My Dual 7800 A/V and Power Mod (Pic Heavy)

[kevtris]

Well since everyone's talking about Atari 7800 mods, I decided to make one of my own. I took the other mod posted earlier by Puppetmark, with the 1 transistor amp and modified it slightly- namely changed the 75 ohm resistor to 68 ohms since I had it.

 

The aim of these two mods is to make your 7800 infinitely more useful while retaining a "stock" appearance. There's no hot melt, glue, epoxy or electrical tape needed! The plastic 7800 enclosure is not cut or filed at all!

 

There were several different purposes for the mods I performed:

 

The A/V mod obviously adds A/V out and replaces the stock RF only output. The modulator's RCA jack is reused for video, and a second RCA jack is added where the channel select switch used to be for audio. The mod does indeed worth with Ballblazer and other POKEY games.

 

The power mod performs two separate functions: The first is to replace the original proprietary power socket with a more standard "barrel" style plug. The original 7800 enclosure appears to be designed to accomodate these and they probably had them in mind before changing to someone proprietary.

 

The other major feature of this power mod is reverse polarity protection. If somehow power is reversed (wrong adapter, etc) it will not fry the 7800!

 

Here's a picture of the finished mod. There's no other changes to the outside appearance except on the back.

 

 

 

 

For the A/V mod you will need:

 

1 68 ohm resistor

1 2.2K resistor

1 3.3K resistor

1 2SC945 transistor (this is an extremely common transistor found by the dozen in old VCRs and TVs or it can be ordered)

1 RCA jack

1 piece of 14 gauge house wiring (no insulation)

1 piece of hookup wire, I used 26 gauge PVC jacketed phone wire

 

 

For the power mod you will need:

 

1 coaxial power socket for power (the kind that fits barrel plugs)

1 1N4001, 1N4003, etc. 1A diode

1 ferrite bead, no wire (optional)

 

 

Tools used:

 

Dremel

1/8" drill bit

desoldering tool or braid

soldering iron

sharpie marker

 

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Part 1: the AV mod!

 

 

Step 1: remove the modulator! I removed it using a desoldering gun, but wick should work. Try not to damage it since we're going to re-use parts of it. You can cut the 4 pins coming off the modulator's PCB if you like, these are not being reused, only the metal shell.

 

 

Step 2: Take the bottom off the modulator. It just pops off; make sure the solder is off the 4 metal pins on the box portion.

 

Step 3: Remove the PCB. Desolder the little coil from the RCA jack. Next, wick off the solder holding the PCB to the box (2 places).

 

Step 4: stick a wire through the hole indicated in the box, and solder the end to the inner contact of the RCA jack.

 

Step 5: Snap the bottom back on the box and set it aside.

 

 

 

Now it's time to remove the parts off the PCB we will not be using and replace them with new ones.

 

Step 1: Remove Q1, L2, L3, C3, C4, C5, C9, R3, R4, S7 as indicated. I desoldered these my desoldering gun, but you could use wick. Try not to damage the holes since we will be reusing some of them. In a pinch you could cut the parts off and remove the leads one at a time.

 

Step 2: Desolder ONE end of R5 and R6, then bend R6 over to the right so that the "free" end goes into the bottom hole of L3. Bend R5 over next to R6 and solder it to R5 on the L3 end.

 

Step 3: Install a 2.2K resistor into C4. Install a 3.3K resistor into R4. Install a 68 ohm resistor (I used 1/2 watt, but 1/4 watt should be fine) from the top hole of C9 into the top hole of C5. DO NOT cut off the 68 ohm resistor lead that is poked through the top hole of C9. We're going to hook that up later. Install the transistor into Q1. Note that it goes "backwards" from the PCB markings. The flat side faces AWAY from the flat side shown on the silk screen (PCB marking).

 

 

Next, solder the modulator back down and connect the wire coming out to the top pad of L2.

 

Connect a wire from the top hole of the modulator's connections (near where it says "RF1" on the board) to the end of the two resistors going into the bottom hole of L3.

