A3000 Chip swap

[toddbailey]

hey all,

I've notice that a series of 74F646 and 74F245 on a A3000 put out a lot of heat.
Looking aty the specs these really suck a lot of current, approx 200 ma or a power dissipation of about 1 watt ea. I was doing some research and found that the 74HCT series will make a suitable replacement while consuming a lot less power 70 ma and producing much less heat.

any one else thought about or performed this upgrade and had good or bad success?

The biggest drawback I see is you got 10 22 pin ic and all are soldered directly to the board

[Jope]

Look at the data sheets very carefully to determine the propagation delay at least.. There may be other things to consider that I'm not aware of.

[RedskullDC]

Hi Toddbailey, et al.

Originally Posted by toddbailey

hey all,

I've notice that a series of 74F646 and 74F245 on a A3000 put out a lot of heat.
Looking aty the specs these really suck a lot of current, approx 200 ma or a power dissipation of about 1 watt ea. I was doing some research and found that the 74HCT series will make a suitable replacement while consuming a lot less power 70 ma and producing much less heat.

any one else thought about or performed this upgrade and had good or bad success?

As Jope said, you need to look at the datasheets carefully.

HCT parts are generally used to interface HC parts to TTL/LSTTL.

Check this out:
http://www.nxp.com/documents/user_manual/HCT_USER_GUIDE.pdf

Immediate things that stand out are:

• Much lower max frequency: F=125MHz (typical) vs. HCT 85MHz (probably not important in Amiga design)
• Fanout of the F parts is generally much higher. HC=10 standard outputs (LS loads), F=50 (Important if you have a few expansion cards in your 3000).

The second point is probably a deal breaker

Cheers,
Red

[mech]

Originally Posted by toddbailey

hey all,

I've notice that a series of 74F646 and 74F245 on a A3000 put out a lot of heat.
Looking aty the specs these really suck a lot of current, approx 200 ma or a power dissipation of about 1 watt ea. I was doing some research and found that the 74HCT series will make a suitable replacement while consuming a lot less power 70 ma and producing much less heat.

any one else thought about or performed this upgrade and had good or bad success?

The biggest drawback I see is you got 10 22 pin ic and all are soldered directly to the board

The others make good points, those particular parts were used for a reason. I wouldn't worry about changing them as they have ran fine for nearly 30 years
The may run warm but it is completely normal.
Changing the 4 PALS to Gals is something that is practical to do if you havent.

[toddbailey]

Originally Posted by Jope

Look at the data sheets very carefully to determine the propagation delay at least.. There may be other things to consider that I'm not aware of.

good advice, the obvious difference is the 74hct646 has a upper freq limit of 85 mhz while the 74f646 is about 105 mhz, as I recall. But the Amiga A3000 uses a 50 mhz clock so that shouldn't be a factor, prop delays might be issue however, have to wonder how close or rather far apart is good enough and at what point a problem will occur.

I guess the best place to start is looking at the schematics and see how these are wired into the rest of the system.

yeah, fanout to the expansion cards could be an issue, still it would be a awesome mod to replace chips that use a lot of power (amps) and the heat generated.
but if it don't work after the mod the effort, time and cost is pointless.

I'll report back once I have more info.

[pandy71]

Don't forget about 74ACT, 74BCT and 74FCT series - they exist and usually offer significantly better parameters when compared to bipolar.