Commodore 64 – Basic ROM, CIA replacement and Recap

This Commodore 64 is turning on and outputting a display but is not booting as expected. Instead of the usual 38911 basic bytes free message, it’s simply showing a blank screen with no cursor.

Inside the board was partially socketed but the Basic and Kernal ROMs were not so I started by socketing those as its helpful for whoever may own this board next and I suspect the Basic ROM may be gone here as that would cause this issue.

With the board socketed I start by switching out the Basic ROM and this somewhat fixes the problem. I now have it booting but there is still no cursor. That points to the CIA 6526 chips, they are responsible for handling keyboard, joystick, cursor, cassette and all manner of input/output functionality. Turns out its U1 and I don’t even need to test U2. I have the proper boot screen complete with flashing cursor. Just for good measure I load up Flimbo’s quest to test the joystick, keyboard, cartridge slot and sound. Everything works perfectly. Job done! Well, not quite…

This is a 250407 board and they have a lot (and I mean A LOT!) of electrolytic capacitors. I always like to replace the capcitors if they havent been done already, and this board definitely has not. A short while later and the board is completely recapped:

I’m going to clean up this board a little with some isopropyl alcohol and add heatsinks to the SID, CPU and PLA before using it this weekend to play some Sam’s Journey to ensure its running fine.

Amstrad CPC464 – switching to 4164 RAM

I got this poor Amstrad sold as seen for a very low price, and I can see why. It’s a total mess, filthy on the outside (greasy too eugh!), even dirtier on the inside and it won’t boot. The board looks like its been hacked at too, with a missing 4464 ram and a socket where it should be. The tape drive looks completely knackered with the reset counter not working and the record button seems twisted and broken.

The first thing to notice was that the on/off switch is broken and only very rarely allows the machine to turn on. Shorting the contacts on the switch with my tweezers brings the machine to life, albeit with a broken blue bordered boot screen:

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This board has 8 slots along the bottom for 64kx1 RAM (4164) and 8 missing capacitors, so rather than waste my time hacking at the 64kx4(41464) area I desoldered the remaining RAM and used my lovely new Antek 760RW hot air rework station to add sockets for the 64kx1 RAM and added some Oki chips in them, and some ceramic 100nf capacitors int he empty spaces below. I guessed the capacitor value by matching the two ceramics near the old RAM.

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Honestly I wasn’t expecting much at all. More than anything I just wanted to play with my new toy as I hadn’t had a chance, and I was just thinking this is a potentially scrap board so why not give it a try. But when I shorted the pins:

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It actually fixed the issue! Keyboard is working great too. Sometime in the future I’ll give this a huge cleanup, recap, change the power switch and have a go at the cassette drive repair. But for now I’m chalking this down as an unexpected win.

 

3x Commodore 64 repair

Today three Commodore 64 breadbins arrived and all need a little repair work. Turns out not that much.

Computer #1

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This 64 is in pretty good shape and booted perfectly although the LED did not come on. I like to do a quick initial test by loading a demo from my Ultimate II cartridge, it tests the serial interface and the cartridge slot as well as a few keyboard and joystick functions in the menus. The demo tests the VIC and SID chip quite extensively and will usually show issues pretty quickly. My demo of choice is Comaland by Censor Design and Oxyron.

This 64 has no sound, potentially a SID chip is required, annoying as its the most expensive part but hey ho. Time to open it up and take a look.

So the LED not working is obvious, this machine has been dismantled before (to be fair after 30 years in the wild it’s hard to find one that hasn’t!) and the LED connector not put back correctly. Easy fix!

The insides dont look too bad and it looks all original judging by the date codes on the chips.  I can’t be sure but the caps look like they might have been replaced as they don’t match. I’ll know for sure when I take the board out to clean, which I’ll probably do tomorrow as I want to get all three of these machines tested today.

SID chips are almost always socketed, so I decided to test the chip from Computer #2 to see if it works. And it does, I’ll close this up until cleaning time. One down!

 

Computer #2

This breadbin is in pretty dirty outside and in, just look at that dust! It’s also now missing a SID chip thanks to computer #1. Turning this machine on produces no picture at all, not even a black screen. First place that I  check and hey presto there’s the problem, the internal fuse is blown, after replacing it the machine springs into life.

After quickly “borrowing” the original SID from computer #1 and booting the Comaland demo, I manage to verify the computer seems to be running great now.  The keyboard seems a little unresponsive and probably needs cleaning, trying a known working keyboard verifies that its not the board

I’ll have to source another 6581 SID at some point to get this fully running. For now it’s time to close up until cleaning time. Two down!

Computer #3

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This is in the best condition of the three breadbins. The case is very clean and the both it and the keyboard have almost no visible yellowing. It’s nice to see well looked after machines like this! Turning the machine on produces only a black screen. Time to open it up:

Wow. The inside is incredibly clean too, this is machine has had some love. All the big chips have been socketed too, which is great because I want to try replacing the PLA. With a black screen I’d normally try the VIC chip first as its socketed and the rest usually aren’t, however with the PLA being socketed I may as well try that first. The PLA on the breadbin models is notorious for breaking and black screen on boot-up is one symptom.

It’s really my lucky day. The new PLA works a treat and the machine boots up perfectly. In fact the picture quality is amazing, probably the clearest picture I’ve seen yet without a Lumafix or Reloaded board. Testing Comaland once again the VIC and SID chip behave as expected and I close the machine up until cleaning time. Three down!

All in all its been a great night for repairs, three broken Commodore 64s and they all were fixed incredibly easily. In fact I didnt even need to turn on the soldering iron. Next step is cleaning them properly, replacing the capacitors, finding an extra 6581 SID for computer #2 and adding internal voltage savers. But that’s one for another post, another time!

