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Old 12 November 2017, 13:19   #1
Lanksu
 
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Amiga 500 PSU repair help

Hello,

I've come across a light version of the psu and noticed that two of the resistors have burnt out. The color coding is gone so perhaps someone more knowledgeable with this stuff might be able to determine what resistors I need to fix it?

I also have a heavy psu, it powers my amiga so that it boots up, but the floppy drive doesn't work, no drive head moving sound either.

Thank you in advance for any help
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Old 12 November 2017, 13:31   #2
ptyerman
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The floppy drive problem is more likely to be the drive itself than the PSU. There is several threads on this forum about floppy drive problems and what to look for, have a read through and see if anything comes to mind. Easily overlooked also is to check the drive cables and connections, you wouldn't believe how many times that has been missed!
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Old 12 November 2017, 13:49   #3
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Hey thanks for a quick reply

I forgot to mention that my amiga has been tested with a known working psu and with that, the floppy drive worked. I've read now that the floppy drive uses 12V and the rest mostly 5V from the psu (if I got it right).

So perhaps the heavy faulty psu doesn't output 12V correctly..
 
Old 12 November 2017, 14:08   #4
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Yes it's possible, they are getting old and tired now, (bit like me). . If it is the PSU, the good thing is that new replacements are available, the cheaper options are converted PC ATX PSU's. It would be worth you getting one if you intend to use the Amiga regular.
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Old 12 November 2017, 14:21   #5
Lanksu
 
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Heh, that is a good alternative actually, thank you. I'll look into ATX PSUs if I can't get these ones fixed.
 
Old 12 November 2017, 15:37   #6
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There is some bare PSU's made by Meanwell and others that will fit inside the original Amiga PSU case. There's a few users on here that have purchased and fitted these, there is a thread about it somewhere on here if you want to look for it, I'm just off out so can't look at the moment.
These are more expensive than the ATX ones, but they do keep things looking original and neat.
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Old 12 November 2017, 15:56   #7
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Meanwell PSUs are actually cheaper than *decent* ATX PSUs. You can find cheaper ATX PSUs, but I wouldn't trust them with an old PC, let alone a precious Amiga. The PT-65B is ideal for replacing the guts of an A500 PSU, and is available here for £18.32 + VAT and shipping. There's also the MPT-65B which is similar but rated for medical devices, and so is of a higher standard (lower leakage current etc.). That's still only £26.45 + VAT & shipping. If you wanted to do a cleaner job, the crimp connectors for the cabling will add a bit to the price, but the original leads can also be soldered if you're not too worried about that. If you need more electronics stuff, Mouser will ship for free on orders over £33...

You're right, the floppy drive uses 12V, but the logic circuits use 5V and will work fine with a poor 12V supply. Serial, audio and some floppy drives will malfunction without 12V however.
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Old 15 November 2017, 17:46   #8
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Thanks for the suggestions

After browsing some more it seems the consensus on this site is to just replace the innards of these old psus instead of repairing them and worrying about what will break next.

Mean Well PT-65B sounds like a good candidate and I can get two of those + molexes and crimp connectors pretty cheap overall from here.

Now all I'm worried about is figuring out what wire connects to where.
 
Old 15 November 2017, 17:59   #9
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Cool! The wiring will be relatively straightforward, download the datasheet and have it in front of you just to be sure of what you're doing. The slightly more awkward part is securing it inside the case. Some creative use of nylon spacers and brackets might be doable.
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Old 15 November 2017, 18:17   #10
Lanksu
 
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I'm thinking of soldering off the components from the old pcb boards and using them to somehow secure the new psus into the cases, having something in between the old and new pcb so it's solid.

Do you mean that apart from the grounding wire, I do not need to think too much about what wire goes where from the old psu cables? I think the yellow and yellow-green wires are ground? (if looking at the original post's pic)
 
Old 16 November 2017, 00:13   #11
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I never trust wire colours when it comes to things like this - you never know if one particular contractor decided to use different colours. The datasheet for the PT-65B will show you which pin is for which input/output, so use a multimeter to find out which colour in the Amiga's old lead corresponds to which pin:



The shields and mains earths should be connected to the power supply at the mounting hole as shown in the data sheet.

Edit: Actually, if your PSU has the outputs marked on the PCB, then you're probably ok to go with those markings, connecting them to the equivalent on the PT-65B output. On the input side, yes, the green/yellow is earth, the brown (black after the switch) is live and the blue is neutral. These need to be connected as specified on the PT-65B datasheet.

Last edited by Daedalus; 16 November 2017 at 00:20.
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Old 19 November 2017, 12:32   #12
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I did the meanwell conversion after my vampired 600 started to crap out randomly - It had been recapped so i checked my psu voltages and sure enuff the 5v had snook way out of spec. now it runs solid using the meanwell - solid other than when i crash it with my amature asm skills lol
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Old 19 November 2017, 14:00   #13
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Can't give you resistor values as my Amiga PSU's are currently not easy reachable - if picture is not distorted then one of output el capacitors looks like slightly poofed - maybe this was main problem... if your intention is to repair this PSU then try to replace capacitors first and resistors, check also transistors and provide power(voltage) trough some current limiter - ordinary incandescent bulb 75 - 100W can be used as current limiting resistor (in valve tubes era such bulb can be called barretter).
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Old 22 November 2017, 17:59   #14
Lanksu
 
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It's alive!

Hey,

Thanks for the replies.

I've now converted the light psu to using a meanwell pt-65b and it works! Damn I was nervous when turning the power on if I'd made a mistake or not

Now to part two, making the old pcb into a mounting plate.
 
Old 22 November 2017, 20:48   #15
Lanksu
 
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Thanks everyone for your input and advice, especially Daedalus

Here is the end result.
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Old 22 November 2017, 23:25   #16
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nice work, and nice reuse of the old PCB as a mounting platform
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Old 23 November 2017, 00:22   #17
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Welcome, and as dalek says, excellent move with the old PCB - I may well borrow that idea for the future Thanks!
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Old 07 August 2019, 10:37   #18
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Quote:
Originally Posted by Lanksu View Post
Hello,

I've come across a light version of the psu and noticed that two of the resistors have burnt out. The color coding is gone so perhaps someone more knowledgeable with this stuff might be able to determine what resistors I need to fix it?

I also have a heavy psu, it powers my amiga so that it boots up, but the floppy drive doesn't work, no drive head moving sound either.

Thank you in advance for any help

I have the same power supply but works,on the PCB is a little burned is there anyone who knows what value corresponds to those two resistors?
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