Do not be afraid and use common sense.

That is my only qualifications and they are enough fix many problems.

I did read a course in analog circuit technology in school almost 20 years ago but I slept through the whole thing and I have never worked with electronics on a circuit level. You will learn more from reading forums like Vogons and by watching Youtube videos.

When it comes to tools a DMM and a good soldering station are the two most important things. I have also bought a cheap desoldering station to make it easier to desolder stuff.

Thanks skyscraper, It's encouraging to know it comes naturally to some!

As for common sense and not being afraid...some guidance on how to decipher the hieroglyphs that is PSU power ratings would be marvellous, as it's all Greek to me!

When it comes to PSUs P(W) = I(A) x U(V) is the basic formula. Its derived from Ohms law, U=I*R. I=U/R. R=U/I. and P=I^2*R. I=P/U. R=U^2/P. U=P/I.

If an AT PSU is rated at 25A on the 5V rail you can draw 25A(I) x 5V(U) = 125W(P) from the 5V rail.

...if you say so. I'll have to go and run that formula myself and so how it works.

This is an example though - you mention rails. What are rails? And if I want to access the 5V rail, how would I make an appliance do that?

gladders wrote:

...if you say so. I'll have to go and run that formula myself and so how it works.

This is an example though - you mention rails. What are rails? And if I want to access the 5V rail, how would I make an appliance do that?

Most PSUs only have one rail (circuit) for every voltage. The 5V rail is normally all red cables, the 12V rail is yellow cables and ground is black cables.

Okay. So.

As an example: my new 8086 I got has a mains supply rating of "100-240V ~ 2.1A" Therefore, with Ohms law, that's 2.1A x 240V = 504W. Is that right?

gladders wrote:

Okay. So.

As an example: my new 8086 I got has a mains supply rating of "100-240V ~ 2.1A" Therefore, with Ohms law, that's 2.1A x 240V = 504W. Is that right?

That is the mains (AC) "power in" part.

For computers the "power out" (DC) part is what is interesting. The relation between power in and power out wattage is the efficiency of a PSU and the efficiency was really bad 30 years ago. The rated MAX mains (AC) power draw rating is also never representative of the actual power draw during full load as the rating needs to account for short spikes in the PSUs mains power draw.

A dirt cheap copy of Scott Mueller's book "repairing and upgrading PC's" from amazon or ebay or local second hand book shop might be a big help for you. Older edition the better I expect. It has a LOT of info regarding everything about this hobby. I only have the 12th edition, that takes you up to Athlon and PIII comfortably, think mine was £3 shipped.

gladders wrote:

Okay. So.

As an example: my new 8086 I got has a mains supply rating of "100-240V ~ 2.1A" Therefore, with Ohms law, that's 2.1A x 240V = 504W. Is that right?

Not in this case. Because it works at anything between 100VAC and 240VAC, it is a switching power supply. The 2.1A is the maximum rated current it will ever draw during normal operation.

The power consumption of the switching power supply does not depend (much) on the mains supply voltage, so power is (about) constant even if mains supply varies.

So, at 100VAC mains, it can draw up to 2.1A * 100VAC = 210W.
If you connect it to 240VAC mains, it draws much less current to get 210W, namely, 210W / 240VAC = 0.875A.

I approximate the PSU is rated to provide output power at somewhere between 150 and 200 watts.

A counterexample, if it would consume 504 watts of mains power, at 100VAC mains it would require 5.04 amps, but it only says 2.1A on the PSU, so it can't be a 504 watt supply.

BSA Starfire wrote:

A dirt cheap copy of Scott Mueller's book "repairing and upgrading PC's" from amazon or ebay or local second hand book shop might be a big help for you. Older edition the better I expect. It has a LOT of info regarding everything about this hobby. I only have the 12th edition, that takes you up to Athlon and PIII comfortably, think mine was £3 shipped.

Great idea! I'll look out for it.

Okay, so take two:

The "rated output" is +12V 0.5A+5V 2A-12V 0.1A-5V 0.1A

So, um...which is it? Is it all of them? Or does it depend on special kinds of cables?

gladders wrote:

Okay, so take two:

The "rated output" is +12V 0.5A+5V 2A-12V 0.1A-5V 0.1A

So, um...which is it? Is it all of them? Or does it depend on special kinds of cables?

In this case you can probably just add the max wattage of the different rails to find out the total max DC wattage but that isnt always the case.

