Atari 800 PSU

[scelbi8h]

Years ago I got an Atari 400 that came without any PSU. I purchased one on Amazon that luckily came with the same connector. It's a 230V/9VAC and when I measure it I get 11.7V on the output (without any load).

 

When I purchased my Atari 800 from the States (I'm located in Spain), it came with its original PSU. I've been using it with a step-down voltage converter, which gives an output of 110V like many of you guys have there on the other side of the pond. After the transformer I get 9.5V. Of course these values are without a load, just measured with a tester on the cable.

 

My question is if it's safe to use the 11.7VAC on these computers or I should stick to the original Atari PSU. I know 11.7V is not much and it will be less than that when connected to the computer. But these are computers from the 70s and I don't want the old rectifier circuit inside them to get hotter than it should. 

 

For me it would be more comfortable to use the European PSU and not to be depending on the step-down voltage converter. 

 

I tried both, and I see no difference in video quality or something. I say this because having 9.5V without any load on the original PSU looked a bit low to me. 

 

What do you think? ?

[bfollowell]

That sounds awfully high to me. 11.7VAC is 30% higher than these machines are supposed to take and that's considerably more than spec. Has your 400 been running fine on this voltage? I know the 400/800 machines were built like tanks and they'll take a lot of punishment, but that amount of over-spec voltage sounds pretty high to me, even with no load. Hopefully, someone with more 400/800 experience will pipe in with their opinions.

 

[scelbi8h]

22 minutes ago, bfollowell said:

That sounds awfully high to me. 11.7VAC is 30% higher than these machines are supposed to take and that's considerably more than spec. Has your 400 been running fine on this voltage? I know the 400/800 machines were built like tanks and they'll take a lot of punishment, but that amount of over-spec voltage sounds pretty high to me, even with no load. Hopefully, someone with more 400/800 experience will pipe in with their opinions.

 

I don't think that's too high, because being AC it's supposed to get lower when a load is connected. I even think the 9.5V is a bit low, but I may be wrong.

 

All my C64 and C128 PSU I have around give more or less that value on the 9VAC side. If fact, I just measured a new built C64 PSU and an old one, and I got 11.5 and 11.9V.

 

My Atari 400 worked without problems with this PSU, but I can't say I used it much, just a couple of times to see if it works. I plan to use it now after ordering a RAM upgrade and SCCC board, but so far it stayed ON like for 10 minutes in total since I got it about 5 years ago.

Edited January 4, 2022 by scelbi8h

[sup8pdct]

Does the amazon Psu have  amp output marked?

400 requires about 1.5A

400/800 uses AC to get voltage rails of -5, 5 and 12v. A higher ac voltage means things get warmer on the power supply board.

There will be an upper limit tho 11.5v should be fine.

 

James

[TGB1718]

11.5VAC equates to 16.2VDC presented to the input of voltage regulators, the 7805 and 7812 which produce

the internal +5V,-5V and 12V in the 800 have an input limit of 35 VDC so as long as the PSU you have

can supply enough current is should be quite safe to use.

 

[scelbi8h]

6 hours ago, sup8pdct said:

Does the amazon Psu have  amp output marked?

400 requires about 1.5A

400/800 uses AC to get voltage rails of -5, 5 and 12v. A higher ac voltage means things get warmer on the power supply board.

There will be an upper limit tho 11.5v should be fine.

 

James

Yes, it is a 2100mA PSU

 

4 hours ago, TGB1718 said:

11.5VAC equates to 16.2VDC presented to the input of voltage regulators, the 7805 and 7812 which produce

the internal +5V,-5V and 12V in the 800 have an input limit of 35 VDC so as long as the PSU you have

can supply enough current is should be quite safe to use.

 

 

I see you are multiplying the output by SQR(2). About the original PSU, being 9.5 means it gets the equivalent of 13.4VDC. Isn't it to low voltage for the 7812 regulator? I thought the 78xx needed over 2V from the voltage they are regulating.

 

Well, I see it will be fine to use this 2100mA 9V AC power supply. Anyway while I'm using the 800 and 400 on my desk I'm going to stick to the original Atari PSU plus the step down converter. If I use it in another room or any other place, I will use the Amazon one then (by the way, it's a "RockPower by Warwick" PSU model RP NT8 EU, if it means something).

 

Edited January 5, 2022 by scelbi8h

[TGB1718]

1 hour ago, scelbi8h said:

I thought the 78xx needed over 2V from the voltage they are regulating.

 

I know, but it seems to work ok. maybe the older spec devices were more tolerant

 

[BillC]

4 hours ago, scelbi8h said:

I see you are multiplying the output by SQR(2). About the original PSU, being 9.5 means it gets the equivalent of 13.4VDC. Isn't it to low voltage for the 7812 regulator? I thought the 78xx needed over 2V from the voltage they are regulating.

There are a couple of diodes that create a voltage doubler circuit, if you check the 7812 input you should find well over +20VDC.

[TGB1718]

1 hour ago, BillC said:

if you check the 7812 input you should find well over +20VDC

Thanks @BillC I totally forgot about that