Atari 400 PSU - Help needed

Any 8bit repairs, images, cool things, upgrades etc.
Tomswork
Posts: 775
Joined: Fri Nov 22, 2019 10:05 pm

Re: Atari 400 PSU - Help needed

Post by Tomswork »

Are you sure c202 is in correctly? it is backwards in the schematics relative the the other caps also it looks big is it a 470uf cap

Tom

@PuddyTat
Tomswork
Posts: 775
Joined: Fri Nov 22, 2019 10:05 pm

Re: Atari 400 PSU - Help needed

Post by Tomswork »

Just a question how are you getting the video out of this atari 400 with the rf removed? Maybe the inductors for the rf modulator should be lifted from the circuit for testing l201 and l203

Tom
Tomswork
Posts: 775
Joined: Fri Nov 22, 2019 10:05 pm

Re: Atari 400 PSU - Help needed

Post by Tomswork »

Another question are you holding the lockout switch down when checking?


Tom
PuddyTat
Posts: 23
Joined: Tue Jun 23, 2020 8:23 pm

Re: Atari 400 PSU - Help needed

Post by PuddyTat »

Hi @Tomswork ,
Thanks for the feedback.

CR201 is in with band to the right as per the diode I removed (towards the center of the board). All three of the diodes there (CR201-CR203) are marked the same way). CR202 & CR203 are 1N5391 same as the bridge rectifier circuit.

C201 & C202 are Vishay 470uf 25v caps and polarity is marked on the circuitboard with the -ve to the left edge (pointing away from the center).

S202 is being held down for testing to ensure the power LED lights (which is does), however the RF Modulator is not currently installed for testing as should be redundant. This is a bare PSU only (no load) as I'm unwilling to fit it yet with the tested voltages being so badly off.
PuddyTat
Posts: 23
Joined: Tue Jun 23, 2020 8:23 pm

Re: Atari 400 PSU - Help needed

Post by PuddyTat »

Well, after a LOT of checking, re-checking and checking again, one tiny bit of feedback on another forum started me thinking and led me to the solution. Or rather, showed me what a tit I'd been. I'm so used to DC voltages that I never thought to ensure the -ve lead was on after the bridge recifier... So bring on the jibes, jokes, slagging and insults :lol:

And for those that want to know, there is another step to take if you want to fully test your Atari 400 Power Board without connecting it to the main board.

BillC on the other forum said that the 400/800 have a 33 ohm resistor between Vin and Vout of the +5V regulator which is used to deliver extra current beyond what the regulator provides. If there isn't enough current demand it increases Vout because of insufficient voltage drop and if there is no load/current on the +5V rail, Vout will equal Vin basically.

So I used a 10 ohm, 10W resistor pushed through either the 5a or 5b header and fix the other end to ground (yeah, DC ground before anyone says :P ). This provided enough 'load' to test the 5v rails and mine show rock solid at 4.98v now. -5v and 12v will show correctly without any load resistor.

Thanks to all who helped out and look at it this way, with the diodes, caps and power regulators all done, this should be great for decades to come!
Post Reply

Return to “8 BIT CORNER”