Any Advice on common breakdown Problems on CPC464 with Colour Monitor?

[Puresox]

I have had my Amstrad CPC464 with colour monitor from new, it was bought prior to the offer of the free games pack 84 xmas maybe?
Unfortunately in about 1995 I tried to turn the thing on and no screen came up?
The Keyboard LED works but No power at the monitor!
Amend that No LED works !

Can anyone advise on common faults or any tests that I can carry out to diagnose fault Please

[ralferoo]

If the keyboard LED doesn't come on and you're sure the monitor is supplying 5v, then it's either a faulty power switch, a dry joint on the power connector, or the connector from the motherboard to the tape unit or something shorted somewhere either on the tape unit or the motherboard. The power LED is right next to the power switch (part of the tape unit), so baring a short circuit it should be easy to track down.

[Gryzor]

Second that dry joint on the power switch, quite common.

[Puresox]

I have just stripped down my monitor again to see if I can get to the bottom of this problem, Now I have done a few basic fault finding techniques and , think i may have found a faulty component , I think it is a resistor , but not the normal looking type , it is R501 and is like a white block (Details on it TOPOHM 5S5.6OhmJ), I have desoldered it and have ohms tested and cannot get any readings on it. Hopefully I will be able to get a replacement from maplin or somewhere similar. I am not an electronics buff so, I am not entirely sure that I am on the right lines. Anyone with any know how on this stuff?
Damn I'll be impressed with myself if I have cracked this.

[Bryce]

I assume that with "cannot get any readings on it" means that it's open circuit and not a short circuit. If R501 is blown then the monitor was showing absolutely no signs of life? This is the main power feed to the monitor.
The good news is that this is a very easy and cheap part to acquire: 5W AXIAL 5,6 - 5Watt Drahtwiderstand, Serie 208-8, 5,6 Ohm bei reichelt
The bad news is that if this has blown there was a major short-circuit inside the monitor at some time. My advice would be to order a new one, but before you replace it, give the monitor board a good visual check that nobody dropped something metal onto the PCB that might still be there and check that no parts look burnt and that no capacitors have popped their lid (bulge on the top of any large capacitors). If you can't find anything, measure the resistance across the pins of C505 (with R501 still removed) and let me know what you get.
If all is ok then I'd install the new resistor and turn the monitor on (without the CPC connected and at a safe distance from the monitor). If the resistor fails again immediately (you'll know this has happened, it will probably be a loud bang), then the monitor has other serious problems.

Bryce.

[Puresox]

Hey thanks for the tech info, I haven't managed to sort a resistor yet , but hope too later today all being well. Interesting about short circuit, I can not recall it going with a bang years ago , but I will give the thing a good dust over and take your advice into account.

[Puresox]

I have sourced a component but it is not exactly the same it is 7w 5.6ohmj , will this be suitable?

[IanS]

Higher wattage should be fine. As long as it is the same resistance value and you can physically get it to fit.

[Puresox]

right, managed to replace it , cleaned everything up a little , braced myself and turned on the monitor , saw a little flash in the back . Took it apart and pcb fuse has blown , so it is a case of finding the short circuit now.

[Puresox]

Shit I forgot to measure the resistance of that capacitor, shall I desolder the resistor and test it?


I have desoldered resistor and tested C505 and get 2.5ohms on it. Does this give you any idea's

[Puresox]

Need to work out what components are most likely to fail to a short circuit. it would be great to get this old boy working again.

[Bryce]

The capacitor was just the easiest place to measure, it wasn't where I thought the short circuit would be. It could be anywhere on the PCB, but 2R5 is a serious short circuit. It will be difficult to find the offending part if there are no physical signs.

Bryce.

[Puresox]

Right that's it , It's going away again back into storage for another few years , till I find the will to have another dabble, At least I found one component which was at fault , maybe I'll pick up some more knowledge a long the way. I really want to understand how to fault find on PCB's better. I know the basics, to look for cold soldered joints , Swollen capacitors  and a few other techniques.

[Bryce]

Unfortunately it wasn't a fault you found, rather the result of a fault. The short circuit killed the resistor, it didn't fail naturally. If all the capacitors look good and you can't spot any burnt resistors or diodes, then it's most likely that one of the ICs has catastrophically failed. Normally the failed IC would get hot and reveal the problem, but in your case the short is so severe that it doesn't draw current long enough to get hot before something else fails. However when this happens you can sometimes see dis-colouring or even a crack in the IC casing. Check them closely, but if nothing's visible you'll probably have to start removing parts to find the culprit.

Bryce.

[Puresox]

I know that bud , I do know that much , but I at least tracked down a component which has given me a further step  along the track to sorting the problem out , I obviously know that isn't the fault cos it hasn't fixed it.without me finding that out, I wouln't know there was a short on the board.

[Puresox]

I need to get a schematic , and at least identify what components there are to make up the board. once I know what each component is then systematically test them in the manner that they need testing , but I thing the likely thing I shall do is get hold of another monitor and compare results.

[IanS]

I need to get a schematic

http://www.cpcwiki.eu/index.php/File:CTM640_Schematic.png

[Bryce]

I need to get a schematic , and at least identify what components there are to make up the board. once I know what each component is then systematically test them in the manner that they need testing , but I thing the likely thing I shall do is get hold of another monitor and compare results.

Oops, sorry Puresox, I was kind of assuming that you had and were referring to the schematic all along.

Bryce.

[Puresox]

Neigh problemo!