Atari 800 (non XL) power supply mod

[rfancella]

Hello all!

I recently picked up an 800 (non XL) without a power supply. It should be a 9v AC, 3 amp PSU. I have a ton of 12v DC switching power supplies. Has anyone used a 12v DC supply with either a buck converter/voltage divider and disabled the rectification circuit? Or possibly swapped out/changed internal components in the power supply section to accept 12v DC?

 

Thanks,

Ron

[reifsnyderb]

I've never tried this.

 

It should be possible to remove the bridge rectifier, rework the circuit a little, and run 12v DC into the circuit.  However, I'd watch R203 as it takes some of the load off of the 7805 voltage regulator.  R203 may have to be replaced with a resistor of a slightly higher resistance to make this happen.  If the power supply board is not connected (and under load) it's output will be higher than 5 VDC due to R203.

 

I have the service manual, with schematics, here:  https://5cfab.com/atari-400-800/

[rfancella]

@reifsnyderb

Thanks for your prompt reply!

Agreed.  Looking at the block diagram, in theory, I should be able to bypass the rectifier - bypass the 12v boost circuit - drop the voltage down to 9v and supply the 5v and -5v regulators.

I'll have to look closer at the schematics to see what's involved.

In the long run I think the machine would be much happier with a switching power supply instead of a linear. 

Edited January 23, 2023 by rfancella

[_The Doctor__]

I do not advise this, the converter is going to be taxed and the Atari might work for a while, but I suspect it'll be injured as things go south, why are we going this route when there are plenty of 9 vac transformers blocks to be had for cheap?

[rfancella]

@_The Doctor__

Just because I have all the parts laying around and a drawer full of 12v power supplies.

[_The Doctor__]

I cry for so many ruined Atari's over the years...

[+bob1200xl]

A 12v DC supply would normally be fed 18v+ AC. Be careful... The 800 power circuits make +5VDC, -5VDC and +12VDC.

 

Bob

 

---you must have a lot of parts laying around!

[Vyvyan B.]

[Peri Noid]

The PSU board generates +5DC, +12VDC and these can be generated with a 12VDC PSU (rather a bit more than 12V to allow 7812 to work). But it also generates -5V, which is impossible to obtain then. Without it, the standard RAM modules will not work.

Edited January 24, 2023 by Peri Noid

[+x=usr(1536)]

12 hours ago, rfancella said:

@_The Doctor__

Just because I have all the parts laying around and a drawer full of 12v power supplies.

Believe me when say that I understand the concept of, "because I can": it's been one that I've followed on more than a few occasions.

 

What isn't clear is what the gain from this would be.  800 PSUs are typically pretty solid, and there are known and available third-party replacements for them if one wants to go that route.  Converting to DC input seems like a lot of work for not much gain in convenience or functionality.

 

By all means go for it; it's your machine and you can do with it as you please.  It's just difficult to see the benefit of doing so.

[TGB1718]

Also, your not going to be able to do this using one Buck/Boost device, as said before +5 and +12 are easy, but the -5V

is difficult to achieve, although not impossible, from what I've learnt, modifying a buck/boost device to produce negative

voltage just requires a few components adding, the down side is the size of the input voltage, the example I saw needed 18V 

input to get -5V output, also apparently the output noise is considerably more and might be difficult to filter without adding

further components on the -5V line.

 

[BillC]

3 hours ago, TGB1718 said:

Also, your not going to be able to do this using one Buck/Boost device, as said before +5 and +12 are easy, but the -5V

is difficult to achieve, although not impossible, from what I've learnt, modifying a buck/boost device to produce negative

voltage just requires a few components adding, the down side is the size of the input voltage, the example I saw needed 18V 

input to get -5V output, also apparently the output noise is considerably more and might be difficult to filter without adding

further components on the -5V line.

 

It shouldn't take much to eliminate the need for -5VDC, AFAIK the 4116 chips on the RAM boards and the power LEDs are the only components which require -5VDC.

[reifsnyderb]

After looking at the responses, I have some more thoughts...

 

I agree that if the RAM is replaced with SRAM and a little more re-work is done, the -5VDC problem is resolved.

 

The +12VDC may not be an issue at all as the XL's didn't supply 12VDC to the SIO and an SRAM memory board wouldn't use the 12VDC at all.

 

As long as the Atari LSI chips (i.e. ANTIC, POKEY, etc.) are not damaged and the case isn't modified, it's all repairable.  My opinion is that given some of the upgrades available, the question of whether a machine is ruined is opinion based.

 

I do agree that it's so much simpler, safer, and cheaper to just use a 9VAC power supply as opposed to all of these changes.  But, as a question of can it be done?  Sure.  Should it be done?  Well, that's a matter of opinion.  Just don't damage the LSI chips.

 

[rfancella]

Thank you all for your replies!

My thought was to remove the rectifier (or take it out of circuit).  Take the 12v regulator out of the loop and feed 12vdc in to that section of the power supply.  Then use a buck converter to feed 9v ac just outside the rectifier.  The issue is that it uses the negative output of the rectifier to derive -5v dc.  That in fact, is a deal breaker.  That voltage is not easily had from anything I have just laying around.

I'll chock this one up to "Mental Ma$turb@tion". 

 

Thanks,

Ron

[Vyvyan B.]

Stop all this nonsense and just buy a proper C017945 power supply off eBay and be done with it. This is how Multi8 ended up with a broken 800. This thread definitely fits into that "encouraging others to break their shit" category.

Edited January 24, 2023 by Vyvyan B.

[rfancella]

@Vyvyan B. LOL

Yea, I'll get a proper PSU for it. 

It will sit nicely on the same table as my Commodores, Amigas and right next to my 1040st.  I don't believe in painting anything, these machines earned the scars they show! 

 

Thank again, everyone, for the comments.

Ron

Edited January 24, 2023 by rfancella