CoCo2 26-3134A with 230V~ ???

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jedie
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Re: CoCo2 26-3134A with 230V~ ???

Post by jedie »

It's alive.

First i see only cursor and a LIST or Enter will result in a freeze with some random characters on screen. :o

Next one i turned ON, everything is fine :)
sorchard wrote:If Q1 heatsink doesn't get too hot then it shouldn't be a problem.
You mean the big, black one? For one transistor?
It runs only a few minutes. I can touch the heatsink. But it is already clear warm.
sorchard wrote:Do you have another multimeter you can try?
Currently, no.
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jedie
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Re: CoCo2 26-3134A with 230V~ ???

Post by jedie »

There some pictures:
Image
Image
Image
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sorchard
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Re: CoCo2 26-3134A with 230V~ ???

Post by sorchard »

That's a nice tidy job. Looks like a perfect fit!

Not starting up first time must have been scary. Hopefully it was just the electrolytics reforming.

Yes, Q1 is the transistor on the large black heatsink. It will definitely get warm. If it gets too hot to touch then you might need a bigger heatsink.

Where does the earth wire go?
Stew
jedie
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Re: CoCo2 26-3134A with 230V~ ???

Post by jedie »

PE (earth wire) goes nowhere. I have cut it.

Yes, the heatsink get hot. Think it can't run hours and hours...

If the Q1 transistor died: Can i easy get a replacement? The specs says Q1 is: 2SD313(E,F) or KTD880(Y,GR)

Is Q1 the only one who can die?
... too many ideas and too little time ... Related stuff written in Python:
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sorchard
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Re: CoCo2 26-3134A with 230V~ ???

Post by sorchard »

Q1 doesn't need to be anything special. There are likely to be many alternatives available. (e.g. D44H8)

It could be bad news if Q1 fails short circuit. This would pass a high voltage on to the rest of the system.

It might be worth considering lowering the voltage coming out of the transformer, either with a bridge rectifier between the transformer and PCB or low value resistors (1-2 ohms ?) in series with the transformer outputs.

Edit:
In addition to the resistors a 5W 8.2V zener diode could be placed between Q1 collector and 0V.
Stew
jedie
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Re: CoCo2 26-3134A with 230V~ ???

Post by jedie »

sorchard wrote:It could be bad news if Q1 fails short circuit. This would pass a high voltage on to the rest of the system.
Hm! :o
sorchard wrote:It might be worth considering lowering the voltage coming out of the transformer, either with a bridge rectifier between the transformer and PCB or low value resistors (1-2 ohms) in series with the transformer outputs.
That sounds to be a good idea.

Power resistor and bridge rectifier are cheap. Maybe i have some resistors.

But, voltage drop by Resistors in series is current dependent. I can measure the current but this varies...

A bridge rectifier seems to be nice. But where to connect it on the PCB? To the normal AC inputs (A1 and A2 that i now used)?
... too many ideas and too little time ... Related stuff written in Python:
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sorchard
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Re: CoCo2 26-3134A with 230V~ ???

Post by sorchard »

jedie wrote:But, voltage drop by Resistors in series is current dependent
It shouldn't matter too much, you just want to generate some heat in the resistors instead of Q1.

A bridge rectifier can be connected to A1/A2 as you suggested. The transformer connects to the AC side and the PCB to the DC side. It doesn't matter which way round + or - are connected, the bridge on the PCB will sort it out.

A better idea might be to take 4 diodes (e.g. 1N5401) and make 2 antiparallel combinations. Each of these combinations would drop 0.7V in both directions. It might be easier to do a tidy job this way.
Stew
jedie
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Re: CoCo2 26-3134A with 230V~ ???

Post by jedie »

sorchard wrote:A better idea might be to take 4 diodes (e.g. 1N5401) and make 2 antiparallel combinations. Each of these combinations would drop 0.7V in both directions. It might be easier to do a tidy job this way.
Can you explane a little bit more about the wiring? What do you mean with "antiparallel" ?

Do you mean something like this:

Image

Or maybe the same "into" A2 wire?


If i made a new order, then i can buy some diodes and a bridge rectifier. Maybe shipping costs are higher then the components...

EDIT: 10x1N5401 costs 0,35 € and bridge rectifier are also <1€ ... But whith rectifier: http://www.pollin.de/shop/p/NDk4OTk4/Ba ... chter.html ???

Maybe i buy the PSU, see: viewtopic.php?f=8&t=4967&p=12294#p12294
... too many ideas and too little time ... Related stuff written in Python:
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sorchard
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Re: CoCo2 26-3134A with 230V~ ???

Post by sorchard »

@jedie

You are right, antiparallel diodes are connected exactly as you have drawn. The same in A2 as well would be good.

A 4A rectifier would be fine. (For example the €0.25 one). If you buy 10 diodes then you will have spares to drop the voltage even more if required.

I've been tidying up the mess in my roof this week and I'm hoping to find my PAL CoCo. Assuming it still works and the power circuit is similar I will measure the voltages on the board and let you know.
Stew
sorchard
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Re: CoCo2 26-3134A with 230V~ ???

Post by sorchard »

sorchard wrote:I've been tidying up the mess in my roof this week and I'm hoping to find my PAL CoCo. Assuming it still works and the power circuit is similar I will measure the voltages on the board and let you know.
I found it hidden behind some bags of old clothes. The PSU circuit looks completely different to your one. I tried to find some PAL schematics online without any luck but I took some measurements anyway:

The transformer has outputs for 0V, 2 x 9 VAC and 2 x 17 VAC.
The 2 x 9 VAC outputs get rectified/smoothed to +10.5 VDC
The 2 x 17 VAC outputs get rectified/smoothed to +/-21 VDC
Protective earth is connected to 0V.

The +/-21 VDC supplies get regulated down to +/-12V by 7812 / 7912 regs.

The +10.5 VDC is regulated down to 5V by a linear circuit based around a 2N6594 PNP transistor. That's a lot of power to get rid of so the transistor gets pretty hot. After about 5 minutes, it's too hot to keep your finger on for more than a few seconds. If you plugged a disk controller in I imagine the transistor would really start to cook. That's commercial circuit design for you :-)

The 10.5 VDC in my PAL coco is significantly higher than the equivalent voltage (8.5V ?) in your NTSC coco, which explains why the transistor and heatsink are much larger in the PAL coco. As long as the transistor doesn't get much hotter than 80 deg C then it shouldn't be a problem. Of course, it will last longer if it runs cooler so lowering the voltage from the transformer will be of benefit.
Stew
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