Atari 400 PSU - Help needed

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PuddyTat
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Atari 400 PSU - Help needed

Post by PuddyTat »

I got myself an 'untested' 400, stripped it, recapped the PSU board and fitted new 78 series regulators. Before re-fitting to the mainboard, I checked the voltages on the connector and I'm getting -12v on the -5v rail, 6v on the 5v 5a and 5b rails, 4v on the +12v rail and -7v on the ground pin!

All caps are the correct way round and of the correct ratings, all voltages are DC and have re-fitted the original regulators - same issue.

Anyone any suggestions on where to look next or am I being stupid somewhere?
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rubber_jonnie
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Re: Atari 400 PSU - Help needed

Post by rubber_jonnie »

Aside from re-checking your work, are you testing the PSU with any sort of load? If not then this may be your problem.

When I test ST PSUs I use resistors from exxos store, but not sure what you'd need for yours, you'd have to look at the load the 400 puts on each rail and calculate what resistors you'd need to mimic that.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
Tomswork
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Re: Atari 400 PSU - Help needed

Post by Tomswork »

Can you share some pictures of the capacitors and regulators. Are the capacitors you installed low esr ?

Tom
Tomswork
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Re: Atari 400 PSU - Help needed

Post by Tomswork »

Ok just looked at the schematics -12v on the -5v is not regulated just handled with diodes off the bridge rectifier my guess is the diodes have failed in the bridge or the diodes used to shut the -12 to -5 has failed

Diode cr201 drops 5v off to ground on on the minus power rail must likely it's turned into a resistor


Tom
PuddyTat
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Re: Atari 400 PSU - Help needed

Post by PuddyTat »

Thanks for the pointers Tom. Will order some replacement diodes and see where that takes me next. Did check the voltages were DC so the bridge I assumed was working correctly. Stupidly should have taken the step on page 110 of the service manual literally I guess - no -5v, replace diodes!
It also suggests replacing CR201-CR203 - Any suggestions as to what these are or a resource I can look them up?

All caps are Nichicon/Vishay as I don't use cheap China versions - I've seen too many failures on them to make it worth while. Will upload a pic later today once I'm home from work.
Tomswork
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Re: Atari 400 PSU - Help needed

Post by Tomswork »

My guess is its a zener diode with a 5.1v drop problem is the current rating 500mw 1000mw. It's hard to tell by the size of the part

Tom
PuddyTat
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Re: Atari 400 PSU - Help needed

Post by PuddyTat »

Found it. Sams ComputerFacts Atari 400 has CR201 listed as 1N5231B and CR202-CR203 being 1N5391. Will get some ordered just in case as well.
PuddyTat
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Re: Atari 400 PSU - Help needed

Post by PuddyTat »

rubber_jonnie wrote: Tue Sep 28, 2021 11:08 pm Aside from re-checking your work, are you testing the PSU with any sort of load? If not then this may be your problem.

When I test ST PSUs I use resistors from exxos store, but not sure what you'd need for yours, you'd have to look at the load the 400 puts on each rail and calculate what resistors you'd need to mimic that.
Apologies, just spotted the reply! (Yes, I do wear glasses. Why do you ask... :lol: )

Didn't test the 400 PSU with a load as like yourself, not sure what it'll draw. However didn't want to put -12v through the system or 6v on the 5v rails! I did triple check I had all the caps the right way round and could see nothing wrong with any of them. Having had a bad 7805 from my current batch is why I swapped back to the originals just in case, however looks like the new ones were fine after all.

9vAC 1A is supplied by a brand new PSU which without load, registers 12vAC. I know the 78xx series can handle that so wasn't too concerned and all the caps are 16v min spec.
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rubber_jonnie
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Re: Atari 400 PSU - Help needed

Post by rubber_jonnie »

PuddyTat wrote: Wed Sep 29, 2021 4:17 pm
rubber_jonnie wrote: Tue Sep 28, 2021 11:08 pm Aside from re-checking your work, are you testing the PSU with any sort of load? If not then this may be your problem.

When I test ST PSUs I use resistors from exxos store, but not sure what you'd need for yours, you'd have to look at the load the 400 puts on each rail and calculate what resistors you'd need to mimic that.
Apologies, just spotted the reply! (Yes, I do wear glasses. Why do you ask... :lol: )

Didn't test the 400 PSU with a load as like yourself, not sure what it'll draw. However didn't want to put -12v through the system or 6v on the 5v rails! I did triple check I had all the caps the right way round and could see nothing wrong with any of them. Having had a bad 7805 from my current batch is why I swapped back to the originals just in case, however looks like the new ones were fine after all.

9vAC 1A is supplied by a brand new PSU which without load, registers 12vAC. I know the 78xx series can handle that so wasn't too concerned and all the caps are 16v min spec.
And I wouldn't expect you to put it through the 400 as a load :) Makes me wonder if there is some sort of load details out there for this purpose though?
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
Tomswork
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Joined: Fri Nov 22, 2019 10:05 pm

Re: Atari 400 PSU - Help needed

Post by Tomswork »

@rubber_jonnie

These are amazing to switching replacements

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35a3e2c7-e7aa-42f6-ab26-6e9d74440860_1.daf28efa48d6b795072fce90f6f9f9cf.jpeg.jpg (23.46 KiB) Viewed 2414 times

Tom
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