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Old 24 July 2018, 13:21   #1
chocsplease
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Unhappy Help please. A1200 has white dots on screen suddenly.

Hi,

I turned on my A1200 today to find the screen covered in random fast moving white dots. It is plugged into a Hitachi 1020p HD TV via the SCART socket and the lead is a correct one with the resistors and (wonder of wonders) a proper 23pin Amiga plug. I actually have 2 of these but swapping cables has no effect. There is a stable picture behind the moving dots with no tearing or anything like that.

Playing around I initially found that removing the sound connections (the plugs) from the A1200's stereo out solved the problem, but shortly after when I turned on again I had the same white dots without the sound connected.

It looks like some kind on interference (?) but turning everything else in the house off has not fixed it, so I am now wondering if the A1200 has a problem.

I cannot find any info on this searching the net so does anyone have any idea what might be wrong? The A1200 was recapped about a year ago and does not have a rtc battery.

Hope someone can help.
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Old 24 July 2018, 14:52   #2
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What is your PSU like? Was that recapped too? Does it have a good ground connection? Noise from bad PSUs can easily make its way through the A1200 motherboard and appear as interference on the audio or video outputs.
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Old 24 July 2018, 15:20   #3
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First of all you should verify the issue is not with the monitor in the first place by plugging another device into it.

If the issue is not present with that other device then the problem lies with the Amiga. If it is still present, then either the monitor is faulty or indeed you could have noise issues on your power line.

Do you by chance have a ferrite choke available around your house?
If so, try wrapping it around your Amiga power cable (right before it enters the Amiga) to see if that fixes it.
(This is what a ferrite choke looks like: https://www.dreamstime.com/stock-ima...es-image535934.)

If it does, then the cause is definitely instabilities in the power source (either mains or the power supply).
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Old 28 July 2018, 13:25   #4
chocsplease
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Hi,

Many thanks for the suggestions. It is the PSU, I swapped it for the one in my A500 and all is well. So I guess this is a recapping issue? It has been V hot recently in the UK, today is cooler and when turning on the A1200 with its usual PSU plugged in the dots took a couple of minutes to appear.

This is a black Escom PSU, which I am guessing has not been recapped. I picked it up a while ago as the A1200 originally came with a heavy PSU which was not powerful enough to run the 68030 accelerator.

How easy is it to re-cap one of these? I am so-so with soldering and not as good de-soldering. I have the kit but lack the experience.

Other than attempting a DIY has anyone used one of the folk on this, or another forum, for such a repair? Feel free to PM with your experiences if you have.

Thanks in advance.
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Old 02 August 2018, 15:09   #5
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Hi,

I opened the PSU today and was surprised as to how much there was in there. I studied electronics a looong time ago so know a bit (but am not great at the soldering game) and there are bits I just don't understand... see pic.

https://imgur.com/L4myDma


My Questions.

What is the blue/grey rubbery stuff between the 2 triacs and the 'heat sink metal'? I would have thought these would be bolted directly to the metal, not sort of insulated from it.

The caps look OK, and since I am getting noise a short time after its turned on I'm wondering if its a problem elsewhere. Either with the 2 transistors, or maybe that IC in the front? In fact what's an Optocoupler doing in a PSU in any case?

Does anyone know where I can get hold of a Schismatic for an Escom PSU? Google is not being my friend here.
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Old 02 August 2018, 17:03   #6
DanyPPC
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This is one of the worse power supply released for Amiga 600/1200.
I had your same problem too, and I had a new generation of compact power supplies installed inside.
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Old 02 August 2018, 17:08   #7
hooverphonique
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Quote:
Originally Posted by chocsplease View Post
What is the blue/grey rubbery stuff between the 2 triacs and the 'heat sink metal'? I would have thought these would be bolted directly to the metal, not sort of insulated from it.

The mounting tab on the transistors/regulators is connected directly to one of the other 3 pins, so (in most cases) it needs to be insulated from the heat sink.
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Old 02 August 2018, 17:32   #8
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Just to add to the above, electrically insulated; those pads are still of course reasonable heat conductors.
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Old 04 August 2018, 15:26   #9
chocsplease
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Quote:
Originally Posted by DanyPPC View Post
This is one of the worse power supply released for Amiga 600/1200.
I had your same problem too, and I had a new generation of compact power supplies installed inside.

Any suggestions as to what new compacts are any good, there are some being shown on the US ebay now - can't find the link right now .


There also new? square power plugs.
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Old 04 August 2018, 17:31   #10
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I suggest this
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Old 04 August 2018, 18:19   #11
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I suggest this
ditto, I wouldn't even give that Escom PSU the time of day.
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Old 04 August 2018, 18:37   #12
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Quote:
Originally Posted by chocsplease
I opened the PSU today and was surprised as to how much there was in there.
Looks like a pretty standard SMPS to me.

Quote:
Originally Posted by chocsplease View Post
In fact what's an Optocoupler doing in a PSU in any case?
Optocoupler's are common in switch mode power supplies. Used to isolate the mains voltage from the low voltage output.
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Old 18 September 2018, 15:59   #13
chocsplease
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Hello,

(Well this is my thread so I guess this does not count as a necro....)

I ordered the suggested psu - http://centuriontech.eu/product/nuovo_amiga/ and it arrived today. I did a quick check on the voltages and they are v close to the Commodore recommended except that the shield pin is not connected.

I wondered if the shield pin was connected on the Commodore A500 PSU that I have been using on the A1200 desktop while I waited for the new PSU and here I entered the twilight zone...

