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TSN 1-2450A instead of LM7905

Posted: Sun May 31, 2020 3:13 pm
by HigashiJun
As you know, our Orics use a LM7905 as a voltage regulator, which can seem strange as there were simpler solutions even in the '80s...

Heat is of course an issue with this regulator which tends to get really hot, and adding a heatsink under the motherboard is a common way to prevent from overheating.

Time has passed since the first Orics were produced but even nowadays people seem to continue using this old LM7905 to replace a failed one, whereas a better solution exist.

This solution is the TSN 1-2450A from Traco Power:

https://www.tracopower.com/int/model/tsn-1-2450a

I do agree its price a slightly higher than the LM, but it's really worth it: our beloved Orics run very cool without a glitch and there's no need to think about a better cooling system.

I first thought I was the only guy using it (nobody on the forum except one member mentioned it) but then I discovered this very informative English website:

http://blog.tynemouthsoftware.co.uk/

where the webmaster also used the TSN to replace a defective LM.

I don't know if some of you guys have replaced your failing LM with this better part, but I can only recommend it warmly to everyone. :D

Cheers.

Re: TSN 1-2450A instead of LM7905

Posted: Sun May 31, 2020 5:56 pm
by mikeb
These do seem like a good "modern replacement" for the 7905, and in fact not only will they run cooler as you observe, but the input current draw will also be lower (as instead of ditching the excess voltage as heat, a buck converter trades the excess voltage for greater output current).

Also, the input voltage range is quite wide, which means fewer accidents with "oops, wrong power supply voltage". Note C8 (100uf/10v) on Oric's board limits the input voltage here, so you'd need to uprate that if you were planning to go higher.

The only real reason for using a 7905 nowadays is -- if you already have them, or for authenticity!

You may want to warmly recommend the module, but you have to admit, a 7905 is a hot little number :)

Re: TSN 1-2450A instead of LM7905

Posted: Sun May 31, 2020 10:57 pm
by HigashiJun
:D

You greatly explained all the other benefits I have omitted, and it adds more "heat" to my warm recommendation...

Cheers !

Re: TSN 1-2450A instead of LM7905

Posted: Wed Jun 10, 2020 9:29 pm
by tingo
The VX7805-1000R might also be suitable if it can supply enough current in the negative configuration - it is so new that DigiKey doesn't have a datasheet yet. It is a bit cheaper than the other candidate.

Re: TSN 1-2450A instead of LM7905

Posted: Wed Jun 10, 2020 10:51 pm
by iss
tingo wrote:
Wed Jun 10, 2020 9:29 pm
... VX7805-1000R.
Datasheet here: VX7805-1000R.

This will NOT work :( because:
- only 500 mA output current:
Screenshot_20200611_003617.png
Screenshot_20200611_003617.png (63.83 KiB) Viewed 1811 times
- and more important the negative regulator is common ground too and the equivalent connection inside Oric is as on picture bellow:
Screenshot_20200611_003750.png
Screenshot_20200611_003750.png (38.89 KiB) Viewed 1811 times
i.e. without track cutting there will be definitely a short connection and lot of smoke :!:

EDIT: the same thing should be checked for TSN 1-2450A too.

Re: TSN 1-2450A instead of LM7905

Posted: Thu Jun 11, 2020 12:41 am
by HigashiJun
I can confirm the TSN 1-2450A was a "plug'n'play" replacement for me.

No need to cut any tracks, just desolder your old LM7905 and replace it with the TSN and voilà...

:D

Cheers !

Re: TSN 1-2450A instead of LM7905

Posted: Thu Jun 11, 2020 5:15 pm
by Vyper68
The Traco power VR mods are very popular in the Sinclair community and it is on my “to do” list for both my Atmos and also my Sinclair 128K “Toastrack”
Great thread :D

Re: TSN 1-2450A instead of LM7905

Posted: Fri Jun 12, 2020 3:00 am
by HigashiJun
It's really worth it... The Oric stays very cool. You should definitely give it a try.

Re: TSN 1-2450A instead of LM7905

Posted: Tue Sep 01, 2020 11:07 am
by Vyper68
I finally bought one of these TRACO regulators and fitted to my Atmos but it wouldn’t boot. Just got the pre-boot screen. Checked the voltages and all good. So took it out and fitted another LM7905 and the Amos is back to normal and working happily. It was almost like when you turn the Oric on at the wall and it doesn’t start up correctly. I tried various combinations of power, direct, via RGB SCART, turned on from the wall. All provided the same screen, so my Atmos doesn’t play nice with this part. So others may have a similar experience.
I have an Oric-1 and I may try that next, but I don’t want to keep removing and fitting regulators on these PCB’s so I’m not sure if I will, I will have wasted £24 if I don’t use it :(
So if anyone has any insight as what may solve the issue I’m all ears, I replaced all the electrolytic capacitors last year so it’s not that.
Failing that If anyone wants it cheap pm me...

Re: TSN 1-2450A instead of LM7905

Posted: Tue Sep 01, 2020 3:02 pm
by HigashiJun
I'm sorry to hear that...

It's quite weird.

Did you receive the right part, ie. TSN 1-2450A and not another one (TSN 1-24150A) ?

Re: TSN 1-2450A instead of LM7905

Posted: Tue Sep 01, 2020 5:18 pm
by mikeb
Maybe the TSN module starts up a little slower than a 7905 and a snappy power supply, (either just how it is, or a deliberate soft-start feature).

Next time you come across this :-

1) Ensure that you are providing a good solid input supply, powered on smartly.
2) When Oric doesn't boot (as you found), with the power applied, short pin 1 and 40 of the CPU together (~RESET to ground) briefly with a wire and
release it.
3) If Oric now starts, it needs a longer reset pulse, because the +5v power supply is now too slow coming up. Replace C21 1uF/10v with something larger (4.7uF/10uF still at 10v) to get a x4 or x10 size reset pulse. And try again.

4) Do not get distracted and build a microcontroller based reset manager module, it's not needed!

Re: TSN 1-2450A instead of LM7905

Posted: Wed Sep 02, 2020 8:37 am
by Vyper68
Thanks Mike I will try that if I fit it again, I won’t probably do this for a while though.

Re: TSN 1-2450A instead of LM7905

Posted: Wed Sep 02, 2020 8:40 am
by Vyper68
HigashiJun wrote:
Tue Sep 01, 2020 3:02 pm
I'm sorry to hear that...

It's quite weird.

Did you receive the right part, ie. TSN 1-2450A and not another one (TSN 1-24150A) ?
It’s definitely the right part HigashiJun, I just have a very temperamental Atmos :lol:

Re: TSN 1-2450A instead of LM7905

Posted: Mon Sep 07, 2020 12:12 pm
by Vyper68
I fitted the 2450A regulator again and the Atmos does boot if you short the Reset pin so I fitted a 4.7uf cap and had no joy but when I fitted a 10uf Capacitor the Atmos boots up so thanks to Mikeb for the tip on replacing the Cap with a higher value.

Re: TSN 1-2450A instead of LM7905

Posted: Mon Sep 07, 2020 1:09 pm
by HigashiJun
Great !

I'm happy your problem is solved !

:D