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Just picked up an SX-64 for $20

T

Tiberian Fiend

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Central Florida
Unfortunately, it's of the non-working variety. When I turn it on, the drive light comes on, but there's no picture, and I can't hear the CRT. Could it just be a bad picture tube? How could I tell if the rest of it's working?
 
There's an AV port on the back so you ought to be able to cable up to a monitor to see if a picture is being made. If you have a cable for it, that is.

As always, I would start with the power feeding everything, see if it's getting enough quality juice to make things work.
 
If you turn the brightness all the way up, you should be able to see the screen glowing, even if the computer isn't putting out a video signal. The CRT uses an internal S-video (luma/chroma) video connection, so if you tap into that using an external video source, you could test the display independently of the rest of the computer. The floppy drive controller and cartridge port circuit boards can also be unplugged for troubleshooting in case they are somehow malfunctioning. Complete schematics for the SX-64 are available online if you dig deep enough.
 
NathanAllan said:
There's an AV port on the back so you ought to be able to cable up to a monitor to see if a picture is being made. If you have a cable for it, that is.
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I don't. Is the AV port the DIN port next to the serial port?

As always, I would start with the power feeding everything, see if it's getting enough quality juice to make things work.
Click to expand...
From the outlet or from the power supply?

vwestlife said:
If you turn the brightness all the way up, you should be able to see the screen glowing, even if the computer isn't putting out a video signal.
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Turning the brightness and contrast all the way up and down was the first thing I tried, and nothing.

The CRT uses an internal S-video (luma/chroma) video connection, so if you tap into that using an external video source, you could test the display independently of the rest of the computer.
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This is different from the 4-pin mini-DIN on more modern televisions, I'm assuming?

Edit: I meant to add, is there some way to make the computer make sounds? I guess I could see if it reads from a floppy disk when I tell it to load a program.
 
Yes, the other DIN port next to the serial port is the monitor port.

Not the outlet, from the power supply internally.

Also looked up who told me, it was this:

February 16th, 2007, 12:59 PM
Quickest way to test for sound:

POKE 54296,15:pOKE 54278,240:pOKE 54273,100:pOKE 54276,33
(max volume, full sustain ADSR, medium pitched note, sawtooth waveform)
Click to expand...

Hopefully you can type it all in right without seeing it.
 
Do make sure to fool with the contrast and brightness knobs -- they have a huge range on this machine (much more than any monitor I've seen) and it is easy to get them set so there's nothing visible on the display. And the CRT is actually fairly quiet.

If you have a C64 game cartridge, booting that would probably test the sound pretty easily too.

Let me know if you need more help.
 
I've tried loading and poking to no effect. The lock key turns on and off, though, FWIW. It seems this C= is in a vegetative state, but I guess that leaves more hope than if it was completely dead. I'll have to pull it apart tomorrow when I have the time to see if anything is awry.
 
So I've done some testing, and all of the 12v and 5v lines check out except the line for the I/O board, which I'm getting >0.5v from. It may be a poor connection; I'm having to insert paper clips into the connectors to complete the connection (multimeter probes are far too large), but I've moved them around and tried re-inserting them, and I'm getting the same result. I'm going to take the power supply apart next to check for blown capacitors.
 
I found out that the I/O line is supposed to be 12v A/C, and it's testing at 10v. I'm not sure if that's significant in any way. Also, I noticed some solder might have flowed over the terminals of some components: there's a huge mess of the stuff coming down from what I'm guessing is a rectifier (it's marked REC 2), but it doesn't seem to be affecting anything.

I've read that I'm supposed to be able to see some sort of glowing filaments in the CRT with the cover off, and it's completely dark. If the computer has other problems, fixing the monitor is my first priority. I'm sure that it's getting power from the power supply now, so what else could prevent the tube from working altogether?
 
Usually the heater (the glowing bit of the tube) is fed from a supply derived from the flyback. If the flyback isn't operating (either it's dead or the HOT is dead usually) then there will be no glow from the filament. Do you hear a high pitched whine from the monitor while powered on? Do you feel any static buildup on the front of the picture tube when first turned on/off?
 
I could be wrong about this, but I believe the monitor in the SX-64 is simply AC line powered. Check the schematic and see. Are you sure the supply to the monitor is good?
 
dumb question probably, but have you checked the fuse on the monitor? F3 is right on the input...

Do you have access to an oscilloscope?
 
I'd never turn away advice, even dumb advice, but that wasn't a dumb question. I've found the fuse and it looks burned out. I'll post an update once I have it replaced. Is this fuse 12.5v, or 125v?
 
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