SX-64 – Blank screen repair

Patient: Commodore SX-64 (serial numbers on the casing and on the internal do not match 🙁 )
Symptoms: “white” screen and takes no input or squeaks at: load “$”,8
Diagnosis: Patient is brain dead
Documentation: Original SX-64 Portable Computer Schematics by Commodore, part no. 314001-05

Diagnostics: CPU board transplant and the patient is alive again, so somewhere in the CPU board we got some headache!

1. First thing i did is swapping CPU board and the SX-64 boots again. So we have to narrow it down.

2. Exchanged all the chips from the working board to the non-working board, still no go.

3. Replaced capacitors C17 and C21 , both 10 µF 25v 85°C, as they seem a little buffed, still no go.

4. After contact with Ray Carlsen i understood anything can he be case at this point, so i started reading on his SX-64 Repair section on Ray Carlsen’s home page.

5. Now we need to get serious. So together with Benadski from RevSpace.NL we performed open heart surgery on the SX-64 Executive Computer.

Wait! First we need to measure to make a more educated guess! This is where the Club Mate comes in handy.

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Now Ray Calrsen’s schematics, the original SX-64 Portable Computer Schematics by Commodore, part no. 314001-05 and not the least the Ray Carlsen’s diagnostics instructions (local mirror) comes in handy.

We used a oscilloscope to determine the basic functions like voltage and clock settings. Eventually after suspecting the 74LS139 we decided to desolder this Decoder/Demultiplexer and place a socket on the board. After exchanging the IC the patient was still brain death.

So back to the drawing board: exchanging all chips again (as the rest of the board seems to be in a working shape). We found the following ICs a defective;

  • UD3 – 251104 – Kernal
  • UD4 – 901226-01 – BASIC ROM
  • UD7 – 6510 – CPU
  • UE5 – 6581 – SID (replaced by 6581R3)
  • UE4 – 906114-01 – PLA

After finding working replacement ICs I felt like the 15 year old boy again when I got the C64 for my birthday. 😉

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Never mind the errors on the diagnostics as there is no additional diagnosis hardware attached, the machine is working OK. 😉

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SX-64 Reset modification

While opening one of my SX-64 executive machines i happen to notice this reset modification. Actually i already suspected some modification was done as the reset button on the front, which normally only does a IEC device reset, acts also as a system reset. The J3 connector is disconnected from the P22 “Reset” pins from the IEC board and direct connected to the ‘blue and grey’ modification cable. It works perfectly!

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Turbo Chameleon 64

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