Friday, June 20, 2014

Amiga Restoration and Repair (Part 2)

The story so far...

My Amiga is no longer yellow but there are nasty things going on inside that case.

Green corrosion on the capacitor contacts
The capacitors in these machines are now over 20 years old and I don't think anyone expected that they would still be being used by anyone.  Failure of these components is inevitable and it is a bit of a lottery as to the effect when they do go.

Symptoms caused by capacitor failure include audio problems, video problems, floppy drive issues, just about anything that could go wrong could be blamed on the capacitors.  Fortunately I caught this relatively early.  Even better, because this is a well known issue with Amiga computers, particularly those with surface mounted components, kits are readily available that contain all the correct values and ratings for the A1200 (and A4000 if you're lucky enough to have one of those).

I ordered my kit from (where else?) for the bargain basement price of £6.95 + vat (+p&p).  This may seem a bit steep for 18 capacitors but if you've ever tried to navigate sites like Farnell or RS Components when all you want is a few bits then 7 quid is really a bargain.  The kit arrived a few days later and I was ready to begin the surgery.

First job was to remove the old capacitors.  I am not that experienced with surface mount stuff and so I did a bit of research (i.e. Google and YouTube) and decided that there are some crazy people in the world.  One particular Aussie guy demonstrated on YouTube how to remove the surface mount caps from his ancient games console (I forget which one) with a pair of pliers and a stiff flick of the wrist.  It looked a great way to rip the pads and tracks off the substrate of the motherboard and so I decided against this method.

The best way was to add extra solder to the pads of the caps and then using two irons, hit both pads for a few seconds and then flick the cap away from the board.  It was at this point that I discovered what people meant by the 'fishy' smell if the caps had failed.  Trying to remove C321 (pictured above) the smell was brutal.  Rotting fish mixed with week old rubbish and cat sick.  Fortunately it dissipated quite quickly but was not very nice at all.

I should say that several key things were needed to do this:

Solder (lead free with flux)
Solder wick (to clean the pads once the caps had been removed)
Two soldering irons (as mentioned above)
Tweezers (those caps are small!)
Lots of time to do it properly...

I almost had a disaster with C321 which was the first cap I tried to remove.  The edge of one of the pads lifted off the board.  It was only about a quarter of the pad so I think I got away with it.  Any more and I would have had to go and get some epoxy to stick it back down which is a very, very fiddly job.  Pretty much all of the caps came off easily and using the solder wick was straightforward to clean up the pads and remove the excess old solder.

Work started - no going back!
At this point I've taken off about a third of the caps. If you're planning on doing this yourself I would really suggest you practice on an old board that has SMD components on it before you start.  I had an old modem which after a few hours of heavy handed practice will never be the same again...

Also, take off the through-hole caps first.  It makes it a lot easier getting to the SMD caps in the big gaggle near the PCMCIA slot.  These through hole components are surprisingly tricky to get off though.  It needed a few goes to get enough heat into the solder to pull them off the board.  One cap leg in particular is soldered to a big earth plane (a big silver bit of the board in any case) which meant that all the heat went there instead of into the solder leading to several heart stopping attempts to get the thing off.

Capacitor  behind the keyboard connector
Here's another pain in the rear.  There is a capacitor right behind the plastic keyboard connector.  And the contacts are perpendicular to it meaning that you need to be a contortionist to get to it.  It's not too bad getting the old one off but this was probably the hardest one to get back on!

A Slight Diversion..

During my Googling about this I came across an interesting YouTube video from a guy in Sweden.  He had a fix for an audio fault on Rev 1A motherboards.  The eagle eyed of you may have seen that the board I have is an early 1A, so early in fact that there are NO clock pins on the board.

The issue is that the sound is really distorted when it is played back through a TV.  Think of a bad mood teenager bedroom with the stereo at full volume.  To be honest, I had noticed this problem but had assumed it was a fault with the Amiga itself rather than a design fault. But a design flaw it is. 

Op-amp with 152 resistors
In this photo there is an op-amp chip (labelled LF347M) which drives the audio output.  The gain of the op-amp is controlled by the resistor labelled '152' which means it is 1500 Ohms or 1.5k Ohms (there are actually two of them - one left channel, one right channel).  This value is too high and means that the gain of the op-amp results in clipping of the sound even at low TV volumes i.e. the Amiga audio output level is just too darn high.

The solution to this is to replace these resistors with a lower value which reduces the gain (it's the way op-amps work - google it!).  A value of 680 Ohms was suggested and proven to work by RetroGameModz on YouTube; the video is well worth a look if you're into retro gaming or computer repair. 
HOW Small??

To give you some idea of how small these things actually are, here's a photo next to a standard size paper clip..

The resistor is size 1206 imperial (approx 3.2mm x 1.6mm).  A good pair of tweezers is recommended.  In fact I  just dropped that resistor into the carpet, never to be seen again...

Anyway, see the picture below for the results of my resistor replacement....

New 681 (680 Ohm) Resistors
With my resistors successfully installed, it was time to return to the capacitors.  Except, there's a problem...

(To be continued....)

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