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Fixing Broken VCS


flickertail
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post-20388-0-89828900-1513577866.jpgDoes anyone here at AtariAge fix VCS systems?

 

About 14 years ago I was in a flea market in Toronto and came across this Coleco knock-off of the VCS. No controllers, no power supply, and nothing otherwise indicating that it would actually work. But it was different, so I bought it, and after some funny looks from customs when coming back into the US, I tried it out when I got home, and unsurprisingly, it was a brick.

 

Today I pulled it out of a box of Atari stuff that I had stored at my mom's. In the hopes that it was just my ineptitude 14 years ago that caused it not to work, I hooked it up again today, but it is still a brick.

 

Anyway, I would like to get this working, if someone could point me in the right direction.

Edited by flickertail
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The first place to start when repairing anything electronic that appears to be totally dead is always the power supply because if that is not functioning any other problems cannot be identified.

I presume that in line with the VCS there will be a 7805 voltage regulator inside so first make sure that it is getting a 9V DC input and producing a 5V dc output, that should tell you if the power circuits and power switch are operating.

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The power supply is a relatively new, modern Radio Shack all-in-one power supply with swap-able tips. It's possible that I'm not use the proper connector, but it's the only one that fits. Does seem a little loose, but it does "click" into place when the power supply is plugged into the Gemini power port.

 

It's a little hard to plug into the power port as it is recessed into the Gemini case more than you would expect.

 

I suppose I can open it up, get a volt meter, and start posting photos of the innards as I try to find the issue. I have very little experience with that sort of thing. It's already not working, so I suppose the worst I could do is give myself a tiny shock.

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The power supply is pretty important. I'm not familiar with the Gemini, but it's possible that the tip you are using isn't the right one. It's important to know where positive and ground are on the tip and if that matches the port. There are people on the forums who would probably work on it for you, they just have to see this thread and reply. If you want to try and troubleshoot it on your own, then posting plenty of pics will make it easier for us to help you along. I know there are a couple of members here who have worked on Geminis before, I just can't recall who they are... The good news is, with a voltmeter, you can track down a power issue really quick.

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Ok, First plug the power supply in and use the meter to check the tip on the power supply to make sure you are getting power there. Also use it to find out where positive and negative are on the tip, since I'm not familiar with the Gemini, I can't tell you which part of the tip should be positive. You might try looking up a schematic for the Gemini board on Google.. If the tip has power, the next step is to plug the power supply into the port on your board and find the solder points for the port and check to make sure you have power going through the port to the board. If so go on to the next step.

 

See the big round capacitor (the thing that looks like a can)? Right beside it is the voltage regulator. It's the black square thing with three pins. It has 3 pins, one is power going in, one is ground and the other is power coming out, which should be 5 volts. Search google for 7805 pinout and you should be able to find a photo showing which pin is which (I can't recall at the moment). If you have power at the voltage regulator, then maybe check the on/off switch to see if it's working correctly, that's about as much help as I can be, since I don't have a Gemini here to mess with.

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post-20388-0-94509200-1513844731.jpg When I test the power supply using the settings shown in the image to the left, the voltage at the tip of the power supply reads 13.5.

 

The power supply is an Enercell 13.5V DC & 30 VDC * 1 AMP AC-TO-DC POWER ADAPTER #273-329

 

From what I can tell searching Google the Atari VCS, Atari 7800, and Coleco Gemini all use the same tip. Though I maybe wrong, if someone knows better, let me know.

 

The power supply is supposed to be a smart power supply, which sends only the power necessary to run the electronic device. This power supply and tip work with my Atari 7800.

 

With the power supply plugged into the Gemini, and using the same multimeter settings in the image, the power port registers 0.12 volts.

 

With the switch on, the input voltage to the 7805 power regulator reads 0.12 volts.

 

With the switch off, the input voltage to the 7805 power regulator reads 0 volts.

 

Based on these readings, should I assume that the power port is bad?

 

 

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I just noticed something with more testing.

 

If I unplug the power supply from the power port, and then plug it back in, with the switch turned on... the voltage jumps up to around 3.60 volts from the power port, through the switch, and up to the input of the 7805 voltage regulator.

 

And then, the voltage drops by 0.01 volts every second until it gets down to around 0.10 volts, then drops off more slowly, but eventually it drops down to 0.

 

Could this possibly be a capacitor issue?

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Without having it in hand to mess with, my theory is that the power supply just doesn't agree with the Gemini. Also, the 7800 has a different port than the 2600, unless someone has changed it on yours. The 7800 power supply has a two prong port with a two hole connector on the power supply. I know that the 2600 can be very picky about the type of power supply used, so it's possible the Gemini has the same issue. I'd try to find an original power supply in order to rule that out before replacing components on the board.

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Oh man, I'm incredibly stupid. Apparently, my 7800 is still at my mom's house. I took a closer look at what I thought was my 7800 and it's actually an Atari Jr. I know they don't really look a like, but if you haven't seen either one in a while... (I just got a box of my atari stuff from my mom's house) and you don't look at it closely... and... well... like I said, I'm stupid. :dunce:

 

Now that you mention it, my 7800 had like a tiny two prong blue plug.

 

Anyway, the power supply works with the Atari Jr. So it explains why the tips are the same. I'll order a vintage power supply and see if I have better luck with the Gemini.

 

Thanks for the help.

Edited by flickertail
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I just noticed something with more testing.

 

If I unplug the power supply from the power port, and then plug it back in, with the switch turned on... the voltage jumps up to around 3.60 volts from the power port, through the switch, and up to the input of the 7805 voltage regulator.

 

And then, the voltage drops by 0.01 volts every second until it gets down to around 0.10 volts, then drops off more slowly, but eventually it drops down to 0.

 

Could this possibly be a capacitor issue?

 

Possibly, as you are using a regulated power supply if the current consumption exceeds the set limit the usual result of current limiting is a reduction in output voltage (thus output power remains constant) although surprised that it would actually go down as far as zero that could be indicative of a short circuit somewhere that is drawing a lot of power.

You could try removing the output pin of the 7805 from the PCB (right pin as you look at the side with the writing on it) and see what happens then, if it still goes down to zero that will tell you the problem lies with the power circuit (input smoothing capacitor/regulator), if it is fine and the voltage on the regulator output pin reads about 5V then the problem is probably after the regulator.

 

Anyway, the power supply works with the Atari Jr. So it explains why the tips are the same. I'll order a vintage power supply and see if I have better luck with the Gemini.

I am sure it works although I would not use it consistently without beefing up the heatsinking. Because that power unit is regulated with a current rating in excess of that the 2600 requires there will be no voltage drop down to around 9V when you connect it to the 2600, that would mean the internal 5V regulator will have twice the voltage dropped across it (8 volts) and thus be dissipating twice the power (i.e. 4 watts @ 0.5 Amps as opposed to 2 Watts). This may be too much for the internal heat sinking arrangements which were designed for less power dissipation and eventually cause the voltage regulator to go into thermal shut down periodically during extended period of use.

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