Help with Repairing an SX-64

[John Cook]

I've had this old Commodore SX-64 in my closet for a while. I decided to drag it out last night and poke around. It would boot up after several tries, but eventually the screen would fill up with garbage and it would stop working.

 

After surfing around and reading this I decided the problem was likely the 906114 PLA. I scavenged one from an old "regular" C64 (desoldering sucks by the way) and put it in place of the one in the SX-64 which is, mercifully, socketed. It took a few on/off attempts, but finally I got the familiar 38,911 bytes free screen. Yay!

 

But I noticed right away that the keyboard was erratic. Several of the keys took many attempts to work and sometimes they'd print double. Then after a few minutes the screen garbled again and I was back at square one. Boo! Since then I haven't been able to get it to work at all beyond some funky colors and garbage on the screen. I even tested with a cartridge in place, but it didn't change anything.

 

I suspect now that it may be a bad RAM chip or two. But these are not socketed which makes the work a lot slower. So I wanted to check with you gurus first to see if this was the most likely problem.

 

I made a quick video of me trying to boot it up several times and threw that

.

 

Any ideas?

 

Thanks,

John

[mamejay]

Hi there. Not sure if this has anything to do with it but I recall years ago my C64 would not start up. i have to restart the thing over and over again.

A mate of mine who was fairly tech savvy had a look and tested the power brick and told me that that was on the way out. That is why it would work sometimes and not others.

 

Check out the power voltages and see if this could be the problem.

[A.J. Franzman]

C-64s are notorious for power supply failures. Also, the most common designs of the power "brick" are epoxy-potted and considered by most folks as non-repairable. If, on the other hand, you happen to have one of the power bricks that is put together with screws, and you can actually see all of the electronic components when you open it, do not discard it when it goes bad -- that's the model that is easily repairable.

 

However, I suspect that the "portable" (*cough* *wheeze*) SX-64 doesn't use the same scheme as the regular C-64; it should have an internal power supply, with a regular AC line cord going in the back, correct? In that instance, the power supply(/ies) will be inside the case somewhere. But should still be easily repairable, IMO it's very unlikely that Commodore would have epoxy-potted something inside that case.

[Rybags]

Keyboard + screen problems might mean one or both of the CIAs are faulty.

 

They're the same chip, so you could swap places and see if the problem changes.

[John Cook]

Keyboard + screen problems might mean one or both of the CIAs are faulty.

 

They're the same chip, so you could swap places and see if the problem changes.

Ah, thanks for the tip. Going back and reading, I see that I missed those symptoms. I'll give that a shot tonight. If that doesn't do it, on to the power supply.

 

Thanks again!

 

John

[John Cook]

I decided to go for broke and desoldered both CIA chips from the sacrificial C64. Swapped those in and........ no change. I so thought that was the problem.

 

Somewhere up in my attic I have a working SX-64 squirreled away. I'm gonna go find that and perform a power supply swap to see if that's the problem. I'm not sure if I'd prefer that or the RAM being the problem.

 

Thanks,

John

[R.Cade]

I've repaired a lot of those over the years and I can't recall a defective SX power supply or RAM failure, which are very common on the regular desktop models. Surprising (from Commodore) but they seem very rugged.

 

For whatever reason, usually the PLA and one or more of the ROM chips are the fault on the SX. You've swapped the PLA with a known good one (retest to be sure) so try swapping ROMs. The kernal is the most common failure, and can be swapped for the C64 ROM, you'll just get the blue colors instead of green/black SX. Watch the pin alignment, as it's a 28-pin on the SX and only 24 on the standard C64.

 

Other than that you can make sure that the boards are seated together 100%- it's a 90 degree pin/socket setup from the mainboard to the power/drive boards and that being loose can cause your symptom as well. Push on the board on the right-hand side until you can't see the pins to make sure it's tight. Good luck!

Edited July 27, 2010 by R.Cade

[Video]

I hope you get your system fixed. I'd love to get an SX myself, I'd probably sport that every day at the place I get on the internet, playing Mule between bouts of posting or something

[nathanallan]

The resemblance of this thread and the one at vintage-computer is uncanny. Are you the same person?

 

If not, please refer to the thread there also for reference. Fixing things can be fascinating, educational and frustrating all at the same time.

[channelmaniac]

The resemblance of this thread and the one at vintage-computer is uncanny. Are you the same person?

 

If not, please refer to the thread there also for reference. Fixing things can be fascinating, educational and frustrating all at the same time.

 

The keyboard problems on the SX-64 won't be CIA related, instead it will be dirty keys which is a common thing on those.

 

Disassemble the keyboard and clean the gold/copper contacts on the PC board itself with a pink pencil eraser (NOTHING more abrasive than that!!!) then clean with alcohol. Reassemble and they keyboard will work well once again.

 

Just beware that the keyboard is a real PITA to take apart to clean.

 

Have you looked down inside the cartridge slot to see if there's junk shorting out the contacts? That's a common issue.

 

Also, before you declare bad RAM chips, try swapping out the 3 ROMs for the ones in the old C64 you have. You could have a bad Kernal ROM, etc...

 

Do carts play in the system OK? If so, look at the BASIC ROM.

[John Cook]

Thanks for all the tips. Work has been a little nuts this week so I had to put my project on hold for a few days. I'm going to try swapping the ROMs tonight.

 

Heh, I assure you that I'm a different person than the one on vingate-computer.com. Apparently I'm not alone in having a bad SX.

 

I've tried the cartridge trick, I get the same behavior. That port looks pretty clean, but I might lug this out to the air compressor if the ROMs don't work.

 

John

[John Cook]

I got the "good" SX out of the attic and the keyboard was no longer working. Took it apart and it was just that the I/O board had worked itself loose from the CPU board -- all is good with that one again.

 

So this is interesting... the good SX has a kernel ROM that's a real chip. The bad SX's kernel ROM is clearly an EEPROM with a sticker over the UV window. Not sure how unusual this is, so I took a pic:

 

 

Anyway, back to work...

[A.J. Franzman]

That's EPROM, not EEPROM. EEPROMs don't need windows; "Electrically Erasable Programmable Read-Only Memory".

Edited July 30, 2010 by A.J. Franzman

[nathanallan]

OMG these acronyms make me wanna pmho (pull my hair out) sometimes!

[John Cook]

Right - EPROM. I would have said that correctly in 1986 or so but I've been in software too long now.

 

I finally got it working. The problem turned out to be the original PLA chip I replaced -- kudos to R.Cade who suggested I re-check that 3 days ago. I must have fried the one from the breadbox C64 when I desoldered it (or maybe it had died since I last fired it up? nah, it was probably me).

 

Also after Googling around I found that an EPROM kernel on an SX-64 is not too unusual.

 

Anyway thanks again for all your help! Now I just gotta scrounge around for another PLA. And a handle because one of these SX's is missing it. And I gotta clean these dirty, dirty keyboards.

Edited July 30, 2010 by John Cook

[nathanallan]

Pictures of it working or it never happened

 

Glad its working, these old machines need love sometimes.

[John Cook]

[saffron1]

Hi All,

I have diagnosed my SX 64 and it has a faulty Kernal Rom.

Can anyone help me get a replacement.

Cheers