Atari 2600 Modifications

[TuzenTCA]

Hello,

 

Recently I purchased another Atari 2600. It has apparently been modified and does not work. I took the case apart and noticed that the 7805-power reducer was replaced. It was poorly replaced with a LM78M05CT power reducer. I know that the 7805 drops excess voltage down to a +5 volts, but will this LM78M05CT drop it down to the safe power level? I don't seem to think they are at all the same because I have compared this Rev. 14 Board with the LM78M05CT, to a Rev. 16 board (both 1980) with the original 7805, yet it does have the numbers 7805 in it (LM78M05CT.) I was wondering if this might have fried the board. If you can shed any light onto this subject about whether this is a safe part to use or if it is dangerous please post a reply.

 

Thanks,

 

James Henry McAninch, III

 

p.s. The LM78M05CT is not screwed to the board like the 7805 was. Could this be another problem?

[CPUWIZ]

The LM part should work fine but all 7805's need to be mounted to a heatsink with proper thermal grease.

[TuzenTCA]

Hello,

 

Thank you for your response about the 7805. I see where the thermal grease used to be but it looks more like charred corrosion now. Say it will work? Ok, I'll get a screw and some thermal grease and fix that problem. Thanks again for your quick response.

 

Sincerely,

 

James Henry McAninch, III

[CPUWIZ]

There is a good chance the 7805 is smoked now and you may need to replace it.

[Bruce Tomlin]

Not that it's hard to find spare 7805 regulators if you have a good junkbox. Check the voltage coming out of it to make sure it's 5 volts. If it's outside of say 4.75 to 5.25, replace it. Especially since you said it looks crispy.

 

Just for reference, the "M" in 78M05 means "medium power". An "L" means "low power", and 78L05 regulators are usually found in a transistor packaging. I don't know what the power ratings of M and L are, but a regular 7805 is 1 amp. 79xx regulators are for negative voltage regulation (for dual-polarity power supplies).

 

And it's a power regulator, not a "reducer". It makes sure that the power coming out of it is exactly 5 volts (within some percentage range I don't know), and without any external parts, aside from filter capacitors. A 7805 needs 8-10 volts input to give good regulation.

 

Definitely it needs to be on a heat sink for any reasonable power draw through it.

[TuzenTCA]

I said the thermal grease looked like crispy corrosion. This is a new heat sink. I know the thing ain't fried, I think the Atari might be though. I know it is a power regulator, yet at the time "reducer" seemed fine, considering it "reduces" the power from 9-12 volts to a safe +5, while still regulating it above 4.75 or whatever. Thanks for the tip about the L and M though.

[Blinky]

p.s. The LM78M05CT is not screwed to the board like the 7805 was. Could this be another problem?

 

As a minimum It must be screwed to the board so it can dissipate its heat.

A heat sink would be even better and is used on several(most?) boards. The 78M05 has 500mA rating and It has a thermal overload protection. Just check how fast it heats up and that there where no shorts made when the 7805 replaced.

 

If the 7805 was replaced because a adapter was used with the wrong polarity or too high rating (>12V 500mA) then that 2200uF Capacitor might need to be replaced too.

[TuzenTCA]

Thanks for all the input!

 

I think it will work as I correctly soldiered it to the board, and put some thermal grease in under it. One concern is that the Atari itself may be fried because the regulator went bad in the first place. Another concern of mine is that I no longer have the original power pak, yet I have a very similar off brand from Wal-mart with like 15 multitips. The concern about that is the plug in on the board seems to be loose and I have never officially got this Atari to work, so i think that thge connector that is on the board maybe bad. In all honsety I should probly junk this atari but I simply cannot bring my self to do it, and, in the future as a "Retro-Project" I was thinking about installing this particular unit into my Yukon's center console as a joke, yet to still have it work and display on a screen, Alpine maybe?

 

Thanks Again,

 

James