A600 PSU question on resistors.

[toffo]

Hey all,
Just got my A600 today and I'm so thrilled Haven't turned it on at all yet, just checking the bits around it to see what I could do to futureproof it

I'm in the process of cleaning it and it looks quite ok I guess. Caps seems to be in good condition, might already be new? Any ideas?








Anyhow, when I opened up my PSU I saw that there was a two resistors that have been running hot as can be seen on the following pictures. I thought of replacing these with some new ones that have a higher wattage tolerance. I just need to know what value these should be.

When I measure I get ~47 ohm, if someone perhaps have schematics or maybe could measure their resistors I would be really happy.




Let me know if you could help me out here!

Thanks in advance!

[rkauer]

You noticed those resistors are away from the board, right?

That's for a reason: they act as a voltage regulator of sorts. When they get hot they decrease the voltage supplied. Old electronic nasty trick to regulate a PSU.

[kipper2k]

It looks like the capacitors on the motherboard are original with the exception of maybe the can that is laying on its side. Remember, the capacitors don't have to look bad to actually be bad. Most capacitors are rated between 1000 - 3000 hours so in all probability they are old. I'm basing the fact that they are original by looking at the solde jobs on the caps, they look to be factory soldered.

[toffo]

@rkauer
Thanks for the info, I didn't think about that. Still I never like warmed up circuit boards

@kipper2k
Yeah true, I should probably change them as soon as I can
About the factory soldering, it probably isn't the case here, but it actually is quite easy to "fake" that (depending on what solder you use and such). But surely isn't the case here

Thanks for the replies guys!

Edit..
By the way, are there any lists for the caps on the A600 Rev 1.5? Just making it easy to get the new caps. I saw someone selling kits here on the forum I think, but as I can get them from my company I thought I would do so instead

Normally it's only the electrolyte that you change right? or have you guys come up with some other stuff to do as well?

[8bitbubsy]

The bent/long-pins capacitors are probably a result of the previous owner trying to change them, but not getting enough heat to put the pins all the way through the ground plane. It should be fine, leave it like it is.
Regarding changing of the caps, I'd say that the surface mount (SMD) ones are the ones you should pay attention to. They are doomed to leak their blood all over some day, and it will eat what it finds over the years. You should of course change the through-hole ones too, but the SMD ones are way more important.

Also, the mobo looks very good, no major SMD cap leakage... But a lot of dust and dirt!
Tip: Put the motherboard in the shower and turn on hot water and let it get clean. Don't use soap, just hot water and maybe a soft brush (or toothbrush) to remove the dirt that is stuck while it's getting wet. Pay attention to the SMD caps, they are more fragile than they look -- e.g. don't put force on them or else they'll break right off.
It will shine like it was new afteron. Just let it dry for a day or two in a rather warm room.

[Zetr0]

@toffo

A great little machine is the A600 =)

Now, I can confirm that the PSU you have is stock and there has not been any modifications done to it.

Ironcially I have one open on my workshop table and it is identical - even to the floating resistors (3x 47 Ohm metal film 5% tolerance).

As rkauer has stated this is a bit of a dirty trick to regulate a power line (hence the use of metal film resistors)... I will say that looking at yours there has been some serious juice being pulled from it!

In terms of just an A600 I seriously doubt you would need to change anything - if however you were trying to drive an A1200 with an accelerator and a hard-disk then I would suggest finding an alternative supply.

good luck with your hardware the A600 is a bit of an un-sung hero in my humble opinion.

[Daedalus]

Originally Posted by toffo

When I measure I get ~47 ohm, if someone perhaps have schematics or maybe could measure their resistors I would be really happy.

Tut tut, do you not know your colour codes off by heart? Anyway, good find, hope she serves you well. Like others have said, that PSU will do great with the A600, no need to worry about it - it's designed to get warm like that...

[toffo]

Daedalus,
Yeah sure it has been designed like that, but still the components are old, so no harm in changing them right?
No of course not, I used a multimeter is that the name in English? Anyway, a meter :P
.. Oh, did you mean..? hmm.. I could of course check that against the chart yeah, and it seems to be 47.5 ohm 1%, which seems to be ok then I guess. But a close up shows that it has been going quite hot.



Zetr0,
Yeah I loved it back then and I love it right here and now Didn't know it was three, I guess I didn't look around that much, just saw the warmed up circuit board and checked what components were there

I will definitely not use this for anything else than the A600 with perhaps the addition of a CF drive and perhaps memory expansion, but no more than that.

8bitbubsy,
Yeah I know you can clean it like that, but I don't quite like it :P and I don't have a warm room or an hot-air gun available. We've done that with several circuit boards at work, but I can't trust myself with my own stuff :P


All in all, I just want to make sure that the hardware stay alive for some more years and changing the resistors (and the caps inside the A600) shouldn't do any harm, just prolong it's life, wouldn't it?

I mean, it can't do any harm? After all, that's what I've been working with when repairing TV's and such; replacing old and worn-out components. Even if they may look fine also


Also, are there any caps list for the A600 Rev 1.5?

[Daedalus]

Originally Posted by toffo

Daedalus,
Yeah sure it has been designed like that, but still the components are old, so no harm in changing them right?
No of course not, I used a multimeter is that the name in English? Anyway, a meter :P
.. Oh, did you mean..? hmm.. I could of course check that against the chart yeah, and it seems to be 47.5 ohm 1%, which seems to be ok then I guess. But a close up shows that it has been going quite hot.

