Atari TT / MegaSTE ATX-PSU adapter

Repairs & upgrades.
User avatar
JezC
Posts: 2081
Joined: Mon Aug 28, 2017 11:44 pm

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by JezC »

@Steve From what I remember, the power dissipation with ECL is pretty high all the time, so it is less dependent on switching frequency than say CMOS. So, just having it connected and running may be a good enough test.

Huge oversimplification I know, but effectively the current always flows and is just switched down the different transistors for the logic transitions.

I've not used it for the last 25 years or so...and apologies if I'm telling you what you already know.
Steve
Posts: 2570
Joined: Fri Sep 15, 2017 11:49 am

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by Steve »

@JezC
Cool thanks, luckily I do have a TT ECL > VGA adaptor so I'll break that out for the test. Just means opening up my TT again, which I dislike doing ;Z But! it is important that I test this before giving it to anyone else.
Steve
Posts: 2570
Joined: Fri Sep 15, 2017 11:49 am

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by Steve »

@czietz
I'm very happy you mentioned the cool temperature of the 7905 being strange. After more testing I found that my PicoPSU is not outputting the correct -12v, more like -5v. Checking the outputs showed -5v on the -12, and -2.5v on the -5. I removed the Pico and plugged in a real ATX psu and now the 7905 gets very hot, even without my TT in ECL mode, checking voltages shows a correct -12v and -5v.

So I now think I will fit a heatsink to the 7905 and the adapters are working perfectly, but I will need to figure out why my Pico isn't outputting a proper -12. I am wondering whether it cannot provide the required amount of current and somehow that causes a voltage drop? Is that possible @exxos ? (calling the power supply guru lol) I know the 7905 is rated at 500ma and I wonder if this is enough for the TT ECL circuitry. This is the 7905:
https://www.mouser.co.uk/ProductDetail/ ... Frqg%3D%3D

(I have tested the TT in ECL mode with the proper ATX PSU and it is working)

The Pico I was using isn't a cheap one either, it is supposed to be one of the best:
https://www.mini-box.com/PicoPSU-120-WI ... wer-supply
In the mean time I have contacted mini-box support to see what they say...

I might look for non-Pico options at this rate...
czietz
Posts: 548
Joined: Sun Jan 14, 2018 1:02 pm

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by czietz »

Steve wrote: Wed Oct 28, 2020 10:39 am @czietz
I'm very happy you mentioned the cool temperature of the 7905 being strange. After more testing I found that my PicoPSU is not outputting the correct -12v, more like -5v. Checking the outputs showed -5v on the -12, and -2.5v on the -5. I removed the Pico and plugged in a real ATX psu and now the 7905 gets very hot, even without my TT in ECL mode, checking voltages shows a correct -12v and -5v.
Yes, I think @JezC is right and the ECL circuitry consumes more or less the same power regardless of whether you're actually in TT high or not.
Steve wrote: Wed Oct 28, 2020 10:39 am So I now think I will fit a heatsink to the 7905 and the adapters are working perfectly, but I will need to figure out why my Pico isn't outputting a proper -12. I am wondering whether it cannot provide the required amount of current and somehow that causes a voltage drop?
The PicoPSU manual on the webpage mentions a maximum load of 100 mA on the -12 V rail. In a previous thread in this forum I wrote that the ECL chips alone need about 200 mA; though I don't recall if I measured it back then or just estimated from the datasheets.
Steve wrote: Wed Oct 28, 2020 10:39 am Is that possible @exxos ? (calling the power supply guru lol) I know the 7905 is rated at 500ma and I wonder if this is enough for the TT ECL circuitry.
Without heatsink, you'll hit the thermal limit of the 7905 before reaching the 500 mA.
Steve
Posts: 2570
Joined: Fri Sep 15, 2017 11:49 am

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by Steve »

@czietz
Thanks for everything, so I guess we can conclude that this particular PicoPSU isn't acceptable for a TT if it only outputs 100ma on the -12v. No wonder.
edit: I found their comparison page:
https://www.mini-box.com/site/ComparisonChart.htm
Seems none of them are suitable.

