Diary of a Black Screen Assy 250403 REV D

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mrr19121970
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Diary of a Black Screen Assy 250403 REV D

Post by mrr19121970 »

I was given an assy 250403 REV D 'for spares' as it wasn't working, only black screen with feint vertical lines in pairs on the screen. All the chips were soldered directly on the board. No chips got hot, I forgot to try blind disk commands, no cartridge worked I had +5v on the reset line. Today I decided to take the challenge.

The fuse was missing, I thought easy fix. T3.15a fuse in. Didn't help.

1st off the board was the mostly overlooked UB4 7406 Hex Invertor which tested ok in the IC tester. Added socket.
2nd off the board was the UC12 901488-07 KERNEL ROM, surprisingly tested OK on a donor board. Added socket.

now I was scratching my head. What can it be?

3rd off the board were UAB1 & UAB3 CIA and set to the side, still black screen. Now I thought oh, I forgot the blind disk commands...
....so the UB7 6561-101 came off the board and tested OK on a donor board

Time to consult Ray Carlson

UC4/UC5 74LS138 MEMORY/BLOCK CONTROL and UD8/UE8 74LS245 TRI-STATE LOGIC could cause a black screen too. All 4 tested fine in the IC tester.

What now? I didn't have much left on the board:

UB6 NE555 Timer : I have +5v on RESET, must be OK
UB9 M53202P MASTER CLOCK OSCILATOR : I have vertical lines
UC2 74LS02/UC3 74LS04 LOGIC : Screen colour failure only
UC6 74LS138 I/O : not related
UD1 CD4066 QUAD SWITCH : Splits colour RAM
UD7 CHAR ROM : can't cause black screen?
UD9 74LS133 : I have no idea what this does
UE1 TMM314APL-3 COLOUR RAM : can't cause blank screen ?
UE10 6502 : would cause no screen, not black screen ?
UE11 BASIC ROM : can't cause black screen?
UF8 74LS245 : I have no idea what this does

This leaves only 4 lucky ICs?

U14/U15 HM6116P-4
UD2/UE2 TMM314APL-3

I decided the 2x 2114s were most likely to be broken over the 6116. One tested BAD in the colour RAM slot of a donor board and the 2nd was OK.

Now I need to socket the remaining 10 ICs removed and test again. Low and behold the famous start up screen appears again. Another VIC20 saved from the scrap heap.

Here are my questions:

The fuse is clearly only for the 9vAC, as even without it the starup screen appears. Why is this ? What is the 9vAC even needed for? A C64 has no clock and no picture without 9vAC.
Could I have got to the UD2/UE2 faster/better/smarter?
Any tips?
Kakemoms
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Re: Diary of a Black Screen Assy 250403 REV D

Post by Kakemoms »

Well, the 9V AC isn't used inside the machine (unless you have a 2-pin power connector), only 5V DC is. You do have to have 9V AC on the cassette port for (many) tape drives to work.

2114 do tend to fail, but mostly on older C equipment. Ray's guide states memory block (as you pointed oit), so RAM would be a part of that (not only the 245 buffers).
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Re: Diary of a Black Screen Assy 250403 REV D

Post by Overdoc »

On a black screen machine, I first check if anything is getting hot ?
If everything runs cool, then it helps a lot to try a ram-test kernal instead of the original kernal. It runs completely independent of any ram (uses only the CPU's registers A,X,Y). If a ram chip is dead, but not shorted (if shorted, it will get hot), then chances are high that the ram test kernal will display a picture.
The ram test kernal writes all characters into the first page of each of the 5kB.
So, when you look at the first page at $0000, then you can identify which of the two 2114 rams is dead when you see wrong characters displayed (just check which bit or bits are wrong)
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mrr19121970
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Re: Diary of a Black Screen Assy 250403 REV D

Post by mrr19121970 »

Where can i get this kernel?
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Re: Diary of a Black Screen Assy 250403 REV D

Post by Overdoc »

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eslapion
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Re: Diary of a Black Screen Assy 250403 REV D

Post by eslapion »

Kakemoms wrote:You do have to have 9V AC on the cassette port for (many) tape drives to work.
I don't think so.

The schematics of all VIC-20s I am familiar with contain a power transistor which perform a crude 6Vdc regulation from the unregulated rectified 9Vac.

If you feed DC instead of AC then the power capacitor will still charge to a level high enough for the power transistor to do it's job.
mrr19121970 wrote:The fuse is clearly only for the 9vAC, as even without it the starup screen appears. Why is this ? What is the 9vAC even needed for?
The 9Vac is fed to the user port as some special devices require non-standard voltages. Modems and the Promenade C1 require that.

The 9vac is also rectified in the VIC-20 to create a higher DC voltage for the datasette motor BUT you could also feed DC for this particular usage and it would be just as good. 8Vdc is good enough.
Be normal.
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Re: Diary of a Black Screen Assy 250403 REV D

Post by mrr19121970 »

I was just trying out in VICE. However xvic.exe won't start with this KERNEL
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