Timex 2068

[harryjumper]

Hello,

I have a Timex Computer 2068 (black) with a strange behavior.

When connected with the cartridge Spectrum emulator, it turns on normally, loads games, and the keyboard works.

Without the emulator, the initial copyright screen appears, but after a few seconds, it starts drawing black and green vertical lines (photo attached).

Anyone familiar with this problem? Could this be a ROM issue?

When I run a diagnostic card, no problem is detected, including the ROM passes the test.

[XavSnap]

Hi,

Try to put a cardboard or piece of paper (1mm thickness and electrical insulator) in the ROM edge connector, and restart the computer.

If the computer run the BASIC ROM, you had to change the femal edge connector on the mother board.
Two pins seem folded, and are shortcuted in the connector when the ROM card is out off the rack.

[harryjumper]

Hi,

Try to put a cardboard or piece of paper (1mm thickness and electrical insulator) in the ROM edge connector, and restart the computer.

If the computer run the BASIC ROM, you had to change the femal edge connector on the mother board.
Two pins seem folded, and are shortcuted in the connector when the ROM card is out off the rack.

Thanks for the answer XavSnap.

I've tried your suggestion with the paper, but the problem remains. The pins also look OK, not touching each other.

Could this be the internal Timex ROM that is failing? Do you or anyone else recommend an EPROM burner?

The computer doesn't produce sound when loading or running a game with the emulator, but that might be a separate issue.

Thanks.

[XavSnap]

This screen seem to be a faulty RAM, but the diagnostic ROM is ok.
Try to check the RAM power supply, and change the faulty capacitors if there are any.
Check the BASIC ROM pins if on chip support.(unplug and replug it)
Check the RAM chips too.

Schematic:
https://www.timexsinclair.com/wp-conten ... ematic.jpg

[XavSnap]

On the Spectrum emulator, try to feed the RAM with a DIM.

10 DIM A$(15500)
RUN

And try to test upper values.to get the "Out of memory" error, to test the upper memory.

[1024MAK]

Some questions:

Have you carefully inspected the board using a good light and a magnifying glass? You are looking for any poor, dry or otherwise bad solder joints. Or any contamination.

Also look for any damage to PCB tracks or causes of short circuits such as solder splashes (the fine hair like strands). Or damage to any components.

As Xav says, if any of the chips are in sockets, carefully remove them (ask if you haven't done this before, don't just try to pull them out for you risk bent or broken pins). Inspect both the socket contacts and the pins of the chip(s).

Mask ROM chips are some of the most reliable (apart from some used in Commodore machines) chips used in 1980s computers. These are the type without a window and which have manufacturers custom part numbers. These typically work fully or fail completely. That doesn't mean that failure doesn't happen, but it's not a common problem.

DRAM chip failure, especially partial failures are however rather common. I'm not familiar with the 2068. Is this a USA or a European model? What type of DRAM chips are fitted in your board?

How long have you left the diagnostic card running tests for? Some DRAM faults can be intermittent.

Mark

[harryjumper]

This screen seem to be a faulty RAM, but the diagnostic ROM is ok.
Try to check the RAM power supply, and change the faulty capacitors if there are any.
Check the BASIC ROM pins if on chip support.(unplug and replug it)
Check the RAM chips too.

Schematic:
https://www.timexsinclair.com/wp-conten ... ematic.jpg

Thanks for the schematics, but they appear to be for a different version. Can't find the capacitors. Maybe the schematics are from the US Timex model?

On the Spectrum emulator, try to feed the RAM with a DIM.

10 DIM A$(15500)
RUN

And try to test upper values.to get the "Out of memory" error, to test the upper memory.

I've run the test and the result was "0 OK, 10:1"
I'm not familiar with how to test further the RAM with upper values.

Some questions:

Have you carefully inspected the board using a good light and a magnifying glass? You are looking for any poor, dry or otherwise bad solder joints. Or any contamination.

Also look for any damage to PCB tracks or causes of short circuits such as solder splashes (the fine hair like strands). Or damage to any components.

Doesn't seem to be any damage or splashes in the PCB. I will have to look closer.

As Xav says, if any of the chips are in sockets, carefully remove them (ask if you haven't done this before, don't just try to pull them out for you risk bent or broken pins). Inspect both the socket contacts and the pins of the chip(s).

Unfortunately, all chips are soldered directly, no sockets. Doesn't seem any intervention was done in this board. It all looks stock.

Mask ROM chips are some of the most reliable (apart from some used in Commodore machines) chips used in 1980s computers. These are the type without a window and which have manufacturers custom part numbers. These typically work fully or fail completely. That doesn't mean that failure doesn't happen, but it's not a common problem.

Attach a picture of the ROMs.

This is the European 2068 model. DRAM chips are 4416 (picture attached).

How long have you left the diagnostic card running tests for? Some DRAM faults can be intermittent.

I've done a soak test with the diagnostics card with 55 interactions. No error detected.

Some conclusions about the computer's behavior:
- FAIL with internal timex ROM (vertical lines and keyboard doesn't respond)
- OK with external emulator ROM (games, kb, etc)
- OK with external diagnostic card ROM (kb, tests, etc.)

The RAM is ok when the internal ROM is not in use. The computer only fails when the internal ROM is required.

I found a similar behavior on this spanish site, where the computer didn't run with the internal ROM, but worked fine with the external cartridge ROM. Problem fixed by replacing both internal ROMs.