Of course, the adaptors are still perfectly fine. I've ordered heatsinks for them and I will research smaller ATX supplies.
Edit: I may have sourced some good & small psu's that can output 500ma on the -12 which would be perfect. Once I receive them and do some testing I will update the thread. Checking the original PSM-5341 specifications it outputs -12v 0.3a and 5v 0.4a. So this new PSU/adapter will be able to output -12v 0.5a and -5v 0.5a. So we should be good.
Steve
Posts: 2570
Joined: Fri Sep 15, 2017 11:49 am

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by Steve »

Update on my endeavours:

Sourced new small ATX PSUs small enough to fit in the TT case with the -5v adapter. It is an FSP250-50GUF with the following specs: +5v 16a +5vsb 2a +12v 18a -12v 0.5a. It is a really nice little PSU with many modern features and protections. It includes its own fan controller which measures the heat and adjusts as necessary. So quite nice for an almost silent TT when you have a decent fan. The TT pulls so little power in comparison to what the PSU can provide that it runs very cool and silent. I also sourced a perfect heatsink for the 7905. So now the 7905 which is rated up to 500ma is able to get as much power as it needs from the PSU. ECL mode is working fine.

For reference I was inspired by this old website:
http://www.mr2.net/marknias/atari_tt030 ... r%20Supply

PXL_20201030_131152152.jpg
PXL_20201030_131152152.jpg (191.89 KiB) Viewed 4710 times
PXL_20201101_171317629.jpg
PXL_20201101_171317629.jpg (326.23 KiB) Viewed 4715 times
PXL_20201101_170956345.jpg
PXL_20201101_170956345.jpg (271.83 KiB) Viewed 4715 times
PXL_20201101_185554590.jpg
PXL_20201101_185554590.jpg (451.7 KiB) Viewed 4715 times
User avatar
derkom
Moderator
Moderator
Posts: 1208
Joined: Sun Jul 29, 2018 6:45 pm

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by derkom »

Excellent work. I'm following your progress here with anticipation of building something similar. I've given my TT PSU the Noctua fan treatment, and stuck a 68 Ω resistor in there to lower its speed, and it's fairly quiet now (certainly compared to stock), but something even quieter would be nice.
mikro
Posts: 474
Joined: Mon Aug 28, 2017 11:22 pm
Location: Kosice, Slovakia
Contact:

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by mikro »

Before lowering the speed I would recommend to measure temperature inside the metal cage - after that, I assure you wont feel comfortable lowering that speed anymore. ;-) It's really super hot there, sometime I fear whether even that one fan for such a huge box is enough.

Steve - great find!
User avatar
derkom
Moderator
Moderator
Posts: 1208
Joined: Sun Jul 29, 2018 6:45 pm

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by derkom »

mikro wrote: Mon Nov 02, 2020 7:07 am Before lowering the speed I would recommend to measure temperature inside the metal cage - after that, I assure you wont feel comfortable lowering that speed anymore. ;-) It's really super hot there, sometime I fear whether even that one fan for such a huge box is enough.
I stuck a temperature probe in there at various stages. The Noctua improved things over stock, and reducing the speed brought it back up over stock, but only a little. My TT has had the really tight fan grille cut out and replaced with a very loose grille, which improves airflow a lot. I suspect that makes the difference.

Definitely still would like to stick a modern PSU in there, so I'm looking forward to doing that. I was originally planning to go the picoPSU route, but then made the same discovery as @Steve, and realised I was going to be able to create enough -5V that way. :?
User avatar
derkom
Moderator
Moderator
Posts: 1208
Joined: Sun Jul 29, 2018 6:45 pm

Re: Atari TT / MegaSTE ATX-PSU adapter

Post by derkom »

If you consider it a completed design at this point, do you mind sharing the PCB you've built for the -5V regulator? When I first started looking at doing this kind of project myself (inspired by the same old page you linked), I was anticipating building something on protoboard like I did when building the new PSU for the Mindset a few months ago, but a tidy real PCB would be nicer. :D
Post Reply

Return to “PSU (power supplies)”