Tuesday, June 21, 2016

A1200 Capacitor Trouble (again)....

So on the A1200 that I repaired the other week, I noticed a very slight flicker in the video output. It wasn't much but it was noticeable. It looked like the colour 'green' would disappear for a split second and then it would come back.


I took the motherboard out and had a good look over the topside. Nothing out of the ordinary but I think that it definitely needs to be re-capped. In any case, there's nothing obviously wrong on the top.

Turn it over and look at the back. My attention is immediately drawn to this:


Holy crap. Another failed capacitor... This is a general power de-coupling cap that has clearly failed in a fairly spectacular way. I'm surprised I didn't see this when I did the previous repair but, to be honest, it could've happened after I did that.

Anyway, as it happens, I have a few of these value SMD caps in my box so I just replaced it:

Having put everything together, I plugged the A1200 back in and switched it on. This is what I got:

Now, I may not be an expert electronics engineer but this does not look like an improvement.

I dug the board out again and got the USB microscope on it for another survey, this time I was looking for ANYTHING that might look out of the ordinary. It didn't take long...

These two resistors have been replaced by someone:

361? 360 ohms? Are you sure?
According to the A1200 schematics for Revision 1 type boards, these resistors should be 2000 ohms, not 360 ohms.

R217 is very badly soldered and looks like it has been messed with:

 Bad Solder Joints

R345 has been replaced:

681? 680 ohms? Are you sure?
According to the A1200 schematics, R345 should be 22,000 ohms. Yes, that's right, someone has replaced a 22K ohm resistor with a 680 ohm resistor. What the hell? If anyone has any idea why somebody would do this, feel free to comment below.

 This one (and another nearby - close to the op-amps for the audio) have also been replaced:

Correct - but REALLY badly soldered..
This last one (and it's pair) are part of a known fix for the audio on these earlier models. The original schematics had these at 1.5k ohms but that value is too high for the op-amps and the sound comes out distorted. 681 - or 680 ohms is the correct value. Soldering standard sucks though..

Finally, this:

This little group of three 1206 size components are capacitors. They should all be exactly the same as they are part of the de-coupling circuits on the video encoder, with one capacitor each on the Red, Green and Blue inputs. So why are there three different colours and why are they all soldered differently? Bottom one looks like factory, second one up is a reasonable manual attempt and top one has way too much solder on the right side. Could this be the cause of the video problem?

Fortunately, these are all 22nF which just so happen to be the only value of SMD caps that I have (and was also the same as the one I replaced at the top of page!). So, out came the soldering iron again. And now we have this:

More new shiny...

And that completely cured the video problem. Rock solid. I left Frontier playing its demo for nearly six hours and no problems. Another great result.

Just waiting for the 'real' caps to arrive so I can do a complete job on this A1200 now. :)

Wednesday, June 15, 2016

The Scarlet Amiga - Part 2

Sooooo. The Scarlet Amiga is working. It's a KS1.3 early(ish) A500 with a Samsung keyboard, a tidied up A501 half meg memory expansion and, following a lot of careful calibration, a working floppy drive. But we still have that awful sloppy red paint to deal with.

Following a few test scratches, the paint has two or three distinct layers. On the top is the red oil based gloss paint. I suspect that this is the same as some might use to paint skirting boards. Underneath that is a softer white-ish layer that I think may be some sort of spray paint, presumably used as an undercoat. In some areas there is also a dark green 'smudge' of what might be paint but could equally be wax crayon...

I googled 'remove paint from plastic'. One suggestion was to use vegetable oil on it, which should lift the paint from the plastic without causing any damage. I did actually try this as it would be non-destructive and if it worked would have been very cheap.

It didn't work. It just made the case oily.

Next up, isopropyl alcohol. This was a long shot as it's more a de-greaser than a paint remover.

That didn't work either. It just made the case shiny.

Next. Nail varnish remover (acetone basically). Mrs Crashed has (or had) a bottle of purple nail varnish remover which, to be honest, was a bit stronger than I anticipated. I put some across the top part of the case, near the AMIGA logo and within 10 seconds the paint had shrivelled and cracked. A quick scrape and a hug swathe of red paint was gone. BUT, where the acetone touched the bare plastic, the plastic started to melt and deform (she puts this stuff on her finger nails??) so, despite the promising results, I didn't continue with this.

By now we have a case that looks like this:

And so it starts...

More work has been done here - mostly around the inside of the keyboard

While trying to pick out the Amiga logo my hand slipped and, despite my best efforts, I failed to injure myself but I did put a large scratch in the paint. It was immediately apparent that large areas of the paint could be scraped off relatively easily using a handy scalpel with a flat square blade. This is the result of a couple of nights of scraping:

Getting there....

A smaller blade allowed me to get into the vents to some degree and soon, almost the entire top was back to an almost normal colour. A few more nights scraping and we've reached this point.

Vents waiting to be cleared

Almost done...

Fun fact. Do you know how many small ridges there are on the back of an Amiga A500 case? Seventy five. SEVENTY FIVE. S E V E N T Y  F I V E. Each one had to be scraped out by hand. 