The A500 PSU has 14.5v(!) on the plus 12V pin, 4.8V on the plus 5v a connected shield and nothing at all on the neg 12V zip, nada.

Given that the A1200 has been working fine with this A500 psu, complete with sound etc and the -12v is supposed to power the RS232 and the audio I am somewhat baffled.

Can someone explain:

What the -12v line is actually used for as my A1200 does not seem to need it.

What the shield pin is for and if it should be connected to the plug shield as the new PSU's isn't

And if there is some kind of over volt protection inside the A1200 for the +12, or have I been gently cooking things at a reglo 14.5V ?

Many thanks
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Old 18 September 2018, 17:06   #14
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Quote:
Originally Posted by chocsplease View Post
The A500 PSU has 14.5v(!) on the plus 12V pin, 4.8V on the plus 5v a connected shield and nothing at all on the neg 12V zip, nada.
Okay, first thing to check is whether the PSU is under load or not when you measured it. Were you measuring the voltages with the Amiga connected and turned on, or just checking the plug? You might not be getting accurate readings without a load present.

Assuming it was powering an Amiga at those voltages:
Quote:
What the -12v line is actually used for as my A1200 does not seem to need it.
-12V is most certainly required for serial and audio on the A1200. Your audio will still "work" without it, but it will be heavily distorted and clipped around 0V, resulting in a very nasty sound quality. Serial ordinarily wouldn't work, but you might get some very tolerant receivers at the other end that can deal with an asymmetric signal.

Quote:
What the shield pin is for and if it should be connected to the plug shield as the new PSU's isn't
The shield is connected to the shielding on the Amiga side's cable, and the RF shielding of the computer. It connects all these parts to the Earth system of your house when used with a standard Commodore PSU. It's not essential for the working of the computer, but I would always connect it when it's there, and might help reduce any noise or interference connected with the Amiga. It's also a safety thing in case something terrible happens inside the PSU and mains power is allowed through to the Amiga. Whenever I'm building a PSU I'll always connect the shield as it was in the original supply.

Quote:
And if there is some kind of over volt protection inside the A1200 for the +12, or have I been gently cooking things at a reglo 14.5V ?
No, there's no overvoltage protection in the A1200, though the chips involved are fairly tolerant of supply voltage so it's probably not the end of the world. I would still be concerned about it however, it's a little too high for my comfort. But bear in mind that it's common for PSUs to output higher voltages than their rating when they're not connected to anything.
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Old 18 September 2018, 18:03   #15
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Huge thanks for such a quick and comprehensive reply Daedalus.

Oops, no the A500, nor indeed the new A1200 PSUs were measured under load, so maybe the 14.5v on the plus 12v for the A500 is less of a concern. That said the new PSU was slightly under volt on all pins with no load, +11.88, -11.9, +4.8. Is this to be expected with newer psus?

I would have thought the -12v on the A500 psu would have given something even unloaded? The A1200 has been running a serial to PC cable for file transfer and that seemed fine, even at v high speeds. I haven't noticed any sound distortion either, so this is just odd.

Can you suggest/ point me at a suitable load circuit or am I just over thinking all this?

I guess it won't be too much of a job to unclip the plug and solder a jumper wire between the shield pin and the plug shield - would that work? Or have I miss measured. I did a continuity check between the plug shield and the pin and that showed an odd resistance, (56 ohms) not a direct connection (0 ohms). My DVM has a continuity check (it beeps) and was silent when I did the test.

Just to make sure I understand, there should there be a wire from the shield pin back to the psu; yes/no? Would a continuity test on that pin and the mains plug earth confirm this?

Many thanks again and I hope you can help with the above.
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Old 18 September 2018, 21:42   #16
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OK a quick update on the above.

I checked for continuity between the power plug's earth and the Amiga A500 psu's square Amiga plug shield and the shield pin. Both are connected to the power earth.

There is no such continuity on the new A1200 psu. I can thus only assume that the psu does not have *any* connection to the power earth, or if it does it is *only* up until the psu itself and not beyond to the Amiga plug.

Since the A1200 has a metal shield in it and this will not be earthed I am v wary about using this new PSU.

Has anyone else bought one of these - http://centuriontech.eu/product/nuovo_amiga/ ?

If so are your's earthed as they should be?

I have contacted the seller.
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Old 18 September 2018, 21:52   #17
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It's sometimes hard to predict what exactly a PSU will do when not under load. I've seen some that drift up and down around 1 or 2 volts, and others that go beyond their specification by up to 50%. So I can't say for sure whether it's expected or not, but I wouldn't trust the readings without a load.

A good point for measuring 5 and +12V is on the floppy power connector. -12V is available at the negative side of C822, which is right beside the floppy power connector on the A1200.
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Old 18 September 2018, 21:54   #18
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As for the ground connection, it's not essential really, and Amigas were supplied with 2-pin non-earthed cords in many places including the US. The PSU ground and shield are connected together in the Amiga anyway via an inductor, so it's not the end of the world.
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Old 19 September 2018, 10:36   #19
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Are all outlets earthed in the UK? I think half of all houses I've lived in have been unearthed apart from kitchen outlets.
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Old 19 September 2018, 10:43   #20
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Are all outlets earthed in the UK? I think half of all houses I've lived in have been unearthed apart from kitchen outlets.
Yep - all outlets are three pin and individually switched too. There's also a spring loaded plastic guard built in that slides the L and N holes open as the earth pin goes into the socket. So even for devices that don't need earth, you have a plastic pin like this:

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