Commodore VIC-20 video issue + recap

This VIC-20 has a weird switch connected to it and internally has a speaker attached. The switch seems to be some kind of s-video mod but the video output is messed up:

 

With old computers like this it’s a good idea to replace the capacitors so I give that a try first as the computer does seem to be putting out a signal. Replaced the big caps on the motherboard and also the caps in the RF modulator.

And what do you know this seems to have done the trick. I guess one of the RF modulator caps must have been bad. At some point soon I’m going to remove the switch mod but leave the internal speaker if possible. Also this case is in dire need of some Retrobrite love!

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Sinclair ZX Spectrum 48k keyboard repair and composite mod

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This spectrum 48k seems to work fine except for the 6,7,8,9 and 0 keys. This is almost certainly going to be the keyboard membrane and I just happen to have a spare one (newly manufactured membranes available from sintech.co.uk). While I’ve got it open I’ll do the well-known composite mod to give a better picture by modifying the RF output to give a composite signal instead.

The composite mod is very easy to perform and only requires basic soldering skills. First I disconnect the resistor from the output connector shown circled in the picture above. Next desolder the two wires from the PCB connected to the RF modulator on the left side (also circled above). Once all these are disconnected I cover with heat shrink-wrap so it can easily be restored at a later date.

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The next step is to connect the top most hole on the where I disconnected the wire, to the output connector inside the modulator. This can be done with a simple piece of wire but supposedly a better picture can be achieved by soldering a 100uf capacitor between them (positive side to the PCB), so this is what I’ve always done:

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With the composite mod complete I move on to the keyboard. First I need to  confirm that all keys work from the motherboard side using a piece of wire to connect pins from KB1 to pins on KB2 using a lookup I found on Google to test the 6,7,8,9 and 0 keys. ALl keys work this way including the offending numbers. This means I need to replace the membrane.

Using a heat gun on a low setting so as not to melt the rubber keys I gently heat the metal faceplate to melt the glue a little and allow me to free up the faceplate from the case. With this removed it’s an easy task to switch membranes, I take the opportunity to clean the rubber keys while I’ve got it open with some isopropyl alcohol and an old toothbrush. The second picture above shows the old (top) and new (bottom) membrane, the old membranes get so brittle after 30 years they break incredibly easy and it’s almost always going to be this when you have keyboard issues.

After putting everything back together using a hot glue gun to re-adhere the faceplate. I plug in and turn on the computer and try to type the problematic numbers again and it’s a success!! Look at that picture quality over composite, well worth the little bit of effort.

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Sinclair ZX Spectrum +2A RAM repair (eventually!)

I’ve had this Spectrum +2A for some time but it never booted correctly. It took me a while to even get power to it as I didn’t have an original PSU, in the end I made a makeshift adapter for my Amiga PSU that converted from the square Amiga plug to the DIN plug the +2A requires. I’m beginning to hate systems that require negative voltage or an AC supply, makes my life more difficult.

The system would either boot with a blank screen as shown above or with random vertical bars of various colors. This usually points to a RAM fault but with no spare 64k 4bit ram chips available nor any socketed in my other systems I couldn’t immediately test this theory. After ordering a new supply I got to work blindly replacing and testing what I could:

  • The Z80 cpu is always socketed so easy to test this and I have a few of those, but alas no luck here.
  • Next up I replaced the 74HCU04 and the TEA2000 chips socketing both for good measure, no luck here either.
  • I didn’t think capacitors would be causing a problem here but replaced them all anyway as its good practice on these older systems.

In all honesty I knew it was going to be the RAM as the chips looked slightly charred suggesting they had overheated at some point:

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Sure enough when the new chips arrived today and I replaced them, I was greeted with the familiar +2A boot screen. Loading up a game from tape worked successfully if a bit noisily (need to oil the cassette mechanics next). Another machine to add to the collection yay!

Texas Instruments TI 99/4A keyboard repair hack

I received a TI 99/4A with a keyboard issue, it booted fine but the keyboard was not responding. Mashing the buttons a while would eventually press the required “any key” to bring up the options, this seems to imply there’s not a whole lot wrong. Time to open it up.

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This thing is ridiculous, I’ve never seen so many screws to get into a machine. Everything inside is fitted to the top of the case rather than the bottom so once you open the case (7 screws) you then have to remove the power supply board, mother board and keyboard (9 screws). Even then to get at the actual motherboard there’s another 3 bolts holding the heat shielding together!!

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So many screws!

 

Most retro home computers like the TI 99/4A use a simple matrix of buttons to connect the keyboard to the motherboard. It’s an easy task to find the pinout with some Googling and use this to test the keyboard header on the motherboard by shorting two pins at a time. Doing so with this motherboard showed that every key worked and therefore the fault must lie within the keyboard itself.

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TI 99/4A keyboard header.

To open up the keyboard another 15(!!) screws need to be removed and the alpha lock desoldered. This allowed me to test the membrane without the keys. The membrane worked correctly, meaning the keys mustn’t be exerting enough force on it to activate the key presses.

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After puzzling for a while over a solution I came up with a relatively elegant one! Using a hot glue gun I created a small blob on each key pad on the membrane, once dried the glue becomes very solid and creates a perfect dome on each area meaning the keys have less effort to press the membrane. Testing with the keys this appeared to mostly work, some keys still wouldn’t work. These keys I discovered seemed to not require the glue domes and so I removed the domes on all the non working keys. Voila! Keyboard fully functional!

I have to admit my glue gun was a relatively recent purchase but it has proved to be invaluable for so many wildly different reasons, this is definitely another one I didn’t foresee.