It took over a week to get all of the solid red paint out of those ridges. If I'm honest, had I started by doing these ridges first it would have gone in the bin and I would've been searching ebay for a new case. But that would be cheating. :)

Red Gloss GONE
I realised some time ago that this case is not going to be suitable for selling. Even if I'd managed ti get all the paint off with no scraping or melting plastic, it was still in relatively poor condition.  Anyway, after all the effort I have put in I have decided that I will keep it for myself. 

There is still a lot to do on this case before I consider it finished. I intend to sand it down to remove the last bits of white/red paint and then I shall look at polishing it. This means it will lose it's customary patterned finish but, what the hell. It's my Amiga. :)

More updates soon..

Tuesday, June 14, 2016

The Scarlet Amiga

I saw this Amiga on eBay. I could not let it go. I had to save it...

A package arrived. Cautiously, I unwrapped it and, miracle of miracles, it had survived the journey with MyHermes. Good show!

A Mysterious Package

Bubble Wrap Goodness

Who can see what's not quite right?

First things first. Does it have a trapdoor expansion? Yes, an A501 still in it's shield. This almost certainly means the battery is still attached. This has to be dealt with first. Batteries that are in equipment this old will leak and they are not known for containing nice neutral compounds that are safe on PCBs and components. They contain chemicals that destroy PCBs so I have to get inside this and quick.

A501 With Shield (and Evil Battery)

I tried to de-solder the shield properly but my iron could not get hot enough given that the whole shielding acts like a huge heat sink. I resorted to cutting it off. Sure enough, the battery was still there and there was some corrosion. It wasn't too bad though, and a quick splash of vinegar (no lemon juice available) and a gentle scrape with a screwdriver got rid of the green stuff.

Battery Removed & Corrosion Cleaned

The Affected Area - I've seen worse...

Next, clean the 'slightly' grubby keyboard. This thing was filthy, with a capital 'fetch me Kim and Aggie!'. It looked like it had been under somebody's bed for a good number of years given the amount of dust and fluff and other crap that caked the whole thing. I first used a small screwdriver to scoop off the biggest bits of fluff (nice) then prised off each key carefully.


Number Pad & Cursor Keys Removed

It doesn't get much better with the rest of the keys removed...

By now I had realised that this is an unusual type of keyboard. Most A500s have a Mitsumi manufactured keyboard but this one is actually different. It does not have the usual spring under each key - which are very easy to lose - rather the spring is under the plunger of each key. Also, instead of a single membrane connector this is the rather unusual double membrane type. The keys are also a very subtly different shape, being a bit more angular at the edges (and they are also made of thinner plastic - especially the space bar). As I understand it, this type of keyboard was manufactured by Samsung.

Normal A500 Keyboard by Mitsumi

Samsung Keyboard - note the double membrane connectors

This Samsung keyboard makes me far happier than it should do. It may not be that rare but it is unusual and so it makes me happy. :)

Each key was washed in warm, soapy water then left to air-dry thoroughly before being put back onto the keyboard. This is not my favourite activity, after all there are over 100 keys on these things... It also gave me the chance to put a few things right. Firstly, the 'Tab' key was upside down. Also, the bottom row of keys were in the wrong order. Instead of the usual 'zxcvbnm' they were arranged as 'zvcxbnm' so someone had clearly been messing around with this Amiga.

Keys. Lots of them.

All back together and CLEAN!

At this point, I'm ready to try and switch this thing on. Trapdoor expansion? Check. Keyboard? Check. Floppy drive? Check. Hideously painted top case hidden out of view? Check.

And....... red screen. ROM fault. Bugger.

I have a spare 1.3 ROM so I thought that this would be an easy fix. It turns out that it was even easier than that. The previous user had obviously been inside the case and, for reasons unknown, had taken out the ROM chip. And then put it back. The. Wrong. Way. Round. A quick switcheroo and try again.

Success! We have a Kickstart 1.3 boot screen. Now to try and boot workbench. But then the next problem rears its ugly head. The workbench disk is not recognised by the floppy drive. This IS a big problem. This symptom normally means that the heads need to be aligned which is no easy task. It requires that the screws holding the heads (or head motor - depending on drive model) need to be loosened and the heads moved by a fraction of a fraction and then the screws tightened again before testing to see if there's any difference. Repeat until the end of time...

Inside the Floppy Drive

Adjustment screws either side of the main motor

Fortunately, a chap called Thomas Richter (who is still active on the Amiga forums) created a clever program to assist with this. What happens is that a blank disk is written with a series of basic sound tones across several tracks with the highest pitch being track zero. Between the tracks is just noise (or silence). The small utility then reads the disk and outputs the sound based on the location of the heads on the disk. While the disk is spinning the heads are adjusted until the tone from track 0 can be heard and then locked into place. It sounds great in principle but does it work in practise?

Yes, is the short answer. The longer answer is that it took a lot of patience and a fair bit of too-ing and fro-ing but, ultimately, I got the floppy in this Amiga to read disks. It took about an hour or so to get it 'just right' but, at the end of the day, it was time well spent. The drive now reads all disks including the Workbench, all original games and even some floppies that last saw the light day about 15 years ago.

So that's the basics covered. Now, what the hell am I going to do with that awful paint job?