside yet? That needs to be your first step. I might be as simple as a blown capacitor - that's a pretty easy fix.

 

A bad chip - or several - could be easy too. The usual routine is take a chip from the system that won't work (call it the bad system) and transplant it into a working (good) system. It doesn't have to a 1200XL. I have a couple donor systems - one is a beat up old 600XL I got on eBay for literally $5 plus about ten bucks shipping, and another is an 800XL I got more recently for the same purpose. Anyway, test chips from the bad system in a good system to help ID which one - or which ones - isn't/aren't working. There's almost no likelihood that a bad chip will damage an otherwise good system.

 

However - and this is important, especially in a system that's been zapped - don't test known good chips in a bad system unless you absolutely have to. The only 1200XL-specific chip you need to check - if all else fails - is the MMU. You can test that one in another good 1200XL if necessary.

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Start with the RAM. It's the first in line from power, I believe. Then work your way around the big guys.

Edited by Paul Westphal
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If you do manage to get this working you may want to use a label maker or something to put a label on the back that says "DO NOT USE AC, 5V DC ONLY". I'm assuming there is no diode whatsoever after the mod since already 5V DC coming in and if used one diode to try to prevent the AC mistake would be a problem itself because then you take the voltage down to about 4.3 to 4.4V due to voltage drop of the diode so probably power just straight off the DC adapter. So basically no protection whatsoever and every chip on the board that takes 5V DC had AC on it(and a higher voltage at that), every electrolytic cap could be an issue, basically everything parallel with the power supply designed for DC, not good. Hopefully won't be as bad as I'm thinking but honestly I think this unit is going to be fubar.

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When you get it working again I suggest you find a replacement power connector that the standard 9VAC power adapter won't connect to, and put a matching plug on the 5VDC power adapter you use with it.

 

The power connector would need to have a compatible footprint for soldering to the PCB, and also polarized to prevent connecting with negative voltage.

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If modifying a 1200XL for 5VDC, nothing but a 5-pin DIN connector should be used. That won't prevent a Commodore PS from being used, but ...

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Have you opened it up to look inside yet? That needs to be your first step. I might be as simple as a blown capacitor - that's a pretty easy fix.

 

A bad chip - or several - could be easy too. The usual routine is take a chip from the system that won't work (call it the bad system) and transplant it into a working (good) system. It doesn't have to a 1200XL. I have a couple donor systems - one is a beat up old 600XL I got on eBay for literally $5 plus about ten bucks shipping, and another is an 800XL I got more recently for the same purpose. Anyway, test chips from the bad system in a good system to help ID which one - or which ones - isn't/aren't working. There's almost no likelihood that a bad chip will damage an otherwise good system.

 

However - and this is important, especially in a system that's been zapped - don't test known good chips in a bad system unless you absolutely have to. The only 1200XL-specific chip you need to check - if all else fails - is the MMU. You can test that one in another good 1200XL if necessary.

 

I already took a look but not sure if I can tell if a cap is blown...everything looks normal to me. Attached is a pic of the motherboard. Only one big capacitor that is part of the mod--a 2200uF 16V as you can see in the second picture.

 

I'll take the advice and begin testing chips in a beater 800XL I have on my next day off, starting with the RAM.

 

As far as the connector is concerned, that's how I received it and it's just unfortunate that the original power port was used and wasn't made foolproof.

 

Thanks everyone for the input!

post-47471-0-59311500-1496639652_thumb.jpg

post-47471-0-47147100-1496639755_thumb.jpg

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