(Topic ID: 211624)

Gottlieb System 3 vertical lines on display

By arolden

6 years ago


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  • 14 posts
  • 3 Pinsiders participating
  • Latest reply 5 years ago by arolden
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#2 6 years ago

Hi,

U3 is the DSPROM, it's a 512 KB - 27C020.
As the board boot, you can be sure that the GAL U8 is OK (as it decode the address map for RAM/ROM/VIDEO, if it's burned, nothing work ! but it's not the case).
The RAM (U4 - 6264 - 8KB), is shared between the video chip (U2 - 68B45) and the cpu (U1 - 65C02). It serve as general purpose memory, and video memory.
To access the RAM, an "address switch" is done by U9/U10/U1 (74LS157).

When there is row or column missing, most of the time the culprit is the RAM (U4).
The video chip, 68B45 can also be involved (default in addressing the RAM) or the switches (U9/U10/U1). But this is quite rare, check before for the RAM, it's easy to replace by a new one. Unfortunately, it's not mounted on socket.

#5 6 years ago
Quoted from arolden:

Any idea why they are different and why they are not 6264s?

It's simple, it's just a matter of procurement by the suppliers.
The manufacturer of the board will put the components he has on hand, according to his stocks. The origin and the brand may vary according to the wholesale prices, and also according to the availability on the market.

Reference 6264 is a generic to denote a CMOS static memory of 64 Kbits (8 KB).
Each suppliers uses his own reference. There may be differences, but they are always minor and mainly concern the power consumption.

Access time does not matter. -10 means 100 ns, -15 means 150 ns, and -80 mean 80 ns.
As, on the video board, the 65C02 and 68B45 are clocked at 3.579 / 2 = 1.8 Mhz, even a "slow" memory at 150 ns will fit nice.

Most of the time, they use reference 6264: HM6264 from Hitachi, CY6264 from Cypress, MK6264 from ST, HY6264 from Hyundai, U6264 from ZMD. But some are well known for using slightly more exotic references : MB8464 from Fujitsu, TC5564 from Toshiba. And sometimes, they are very far from this reference : LH5168 from Sharp, GM76C88 from Goldstar
But all are compatible and can be interchanged.

1 month later
#10 5 years ago

OK, looking at your pictures and as you replace the U4 RAM (considered as safe), the issue should be in the U13 area.

On the pictures, you can notice that 1 pixel on 8 in permanently lighted.
The first 7 pixels are ok, and the last 8th is always lighted.

On the video board, the datas are addressed by the 6845 CRTC from the memory (this chip don't care about the data itself, and is used only for addressing the video RAM). Basically, the datas are serialized/clocked and sent to the display using the RDATA/RCLK signals. The data to be transmitted to the display are selected by U13. This chip is a "data multiplexer" and select one bit among eight bits. The address is given by MA0..MA2 and the source is D0..D7 from the memory:
2018-04-26_12h08_29 (resized).png2018-04-26_12h08_29 (resized).png
The "Y" output reflect one of the eight entries (D0..D7) selected by the 3 bits selectors (S0..S2, aka MA0..MA2). Note also that, there is a pullup resistor (R7), so if an input is not wired, the default value at Y is "1"... so a lighted pixel. But on your board, R7 is not mounted (this resistor is not mandatory).

As there is one pixel permanently lighted, the issue is that at a time, the "data bit" is not transmitted but set to "1". As the first 7 bits are OK, and the last 8th is always 1, the culprit line should be D7 (or D0 - I'm not sure of the direction !). This may come from:
- a deffective RAM, but as you change U4, this option should be rejected
- a deffective U13, with a burned entry (D0 or D7
- a broken line at U13 inputs (D0..D7)
The problem could also be from the selection entries (S0..S2), either from deffective U13 or a bad adressing from the 6845, but I doubt for this option, because the ouput will be not always to "1".

So, I'm quite sure that the issue is on the D0/D7 lines, at U13 inputs.

Check with an ohmmeter, the continuity of the D0 (and D7) lines :
- check between pin 1 of U13 and pin 11 of U4 (D0)
- check between pin 8 of U13 and pin 19 of U4 (D7)
If none of thoses lines are broken, this mean that the culprit is the U13 itself, and thus, replace the 74HCT354.

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