Help with Spectravideo 738 VRAM problem...

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By mxmy

Resident (37)

mxmy's picture

08-07-2018, 04:03

So I'm trying to upgrade my Spectravideo 738 to an MSX2, but after having installed sockets where the VRAM was originally and placing some NEC D41464C RAM into the sockets (and adding 0.1uf capacitors in the appropriate places), I'm not getting a glitched screen. The computer itself otherwise appears to work (caps lock is working fine and I hear beeps as I type), but the video display is completely wrong.

Any help is much appreciated, it was working before now Sad

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By mxmy

Resident (37)

mxmy's picture

08-07-2018, 19:41

Apologies, the post above should have said "I'm not getting a glitched screen"

The screen does two glitches, one looks like the image above, the other glitch I believe is is the same but in a different screen mode. I've ordered some 4464 RAM to see if that makes a difference...

By Louthrax

Paragon (1824)

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08-07-2018, 19:53

You could try to dump the video memory content to a file using BASIC's BSAVE command, and then check that file on PC? Of course that's assuming you have a working SD card device of floppy drive. Also, initialize the VRAM contents to a certain value before dumping. You can create your test program on PC in ASCII mode and save it to AUTOEXEC.BAS so that you don't have to type anything on MSX. Identifying the "bad" addresses could give you a hint on the faulty chips...

By Jipe

Paragon (1309)

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08-07-2018, 20:23

check the wire of VRAM , you may have broken tracks by desoldering

By mxmy

Resident (37)

mxmy's picture

09-07-2018, 05:30

Have checked the VRAM tracks and it all looks good. As I'm getting some 4464 RAM over the next few days I'll wait until I've tried the new RAM before I do any further testing. I also added some 0.1uf decoupling ceramic capacitors to the board for the VRAM, but those also seem fine.

By Jipe

Paragon (1309)

Jipe's picture

10-07-2018, 10:42

what is the type of 41464, normally for the VDP it takes 41464-12

By Wild_Penguin

Champion (460)

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11-07-2018, 12:37

Usually the speed does not matter (the part after "-"), if it is fast enough. I think the 120ns is the slowest on the market. Correct me if I'm wrong, on either of my two claims :-)

By mxmy

Resident (37)

mxmy's picture

12-07-2018, 20:38

According to this datasheet 4464 comes also as a 150ns variant, but mine are 12's, so 120ns.

Tried the 4464 ram chips today and the problem came up again, albeit with a different repeating pattern of blocks of white or blue.

HOWEVER... I then decided to pull out the decoupling capacitors and try the 41464 RAM and it WORKED! Then I tried the 4464 RAM and it worked too! To then I placed a capacitor back on the left-hand slot (in C12, without soldering it in) and the glitch occurred once more. tried the capacitor on the other slots and no problems. Tried different types of capacitors at and again the same glitch So it's definitely one of the connections on capacitor C12. I'm pretty sure there's a short somewhere on one of those connections, but I'm not sure where... yet. Any suggestions are most welcome.

By mxmy

Resident (37)

mxmy's picture

13-07-2018, 02:36

Ok great news: My video now works! Found a trace underneath the socket that had somehow managed to get soldered wrong. Have removed the offending short and will install a VRAM socket as soon as I get a new one.

By Grauw

Enlighted (7248)

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13-07-2018, 02:46

Hurray! Big smile

By mxmy

Resident (37)

mxmy's picture

19-07-2018, 08:32


New socket installed, and VRAM is in place! *dancedance*

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