More fun with the Commodore Plus/4

[Compgeke]

Today I found another Plus/4 in box for $15, as-is, untested. I figured might as well take a risk as worse case it doesn't work, best case it does work. Well, after I got it home to test it I found it does not turn on, at all. Checked all connectors, power cord is working, don't see anything wrong.

Testing the original one I bought a while ago I didn't get anything, but after swapping chips around still nothing. After a few power ons and offs I managed to get something, the screen below. After a reset I got nothing again, so I at least know something is working, but what isn't though remains a mystery. Anyone have any ideas? If I'm correct, this here is beginning to load, but failing to. Based on the fact that I see this I know the TED chip at least somewhat works, although I'm wondering if I have a bad ram problem now.

 

[ClassicHasClass]

Hard for me to tell in your photograph, but the colours look approximately correct (should be black characters on white with a lavender border). However, it could still be the TED, since those would only say that the TED's built-in registers appear to work. That said, it could also be RAM. The fact you get the right colours implies the CPU and the ROM are fine.

 

[crock]

Yep. The fact that you get the right colours tells you things are basically working. As you have a number of @ characters where spaces should be, you could try the DRAM for D5 which is U16. As it's every other character, it could also be an addressing issue, in which case I might suspect one of the 74LS257 multiplexers.

Do you have the ability to burn an eprom? If so, burn one of the Diag264 kernal images from here.

It should be able to identify any RAM addressing or device issues pretty quickly.

Rob

 

[billdeg]

From my testing I found that the tell-tale sign of a bad TED chip is a screen full of "unfocused static-y horizontal dashes" with an dark orange pallette colored background.

Your error is a processor or RAM or something other than the TED, if you ask me.

How hard is it to get a TED chip today?

Initiate fantasy flashback music....I checked, only one of the three local Commodore authorized dealers within 10 miles of my house even has TED chips anymore. Mostly they just sell new Amigas and C128D's. I don't think they're going to keep selling the Plus/4s much longer, the C64/128 has more software anyway.

 

[crock]

From my testing I found that the tell-tale sign of a bad TED chip is a screen full of "unfocused static-y horizontal dashes" with an dark orange pallette colored background.

The TED can fail in a number of exciting ways still leave it basically functional but by far the most common scenario is a black screen. This usually means you have h-sync and v-sync, but no video information.

Some of the other scenarios include the failure of the screen blanking and 24-row mode. This means you get garbage rendered to the screen in either of these modes. The other is the failure of the keyboard latch, which due to the way the kernal scans the keyboard, doesn't stop the keyboard working but does stop the joysticks working properly.

How hard is it to get a TED chip today?

They crop up on ebay once a week or so (at least in Europe) for a BIN price of €15-€30. Personally I would take a gamble on buying another non working 264 machine as the CPU's fail just as commonly as the TED's.

Rob