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Author Topic: Broken 12V (Read 646 times)

Offline dangerman

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Broken 12V
« on: 2019.March.17. 18:41:19 »
Last weekend, I started having problems with my Enterprise 64 and I realised there were problems with the 12V circuit. Thanks to previous posts on Enterprise Forever, I discovered the most common fault is with TR2 and C9. So I replaced them both and it started working again. I thought this had fixed the problem.

However, yesterday I plugged in the Enterprise and disaster happened! Smoke came from the case and TR2 was completely melted, and the wire to coil L1 had broken as well. :cry:

Thankfully the rest of the computer seems to work if I wire in an external 12V source... so it's only the 12v circuit that's blown. That is a big relief!

Anyway, I replaced TR2 and C9 again and patched the wire to coil L1, but when I tried power it up - more smoke started coming from TR2, so I unplugged it quick. I think TR2 must be short circuited somehow. Looking at the schematics and using my very limited grasp on electronics, I guess the culprits are going to be C10, TR3 or the coil L1. Does anyone know how I might narrow this down? I have a multimeter but don't have access to an oscilloscope. Is it even possible to get a replacement coil???

All help gratefully received. :(

Offline pear

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Re: Broken 12V
« Reply #1 on: 2019.March.17. 20:00:54 »
The coil can be wound yourself.
I wrote about it here.

Offline dangerman

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Re: Broken 12V
« Reply #2 on: 2019.March.17. 22:20:38 »
Thanks for the tip, Pear. I'll get some 0.25mm enamel wire and give it a go myself.

Is there any way to confirm whether it is the coil that is causing the problem? Could it be another component that is causing the short?

Online Zozosoft

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Re: Broken 12V
« Reply #3 on: 2019.March.17. 22:56:42 »
The best solution buy EPower from Pear, and replace completly the old power circuit. But I don't know he currently have it in stock :oops:

Anyway after I bulit in EPower to many machines, I have spare coils.

Offline dangerman

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Re: Broken 12V
« Reply #4 on: 2019.March.23. 12:15:36 »
Good idea about EPower, although I think I don't really want to replace everything. The 5V is working fine.

Do you need your spare coils, or would you be able to send me one? It would be great if I could fix it.

Offline dangerman

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Re: Broken 12V
« Reply #5 on: 2019.July.14. 23:29:55 »
I finally found some time to look into fixing my EP. But I'm a bit out of my depth with analogue electronics! Things are definitely improving, but the input to the 12v regulator 78L12 is currently only at 10V

I got some replacement parts from an electronics store. I tried to find the closest replacements that I could, but the wire that I bought to repair the coil is 0.236mm, not 0.25mm. I left the inner winding on the coil (the red wire) as that didn't seem to be damaged. Maybe I should have rewound both the windings.

The other problem is that I asked for BC337 transistors and they have given me BC337-40 transistors. Looking at a datasheet the only difference that I have found is the minimum DC current gain.

DC Current Gain hFE (IC = 100 mA, VCE = 1.0 V)
BC337            Min 100 Max 630
BC337–40      Min 250 Max 630

Can anyone tell me what the implications are. Is it okay to use that transistor?

If the transistor is okay, I'm thinking I'll just add a few more windings to the coil to see if I can increase the voltage...

Offline BruceTanner

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Re: Broken 12V
« Reply #6 on: 2019.July.15. 11:07:38 »
I have used different BC337s with no problem. But I can't remember if they were -40s or something else, sorry!

Online Zozosoft

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Re: Broken 12V
« Reply #7 on: 2019.July.15. 11:32:58 »
As I wrote it previously: I also using -40 version at repairs.

Offline dangerman

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Re: Broken 12V
« Reply #8 on: 2019.July.15. 13:07:45 »
Thanks - that's good to know. So hopefully the transistor is good.

I'll try adding some more windings to the coil. I'm aiming for 14.5V input to the 78L12 regulator.


Offline dangerman

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Re: Broken 12V
« Reply #9 on: 2019.July.20. 09:55:43 »
Finally got it fixed and working.

It was nothing to do with the coil. There was some kind of a problem with the BC337-40. I replaced it with another one (also BC337-40 as that's all I have) and it now works. :smt041