A5000 - diving in at the deep end!

[exxos]

Been trying to trace it through But I don't think this is right... I'm not following where the 5V originates from...

PXL_20221122_085041687.jpg

Was thinking it looks like the output stage of this power supply But difficult to tell.

https://exxosforum.co.uk/atari/last/psu ... 30_PSU.pdf

[exxos]

Did you change the smaller value capacitors ?

If you look at the link in the above post.. Look around C20 on the output side.

It would seem for some reason your power supply things the output voltage is higher than it actually is and then it adjusts when it warms up. So sounds like a capacitor sort of issue.

As C20 is part of the feedback circuit could be tainting results.

EDIT:

Looks like it might be C30 on your PSU... What value is it ?

[rubber_jonnie]

I replaced all the electrolytic capacitors, but none of the others.

I'll have another look, but so far don't see it.

I powered on under load, so cold, after lunch. and 12v started at about 10.2v and took about 30s to come up to about 12.16v. The machine booted, but I had to do a restart before it would see the HDD.

5v started about 4.85v but came up to 5v rapidly. I'm going to print out the PCB and start to work out what goes where and then see if I can get a schematic done as I think it will help overall, even if I can't ID some of the components, and there are a lot of inductors.

FYI I have reflowed the inductors in the 12v line and I did find a cold joint on the smaller transformer on the mains side so I'll try it again now that's all done.

[exxos]

5v started about 4.85v but came up to 5v rapidly. .....and 12v started at about 10.2v and took about 30s to come up to about 12.16v

Very strange. All that happens is the transformer is pulsed, and as both 5V and 12V windings are in the same core.. They both get energy.. They are both driven of the same feedback line... So one voltage cannot really be lower than the other.

It's like having a regular transformer with 2x12v windings and one of them outputting a lower voltage than the other and slowly rising up to the correct level.. It is certainly not a normal thing to happen.. It's why I wonder if there was a internal short on the transformer.

FYI I have reflowed the inductors in the 12v line and I did find a cold joint on the smaller transformer on the mains side so I'll try it again now that's all done.

Fingers crossed it is that simple. But indeed I would definitely re-solder all the transformer and coils on the board, even if they look good.

There are likely bunch of resistors as well which are all the feedback resistors for the 5V and 12V circuit. Probably a good idea to re-solder them as well.

[rubber_jonnie]

@exxos So, I just powered on again after about 30 minutes and once again it took about 30-40s for both 5v and 12v to get up to normal values. Both start low but 4.85v is enough to power on the mainboard, but the HDD starts too late as 10-11v isn't enough to spin it., so the machine boots to ROM and after that I can power cycle the machine and it will boot normally.

In fact I just checked after 10 minutes and it still boots normally. It's only after a long period of standing that it won't start properly. I have one thing left, I will replace the FEP16CT with an MUR1620CT which is a closer match than the 2045 in there at the moment, it just arrived in the post and will be a quick swap.

[exxos]

@exxos So, I just powered on again after about 30 minutes and once again it took about 30-40s for both 5v and 12v to get up to normal values. Both start low but 4.85v is enough to power on the mainboard, but the HDD starts too late as 10-11v isn't enough to spin it., so the machine boots to ROM and after that I can power cycle the machine and it will boot normally.

hmmm... So 5V and 12V are taking the same amount of time to stabilize then ?

Try just changing C30 for a lower value.. Assume its like 3.3uF ?

[rubber_jonnie]

@exxos So, I just powered on again after about 30 minutes and once again it took about 30-40s for both 5v and 12v to get up to normal values. Both start low but 4.85v is enough to power on the mainboard, but the HDD starts too late as 10-11v isn't enough to spin it., so the machine boots to ROM and after that I can power cycle the machine and it will boot normally.

hmmm... So 5V and 12V are taking the same amount of time to stabilize then ?

Try just changing C30 for a lower value.. Assume its like 3.3uF ?

I just dropped in the MUR1620CT, but it was only 10-15 mins on the bench, so will leave it for 30mins to 1hr see what it does after that.

If that doesn't change I'll have to see if I can find the equivalent to C30 and try an alternative.

[rubber_jonnie]

So, there is a possible glimmer of hope for the A5000 PSU :)

The feedback circuit that uses an SHR to drive the PC111 optocoupler has now had the capacitor (C30 in the TT/MSTE 5341 PSU schematic) replaced with yet another new one and also the TLP431C SHR replaced with new and initial results are encouraging.

First power up worked just fine, so I'm giving it an hour or two over lunch before I try it again to confirm it's working, but I am a little optimistic.

Further update later on today :)

[alexh]

Thanks. As an A5000 owner (albeit in bits) this is a job which awaits me I'm sure.

[rubber_jonnie]

Thanks. As an A5000 owner (albeit in bits) this is a job which awaits me I'm sure.

It certainly might if you're having a slow start on the PSU.

As it goes, there was more than just that wrong, so replacing the other parts was probably just good maintenance, but if you do run into it, I'd definitely go for the component marked SHR and C20 next to it as a quick place to start, see if it does resolve the problem.

The A5000 is now reassembled and after leaving for an hour plus during lunch and then walking the dog, it powered on and though the HDD didn't spin up immediately, it did so within about 10 seconds, so more than quick enough to be used as a boot device.

The voltages were good too under load on a cold boot (Off for more than 30 minutes), 11.8v and 4.9v, so a big improvement there.

Such a tiny component and such a huge effect. Next test will be to see how it fares overnight.