Well, those colours actually mean 470/10 = 47 Ohm, +/-1%, but it's all good. Yeah, multimeter is the easiest way alright, you just have to be careful about measuring things in-circuit, as the value can be changed by other components in the same circuit, for example, two of those resistors in parallel would make one read about 23 ohms...

It's true, there probably isn't much harm in changing them, as they are a bit old looking, but they do say "if it ain't broke, don't fix it!" Of course, us engineers see that a little differently... "If it ain't broke, it doesn't have enough features yet..."

[8bitbubsy]

Originally Posted by toffo

8bitbubsy,
Yeah I know you can clean it like that, but I don't quite like it :P and I don't have a warm room or an hot-air gun available. We've done that with several circuit boards at work, but I can't trust myself with my own stuff :P

Fine, let your Amiga PCBs drown in dirt and dust until it's so dirty it's conductive (yes, in extreme cases dirt and dust can be conductive).

Originally Posted by toffo

All in all, I just want to make sure that the hardware stay alive for some more years and changing the resistors (and the caps inside the A600) shouldn't do any harm, just prolong it's life, wouldn't it?

I am pretty sure those worn resistors can live some more years, if you really want this machine to stay in pristine condition for more years then man up and clean the PCB and look for dried solder points.

EDIT: Oh, and change the SMD capacitors of course.

[toffo]

Haha, don't take it like that 8bitbubsy!!
I'm of course cleaning it out, but I'm not putting it under water :P In fact it's already cleaned fyi

I for one should know what dirt and dust can do to circuitboards after some time, trust me, we've had a lot of fun with that at work

I will however put it under some retr0bright similar stuff to get it nice and shiny. But washing the circuit under water just aint gonna happen

So please, dont be mad at me

Daedalus
Well measuring the legs on a resistor most often gives the correct numbers. At least while not powered. I also know about parallel and serial connections I just wasn't too sure about replacing with another value.

I like the saying also, haven't heard the second one before

[prowler]

I have found a bookmark which was enclosed with an old Farnell catalogue. It doubles as a chart for resistor and capacitor colour codes.

For those who would like to see it, I attach scans of both sides below.

[toffo]

Nice big scans, much better than the ones I'm using Perhaps I should get around and get a new poster.. But that takes time and.. I'd like to spend time on my Amiga :P

[Hewitson]

Absolutely nothing wrong with washing boards with water, I put PCB's under the tap all the time..

Mind you, I'd be willing to bet the Australian climate is slightly more suitable than Sweden's for drying them out!

[Loedown]

Originally Posted by Hewitson

Absolutely nothing wrong with washing boards with water, I put PCB's under the tap all the time..

Mind you, I'd be willing to bet the Australian climate is slightly more suitable than Sweden's for drying them out!

You live in Melbourne and you're trying to convince people that our climate is more suitable, something about 4 seasons in a day

The resistors have dry joints / crystalline cracking, this is evident in your first shot. They may measure 47.5 R but they will be 47 R, it's a standard resistor value. Make sure you make your replacements stand off the board like the originals and upping the wattage rating won't hurt either, so if they're 0.5W rated go for 1W rating, space isn't a premium.

You may also want to upgrade the interconnect between the two resistors using wire and more solder, better heat resistivity and the joints won't crack so readily again.

[Eamoe]

Originally Posted by Loedown

You may also want to upgrade the interconnect between the two resistors using wire and more solder, better heat resistivity and the joints won't crack so readily again.

Do you mean adding 1 or 2 wires across the extending legs of both? Something like a handmade heatsink?

[Zetr0]

@Eamoe

while a heat-sink isn't a bad idea, fitting them to a set of resistors will prove problematic. Instead what Loedown is proposing is reduce the heat at the contact with the PCB by essentially increasing the track width.

If you notice in Toffo's pictures of the underside of the PCB there is some browning of the fiberglass PCB, this is because there is a lot of voltage being resisted and hence current building up as heat. As a side effect you then get discolouring of the PCB at these contact area's

Looking at the PSU in this case there obviously has been some "pull" on this PSU for some periods of time, I would hazzard a guess that this was probably driving a more power hungry device like an A1200, perhaps with a Hard-disk and memory upgrade.

So what Loedown recommends is to widen the traces at the points that they heat up using good quality wire and solder.

This will increase the contact point - as a consequence it wont heat up as much, which as Loedown also points out reduces the risk dry solder joints and cracking / fissures created from too much heat.

hope that helps explain it.

[Eamoe]

Does. Thanks Zetr0!

[Loedown]

Originally Posted by Zetr0

@Eamoe

while a heat-sink isn't a bad idea, fitting them to a set of resistors will prove problematic. Instead what Loedown is proposing is reduce the heat at the contact with the PCB by essentially increasing the track width.

If you notice in Toffo's pictures of the underside of the PCB there is some browning of the fiberglass PCB, this is because there is a lot of voltage being resisted and hence current building up as heat. As a side effect you then get discolouring of the PCB at these contact area's

Looking at the PSU in this case there obviously has been some "pull" on this PSU for some periods of time, I would hazzard a guess that this was probably driving a more power hungry device like an A1200, perhaps with a Hard-disk and memory upgrade.

So what Loedown recommends is to widen the traces at the points that they heat up using good quality wire and solder.

This will increase the contact point - as a consequence it wont heat up as much, which as Loedown also points out reduces the risk dry solder joints and cracking / fissures created from too much heat.

hope that helps explain it.

You're now my personal PA and translation agent