 

 

Step 4: Here's where the end of the 68 ohm resistor gets connected. Bend it around as shown in a half circle and solder it to the end two part leads poking through in the row. This is ground (both are so I just soldered it to both).

 

Step 5: solder a short jumper wire between the bottom holes of R3 and C9.

 

 

 

 

 

Now we're going to install the RCA jack for the audio. I bought these ones from Electronic Goldmine (their part number is on the bag) back in 1996 or so, so I am not sure if they still have 'em. nearly any RCA jack will work. change hole locations in that case.

 

 

Step 1: Put the RCA jack on the board for a test fit. I used the middle hole of the switch holes to put the signal pin of the RCA jack. This means I have to drill 3 holes to mount the jack.

 

Step 2: I am using the two holes from the switch that are closest to the edge of the PCB for the RCA's mounting tabs, along with one I marked with a magic marker (the blue dot near where it says "S7").

 

Step 3: Using a 1/8" drill bit, I drilled three holes: the one I marked and the two from the switch.

 

Step 4: Flip the PCB over and scrape the solder mask (green coating) off the the big fat ground trace that was drilled through. This will give the solder we're going to add a place to stick.

 

 

Step 5: Push the RCA jack through the holes drilled.

 

Step 6: Bend the tabs over to hold the jack in place.

 

Step 7: Obtain some 14 gauge house wiring and strip it down to the bare copper. Bend the end in a hook like in the picture so that the middle of the bend touches the middle tab of the RCA jack, and the left end of the 'hook' touches the left tab of the RCA jack.

 

Step 8: solder the left tab to the thick wire, then solder the wire to the middle tab and ground route, and finally cut it to length on the right and solder it to the right tab on the RCA. Use plenty of solder like I did.

 

This last step ensures a very strong secure mounting, and the RCA jack will break before it will come off or damage the PCB.

 

The RCA jack should project through the former channel select hole.

 

So now your 7800 has a bitchin' A/V mod!

 

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And if you are sick and tired of the crappy custom power socket on the 7800, here's how to fix that, too!

 

 

 

Step 1: Remove J5 and FB2.

 

Step 2: Obtain a coaxial power jack. This one takes 2.5mm barrel plugs which is very common. (think Genesis, TG-16, NES, etc).

 

Step 3: Mark the holes we need to drill on the PCB by putting the jack on there and marking them with a marker. I used the middle mounting hole from the original 7800 jack as one of my mounts.

 

Step 4: Using a 1/8" drill bit, drill the three holes (the two marked and the existing one).

 

Step 5: Cut the trace on the top of the board as shown in the purple squares- This is Vin and we do not need it shorting out to our socket we're installing!

 

Step 6: Install a 1N4001 (or 1N4002, 3, 4, etc) 1A diode where FB2 was, cathode (banded end) facing the large capacitor.

I installed a ferrite bead on the anode lead but this isn't really necessary unless you have some.

 

Step 7: Install the plug into the holes drilled and bend the tabs over on the back side of the board to hold it.

 

Step 8: Using a knife or dremel, cut the ground route that goes to the original 2 pin power connector as shown in the box on the bottom right picture. This would short out our input voltage otherwise.

 

Step 8: I soldered the positive tab from the connector (the one on the end nearest the 2 pin connector's holes) to the 2 pin connector's holes with a generous helping of solder.

 

Step 9: Using the house wire, I soldered the remaining two tabs from the power connector to the ground route near by. This gives good mechanical support so it won't break off.

 

 

 

Here's a bit better view, sorry it's kinda hard to see.

 

The final step is soldering a jumper wire from the transistor on the left to the lead of the capacitor on the right. This replaces that route we cut under the power connector.

 

That's it!

Edited September 21, 2013 by kevtris

[SoundGammon]

Like your idea for the power supply mod!

[WERY]

Well done, I am impressed. You put down a hard work to perform this. I made my AV-mod very easy by just connecting the audio and C-video leads to a board I built. Maybe I should try this one in future, it looks quite good =)