modelrail.otenko Trains, computers, vintage, retro, model railways… the lot…

Commodore 64C Set – Refurbishment

It's been while since I last had one of these units. A member of an FB group mentioned they had a C64 + Floppy Disks up for grabs, but only offered for someone to come and collect it. It was a 40 minute drive for me, and a good price, so I went for it. I'd asked for pictures whilst negotiating, but none were provided... so I had no idea what I was in for.

Turns out I was in for a great surprise! Included was a Commodore 64c with Commodore 1802 PAL Monitor. Also included was a 1541-II Disk Drive (with a box of disks!) and a Star NX-1000C Dot Matrix printer. I gave it all a cursory once-over and then applied power... but as you can see from the shot above, the happiness was short-lived.

Commodore 1802 Monitor

This unit was only producing a flat squished pattern. The image could be made-out in the squished pattern on the screen, but there was no dial to adjust vertical height... so something had to be going wrong inside. I spooled up What broke on this Commodore 1802 monitor? from Adrian's Digital Basement and left it playing whilst I started dismantling and reviewing the monitor's guts.

Hilariously, we both pulled out the same loose capacitor at the same time... couldn't believe it.

It was an absolute mess where that capacitor was. Its juices had wrecked a few resistor legs as well.

Anyway, the problematic capacitor was replaced and the vertical picture issue was fixed!

Unfortunately, there was no colour. Every site online pointed to a dead R225 (some places incorrectly mention this as R255), and so I looked at R225 on this board...

Crusty! I had no 1w 5.1Kohm resistors in stock, so I paralleled four 22kohm 0.25w resistors.

Did it work? Not really.. I started getting weird colour bars from the composite (ERROR!, this was an assumption) output of the video cable from the C64C.

So I went on a capacitor rampage and replaced everything on the board. Note that the photo below is whilst I was half-way through... trying to clean the gunk off.

This didn't fix it, so I reviewed the data on the Commodore Monitor Information Site. Unfortunately, the Commodore 1802 (tall) was the NTSC version and there was no PDF for my PAL version. Regardless, the schematics sort-of lined up... I just had issues trusting the resistors? Let's have a closer look at that (supposed) 5.1Kohm:

That resistor above has the visible colour-bands of brown red orange gold? It's meant to be a 5.1kohm? Using Element 14's Resistor Calc, I tapped in the colours I could see and was told it's actually a 12kohm resistor? Double? What gives? I then looked at R329 as seen in the photo above-above... it's meant to be a 560ohm resistor, but the colours are showing green-poo-poo-gold? Brown? Red? No idea... but that second colour is meant to be Blue! So wait... these colours have actually cooked so much that their pigment has changed?

I went back to Jaycar and bought 1w resistors that I could actually use to do proper serial maths on... instead of the parallel maths above...

560ohm was a 1:1 install... 5.1k was 1.8k + 3.3k.

Still a crap picture from the C64. A good lesson here is that you should be absolutely sure that your video signal source works... don't just assume that 40 year-old tech can produce a proper composite signal. For this reason, I tried my Playstation 2:

Oh nup, it works... it's me. I'm stupid. Every time I've written composite above, I need to be punished.

That Momement You Realise You're Stupid

This unit was built in 1987. It's a complete set. It plugs together. Why would the unit have a DIN-8 to RCA cable with Yellow+Red+White plugs if they didn't expect you to plug them into the matching sockets on the monitor?

I had assumed all along that they were stereo sound... because... 1995 called and that was the standard. Of course, that's an incorrect assumption as this machine pre-dates any such standards. So... what happens when you plug in the correct colour plugs into the correct colour sockets and switch to the "SEP" mode (i.e. separated Chroma and Luma, as per the plug names)?

LOL. Didn't I feel a little silly? Turns out a C64 DIN-8 Video cable has Yellow=Luma, Red=Chroma and White=Mono-Audio.

Floppy Drive

I tested the voltages out of the power supply, finding that the 5v line was down near 3.2v. Plugging it in and turning on the drive just saw the disk spinning and all lights on.

I found the nearest spare power supply with the relevant 12 and 5v and wired it in. It worked perfectly!

On the original drive power cable, red was 12v, green was 5v and black was ground... but always test, test and re-test before plugging anything together!

Star NX-1000C

Supposedly this thing is colour. I powered it on and it emitted an annoying beep at first. Turns out it just wanted paper. To test, you just need to hold down the online button as you power it on...

Well I'll be... it just worked. It even printed from GEOS. Time to find some tractor-feed dot-matrix printer paper! A quick google tells me that wont be cheap!

GEOS

Rummaging around in the random box of floppy disks proved to be fruitful. There was a GEOS Applications disk, but no base boot disk. I randomly selected a blank disk with no label from the box and tried LOAD "*",8,1. Would you believe it? GEOS booted!

Unfortunately, the desktop wasn't grey... instead it was a hideous shade of smeared pink. All images I could find on the net showed that GEOS should have a light grey background, similar to a macintosh boot. I did find an eBay auction that had similar issues as mine.

I thought it may be chroma/luma, so I rigged up a composite video cable and tested it:

Seems I might need a LumaFix for this... it's on order. Of course, it might just be how it's meant to be displayed.

S-Video Conversions

I nearly fell for the same fallacy again... make sure you can trust your inputs! The 1802 monitor has S-Video style inputs, but it's using RCA plugs. This makes it hard to both connect something else to this 1802 and connect the C64 to another S-Video display. So, what to do? Build an adapter...

Hideous, right? I read here that I needed a 300ohm resistor inline in the Chroma, so that was wedged in. I then whipped out my trusty Sony PVM LCD and ... well, the C64 looked stunning.

The jailbars are there, so I'm looking forward to receivng the LumaFix... but even without it, this monitor goes very well with the C64! So, with the adapter in hand, I flipped the game and routed my Playstation2 through S-Video and into the C64 1802 Monitor.

Shite. So. What's going on here? I know I twiddled all of the potentiometers inside the monitor to oblivion... but those colours are an entire primary-colour off. Just in case you're wondering...

Right behind the 'external' contrast potentiometer is the "sub-brightness" control. "Tint" is R507, next to the grey-box mid-picture. Anyway, something was off, and I had a hunch! I loaded up Sim-City to check how green the grass was and it proved to be grey! The entire green channel was missing? After an hour of multi-metering... I accidently knocked Q555 on the board that plugs into the back of the picture tube and green came back!

Yup, that's Q555, sitting proud at the top of the PCB. I must've bumped it during capacitation and broken the solder joints. A quick re-solder and ....

It's glorious!