12V x 0.5A = 6W on the 12V rail sounds very low to me as does 5V x 2A = 10W on the 5V rail. Take a picture of the sticker on the PSU and post it here. 😀

since you want to do pcb repair, a nice triple combo of solder station, hot air gun and desolder gun is good.. I have hakko 888 (analogue dial, but I think all current ones are digital), hakko 808 for desoldering, makes through hole desolder super quick and easy, and cheap noname hot air gun from ebay.

then you get into test gear acquisition syndrome and start getting scopes, and real logic analyzers, frequency counters and next thing you know you have 10 hand held multimeters and 5 bench meters and a wife telling you not to buy more stuff!

understanding this stuff goes to another level tho, 5v, 3.3v, cmos, ttl etc.

gladders wrote:

Okay, so take two:

The "rated output" is +12V 0.5A+5V 2A-12V 0.1A-5V 0.1A

So, um...which is it? Is it all of them? Or does it depend on special kinds of cables?

It lists the maximum current for each voltage.

But, that sounds pretty low.
I understand this is for an 8086 computer, but if you count those that's only about 14 watts.

For a 230W AT power supply, the ratings would be more like this:
12V: max 9A = 108W
5V: max 23A = 115W
-12V: max 0.5A = 6W
-5V: max 0.5A = 2.5W

Edit: whoops fixed the watts in above table.

gladders wrote:

So some of you may notice I'm relatively new and many of my posts are probably embarrassingly elementary to you all. I love this hobby, I love computers, but I am pretty ignorant of electronics, which is kind of a big part of this. As bad as being baffled by power ratings on the back of PCs and too scared to plug in a cable for fear of destroying old machinery!

What tools, books, training courses, would you recommend I do, read, embark upon, to get myself up to speed with this type of work? Ideally I'd like to become reasonably competent with understanding appliance power ratings, PCB repair, and soldering things like clock batteries and chips.

I live in London, for reference.

Start, by reading some basic books about electronic's. Then you could look for stuff like solder-techniques that this guy is using.
https://www.youtube.com/user/EEVblog I know he is an professional, so you just have to notice on how he is soldering.

There are a lot of stuff out there..... As an example (some might cost money, some might be free) you can look for stuff on
google books. I have taken the liberty to do a search for you.

https://www.google.dk/?gws_rd=cr,ssl&ei=sDXnV … +repair&tbm=bks

Hope all this will be somehow usefull for you. Cheer's

EDIT:
Forgot to mention that most of us are autodidact on all these thing's. Not shure if if "autodidact" is the right word.
All I am saying is, that most have learnt on our own, and had help here on Vogons.
Shure here are a lot of people who have learned by taking cources and learnt in school's.
Just stay here on Vogons, and absorbe tiny bit's of information, read some books or PDF's and learn electronics
in a nice and steady pace. You will eventually, in time, learn what you need to know.

brostenen wrote:

most of us are autodidact on all these thing's

Probably that's true. You need to learn some basics, then just mess with old broken hardware for a year or two and you should be able to do a lot of things.
You have to improvise a lot and use common sense when repairing old electronics with no spare components available.

Here are three examples of what I did, those things are not done professionally by any means, but there are no information on repairing certain parts in service manuals, often they want you to replace entire component - but it may be impossible because there is no new stock.

Re: Success repairing 286 battery leak - repairing PCB
I fixed 5.25" Mitsumi floppy disk drive - repairing floppy drive (tricky one)
Re: Unidentified Slot 1 mainboard + onboard sound problems - permanently disabling (broken) onboard sound card

And reading datasheets of broken components could be helpful - for example, when you have broken traces on PCB and you are confused and don't know where you should solder some jumper wires, it can be also helpful if you have some rare OEM expansion card with no information about jumper settings - you can look up the datasheet of the main component and try to figure the settings by yourself.

Thanks all, good advice! I'll do my best to learn from you all, as long as you don't mind a raft of potentially silly questions on my part!

I've learned quite a few things from this YT channel: https://www.youtube.com/user/Afrotechmods

gladders wrote:

Thanks all, good advice! I'll do my best to learn from you all, as long as you don't mind a raft of potentially silly questions on my part!

Well.... As the saying goes: There are no silly questions, only stupid answers.

Unless of course. The person wich are sitting in the booth, collecting the road tax fee, asks "What will it be". 😁 😉