Amplifone HV resistors mod

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I never really liked the big extra resistors that Atari "designed" into the HV-pcb. First of all, positioning them in an acaptable way is pretty hard. I've seen some put upright and I've done some other "creative" positioning, but whichever way you look at it, it's clumsy.

Apart from "looking bad" there was also something else that annoyed me a lot more: They get pretty damn hot in use, and they are very close to the (brand new) caps on the board. That means they're nicely aging the caps right away again, something we don't want.

I had played with the idea to use some transistors instead of resisitors. There are designs with 78xx and 79xx voltage regulators that allow for more amps to be delivered using transistors and it seemed like a more sensible way.
However, that would require some more modifications of the circuit on the board which I didn't like. Also, Mark Spaeth pointed out that transistor or resistor, the power was still going through either of them and thus the heat would still be generated anyway.

So, next I thought about getting two wire-wound resistors that are designed to handle some serious loads and have a metal "fan" body. These are transferring heat a lot better than the ceramic one's. Now, these baby's can be mounted on a heat-sink in the way like you do with transistors and voltage regulators.

Since the heat-sink that is used by the 7824 and 7924 is not getting hot at all, I figured it could handle a little extra heat from the resistors.

Thus, I mounted the resistors on it. Had to drill some holes and I used a thin film of thermal grease. There's no need to electrically isolatie the resistors as the body is not in electrical contact with the resistor itself. I ran some thick solid core wires through the original resistor holes and soldered them as were the original one's.

Since I'm still waiting for new yoke parts (David ?) I can't really stress test it, but I started a couple of uncontrolled games and let it run for half an hour. The resistors and heat-sink feel like skin-temperature, so I guess it worked out really well.

More stress testing will be needed (death star explosions!) but I'm pretty confident that this is a great solution. Let me know what you guys think.


There are some pics right here:
web.jpg


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I think that's a great mod. I will be doing something similar to my HV board when I get around to rebuilding it. Stuffing those resistors into the position they did with the official Atari mod was a bad idea with the proximity to all of the other components.

I use the same style resistors on both Sprint 2 boards I've brought back from the dead.
 
Someone on BYOAC mentioned that there are also "power" resistors available in TO-220 housings. That's great for mounting to a heat-sink ! Would be a very neat solution, no wires running over the heat-sink.

Check it out here:

http://www.caddock.com/Online_catalog/Mrktg_Lit/MP9000_Series.pdf

Never knew these existed or I would have got those.

They're available at mouser.
 
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